DEGREE PROJECT IN MATHEMATICS, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Emulation of Recoil in Pyrotechnic Countermeasure Dispenser System
EBBA LINDGREN
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ENGINEERING SCIENCES
Emulation of Recoil in Pyrotechnic Countermeasure Dispenser System
EBBA LINDGREN
Degree Projects in Systems Engineering (30 ECTS credits) Master’s Programme in Aerospace Engineering (120 credits) KTH Royal Institute of Technology year 2020 Supervisors at Saab AB: Knut-Oloj Jönsson, Marcus Birksjö Supervisor at KTH: Per Enqvist Examiner at KTH: Per Enqvist
TRITA-SCI-GRU 2020:234 MAT-E 2020:064
Royal Institute of Technology School of Engineering Sciences KTH SCI SE-100 44 Stockholm, Sweden URL: www.kth.se/sci
Abstract
Developing countermeasures dispenser systems requires many and careful tests. When it comes to testing products with pyrotechnics, testing can often be very complicated and expensive. This might lead to no testing at all due to time or resource shortages. Products to be used in the military requires further testing and even more thorough reviews to meet the strict demands placed on the products. In order to enable more tests of pyrotechnic flares in the countermeasures industry, this degree project aims to increase theability to perform tests without the need for pyrotechnic means. This was done by designing, constructing and optimizing a recoil emulator, an apparatus that imitates the force-time curve obtained by pyrotechnic flares without the need of pyrotechnic means. The construction of the recoil emulator was conducted at a department that develops countermeasure systems at Saab Surveillance in Järfälla. The apparatus aims to be used in the future for testing and verification of product series of countermeasures dispenser systems. The design of the apparatus was based on a test result provided by a flare manufacturer of an arbitrarily chosen flare, typical in the countermeasures industry. Based on the provided test result, three measures were chosen that together describe the fundamental and essential characteristic parts of the recoil motion behavior of pyrotechnic flares. These three measures are in the thesis called recoil measure and defined as the Peak Recoil, the Impulse, and the Peak-Width. To be able to verify the recoil emulator, the three recoil measures were implemented in an error model, which was based on the squares of error. In order to make the emulator imitate the desired recoil motion behavior as pleasant as possible, the error model was implemented in an optimization model. By minimizing the error of data points from each of the recoil measures obtained from the real test provided by the manufacturer with results obtained from the recoil emulator, the emulator was verified and optimized accordingly. Results showed that the selected design of the recoil emulator resulted in a force-time curve that principally mimics the curve given by the real tests. The conclusion from the project was, therefore, that it is possible perform tests on countermeasures systems without pyrotechnics when considering the impact of recoil. Further development of this thesis could be to improve the construction of the recoil emulator and perform more research on flares and damping materials. Other future work could be to implement the emulator in existing test and validation processes at companies within the countermeasure industry. Keywords: Countermeasures, Flares, Recoil, Optimization, Pyroshock Testing
i
Sammanfattning
Att utveckla motmedelsprodukter kräver många och noga utförda tester. När det kommer till testning av produkter med pyroteknik kan testerna ofta bli väldigt komplicerade och dyra eller inte göras alls på grund av tid- eller resursbrist. Produkter som ska användas inom militären kräver i många fall ytterligare tester och ännu noggrannare genomgångar för att kunna klara av de tuffa krav som sätts på produkterna. Den här avhandlingen syftade till att möjliggöra fler tester för pyrotekniska medel inom motmedelsindustrin. Detta har gjorts genom konstruktion, design och optimering av en rekylemulator; en apparat som immiterar den kraft-tid kurva som erhålls av pyrotekniska facklor, utan att använda pyrotekniska medel. Konstruktionen och utvecklandet av rekylemulatorn gjordes på uppdrag av en avdelning som utvecklar motmedelssytem på Saab Surveillance i Järfälla. Syftet med emulatorn är att använda den i framtiden vid testning och verifierering av produktserier av motmedelssystem. Designen av apparaten utgick från testresultat som tillhandahållits av en fackeltillverkare av en godtyckligt vald pyroteknisk fackla, vanlig inom motmedelsindustrin. Utifrån testresultaten togs tre mått ut som tillsammans beskriver fundamentala och viktiga karaktäristiska delar av rekylkraftsbeteendet hos pyrotekniska facklor. Dessa tre mått kallas rekylkraftsmått och definieras som rekylpeaken, impulsen, samt peakbredden. För att kunna verifiera rekylemulatorn implementerades dessa tre rekylkraftsmått ien felmodell, som baserades på det kvadratiska felet. För att få emulatorn att imitera det önskade rekylkraftsbeteendet så bra som möjligt implementerades en felmodellen i en optimeringsmodell. Genom att minimera felet av datapunkter från varje rekylkraftsmått som erhålls från resultatet av både det verkliga testet, tillhandahållna av tillverkaren, samt med resultat erhållna från rekylemulatoren kunde emulatorns valideras. Resultaten visade att den valda designen av rekylemulatorn resulterade i en kraft- tidskurva som huvudsakligen efterliknar den kraftkurva som ges av de verkliga testerna. Slutsatsen från projektet är därmed att det är möjligt att utföra tester på motmedelssystem utan pyroteknik när det kommer till påverkan av rekylkraften. Vidare utveckling av denna avhandling kan vara att förbättra och utveckla rekylemulatorn samt utföra mer forskning kring dämpningsmaterial samt facklor. Andra framtida aspekter av projektet kan vara att implementera apparaten i existerande test- och valideringsprocesser på företag inom motmedelsindustrin. Nyckelord: Motmedelssystem, Facklor, Rekylkraft, Optimering, Pyroshocktestning
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Acknowledgments
As a writer, I would like to send many thanks to people who made this master thesis possible. A big thank you to Saab for offering me this exciting and challenging degree project. Thanks to the entire CMDS section, who supported me and provided both fun and advanced tips and suggestions on ideas. I would like to express my special thanks to my mentors, Knut-Olof Jönsson and Marcus Birksjö, who supported and cheered on me during this trip. Also sending a special thanks to the ever helpful Tommy Eriksson, for your help in building the recoil emulator and supporting me in the workshop. Thanks to my technical manager at Saab, Christer Zätterqvist, for all your scientific ideas and inventions. Thanks also to Mats Danielsson, who took me into the department and solved the administrative problems. Thank you to my supervisor and examiner at KTH, Per Enqvist, for your guidance through the spring, your expertise in mathematics, and patience during the Zoom meetings. It has made this bewildering spring a little easier. A final thank you to my always supportive mother and partner. Thank you forbeing there and always being there for me. You have made me the engineer I now leave school for. Thanks for all the support over the years, thanks for always saying that I should not give up, no matter how difficult the education was. This has been an amazing journey. And now it’s over. Thanks!
Ebba Lindgren Stockholm, June 2020
iii Acronyms
AAM Anti-Aircraft Missile or Air-to-Air Missile ABS Acrylonitrile Butadiene Styrene AECM Airborne Expendable Countermeasure AM Additive Manufacturing CALTECH California Institute of Technology CMDS Countermeasure Dispenser System DAQ Data Acquisition DoD Department of Defense EW Electronic Warfare IR Infrared JPL Jet Propulsion Laboratory MANPADS Man-Portable Air-Defense Systems MIPS Mechanical Impulse Pyro Shock Simulator MTV Magnesium, Teflon and Viton NASA National Aeronautics and Space Administration POD Plug On Device RF Radio Frequency SAM Surface-to-Air Missiles SDS Smart Dispenser System SRS Shock Response Spectrum UV Ultraviolet
iv Contents
1 Introduction 1
1.1 Background ...... 1 1.2 Motivation ...... 3 1.3 Aim, Purpose and Goal ...... 3 1.4 Research Questions ...... 4 1.5 Methodology ...... 4 1.6 Ethics and Stakeholders ...... 5 1.7 Delimitations ...... 5 1.7.1 Delimitations on Available Information ...... 5 1.7.2 Delimitations on Modeling ...... 6 1.7.3 Delimitations on Designing ...... 7 1.8 Outline ...... 8
2 Theoretical Background 10
2.1 Pyrotechnic Flares ...... 10 2.1.1 Schematic View of a Pyrotechnic Flare ...... 10 2.1.2 Recoil Motion Sequence of Flares ...... 12 2.2 Theory of Recoil Motion ...... 13 2.2.1 Theoretical Assumption ...... 13 2.2.2 Force Modeling of Flare ...... 14 2.2.3 Mathematical Model of Recoil ...... 15 2.2.4 Recoil Motion Profile by Le Duc ...... 18 2.3 Mechanical Shock ...... 20 2.3.1 Standard Shock Machines ...... 20 2.3.2 Shock Testing Requirement ...... 21 2.4 Pyrotechnic Shock ...... 21 2.4.1 Pyroshock Applications and Validation ...... 21 2.4.2 Characteristics of Pyroshock ...... 22 2.4.3 Simulation and Testing of Pyroshock ...... 23 2.4.4 Mechanically Excited Far-field Test Machines ...... 24 2.5 Previous Related Work ...... 26 2.5.1 Recoil Related ...... 26 2.5.2 Mechanical Shock Related ...... 26 2.5.3 Pyroshock Testing with Mechanical Impact Related ...... 27 2.5.4 Comments on Previous Related Work ...... 28
v CONTENTS
3 Method 29
3.1 Overview of Method ...... 29 3.2 Interpretation of a Real Test Result ...... 29 3.3 Measures of Recoil Motion ...... 33 3.3.1 Peak Recoil ...... 33 3.3.2 Impulse ...... 34 3.3.3 Peak-Width ...... 35 3.4 Optimization Model for Validation ...... 35 3.4.1 Objective Function ...... 35 3.4.2 Constraints ...... 37 3.4.3 Optimization Problem ...... 38 3.5 Design Process of Recoil Emulator ...... 38 3.5.1 Design Requirements ...... 38 3.5.2 Possible Design Choices ...... 39 3.5.3 Controlling the Mechanical Impact ...... 40 3.5.4 Selected Design ...... 41 3.6 Testing the Recoil Emulator ...... 45 3.6.1 Test Fixture ...... 45 3.6.2 Test Setup and Equipment ...... 48 3.6.3 Test Method ...... 49
4 Result 54
4.1 Overview of Iterations ...... 54 4.2 Iteration 1 ...... 57 4.3 Iteration 2 ...... 58 4.4 Iteration 3 ...... 59 4.4.1 Optimal Solution ...... 60
5 Discussion 61
5.1 Analysis of Result ...... 61 5.1.1 Recapitulation of Aim, Purpose, and Goal ...... 61 5.1.2 Reliability of Result ...... 62 5.1.3 Validation of Result ...... 62 5.2 Improvements ...... 63 5.2.1 Test Method ...... 63 5.2.2 Test Equipment ...... 64 5.2.3 Design and Construction ...... 64 5.2.4 Optimization Model and Constraints ...... 65 5.3 Conclusions ...... 65 5.4 Future Work ...... 66 5.4.1 Perform a Shock Response Spectrum (SRS) Analysis ...... 66 5.4.2 Test Another Impact Method ...... 66 5.4.3 Structural Analysis of the Exposure to Pyroshocks ...... 66 5.4.4 Determine damping Factors ...... 67 5.5 Implementation Plan for Recoil Emulator ...... 67
vi CONTENTS
References 69
Appendix 72
A Iterative Process of Test Method 73
A.1 Iteration 1 ...... 73 A.2 Iteration 2 ...... 74 A.3 Iteration 3 ...... 75
vii
Chapter 1
Introduction
During both peace and war, aircraft are an essential part of a country’s military and civilian defense. Aircraft are used for missions including transportation, aerial delivery, refueling, airborne surveillance, and more. One of many ways to protect aircraft from threats is to use Electronic Warfare (EW). EW represents the ability to use the electromagnetic spectrum signals, which includes radio, Infrared (IR) or radar, to sense, protect, and communicate [31]. Included in the EW suite of an aircraft is the possibility to fire off expendable decoys, known asthe Countermeasure Dispenser System (CMDS). The decoys disrupt enemy signals by reflecting, transmitting, or emitting different types of signals depending on the type of the threat sensor. When the decoys are fired off the aircraft, the resulting recoil can have an impact on the aircraft. To examine the recoil of decoys extended with pyrotechnics, many test firings need to be performed. The testing is very expensive and requires much time, preparation, and trained personnel. [53] This degree project aims to model and construct a recoil emulator without pyrotechnics to emulate the force behavior of pyrotechnic flares. The recoil emulator is optimized based on measures related to a force-time curve of pyrotechnic flare used in CMDS products. The report begins with a background description of EW, CMDS, and flares.
1.1 Background
The possibility to protect valuable platforms from enemy gunnery began before the First World War. During the early usage of countermeasures, warships made smoke to distract enemies [53]. Since then, efforts have been made to produce expendable airborne countermeasures for aviation. The first countermeasure appeared as chaff, small stripes of aluminum foil that forms into a cloud of tiny dipoles, acting as a decoy to the threat radar. Initially, chaff was deployed from the sides of aircraft by crewmen, making it a hit-or-miss mission. The chaff had some effectiveness against specific radar types if it was distributed proportionally, but the chaff often clumped up. As new radar systems evolved, more advanced countermeasure systems were required. The development initiated new threats with more advanced seeker systems operating in different portions of the spectrum, requiring more sophisticated dispensing systems. Such dispensing systems came with a variety of types of expendables to meet the threat radar seeking technology. Today, there are expendable decoys with different payloads to counter for Radio Frequency (RF), IR, Ultraviolet (UV) and laser. [6]
1 CHAPTER 1. INTRODUCTION
When it comes to self-defense of large aircraft, there are different types of threats to protect the aircraft from. The methods of countering depend on the type of threat. The approach of countering Surface-to-Air Missiles (SAM), Man-Portable Air-Defense Systems (MANPADS) and Anti-Aircraft Missile or Air-to-Air Missile (AAM) can be done in three ways [53]. The first approach is to take action regarding the targeted aircraft through design or maneuver. The second method of countering is done in the atmospheric environment between the threat and the target, and the last method regards the missile. Countering threats in the atmosphere can be done with expendable decoys containing payloads such as chaff or flares [53]. Figure 1.1.1 presents an example where pyrotechnic flares are deployed from a helicopter by the platform IDAS, a product provided bythe Swedish defense company Saab AB [44].
Figure 1.1.1: The Saab product IDAS - Integrated Defensive Aids Suite. IDAS is an EW system for self-defense of airborne platforms, to protect civilian and military aviation from ground or air based threats. Collected from reference [44].
Countering systems either use deceptive or destructive methods. Deceptive methods are used to break the threat’s path to the platform and destructive methods are used to eliminate the threat. When countering for SAM threats, off-board countermeasure systems are often used [53]. These are known as expendables deploying from aircraft as off-board things to reflect, imitate, or emit signals from RF, IR, UV, or laser. There are two categories of off-board countermeasures; passive and active. In the passive category, there are chaff and flares, and in the active category, there are different types ofdecoys. Flares utilize passive guidance of missiles by emitting a high-intensity radiant. There are two types of flares; pyrotechnic and pyrophoric [17]. Pyrotechnic flares use a slow-burning fuel-oxidizer mixture that generates intense heat. Pyrophoric flares are made up of small strips that oxidize, ”burns”, in contact with air, creating an intense heat-cloud which distracts the missiles from targeting the aircraft. The ejection and heat make the flare imitate the movement of the aircraft, provided the flare is deployedat the right time and in the right direction. Flares are used for IR-tracking threats, as the flares have a high flame temperature. There are different types of flares, such asstandard flares, spectral flares, or aerodynamic flares. [53]
2 CHAPTER 1. INTRODUCTION
Saab AB has many different types of CMDS products as a part of the business area Surveillance. Such examples are BOL, the electromechanical dispenser, BOP, the pyrotechnical dispenser, and their Plug On Device (POD) systems BOZ and ESTL [42]. The latest CMDS product is the Smart Dispenser System (SDS), which primarily deposits flares of various kinds, containing Magnesium, Teflon and VitonMTV ( ), addressing various IR target seekers [13]. Flares are loaded in a magazine which is mounted in a dispenser and attached or integrated in the aircraft. When flares are deployed from the dispenser, a force is induced and acts in the opposite direction of the flare on the aircraft, known as the recoil force. The recoil can have effects on the aircraft. For instance, if countermeasures are usedon a low-weighted aircraft, such as a helicopter, it is crucial to know the recoil of the flares in order to ensure that the steering, control, or other characteristics of the helicopter are not affected by the force. In this thesis, the recoil behavior of pyrotechnic flares willbe modeled and analyzed from different perspectives in order to emulate the force behavior in an apparatus.
1.2 Motivation
Generally, production companies want to optimize their constructions and fixtures concerning structural mechanics and analysis to lower cost, facilitate design aspects, and manufacture better products. Traditionally, recoil modeling, measurement, and analyses of pyrotechnic flares are exclusively performed with pyrotechnic equipment by shooting multiple flares during testing. This might lead to less testing than desired asa consequence of resource limitations or time restrictions. The motivation behind this thesis is, therefore, to increase the ability to perform more tests on recoil impact without all the preparations needed to perform tests with pyrotechnics. The incentive for this is to make it easier and cheaper to perform more tests with fewer preparations for different fixtures, structures and constructions by having the possibility to ”fire” many flares without actually firing any, to simulate howa CMDS product would react to the recoil from pyrotechnic flares.
1.3 Aim, Purpose and Goal
In order to optimize constructions and fixtures, many tests need to be performed. Performing tests can be expensive in comparison with models, simulations, or emulators. Hence, this project aims to design and construct a recoil emulator without the need for pyrotechnics, and optimize it accordingly based on measures related to the recoil motion curve of a typical pyrotechnic flare used in CMDS products. The purpose of emulating the recoil of pyrotechnic flares is partly to understand how the recoil of flares occurs and behaves. The increased knowledge could, in the future, lead to an increased understanding of how recoil can be reduced or attenuated, for instance, by using different materials or other mechanisms for recoil attenuation. Another purpose of this degree project is to investigate the possibility of substituting the force inducing pyrotechnic propellant system of flares with a recoil emulator, and discuss how wellan emulator could potentially mimic the force-time curve of pyrotechnic flares.
3 CHAPTER 1. INTRODUCTION
In the end, the goal of this thesis is to present a recoil emulator with an analysis of how well it matches the force-time curve of pyrotechnic flares. The analysis will be based on selected measures, which are important when considering the recoil obtained from pyrotechnic flares. The measures will be presented together with an optimization model to make it possible to compare results obtained from the recoil emulator with a real test result, thus, a necessary model to use when validating the emulator.
1.4 Research Questions
In order to fulfill the above-stated aim, purpose, and goal, this thesis will answer the following set of questions:
Question 1 (Q1): How does the recoil of pyrotechnic flares in CMDS arise?
Question 2 (Q2): What does the force-time curve of pyrotechnic flares look like?
Question 3 (Q3): How can the recoil from pyrotechnic flares used in CMDS products be emulated or simulated by the use of a machine or an apparatus without the induced force of pyrotechnics?
1.5 Methodology
As the aim of this thesis is to address a practical research problem by designing and validating a recoil emulator, the most suitable methodological approach of this thesis is to apply Quantitative methods, namely experiments and by using existing numerical data [22]. In a quantitative experimental study, the aim is to produce generalizable knowledge, expressed in numbers and graphs, about the causes of a chosen phenomenon. In this case, the phenomenon is the recoil force behavior of pyrotechnic flares. The force curve of the recoil from the flares will potentially be imitated by the recoil emulator and confirmed in tests by comparing test results obtained from the recoil emulator with existing data. The existing data is provided by a flare manufacturer from an arbitrarily chosen flare, typical for usage in CMDS products. By discussing and arguing for various types of force inducing mechanisms to induce recoil motion behavior, the research question of whether it is possible or not to emulate the recoil motion can potentially be answered. In accordance with the methodology of quantitative methods, the results shall be validated and discussed in terms of reliability. The validation of the recoil emulator will be done by comparing it to existing data. This data is chosen from an arbitrarily firing with the chosen pyrotechnic flare in a controlled environment, where the recoil of the flare is isolated in the measurement to make sure nothing else is affecting the force sensors. In order to further validate the results obtained from the recoil emulator, a literature study on flares is performed to substantiate the strived behavior of recoil from pyrotechnic flares.
4 CHAPTER 1. INTRODUCTION
1.6 Ethics and Stakeholders
Working for a defense and military company can lead to specific ethical issues. As this thesis intends to design, construct and optimize an apparatus that may be used in tests of countermeasure systems, ethical aspects need to be included and considered in the design work. This thesis is done in collaboration with a department of CMDS development at Saab AB, business area Surveillance, in Järfälla. Saab is today a world leading defense and security company. Over 15,000 employees are continually looking to the future with an open mind for innovations to find smart solutions to complex problems to make people and communities safe. It is a basic need to feel safe, and it is Saab’s vision that everyone should have that right. [45] An ethical question that may arise when reading this report is the dissemination of information about how the pyrotechnic systems work and its similarity with weapons. The CMDS products may have some similarities with weapons in terms of functional perspectives and characteristics of force inducing mechanisms. Information on recoil and other pyrotechnic systems used in this report is gathered from public scientific databases, books, and journals. Thus, the information has been retrieved from pages which are available to the public rather than sharing confidential, classified or company restricted information. The author of this report understands the ethical aspects of sharing information, results, and designing a recoil emulator that can be used in the defense industry. To state the ethical problem, the author asks the reader of this report to use the information reasonably, following Saab AB’s vision of keeping people and society safe. Since the project will handle information from Saab that may contain sensitive information, such as values and figures specific to certain products, the values are removed in the report to make it available to the public. The stakeholders of this degree project are generally the defense industry who deals with CMDS products and pyrotechnic flares, and manufacturers of flares. This includes Saab AB, who owns the results compressed in the thesis, as well as the recoil emulator.
1.7 Delimitations
Some delimitations regarding recoil, construction, design, and pyrotechnics are made in this thesis. Below are critical delimitations presented, which where taken into account during the project. Some of these delimitations regard the gathering of information of pyrotechnic flares, and others consider the designing and modeling of the emulator.
1.7.1 Delimitations on Available Information Information about Pyrotechnic Flares Since pyrotechnic flares are mostly used in the defense and military industry, information about them are limited and not easy to find. When searching for information concerning recoil, almost all articles, journals, and books present information about recoil from guns and weapons. An assumption and delimitation of this thesis is therefore that the pyrotechnic flares can be described by having the same recoil characteristics asofagun or a weapon. This assumption is described in detail in Section 2.2.1.
5 CHAPTER 1. INTRODUCTION
Information about Mechanical Shock
In the science of mechanics, there are plenty of journals, articles, and books about mechanical shock in the sense of how it is induced, analyzed and used in the testing of vibration and shock. In this thesis, the focus is on how the recoil force is induced, and not on the vibration and shock analysis that may follow with it. This means that the focus is on the mechanical shock creation of impact, rather than the analysis of the fixtures responding to the shock. To clarify, the focus is on the question of whetherit is possible or not to substitute the explosive pyrotechnic shock with another mechanical shock system.
The focus area of this thesis limits the information about mechanical shock, as in how to create it rather than what happens when fixtures and structures are being exposed to it. This is taken into account in the thesis. For instance, the design of the recoil emulator is inspired by existing shock testing machines, as in how the shock is induced, even though there are not many available articles or reports presenting the construction of a mechanical shock machine adapted to pyrotechnic flares.
1.7.2 Delimitations on Modeling
Modeling Approach of Recoil
When considering recoil, there are several potential perspectives to study. A natural approach could be to consider the theory of pyrotechnics, regarding chemistry and propellant. Since this is a thesis in mathematics, the theory of chemistry and pyrotechnics will not be addressed. A mathematical model presenting the recoil with respect to its pyrotechnical aspects, such as propellant used and chamber volume, is presented. Additional aspects of pyrotechnic or chemistry is not taken into account. The central part of this report is to understand how the recoil is generated and the shape of the force-time curve. As there are mathematical models and test data to confirm this, the chemistry and pyrotechnics are, therefore, excluded.
Force Reactions versus Frequency Response
Since this report is made in collaboration with a department that focuses on the development of countermeasure systems, the focus of the project is on the force impact of shocks rather than the shock wave and frequency arising from the firings. In previous presented work within the area of analysis of recoil, described in Section 2.5.1, the focus is primarily on the induced force and how structures surrounding the shock mechanism is affected by the recoil. When dealing with pyrotechnic flares, the focus is primarily on the recoil. This thesis will therefore focus on the force response of a structure in a structural mechanic way of thinking. This means that the project will not handle an analysis which is based on the frequency of the pyrotechnic flares, but rather the force that is induced during the firing of them. The testing of the recoil emulator is conducted with a test fixture where flares are loaded in its natural state in the dispenser. Thetest fixture is chosen to be used in this way to simulate how CMDS products could respond to repeated tests of recoil, simulated by the recoil emulator.
6 CHAPTER 1. INTRODUCTION
Recoil Motion Behavior to Emulate
An important delimitation when designing the recoil emulator is to state which and how much of the recoil motion behavior that is relevant and ”good enough” to emulate. In this attempt to design a recoil emulator with respect to a real test result, the recoil behavior to be emulated is defined within the in bore period of recoil motion. This is described in Section 2.1.2 and in detail in Section 3.2 and 3.3. The chosen force-time curve that is presented in the method in Chapter 3 is provided by a test result from an arbitrarily chosen pyrotechnic flare, typical for CMDS products world-wide.
Choice of Presented Units
In the science of pyrotechnically induced shocks, many articles and reports present the results and graphs with units of frequency and acceleration. Since this thesis focuses on the recoil motion of pyrotechnic flares, the modeling of the results are based on units that are of relevance for the department where the thesis in conducted. These units are force and time, rather than acceleration and frequency. Since force and acceleration are linearly dependent, with mass being the factor between them, and frequency being the inverse of the period, the graphs will have similar shapes to the graphs presented in previous related work within the area of pyrotechnic shock, described in Section 2.5.3.
1.7.3 Delimitations on Designing Possible Design Solutions
Since the project is time-limited and the workload should be accommodated within a master’s thesis, this means that possible solutions are limited and that all possible solutions will not be addressed. When it comes to the design of the recoil emulator, the design is based on how to create a force with high acceleration and high energy to simulate the pyrotechnically induced force. The force creation can be done in several ways. In this thesis, the focus is on creating the force by inducing a mechanical shock. As pyrotechnic flares induce an explosion, this shock method is not used in the project, since the aim is to construct a recoil emulator without the need of pyrotechnic equipment.
Already Existing Mechanical Shock Testing Equipment
In the world of shock testing, there exists many types of shock testing machines, made for testing of free fall, transportation shocks, vibrations or shake. This thesis will not address any of these existing test machines as these have other purposes, which are mainly for the transportation industry or within structural or material analysis. This thesis discusses and investigates different mechanical shock systems without using existing equipment, such as standard shock testing machines provided by companies specializing in mechanical shock. The choice of not using existing test equipment is mainly due to budget limitations as many of these machines are very expensive, and not easy to manipulate or adapt to the required force in interest. The project will build a mechanical shock machine, which is inspired by existing machines, but modified and adapted to counter for the caseof pyrotechnic flares.
7 CHAPTER 1. INTRODUCTION
Complexity of Recoil Emulator Due to time and budget restrictions, and the restricted number of people involved in the project, the designed recoil emulator is not as complex and well developed as the machines and apparatuses developed by, for instance, National Aeronautics and Space Administration (NASA) or other institutes that specializes in this field of science. As the purpose of this thesis is to investigate the possibility to substitute the force obtained by the pyroshock from the pyrotechnic flares, the emulator is not very complex, but rather based on a conceptual design to provide an answer to the question of whether it is possible or not to simulate the recoil from pyrotechnic flares without pyrotechnics.
Already Existing Pyroshock Testing Equipment Since pyroshock testing is common and used in the space industry, there exists apparatuses and machines which are used to simulate the pyroshock for which space components can be exposed to during the deployment or separation of stages of multistage rockets. Some of these machines are patented, and are therefore not allowed to be copied. These machines are used as inspiration in the design of a recoil emulator, partly in how the shock is induced and other mechanical design choices of the machines. The existing pyroshock machines are in general adapted to the space engineering applications, and can simulate pyroshocks induced during space flights, which means that their characteristics and other aspects are not completely relevant to this thesis of emulating the recoil of pyrotechnic flares as flares are generally not used inspace.
1.8 Outline
The report of this project is divided into five chapters, organized as follows:
• Chapter 1 introduces the project with background, motivation, methodology, and ethical issues that may arise during the project. The chapter also describes three delimitation areas which are applied in the report.
• Chapter 2 provides a theoretical background to the project by describing pyrotechnic flares and their sequence motion behavior. A theoretical approach of recoil motion is also presented, together with a mathematical model describing the recoil motion behavior with graphs. The science of mechanical and pyrotechnic shock is presented with a description of existing test machines for pyrotechnic shock simulation. Follows does a review of previous related work done on recoil, mechanical shock and pyroshock testing.
• Chapter 3 presents how the project was conducted, the method, which is divided into five sections. The first section of the chapter provides an overview ofthe method and is followed by the ensuing sections.
– The first section of the method describes an interpretation of the theory applied to a real force curve of a pyrotechnic flare.
– In the second section, the applied theory of the real test is divided into recoil measures which characterize the recoil motion behavior.
8 CHAPTER 1. INTRODUCTION
– The measures are applied to an optimization model, which is presented in the third section, to be used as a validation model of the recoil emulator. – The fourth section describes different design solutions and control parameters of the recoil emulator, based on inspiration from previous related work. – The last section presents the test fixture, environment, setup, and how the testing of the recoil emulator was conducted. • Chapter 4 describes the results obtained from the iterative test process of the recoil emulator, described in Section 3.6.3. The results are presented with force- time graphs and the corresponding value obtained from the error model used in the optimization model. • Chapter 5 concludes the project by presenting a discussion of the results presented in Chapter 4, together with a description of possible improvements of the project and future work. The chapter finishes by offering a proposal of an implementation plan to include the constructed recoil emulator in test and verification processes at companies within the countermeasure industry.
9 Chapter 2
Theoretical Background
In this chapter, a detailed description of the theoretical background to this degree project is presented together with previous related work. To be able to model the recoil of flares with the aid of other mechanical shock methods rather than pyrotechnics, the behavior behind recoil motion is presented together with a description of pyrotechnic flares. Thereafter follows a description of mechanical and pyrotechnic shock, howit can be induced, measured and used in the designing of a recoil emulator. The last section describes previous related work within recoil, mechanical shock and simulation of pyrotechnic shock.
2.1 Pyrotechnic Flares
In this section, an introduction to pyrotechnic flares and their firing behavior willbe presented. There are many aspects of the pyrotechnics in flares. Most published articles and publications handle the spectral pyrotechnical radiant aspect of flares, as in how the flares work as IR-seekers, for instance. As this thesis is about the recoil of flares rather than their spectral and radiant characteristics, this will not be treated in this thesis. More on the aspect of spectral and radiant characteristics of pyrotechnic flares can be read, for instance, in reference [14].
2.1.1 Schematic View of a Pyrotechnic Flare A pyrotechnic flare is a decoy deployed from an aircraft with the help of pyrotechnics; in other words, black powder. There are various types and sizes of pyrotechnic flares filled with different pyrotechnic propellant and chemicals, where an example of thefilling inside the flare is MTV. Depending on the desired characteristics of the flare, the filling is composed differently. A simplified schematic 3D view of a round flare ispresented in Figure 2.1.1. The outside of the flare is called Flare Housing/Body, which holds the Igniter Assembly, Electric Squib, and the Special Material, also called Flare Material. The flare is closed with the End Cap. The flare material consists of chemicals and differentiates different types of flares from each other by size, mass, and other properties. [25, 53]
10 CHAPTER 2. THEORETICAL BACKGROUND
Figure 2.1.1: Schematic 3D view of a pyrotechnic flare, collected from a manufacturer of flares from reference [25].
The pyrotechnic flares are loaded in a magazine, which in turn is assembled witha dispenser bucket. The magazine and the bucket together compose a CMDS. Figure 2.1.2 shows one of the many CMDS products that Saab AB develops and manufactures, the BOP-L, a smart and lightweight CMDS. In Figure 2.1.2a, the magazine, and bucket are dissembled, and in Figure 2.1.2b, the assembled dispenser is presented. The number of flares that can be loaded in a magazine depends on the dispenser and the sizeandshape of the flares. The type of flare that will be used in the testings of the recoil emulatorin this project is a square flare, typical for CMDS products. In the next section, the motion of pyrotechnic flares is described.
(a) Magazine to the left and bucket to the right. (b) Assembly of magazine and bucket.
Figure 2.1.2: The CMDS product BOP-L, manufactured by Saab AB. The pictures are collected from a product sheet provided by Saab AB in reference [43].
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2.1.2 Recoil Motion Sequence of Flares
The study of the motion of the inside of a pyrotechnic flare cannot be described as a matter of directly applying Newton’s laws of motion to the flare material. The gas produces the motion from the propellant in the ignitions system and depends on the propellant used and the flare material itself. The time of the recoil motion canbe divided into three parts. In Figure 2.1.3, the three periods are presented together with a functional sequence of the deployment of a flare, where each period is represented schematically. [8]
Figure 2.1.3: The three periods of recoil motion together with a functional sequence of a flare from when it ignites to when it leaves the dispenser. The picture is collected from reference [34] and is combined with each corresponding time period.
12 CHAPTER 2. THEORETICAL BACKGROUND
According to reference [34], the functional sequence of a pyrotechnic flare can be described as follows. In the first period, the in bore period, the electric squib, consisting of the igniting propellant, is ignited by applying an electrical impulse. During this period, the special material inside the flare starts to move until it nearly exits the flare at the muzzle. This makes the special material inside the flare act as a recoil mass with an accelerating motion in the opposite direction to the flare body. In this period, the pressure inside the flare is building up, accelerating the inside special material to make it readyfor ejection. In the second time period, the aftereffect period, the flare material exits the flare body and the motion of recoil mass changes from positive acceleration to negative acceleration. This happens due to the pressure difference between the excess pressure formed inside the flare body and the pressure on the outside, the atmospheric pressure. Themoment the flare material leaves the flare body at the muzzle, the recoil is generated. Thismakes the flare body move in the opposite direction of the flare material. If the flare material would not leave the flare body, for instance, get stuck for different reasons, the flarebody would create a closed pressure chamber, and there would not be a recoil. The reason behind this is that the recoil occurs when there is a pressure difference between the inside of the flare body and the atmosphere. The moment the pressure inside the flarebody is exposed to the atmospheric pressure, the recoil occurs since the pressures want to normalize. In the last period, the inertia period, the pressure inside the flare body is equalized to atmospheric pressure, and the recoil force acting on the breech is zero [52]. During this period, the flare material ignites outside the aircraft. Of the three time periods, the in bore period is most crucial to take into consideration when modeling and emulating the recoil, as this is the period for when the pressure is building up inside the flare. The aftereffect period is essential for the actual firing, as this is when the pressure is normalized. The theory of the pressure build-up inside the flare due to the ignition of the propellant is discussed in the ensuing section.
2.2 Theory of Recoil Motion
There are several ways to theoretically describe the motion of recoil from flares. Two ways are either by looking at test results obtained from firings or by considering the theory of the recoil motion behavior. Below follows a theoretical description of recoil by modeling the forces and by applying an empirical model. To begin the theoretical description, an important assumption is discussed, which will be applied in the theory, primarily in the theoretical model of recoil.
2.2.1 Theoretical Assumption As flares are mostly considered as a defense product, there is not much public information about their behavior and characteristics. Flare manufacturers keep this to themselves as this is classified and confidential information in most cases. Due to this reason,the theory of recoil of flares will be based on an assumption. By assuming that flareshave the same ballistic behavior as a gun, many theories for interior ballistics can be applied. According to experts within countermeasures, this assumption is valid in the context of
13 CHAPTER 2. THEORETICAL BACKGROUND flares and guns having many similar components, such as the pyrotechnic propellant, the behavior of ignition, the barrel, and the ballistics of the projectile. To clarify, although a flare and a weapon have different purposes, their physical behavior, for instance, the pressure build-up and other functionalities, holds many similarities. These can be described theoretically with mathematical models. Many models of interior ballistics are presented in reference [41], where systems of internal ballistics are categorized into different types depending on its exactness. One of these models hasbeen chosen for comparison in this report. The model is presented in Section 2.2.3.
2.2.2 Force Modeling of Flare By applying the assumption that a flare and a gun being equivalent in terms of recoil, where the inside of the flare corresponds to the projectile and the outside, the flare body, corresponds to the barrel, we can state the following. Considering the flare body and the flare material as two individual pieces, presented in Figure 2.2.1, their force equivalence can be described by modeling them accordingly.
Figure 2.2.1: A schematic view for where the flare body and flare material with the absolute distance from the breech plate to each component is described. The figure is collected from reference [52].
In Figure 2.2.1, s describes the absolute displacement between the flare body and the breech plate, and u describes the position of the flare material, or projectile, inside the flare in the flare body, or gun barrel. The total absolute distance from the fixedpointis then described as
x = u + s. (2.1)
The total length of the barrel is the length of the flare, which is denoted with U0. When considering the velocities of the projectile and the flare body we get
du V = (2.2) dt ds v = (2.3) dt 14 CHAPTER 2. THEORETICAL BACKGROUND where V is the velocity of the flare material, and v is the velocity of the flare body. By considering the force equivalence of the flare material and the flare body weget
Fflare material = Fflare body →
mflare material · aflare material = mflare body · aflare body (2.4) where mflare material is the mass of the material inside the flare, and mflare body is the remaining mass of the flare, which is the flare housing, the squib, et cetera. The projectile force Fflare material can be expressed as the recoil force acting on the breech plate of the dispenser, and is further on denoted with B. The force evoked from the flare body can be expressed as the gas force induced by the black powder, or as a projectile force, and is further on denoted with Fp. The breech force, the recoil, B, and the projectile force. Fp, can be expressed as
dv B = m · a = m (2.5) flare body flare body flare body dt dV F = m · a = m . (2.6) p flare material flare material flare material dt
Due to force equivalence of Newton’s law, the force acting on the breech must be equal to the gas force acting on the flare inside. This leads to the expression stated below
dv mflare material dV B = Fp → = . (2.7) dt mflare body dt
2.2.3 Mathematical Model of Recoil One of the theoretical approaches presented in reference [41] is the Le Duc Approach, a historical and practical formula designed by Captain Le Duc in the beginning of the 20th century. The US Navy uses the model since the 1940th [41]. Le Duc invented the formula empirically by grounding it on numerical observations obtained from test firings. Even though the physics and chemistry perspective of the firings are too complex to be capture by a simple formula, the approach of Le Duc captures the essence of the dynamics inside a gun barrel [20]. Worth mentioning is that Bofors, the Swedish arms manufacturer, today owned by Saab AB, also have several approaches, presented in reference [41]. Since the US Navy uses the Le Duc approach and that the model is presented in many previous related work and reference, the model will be applied in the mathematical description of the recoil motion in this project. In accordance with the Le Duc approach, the projectile velocity V and travel of the bore u can be expressed with an empirical formula described as a hyperbolic function. The formula is declared as the Le Duc formula, also known as the Interior Ballistic Formula, and was derived by Le Duc and is published by Challeat in references [9] and [28]. By applying the empirical formula, the velocity V can be expressed as
au V = (2.8) b + u
15 CHAPTER 2. THEORETICAL BACKGROUND where V is the projectile velocity inside the barrel, u is the projectile travel and a and b are constants, also called Le Duc parameters. By taking the derivative of the formula with respect to u by applying the the quotient rule we get ( ) ( ) dV d au a · (b + u) − au(1) ab = = = . (2.9) du du b + u (b + u)2 (b + u)2
By moving the factor du to the right hand side we get
ab dV = du (2.10) (b + u)2 and by taking the derivative of time on the expression above, the following expression of the projectile velocity is derived
dV ab du = (2.11) dt (b + u)2 dt.
By inserting Equation (2.11) into (2.7) and (2.7) into (2.5) we get
mprojectile ab du B = mflare body 2 (2.12) mflare body (b + u) dt
du where = V , and is defined by Equation2.8 ( ). dt Thus, the final expression of the breech force, the recoil, is
ab au B = m projectile (b + u)2 b + u a2bu = m . (2.13) projectile (b + u)3
In order to determine the Le Duc parameters a and b, the following expressions are used. The formulas are determined by results from experimental firings and are adjusted to the used propellant. These formulas are published in references [41] and [51], and are formulated as follows:
[ ] w 1/2 a = α ∆1/12 (2.14) q [ ] [ ] W 3/2 ∆ b = β 0 1 − (2.15) w δ where
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α = potential of propellant, 1 w = weight of charge,
q = weight of projectile, same as mprojectile, ∆ = density of loading, β = powder constant, measure of the ”quickness” of powder,
W0 = volume of the powder chamber, δ = specific gravity of the powder.2 The empirical constants a and b can either be determined by inserting values of the used propellant into Equations (2.14) and (2.15), or by using of end state at the end of the muzzle. The second method requires knowledge about the muzzle velocity, the maximum velocity, and the length of the barrel. In order to determine the recoil, we consider the forces acting on the breech plate of the dispenser by applying the expression for pressure force P
B P = (2.16) Abore where B is the gas force on the breech, the recoil, and Abore is the cross-section of the bore, or of the flare material. In order to determine the maximum velocity of the projectile, we apply
d2V = 0 (2.17) dt2 where the second derivative is defined and described in reference [20]. From Equation (2.17), we get that the maximum breech force is generated when
b u = . 2 The maximum pressure on the breech is, thus, given when the breech force B is at its b maximum, hence when u = . Thus, the maximum pressure can be expressed as 2
Bmax Pmax = (2.18) Abore where ( ) ( ) 2 b a b 2 b 2 4a · mprojectile Bmax = B u = = mprojectile ( ( )) = (2.19) 2 b 3 27b b + 2
16823 for nitrocellulose propellant, a common propellant of the black powder. 2Usually between 1.5 and 1.6.
17 CHAPTER 2. THEORETICAL BACKGROUND which gives that the maximum pressure is
2 4a · mprojectile Pmax = . (2.20) 27b · Abore
If we know that the muzzle velocity is V0 when the flare material has reached the position u = U0, we get the following by inserting the end state into Equation (2.8)
aU0 V0 = . (2.21) b + U0
Rearranging the equation above gives the following expression for b
aU − V U b = 0 0 0 (2.22) V0 and by inserting this expression into Equation (2.20) we get
2 · ( 4a mprojectile) Pmax = (2.23) aU0 − V0U0 27 · Abore V0 which can be simplified to √ U P (U P )2 − 4KV 2U P a = 0 max 0 max 0 0 max (2.24) 2KV0 4 · m where K = projectile . 27 · Abore
2.2.4 Recoil Motion Profile by Le Duc By inserting typical values to the Le Duc parameters a and b given by Equations (2.14) and (2.15), the recoil motion will have the characteristic shape of the force-time curve presented in Figure 2.2.2. The results can be compared with the theoretical curve presented in Figure 2.2.3. In the figures, the y-axes describe the recoil force acting on the breech plate, and the x-axes show the travel of the projectile inside the barrel. The two curves in the figures represent characteristic curves of recoil motion asthey partly show a distinct increase in the pressure, or force, on the y-axis, which is given by the high derivative of the curve. The curves also show that the transition from a positive derivative to a negative derivative is not represented by an abrupt change, similar to a spike, but rather by a ”soft” switch given by higher pressure acting for an extended period. Both curves show how the pressure gradually decreases after the soft transition from the point of maximum pressure, or force, and fades in an almost exponential way to zero pressure. In Figure 2.2.3, the point of maximum breech force, the maximum b recoil, is indicated for when x = . Measures related to the characteristic recoil motion 2 behavior is presented in Section 3.3.
18 CHAPTER 2. THEORETICAL BACKGROUND
The purpose of presenting the theoretical approach of describing the recoil motion behavior is to provide other than practical test results to compare with in the results chapter, Chapter 4. Another purpose of stating a characteristic theoretical recoil motion curve is to understand the behavior and how the recoil emulator can be optimized accordingly. In the following chapter, the theoretical results presented in this section are applied to a real test result of a typical pyrotechnic flare provided by a flare manufacturer.
Figure 2.2.2: Resulting curve of the Le Duc approach when inserting typical values of the Le Duc parameters a and b into Equations (2.14) and (2.15).
Figure 2.2.3: Characteristic recoil motion profile based on the Le Duc approach, where the y-axis presents the recoil force and the x-axis describes the projectile travel. Collected from reference [20].
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2.3 Mechanical Shock
In order to make it possible to induce a force acting as the recoil force obtained from pyrotechnic flares in a recoil emulator, the science and engineering of shocks needto be addressed. A mechanical shock can be described as a physical phenomenon created by a sudden acceleration, for instance, by impact, drop, kick, earthquake, or explosion [26]. Similarly, a mechanical shock event can be described as dynamic loading, for where the duration is relatively short in comparison with the excited system’s natural frequency. Shock events are traditionally, but not required per definition, described as having relatively high acceleration, which is a relative measure that is based on the system’s initial capabilities. [47] In the sense of wanting to reduce, decrease or attenuate the recoil of pyrotechnic flares, the mechanical shock needs to be addressed. If the desire is to reduce the peak acceleration in a system, this must be bought at the price of increasing the deflection. Hence, to lower the peak recoil of a pyrotechnic system, this naturally leads to an increment of the duration of the shock. The increment is due to the fact that the longer the distance, the more time is required to traverse the distance. [47]
2.3.1 Standard Shock Machines The history behind the mechanical shock and the advances within the subject is primarily derived from military applications and seismic engineering. Measuring shock and its impact within naval and civil engineering has been made since the Second World War when the first specific shock machines were developed. [47] At the beginning of the development of shock machines, there were two main types of shock machines. These were categorized into two categories; the pendular machine and the sand-drop machine. The first machine uses a hammer to create a mechanical shock by making the hammer fall in a circular motion. The hammer strikes a steel plate which was fixed to a specimen, a high-impact testing machine. The second shock machine utilizes drop as the shock-inducing system, where a table was set between two vertical guide columns and was free-falling into a sandbox. [24, 26] Today, the test facilities of measuring mechanical shock are, according to reference [10], classified into different categories, which are: • Free-fall machines - which is similar to the early sand-drop machines. The magnitude of the shock is controlled by varying the drop height and the mass of drop weight, which impacts the velocity and the acceleration. The height of the guide columns limit the effect of the impact. • Pneumatic machines - where the velocity is created by a pneumatic actuator and controlled by varying the pressure. • Electrodynamic exciters - where the shock is specified by either the shape ofan electrical signal, its amplitude and its duration, or by a SRS. • Exotic machines - these are designed to carry out non-realizable, abnormal shocks which are specific due to its amplitude and duration. The exotic machines are generally not compatible with standard shock machines as the performance
20 CHAPTER 2. THEORETICAL BACKGROUND
variables do not correlate with the desired shape as the shapes are generally not normal. There are many types of shock machines on the market being used in shock analysis. Some examples are the standard shock systems, high-performance shock systems or a high cycle shock system [27]. Though, it is essential to remember that a shock machine is, independent of the chosen standard configurations, primary a device which allows for modification over a short period of the velocity of the material to be tested. Thereare two principal categories of shock machines which are, according to reference [26], usually distinguished by: • The ”impulse” machine - a device that increases the velocity of the test item during the shock. This means that the initial velocity is zero. • The ”impact” machine - a device that decreases the velocity or changes the direction of the test item during the shock test.
2.3.2 Shock Testing Requirement According to reference [10], there is no general requirement that a shock testing machine should be able to reproduce the same shock environment that is intended to be simulated. Instead, the requirement is that the shock machine should be able to provide a shock test that is acceptable under the shock environment conditions. The requirement states that there is a need for an assurance of the conditions and a comparison to real test results of shock conditions. The acceptance level of the requirement is generally set to a level for where the desired shock should be measured in advance. The shock testing machine should thereby be able to capture important characteristics of the shock or have a damage potential for which is, by analysis, similar to the shock that occurs in the real shock environment.
2.4 Pyrotechnic Shock
In reference [12], there is a described difference between a mechanical shock and a pyrotechnic shock. A shock induced by pyrotechnics differs from mechanical shocks in the sense that there is ”very little rigid-body motion (acceleration, velocity, and displacement) of the structure in response to the pyroshock” [12]. By measuring the time-history of the acceleration of pyrotechnic shocks on structures, the acceleration can be described as being oscillatory, and can be approximated with a combination of decayed sinusoidal accelerations with a short duration in comparison with mechanical shocks.
2.4.1 Pyroshock Applications and Validation In the field of science of shock analysis, a pyrotechnic shock is calleda pyroshock and is defined as a shock with high intensity, as in inducing high frequency and high acceleration. Pyroshocks are often characterized by its high-frequency content (100 Hz to 1 MHz), high peak acceleration (300 g to 300 kg), and short duration (10 µsec to 20 msec). [33] Pyroshocks are common in the aerospace industry and are found in many space components or applications with pyrotechnics. Pyroshocks are induced, for instance,
21 CHAPTER 2. THEORETICAL BACKGROUND during the separation of stages of a multistage rocket during a flight of satellites. Pyroshocks are also found within defense industries in applications or devices such as missiles or pyrotechnic flares [47]. In pyrotechnic flares, a pyroshock is created by an explosion of the black powder in the electric squib. Additional applications of pyroshock devices include gas generators, latches, explosive nuts and bolts, pressure squibs, and air-bag inflators [12]. The devices, components, or applications that induce a pyroshock generates a shock which is characterized by extreme levels of acceleration at very high frequencies [26]. The firings of these pyrotechnical charges generate severe impulsive loads, whichcan cause significant failure in electronic components. Components and fixtures subjected to pyroshocks are challenging to validate in the sense of using predictive models of the dynamic behavior as these are of complex structure. The validation, therefore, relies on testing the design of each component and fixture to check for failure criteria. [15]
2.4.2 Characteristics of Pyroshock During the detonation of pyrotechnics in devices that induced pyroshocks, the pyroshock produces high-frequency transients which propagate in the surrounding environment of the hitting area. These acceleration transients depend on various parameters which specify the transient, and are according to reference [12] defined by, but not excluded to: 1. the type of pyrotechnic source, 2. the geometry and properties of the structures and, 3. the distance from the pyrotechnic source. As these parameters can be combined into infinity with endless combinations, the conclusion of pyroshock characteristics cannot be made. However, typical behavior and characteristics of pyroshock can be made within the area of these parameters.
Near-field and Far-field A typical characteristic of pyroshocks is defined by the third parameter in the list above, as in, where the pyrotechnic source is located with respect to the components or fixtures of interest to test. According to reference [12], pyroshocks may be divided into two general subgroups or categories: • Near-field pyroshock • Far-field pyroshock. In the near-field pyroshock, the pyroshock occurs close to the pyrotechnic source before significant energy has propagated through the fixture as a structural response. Typically the peak accelerations of a near-field pyroshock is much higher than 5000 g and is located closer than 3 cm from the point of pyrotechnic source [15]. The near-field pyroshock is dominated by the input from the source of shock and contains very high acceleration at very high frequencies. The distributed energy in the near-field pyroshock has a wide frequency range and is generally not dominated by a few selected frequencies.
22 CHAPTER 2. THEORETICAL BACKGROUND
In the far-field pyroshock category, the pyroshock of significant energy occurs at agreater distance from the pyroshock environment. This allows for structural responses to develop in the fixtures. Typically, far-field pyroshocks occurs at more than 15 cm from thepoint source and have peak accelerations below 1000 g [15]. The far-field pyroshock contains a lower frequency with lower acceleration compared to the near-field pyroshock. The energy of far-field pyroshock is usually concentrated at a few frequencies.
The acceleration of a pyroshock is characterized by the distance from the pyroshock event, as in if the event is a near-field pyroshock or a far-field pyroshock. In thenear field, where the pyroshock occurs very close to the explosive event, the acceleration of the pyroshock is at a high amplitude and high frequency, usually above 10,000 Hz. The near-field pyroshock transients may have a duration of microseconds or less. In thefar- field, the acceleration can be approximated with a combination of decayed sinusoids with one or more dominant frequencies, which are usually less than 10,000 Hz. [12]
In comparison with mechanical shock, a far-field pyroshock usually have much higher dominant frequencies and often reflect the responses of the structures surrounding the pyroshock environment. The far-field pyroshock structural response can be found in applications where there is a sudden release of energy, such as when a projectile inside a barrel or the special material inside a pyrotechnic flare is fired. The detonation ofthe explosive force device produces high-frequency transients in the surrounding structures, fixtures, and components. [12]
2.4.3 Simulation and Testing of Pyroshock Within the field of aerospace engineering and in the space industry, simulation and testing of pyroshocks with devices and apparatuses are commonly used [33]. Simulations of the shock environment are usually done to be able to expect a similar behavior to essential components in those environments.
The problem with mechanical shocks and pyroshocks is their principal characteristic of shock in the field of their variety. This means that shocks cannot be defined precisely due to their range of variations in terms of acceleration, frequency, and amplitude. Simulations and emulations of mechanical shocks and pyroshocks can, therefore, never exactly be duplicated or copied under the conditions of the shock environment. [12]
As described in reference [12], pyroshock simulation and testing techniques can be divided into two categories depending on how the shock is induced:
• Pyrotechnically excited simulations
• Mechanically excited simulations.
Depending on the requirements of the test, each of these categories is needed to be applied. In reference [12], guidelines of when to use which of the two sorts of test and simulation machines are described. When a test requires control over the frequencies for above 10,000 Hz, in other words, for near-field pyroshocks, a pyrotechnically excited simulation and test equipment are generally needed. For a test that requires control over frequencies up to 10,000 Hz, as in the far-field pyroshock category, mechanically excited test equipment is needed.
23 CHAPTER 2. THEORETICAL BACKGROUND
According to reference [33], pyroshock testing can be achieved by applying one of the three implementation methods for sources:
• an explosive device,
• impact of a structural member upon another, such as a hammer upon a beam, plate, shell or combination of these,
• a vibration exciter or shaker programmed to generate short-duration transient motion.
In the first described source method, a pyrotechnic device test equipment is used toinduce an explosion. These test machine are used to create an actual explosion to reproduce the anticipated and desired shock spectrum. A pyrotechnic device test machine can be highly accurate if the test is made with the real structures and fixtures. The problem with explosive devices is that these are generally costly and requires specialized facilities and expandable fixturing. These are also very hazardous and requires excellent knowledge and control of the test. [35]
As described in previous sections, the simulations provided by shock testing machines cannot give perfect nor exact results. The simulations and testings of pyroshocks are still performed even though they cannot give exact results, as described in Section 2.4.1. The development of shock testing equipment to be used in the simulation of pyroshocks is, for instance, done in collaboration between institutes, universities and private companies. Such collaboration is for instance found between California Institute of Technology (CALTECH) and NASA at the Jet Propulsion Laboratory (JPL). [7, 33]
At JPL in California, USA, there exist several pyroshock testing machines. Historically, JPL has used a shaker or an impact device where a beam or a plate is excited by an explosive device or by a hammer-type impact [33]. As this thesis intends to simulate the recoil and pyroshock from pyrotechnic flares, a far-field pyroshock, without the need for pyrotechnics, a mechanically excited simulation test machine will be designed, discussed and evaluated.
2.4.4 Mechanically Excited Far-field Test Machines A mechanically excited test machine provides a short-duration mechanical impact on a structure that can cause a response similar to what is induced by a pyrotechnic source. Such a machine can usually carry out simulations with lower cost and better control in comparison to pyrotechnically excited test equipment. [12]
Within the mechanically excited far-field pyroshock testing, there are several pyroshock simulation techniques. A few of them are listed in reference [12], described as:
• Standard Shock-Testing Machines,
• Electrodynamic Shakers,
• Resonant Fixtures.
24 CHAPTER 2. THEORETICAL BACKGROUND
Standard Shock-Testing Machines As described in Section 2.3.1, there are several types of standard shock-testing machines. A standard shock machine with a drop-table method for inducing the shock is usually not suitable for pyroshock simulation. Due to the significantly higher velocity change in a drop-table machine, this method of simulation technique produces a severe over-test result at a low-frequency with much higher stress impact than desired. [12]
Electrodynamic Shakers Simulation of pyroshocks can be done by using an acceleration transient produced by an electrodynamic shaker. When using an electrodynamic shaker, the frequency of the desired pyroshock is synthesized in the test machine to match the anticipated SRS as closely as possible. This method of simulation is commonly used when there are relatively complex SRS shapes to be matched, with close tolerances of up to 3000 Hz. The equipment that is used in the tests are limited and restricted to tests with low- energy pyroshock environments. Even if the SRS of the desired pyroshock is matched, the results of the tests are likely to be over-tested due to the high mechanical impedance of the shaker, relatively to the structures to be tested. [12, 35]
Resonant Fixtures Within the test technique of using Resonant Fixtures, there exists a variety of ways to simulate pyroshock environments. All of these varieties utilize the same method of having a fixture or structure which is excited into resonance by a mechanical impact. The mechanical impact can be a projectile, a hammer, an air-gun, a pendulum, or some other device. The desired test item to be simulated for pyroshocks is attached to a fixture or structure and subjected to a resonant response, which simulates the anticipated pyroshock. Each of these varieties has its methods and relative merits. Some of the methods require extensive trial-and-error iterations in order to achieve the desired test results. These test machines are often difficult to tailor to the SRS. Though, once the procedure of reproducing the desired results is determined, the results are very repeatable. [12, 35] A well-developed resonant fixture method, which is widely used in the aerospace industry, is the Mechanical Impulse Pyro Shock Simulator (MIPS). The apparatus is developed by NASA at the JPL and utilizes an aluminum plate and a pneumatic actuator to simulate the pyroshock. The plate is excited into resonance by the shock induced by the actuator. There exist interchangeable impactor heads on the pneumatic actuator, which can be changed to alter for different pulse durations. The heads are made in either lead, aluminum, or steel. The machine produces high-frequency energy, and the results are extremely repeatable. [12, 36]
25 CHAPTER 2. THEORETICAL BACKGROUND
2.5 Previous Related Work
Below follows a description of previous related work relevant to the thesis. Research has been done in the fields of recoil measurement, modeling and analysis, and in thefieldof mechanical shock and pyroshock testing. Some of the references mentioned in the section are used later on as references to method choices, ways of modeling, and in the designing of the recoil emulator.
2.5.1 Recoil Related Most previous work made in the subject of recoil measurement is done on weapons. Several articles, reports, and projects have engineered apparatuses or optimizations models intended for measurement of recoil from hunting riffles, small weapons, or sporting arms. The apparatuses are used during shooting, measuring the actual recoil force from the weapon. In references [19], [21] and [23], recoil measurement apparatuses are constructed and used to measure the recoil of different guns. The methods of these references will be used in the sense of measuring recoil and analyzing the results obtained from the tests. In reference [30] and [52], optimization design structures are presented with models of the interior ballistics of a gun. The simulation and optimization models presented in the articles will be used as references to model the recoil of the flares. Reference [3] presents a recoil motion theorem that gives excellent knowledge of the time of travel of the projectile in the sense of recoil. This recoil motion theorem presents a theoretical approach to manage the time of the recoil, which will be used as a reference to the optimization model. As this thesis intends to construct a recoil emulator that can generate a recoil force without pyrotechnics, the previous related work presented above is, in some sense, not entirely relevant. By the above presented previous research regarding recoil modeling and measurement, the assumption presented in Section 2.2.1 can be supported further. By applying the assumption of flares having the same recoil motion behavior as guns, this enables the possibility of a comparison between the results obtained from the testing of the recoil emulator with the results obtained in above presented previous related work and models of recoil motion.
2.5.2 Mechanical Shock Related In the field of science of mechanical shock, most previous research provide an analysison how different bodies, structures, and materials are affected by vibration and mechanical shocks. Several articles describe field studies, experiments, or inspection of how materials are affected by the exposure to mechanical shocks, for a long or short duration. Examples of research made in the field of mechanical shock are partly made on the pervasive SRS. These researches are generally aiming to analyze the environment of shocks or evaluate the product fragility and potential design parameters of external cushioning for shock protection to be used, for instance, in transportation, production, or fatigue determination for drop testing. Examples of articles and journals in these research areas of shock response and shock protection can be read in references [1], [2], and [18].
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2.5.3 Pyroshock Testing with Mechanical Impact Related Within previous published work of pyroshock testing related to mechanical impact methods, most articles, journals and reports present development, review or evaluations of different test machines or equipment. In reference [15], the authors present a review of a test facility by Alcatel ETCA. The test facility holds several test equipment to be used for simulating the pyroshocks of different types. The facility utilizes a resonance test fixture assembly, which is excited by a mechanical impact or a detonating charge. Three of the test equipment utilizes mechanical impact as the shock generating devices, namely a projectile fired by an air-gun, a sledgehammer, and a pneumatic piston. Themethod of testing the equipment and the design possibilities of generating and controlling the mechanical impact presented in reference [15] will be used as a reference to this thesis in the method, presented in the following chapter.
Presented in references [5] and [11] is the development of pyrotechnic shock simulation apparatuses for space applications. The design of the apparatus in the first reference is inspired by the MIPS simulator and utilizes a resonant plate. The configuration of the apparatus described in the article is of less complexity than the MIPS simulator. Some of the design choices made in the article, such as the solution of the sledgehammer configuration, will be used as a reference and inspiration to the designing of therecoil emulator of this thesis.
In reference [29], the authors have designed and developed a pyrotechnics shock simulation apparatus to be used in spacecraft applications. Again, the NASA invention MIPS is used as inspiration of the apparatus in this report, as both of the apparatuses uses an pneumatic actuator which hits on a resonance plate and creates an impact with high acceleration motion and high frequency. The way of presenting the resulting graphs in the article will be used as reference to this project, for where the authors chose to present the results with acceleration and time on the axes.
Reference [32] presents a similar development of a pyroshock simulation device as presented in reference [5]. Both of these references have developed test machines to be used in spacecraft applications and utilizes the mechanical shock impact method of swinging a sledgehammer on a resonant plate. The description of the test machines, presented in reference [32], will be used in this thesis, as in the way the machine is described and presented in the thesis. The authors of both reports states that they chose the sledgehammer design due to its simplicity, low cost, and large available test area. The design and test configuration used in these articles will partly be used or inspired by in the making and designing of the recoil emulator in this thesis, namely the design of the pendulum sledgehammer.
Compared to references [5] and [32], reference [16] has developed an impact testing machine to be used to simulate pyroshocks with a mechanical impact method of using a Hopkinson pressure bar. What a Hopkinson bar is and how can be used in testing can be read more about in reference [37]. Reference [16] describes and explains how the machine is affected by adding a contact material between the Hopkinson bar and the impact area. In previous presented references, the material to be used between the bar and the hitting area is described as lead, aluminum, or steel. In this article, the developers have used tungsten to affect the response of the shock in the resonant plate. The materials choice might be used to dampen the effects of the impact in the design of the emulator.
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In addition to the references on developed impact machines presented above, there are also several patents on pyroshock machines. The United States Patent Office provides several of these patents on shock testing machines. Some of these designs will be used as inspiration to the designing of the recoil emulator of this project. One of the patents that hold in the designing of a shock testing apparatus was given in 1991 by the United States Patent Office [46]. The apparatus utilizes a pendulum hammer that creates an impact in a longitudinal direction. Another patent of a pyroshock testing machine is given on a pendulum impact tester, presented in reference [40], for where a pendulum sledgehammer is designed to induce a mechanical impact on the specimen. The design of the pendulum will partly be used in the designing of the recoil emulator, particularly on how the scale is attached and the construction of holding the sledgehammer.
2.5.4 Comments on Previous Related Work The difference between the presented previous related work done in the subject of pyroshock testing and simulation is that the machines or apparatuses that are being designed or validated are mostly constructed to simulate the pyroshock of space applications, such as rocket boosters or multistage separations. In comparison with the aim of this thesis of simulating the pyroshock achieved by pyrotechnic flares, the force- time curves may be different as the pyroshocks are of different kind than the pyroshocks that are excited in spacecraft components. A pyroshock induced by rocket boosters or other devices have a ”direct” impact when hitting, rather than the ”build-up” behavior a projectile or a pyrotechnic flare may have. This means that the references with belonging machines and apparatuses adapted for space applications are relevant to some extent, as in how they are designed and validated, but not in the sense of how the force curves look like in general. This behavior of having a delayed peak in the impact testing is what characterized the recoil motion of having a pressure build-up inside a barrel, compared with the almost instant pyroshock of rocket boosters. To conclude, there are not many articles that present relevant work related to this project in the sense of constructing a shock machine imitating the force curve of pyrotechnic flares. The presented work of references above still plays a significant roleinthe inspiration for the design and modeling of the recoil emulator.
28 Chapter 3
Method
This chapter describes the work done in the thesis. The method is divided into five parts. Firstly, the chapter begins by presenting an overview section to provide clarity and transparency of the chosen method.
3.1 Overview of Method
Below is a short overview of the method presented in this chapter. It is divided into five sections and described as follows, where the number represents the section number of this chapter. 3.2 Understanding and interpretation of real test data received from a flare manufacturer on pyrotechnic flares, combined with the theory of recoil from the theoretical background presented in the previous chapter. 3.3 Relevant measures derived from previously presented interpretation, which distinguishes the recoil motion behavior in question. 3.4 An optimization model to be used as validation of the recoil emulator based on the measures from the previous section. 3.5 The design process of the recoil emulator, with additional requirements and possible solutions to adapt and control the emulator. 3.6 The testing of the emulator, with the test environment, set up, used instruments, and a description of the iterative test method.
3.2 Interpretation of a Real Test Result
As described in reference [12], in order to measure the shock and build a shock testing machine, previous results need to be analyzed to understand which parameters that are of importance in the designing of the equipment. In this chapter, an interpretation of the theory of flares is applied, discussed, and used in the development of an optimization model. The model is used in order to understand the behavior of pyrotechnic flares. The optimization model will be used to validate the recoil emulator later on in the thesis.
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The interpretation of theory is made by applying the theory presented in Section 2.2 to a real test result obtained from a previous firing of pyrotechnic flares. The interpretation is made to make it possible to interpret how the flares behave and to be able to distinguish which parts that are essential to take into account when designing the recoil emulator. By combining the theory of recoil motion sequence of flares from the previous chapter, presented in Section 2.1.2 and illustrated in Figure 2.1.3, with the characteristic recoil profile, presented in Section 2.2.4, Figure 3.2.1 is obtained. The figure shows a theoretical result of recoil motion for an arbitrary, characteristic motion curve together with marked periods of the sequence. The figure also presents the pressure build-up inside agun barrel, which in theory, is the same pressure build-up as inside a flare body. Due to the pressure being linearly dependent on the force, as
F P = A where P is the pressure inside the flare body, F is, in this case, the recoil force, and A is the constant bore area of the flare material, or a projectile, Figure 3.2.1 will have the same shape as Figure 2.2.3 which is illustrating the Le Duc approach.
Figure 3.2.1: Characteristic recoil motion curve with time periods in bore period (projectile in tube) and aftereffect period (gas exhaust aftereffect) exposed. The constants pa and tn describe the pressure during the point for when the projectile exits the muzzle of the barrel, and the time it takes before the pressure inside the barrel is normalized, respectively. The figure is collected from reference [8].
By applying the theory behind recoil motion of the periods and sequence of flare on a real test result, the test result can be interpreted and analyzed accordingly. Figure 3.2.2 presents a test result of a force-time curve obtained from a firing made by a manufacturer of flares. The flare used in the test is an arbitrarily chosen flare, typical forusagein CMDS products world-wide. The values and units are consciously removed, as this are not relevant to the case of distinguishing the behavior and the periods of recoil motion.
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Figure 3.2.2: Recoil profile obtained from a test firing of a typical and arbitrarily chosen flare, where the The y-axis present the recoil and the x-axis time.
By extracting the curve from Figure 3.2.2 and inserting the theoretically described periods of the recoil motion and add an interpretation of when the flare leaves the flare body at the muzzle, the result presented in Figure 3.2.3 is obtained. Note: the markings of each period and when the flare exits the flare body are done as an interpretation andarenot confirmed by tests. The interpretation may be inaccurate but used as a baseline inthe following discussion regarding necessary measures, described in Section 3.3.
Figure 3.2.3: Interpretation of a real test result obtained from a firing performed by a flare manufacturer. In the figure is the in bore period called Flare material in flare body, and the gas exhaust aftereffect is called Aftereffect period. On the y-axis is the recoil, denoted with B, meaning breech force, and on the x-axis is time.
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Interpretation of Figure 3.2.3
In Figure 3.2.3, the three periods of the recoil motion are presented, where the in bore period, compared to Figure 2.1.3, is called Flare material in flare body. The flare material in flare body period of the force-time curve looks similar to the theoretical results presented in the previous chapter in the sense of having a smooth build-up with a shape that can almost be described with mathematical models. When comparing Figure 3.2.3 with the theoretical results of Figures 2.2.2, 2.2.3 and 3.2.1, there is a difference at the beginning of the graphs. In Figure 3.2.3, the recoil force is close to zero for a longer time than in the theoretical results before the measured recoil starts to increase. As described in Section 2.1.2 is the ignition of the electric squib included in the definition of the in bore period. An interpretation in this case of Figure 3.2.3 is, therefore, that the time for when the recoil is zero could be the ignition period, which is before the flare material has started to move inside the flare body.
The breaking point of the transition between the Flare material in flare body and Aftereffect period is in Figure 3.2.3 called Flare material exits flare body. This point describes the time when the flare material leaves the flare body at the muzzle. The point can be interpreted to be positioned at this point, as after the point, the force curve is more non-predictable, with force not decreasing in a ”smooth’ way, as compared to the ”soft” force build-up that occurs during the in bore period. Hence, it can be interpreted that the pressure difference is starting to normalize when the flare material exits theflare body.
The following period illustrated in the figure is the inertia period, for where the recoil, in theory, should be zero. In this case, the recoil is not entirely zero during the inertia period, which might be due to noise or measurement errors.
Validating the Real Test
Figure 3.2.3 differs from the theoretical results presented in Figures 2.2.2, 2.2.3 and 3.2.1 in two principal ways, partly in what happens at the beginning of the graph during the ignition period, and the behavior that follows in the aftereffect period. The shape of the force curve that ensues after the point where the flare material leaves the flare body cannot be described with mathematical models since the curve is not smooth. The shape of the curve after the point where the flare material leaves the flare body has an irregular shape as the points do not have a constant gradient and are ”jumpy”. The irregular shape of the jumps may be due to the pressure does not decrease linearly in the flare body for various reasons.
What is essential to keep in mind is that the theoretical results that attempt to describe the recoil force behavior rarely coincide with real tests, as described in Section 2.2.3. The theory can indicate what the behavior can look like but does not describe it accurately. The curve used in the interpretation is an arbitrarily chosen case of a firing and shows how the recoil force behavior looks in reality. The real test result can, thus, be interpreted as valid to be used in comparison with results from the recoil emulator, as the result presented in Figure 3.2.3 is principally similar to the theoretical results shown in Figures 2.2.2, 2.2.3 and 3.2.1.
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3.3 Measures of Recoil Motion
The goal of this thesis is to present a recoil emulator that imitates the recoil motion behavior of pyrotechnic flares as pleasant as possible. In order to make this achievable, there need to be some design parameters to compare the results obtained in the recoil emulator with desired and anticipated results. These design parameters will further on be called measures of recoil or recoil measures, and are chosen by their importance to the recoil motion.
In order to optimize the recoil emulator, the recoil measures are used in an optimization model to determine the error between data points from the recoil emulator and the real test result. The optimization model is described in the following section, in Section 3.4. The selected recoil measure are selected together with experts within countermeasures and are described as the Peak Recoil, Impulse, and Peak-Width. These are, according to experts within countermeasures, useful measures that can describe fundamental characteristics of the recoil motion.
3.3.1 Peak Recoil
The first recoil measure is the peak of the recoil force, hence, the highest value that the recoil curve will reach on the force-axis. The highest value of the force curve can represent the highest pressure that is evoked in the pressure chamber of the flare body. The recoil measure of the peak recoil is added to Figure 3.2.3 and is shown in Figure 3.3.1. The peak recoil is important to consider when designing the recoil emulator, as the peak recoil describes the highest force a structure or fixture has to be designed for, in the sense of structural mechanics. Therefore, to successfully emulate the recoil motion of pyrotechnic flares in the recoil emulator, the peak recoil needs to be as close tothe value of the real test result as possible.
Figure 3.3.1: Peak recoil force added to Figure 3.2.3. The peak recoil is the highest measured recoil force of the curve, and is an important measure to consider when modeling the recoil motion of pyrotechnic flares.
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3.3.2 Impulse
The second recoil measure is the impulse of the system. The impulse represents the momentum of the flare material, hence, the total energy that is induced by the recoil mass during the pressure build-up inside the flare body. The impulse is important to consider when designing the recoil emulator, as the recoil behavior is partly characterized by this. The impulse can be described mathematically by taking the integral of the breech force, the recoil, over the time that the recoil is acting. The impulse is marked in Figure 3.3.2 as the gray area under the force-time curve.
In order to determine the total energy of a system in motion, the impulse needs to be considered. Mathematically, the impulse can be described as
∫ I = Bdt (3.1) where B is the breech, or recoil, force, and dt is the combined time periods of the flare material in flare body period and the aftereffect period.
The reason why the last period, the inertia period, is not included, is that during this period, the recoil force is, theoretically, zero. During the inertia period, the pressure difference between the pressure inside the flare body and the atmosphere should havebeen normalized, thus, potentially measured impulse during this period should be neglected. In the figure, the recoil force is not zero during this period. The reason forthismight be due to noise or other measurement errors, as mentioned in Section 3.2. The area between the curve and the time-axis during the inertia period is, thus, not included in the calculations of the impulse of the recoil motion.
Figure 3.3.2: The total energy induced by a recoil mass is defined by the impulse. The gray area in the figure represents the impulse and is described by Equation3.1 ( ).
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3.3.3 Peak-Width The last recoil measure to consider is the peak-width. The measure determines how long the recoil is ”staying” at a certain level of recoil force, hence, the time that a selected interval of the recoil is acting. The peak-width characterizes the smooth shape of the tip of the force-time curve. In order to imitate the recoil motion of the pyrotechnic flares as accurately as possible, the peak-width must be included as a measure in the design of the recoil emulator. In this case, the time of the peak-width is, in agreement with experts within countermeasures, set to being ≥ 70% of peak recoil for dt time. This means that the recoil emulator should reach a recoil force that is greater than or equal to 70% of the peal recoil force obtained from the real test during a time dt. The peak-width is illustrated in Figure 3.3.3, for where the time dt is marked as the 70% of the peak recoil force.
Figure 3.3.3: Representative description of peak-width, agreed with experts within countermeasures, to ≥ 70% of the peak recoil from the real test during dt time.
3.4 Optimization Model for Validation
In order to validate the results of the recoil emulator concerning the recoil measures presented in previous section, an optimization model is constructed and applied. The objective and constraints are presented below with respect to the above stated recoil measures of the peak recoil, the impulse and the peak-width.
3.4.1 Objective Function To be able to determine the accuracy of the recoil emulator, the three recoil measures are applied in an optimization model. The objective of the optimization model is chosen to be the accuracy of the recoil emulator. The accuracy can be determined by applying a model of the error between data point provided from the the recoil emulator and the real test result. For this case, the error is arbitrarily selected to be modeled as a quadratic, absolute error, also known as the squares of errors. There may be other measures that are
35 CHAPTER 3. METHOD more reasonable to use in data point comparisons, but in this case this measure is chosen arbitrarily. The chosen error model of the squares of error can be described as
2 e = ||∆real test − ∆recoil emulator|| (3.2) where e is the absolute quadratic error of data points between recoil measures from the recoil emulator, ∆recoil emulator, and the real test, ∆real test. In order to make the recoil emulator as similar to the real test result, e should be kept as small as possible. An optimal solution of the recoil emulator is, thus, given when e is zero, hence, when the results from the recoil emulator exactly mimics the recoil measures from the real test. The error e is used as an objective in a minimization model, for where e is to be minimized. The minimization of e is mathematically described as
min e. (3.3)
Since e depends on data points from the recoil emulator and the real test result, e can be described as a cost function which depends on data point from each of the three recoil measures of the peak recoil, the impulse and the peak-width. The cost function is here defined as
min e = f(∆Bmax, ∆I , ∆dt). (3.4) where f is the cost function which depends on the difference between data point of the recoil measures from the recoil emulator and the real test. In the cost function is Bmax the peak recoil, I the impulse and dt the peak-width, where the difference from each data point of the real test and the recoil emulator is described as − • ∆Bmax = Bmax,real Bmax,emu
• ∆I = Ireal − Iemu
• ∆dt = dtreal − dtemu where Bmax,real is the peak recoil from the real test and Bmax,emu is the peak recoil from the emulator, Ireal and Iemu describe the impulse from the real test and the emulator respectively. The variables dtreal and dtemu describe the peak-width of the real test and the emulator, respectively. The cost function can thereafter be described as
|| ||2 || ||2 || ||2 min f = ∆Bmax + ∆I + ∆dt . (3.5)
As each of the three recoil measures have different units, the cost function f must be weighed against each measure. The weights are chosen based on the prefix before the units to reflect the impact and influence of each measure as equally as possible, tonot make one of the measures weight more than another as they are all of equal importance.
The selected weights are in the model denoted wBmax , wI and wdt.
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The final objective cost function for each data point of the difference between datafrom the real test and the recoil emulator can defined as
|| ||2 || ||2 || ||2 f = wBmax ∆Bmax + wI ∆I + wdt ∆dt (3.6)
where the weights wBmax , wI and wdt are known and depend on the used unit in the 1 measurements. For instance, if the used unit is kN, then the weight is a factor 1000 to compensate for the unit and convert it to the SI-unit. Due to confidential information, the weights will not be presented within this thesis.
3.4.2 Constraints
In order to further accurately imitate the recoil motion of the pyrotechnic flares into the recoil emulator, the optimization model needs levels of acceptance to be ”good enough”, or in other words: constraints. These constraints are based on the three recoil measures, and their intervals are arbitrarily selected together with experts within countermeasures. Note here that units are not included as this is confidential information and can therefore not be shared. The following constraints are based on the recoil measures presented in the previous section:
1. Peak recoil: This measure should be kept between an interval of 5% of the peak recoil value expected in the real test as the peak recoil is of great importance in the designing of CMDS products.
2. Impulse: As this measure describes the total energy that is induced in the system, this measure should be as close as possible to the real value. Thus, the interval of the constraint is set to 10% of the impulse from the real test.
3. Peak-width: In order to determine the accuracy of the recoil emulator, the recoil from the emulator should be ≥ 70% of the peak recoil from the real test during a certain time dt. The interval is chosen by experts within countermeasure to be within the 1-2 time units, thus, the peak-width of the results from the recoil emulator should be between 1-2 time units.
These constraints can mathematically be described as the following, where Bemu is the recoil from the emulator.
1. 0.95 · Bmax,real ≤ Bmax,emu ≤ 1.05 · Bmax,real, ∀t ∈ R
2. 0.90 · Ireal ≤ Iemu ≤ 1.10 · Ireal, ∀t ∈ R
3. Bemu ≥ 0.70 · Bmax,real, : dtemu = [1, 2] time units.
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3.4.3 Optimization Problem Combining the objective cost function of f with the constraints, the following optimization problem to use in order to minimize the error e is obtained:
|| ||2 || ||2 || ||2 min e = f = wBmax ∆Bmax + wI ∆I + wdt ∆dt
subject to 0.95 · Bmax,real ≤ Bmax,emu ≤ 1.05 · Bmax,real, ∀t ∈ R
0.90 · Ireal ≤ Iemu ≤ 1.10 · Ireal, ∀t ∈ R B ≥ 0.70 · B , : dt = [1, 2] time units, emu max,real emu (3.7) − ∆Bmax = Bmax,real Bmax,emu,
∆I = Ireal − Iemu,
∆dt = dtreal − dtemu,
wBmax , wI , wdt known constants.
3.5 Design Process of Recoil Emulator
In this section, the process of conducting and designing the recoil emulator is presented and explained. As described in the background of pyroshock in Section 2.4.3, a shock induced by a projectile from a pyrotechnic flare is defined as a far-field pyroshock. Since far-field pyroshocks are most suited to be simulated with mechanically excited simulations, the design of the recoil emulator will be based on such a design to induce the impact. In Section 2.4.4, there are three possible solutions listed on how to simulate the far- field pyroshock with mechanically excited impact methods. As described, the impact method can either be a standard shock testing machine, an electrodynamic shaker, or a resonant fixture. Since most of the references presented in previous related work within the section of pyroshock testing, presented in Section 2.5.3, all used a resonant fixture as the mechanically excited impact system, this is the chosen impact system of the design of the recoil emulator as well. This is due to the amount of presented possible solutions found in previous related work, and the ability to compare results with apparatuses compressed in these articles and reports. The comparison is not made within this these, and can be made in future work. The design process begins by describing further specific design requirements that the recoil emulator must comply with. These are given by the company that demands the recoil emulator from the company in question, Saab.
3.5.1 Design Requirements In addition to the design parameters and mathematical constraints presented in the previous sections, Section 3.3 and Section 3.7, the company in question want the recoil emulator to be designed based on additional design requirements. These requirements follow below in a non-prioritized order, and describe mechanical and physical attributes that the emulator should be design for.
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The recoil emulator in question to be design should: • be built – meaning to not use existing machine of shock testing as they are not easy to manipulate or adapt to desired requirements, • be adaptable for different types of flares – as in having the ability to change the induced force to simulate different recoil forces, • be ”small” in size – not be very large in size, as in taking up a lot of space for storage and during testings, • be a movable test facility or fixture – preferably on wheels or with other possibility to be moved, • be customized for indoor testing, • not be very expensive to build, • be able to do repeated tests.
3.5.2 Possible Design Choices In order to fulfill as many design requirements as possible in the list above, andatthe same time, be based on a mechanically excited impact system, there are some possible design choices to consider. Based on the requirements listed above, with respect to the constraint of inducing the impact with a mechanically excited system, the following possible solutions exists. The possible solutions are taken as inspiration from reference [15] and are listed in Table 3.5.1. Each possible solution of the mechanical impact is presented with a short description of the pros and cons, to make it possible to determine the best choice for the design of the recoil emulator in this case. The pros of begin repeatable can be measured in different ways, where ”very repeatable” means that the impact method is very easy to develop with a, for instance, control system that controls and produces tests automatically.
Table 3.5.1: Suggestions for mechanical impact method to induce shock, with pros and cons presented, compiled with inspiration from reference [15].
Mechanical Impact Pros (+) Cons (−)
Pneumatic Piston Controllable & Very Repeatable Costly & Large in Size
Pendulum Sledgehammer Low Cost, Available & Repeatable Trial-and-Error Iterations
Air-Gun Fired Projectile Similar to Pyrotechnics High Risk & Complicated
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Reasoning about the Chosen Design Since the list of requirements presented in Section 3.5.1 consists of many hard constraints, for instance, the size of the emulator and its adaptability, all of the presented possible solutions in Table 3.5.1 are not suitable as design choices. Based on the requirement of being able to do repeated tests, the design choice of having an air-gun as the impact method disappears as an alternative. The difference between the pneumatic piston and the pendulum sledgehammer isthen the size of the construction. Having a pneumatic piston gives excellent controllability and the ability to do very repeatable tests. The problem with a pneumatic impact method is the size. In order to induce an impact significant enough to provide for the desired amount of shock, the actuator needs to be large. Since a high momentum is induced by high pressure, which requires a large actuator, this affects the size of the emulator, and the pneumatic impact method can, therefore, not be an alternative in this case. The chosen method of impact is, therefore, the pendulum sledgehammer, as this method fulfills most of the requirements in the list. The disadvantage of this impact method is the need for many trial-and-error iterations to determine the desired shock behavior. The disadvantage is taken into account in the designing and the construction and will be addressed by fixing a selection of the control parameters presented in the following section. The fixation of control parameters is done in order to minimize the numberof parameters that can be changed and customized of the impact method. The test method of obtaining an optimal result of the recoil emulator is conducted with iterations. The iterative process of finding an optimal force-time curve of the recoil emulator is described in detail in Section 3.6.3
3.5.3 Controlling the Mechanical Impact Narrowing down to the design choice of having a pendulum sledgehammer as the impact method provides a variety of control parameters to consider. The magnitude and shape of the resulting shock curves achieved by a pendulum sledgehammer can be controlled by changing the following control parameters: 1. Test fixture - type, material, size 2. Impact direction - perpendicular (⊥) or parallel (∥) to hitting area of test item, 3. Weight of impact mass, 4. Impact device speed - drop height or initial drop angle, 5. Distance from the impact point to test item, 6. Anvil material - material on top of flare, 7. Hitting tip material, 8. Damping control - rubber, metal et cetera. According to reference [15] should the hitting tip material be harder than the anvil material. The statement means that the materials on the sledgehammer must be harder than the material on the hitting area of the flare. Thus, the sledgehammer cannot be made out of a softer material than the hitting point of the flare.
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Fixed or Dynamic Parameters Based on the sledgehammer impact method, the above-stated parameters can be controlled and fixed. In order to minimize the number of parameters that maybe changed, a selection of the variables will be fixed from the beginning. Below follows a description of which variables that will be changed or not in the tests: Parameters which are fixed: (1) Test Fixture – Based on having the same fixture as is used in reality - loading the flare inside the magazine and in dispenser. (2) Impact direction – ⊥ to flare as a pendulum sledgehammer only allows for one direction of impact direction. (5) Distance from test item to impact point – Keep constant to try a specific position of the flare - may be changed in further tests but kept constant in this case to minimize the number of test cases and iterations in the iterative process. (7) Hitting tip material – Material of the sledgehammer will not be changed. Parameters which are changed in the iterative process: (3) Weight of impact mass – Weight of sledgehammer is changed in order to customize the impulse and the peak recoil. (4) Impact device speed – Drop height of sledgehammer is changed by increasing the angle from the hitting point (6) Anvil material – The material on top of the flare on the hitting point is changed with damping materials (8) (8) Damping materials – Modeled with different types of rubber to achieve the desired peak-width from the real test.
3.5.4 Selected Design Based on the mechanical impact method of having a pendulum sledgehammer, the selected design of the recoil emulator is presented in Figure 3.5.1. The recoil emulator is constructed with a simple sledgehammer attached to a pivot shaft. The sledgehammer is manually controlled by moving it to a desired angle and dropping it to the resonance
41 CHAPTER 3. METHOD plate and, thus, hits the flare. The movement of dropping the sledgehammer ona flare positioned in the chosen test fixture is representative to reality, for wherethe real pyrotechnic flares are loaded and positioned with the same configuration asused in the test fixture. A detailed description of the test fixture and the used mockupofa pyrotechnic flare is presented in Section 3.6.1.
Components of the Recoil Emulator The recoil emulator is constructed with nine unique components. Each component’s name, component number, and used quantity in the recoil emulator is presented in Table 3.5.2. Figure 3.5.1 describes the placement of each component with numbers and arrows on the recoil emulator as an overview image.
Table 3.5.2: Description of each component used in the construction of the recoil emulator, with number, name and quantity.
Number Component Name Quantity
1 Bottom Plate 1
2 Angle Bracket 2
3 M4 Screw 14
4 Supporting Frame 2
5 Cross-connection 1
6 Pivot Shaft 1
7 Rubber Ring 4
8 Sledgehammer 1
8a Sledgehammer Shaft 1
8b Steel Hammerhead 1
8c Ball-Bearing, SKF 608-2RS 1
8d Degree Indicator 1
9 Protractor 1
The recoil emulator is constructed with a thick aluminum bottom plate (1), or the base plate, which is customized to fit and be attached to the test fixture, described inthe following section. The bottom plate is assembled with the test fixture with a aluminum angle bracket (2) on each side of the bottom plate and four M4 Screws (3) for each angle bracket. Component (4) describes the supporting frame, constructed in aluminum sheet, which is stabilized with the cross-connection (5) at the top. The supporting frame and the cross-connection are mounted with a M4 screw on each side. A detailed description of the mounting and the positioning of the angle brackets and the supporting frame is presented in Figure 3.5.2.
42 CHAPTER 3. METHOD
Between the supporting frames is a pivot shaft (6) positioned. To ensure that the pivot shaft is fixated in the horizontal plane, tight rubber rings (7) are mounted onboth the inside and the outside of the supporting frame around the pivot shaft, displayed in Figure 3.5.3. Assembled on the pivot shaft is a specially made sledgehammer (8), which is described in detail in Figure 3.5.4. Figure 3.5.4a shows an overview of the sledgehammer, and Figure 3.5.4b displays a zoomed image of the top of the sledgehammer.
The sledgehammer is constructed with a sledgehammer shaft (8a), which is made of Acrylonitrile Butadiene Styrene (ABS) plastic with Additive Manufacturing (AM) technology to fit the steel hammerhead (8b) and the inserted ball-bearing (8c).The choice of having a ball-bearing between the pivot shaft and the sledgehammer shaft is to reduce the friction between the materials and to make is possible to transfer as much energy as possible during the dropping of the sledgehammer. The model of the ball- bearing is a standard bearing manufactured by SKF, model 608 2SR [48], with the same hole diameter as the pivot shaft. At the bottom of the sledgehammer is a degree indicator (8d) mounted in order to make it easier to see which angle the sledgehammer is positioned at on the protractor (9).
Figure 3.5.1: The selected design of the recoil emulator. The final construction is attached to a test fixture, which is presented in detail in Section 3.6.1.
43 CHAPTER 3. METHOD
Figure 3.5.2: Bottom plate and angle brackets assembled with the test fixture with the M4 screws, two screws for each supporting frame and four screws for each angle bracket.
Figure 3.5.3: Description of the positions of the rubber rings based on the position of the pivot shaft and the top of the sledgehammer.
44 CHAPTER 3. METHOD
(a) Overview of the special made sledgehammer. (b) Zoomed image of the ball-bearing.
Figure 3.5.4: Detailed description of the specially made sledgehammer with zoomed image of the ball-bearing position.
3.6 Testing the Recoil Emulator
The testing of the recoil emulator was done by hitting a mockup of a pyrotechnic flare, which was loaded in a real CMDS dispenser. Below follows a detailed description of the used test fixture, the test setup, and used test equipment. The iterative process offinding an optimal configuration of the recoil emulator is described in the end of this section,in Section 3.6.3.
3.6.1 Test Fixture In Figure 3.5.1, where the recoil emulator is presented, the used test fixture is visible in the back. The test fixture that is used in the testing is a setback force measurement test fixture, which is constructed based on US Navy standards within the countermeasure industry of flare testing. In this case of testing the recoil emulator, a CMDS product, the dispenser, is assembled in the test fixture. The test fixture is constructed to able be assembled with various types of CMDS products. The test fixture is developed by the Department of Defense (DoD) in the US with the purpose of measuring the setback force of Airborne Expendable Countermeasure (AECM) flares imparted upon the structure of aircraft. The dispenser used in the testing in this thesis is arbitrarily chosen, and is manufactured by Saab.
45 CHAPTER 3. METHOD
Presented in Figure 3.6.1 is a schematic view of the test fixture, where the Dispenser Magazine, Breech Plate, and the forces sensors are positioned. The dispenser is disassembled and mounted with the breech plate in the middle of it. The position of the breech plate in the dispenser makes it possible to measure the force that the dispenser is exposed to, as the dispenser is freely mounted and attached to the test fixture with the breech plate. The screws that hold the dispenser are covered by lubricant to prevent friction and other force absorbing mechanisms. Between the breech plate and the test fixture are four force sensors mounted, which measure the force induced by theimpact of the sledgehammer.
Figure 3.6.1: Schematic view of the used standardized test fixture with test equipment.
Position of Pyrotechnic Flare
As described in Section 3.5.3, the distance from the impact point of the test item is chosen to be fixed from the beginning. By this means that the initial position ofthe hitting point of the sledgehammer is fixed and that a certain position in the magazine is chosen. In the testing of the recoil emulator, a mockup of pyrotechnic flare is used. The mockup flare has the same size and weight as a real pyrotechnic flare, but insteadof consisting of the special material et cetera, the mockup is filled with another material, a filling material. The end cap of the mockup flare is made of aluminum.
In Figure 3.6.2, a schematic view of the position of mockup flare in the magazine and the test fixture is presented. The position of the mockup flare is arbitrarily chosen tobe approximately in the middle of the magazine. Figure 3.6.3 shows the front view of the test fixture with the position of the mockup flare.
46 CHAPTER 3. METHOD
Figure 3.6.2: Schematic view of test setup of the mockup flare position in the standardized test fixture.
Figure 3.6.3: Schematic front view of the position of the mockup flare.
47 CHAPTER 3. METHOD
3.6.2 Test Setup and Equipment In order to measure the force induced by the sledgehammer, the test fixture with the force sensors is connected to test instruments. A schematic view of the test set up is presented in Figure 3.6.4, where the test fixture, consisting of the dispenser magazine, breech plate, and the four force sensors, are displayed. In addition to Figure 3.6.1, Figure 3.6.4 illustrates the configuration of the instruments used in the testing. The figure also presents the original test fixture of the setback measurement fixture developed bythe DoD in the US. The measurement of the force was done by analyzing the signals that the force sensors perceive. The sensors detect the force by measuring the voltage as the breech plate is displaced when the sledgehammer hits the mockup flare. The signals from the sensors are processed in the signal conditioning box, which is connected to a Data Acquisition (DAQ) system. The signal conditioning box is manufactured by the same company as the force sensor, PCB Piezotronics, and is of model PCB482C05. The DAQ is manufactured by Keysight and is of model U2531A. The signals are processed and viewed by hardware installed on a PC laptop, connected by a USB to the DAQ. Each hitting of the sledgehammer is measured individually as tape recording is not used due to its low dynamic range and poor operational performances.
Figure 3.6.4: Schematic view of test fixture with used test equipment.
Force Sensors The four sensors that are mounted in between the breech plate and the test fixture are manufactured by PCB Piezotronics and are of model PCB208C05. The sensor is a multi- purpose force sensor with platform installation, which makes it possible to mount them between plates [38]. The positions of the sensors are presented in Figure 3.6.5, where there are four force sensors between the test fixture and the breech plate, mounted with screws on each side. When the sensors are mounted between the breech plate and the test fixture, there might be an initial compression of the force sensors, resulting in an installation offset.
48 CHAPTER 3. METHOD
This means that the force sensors might detect a force even though the system is in equilibrium. The offset value of the force is taken into account in the final resultsby subtracting the initial value that the sensors detect before the testing is initiated. The subtraction of the initial offset force will give the same shape of the force-time curveas before, only moved to start at zero force. In addition to the offset value of the force sensors that might affect the final result,the accuracy of the sensor may has an impact as well. According to a data sheet of the force sensors presented in reference [38], the sensor accuracy of each force sensor is 95%. By having four sensors, the accuracy of each force sensor is multiplied, giving a final accuracy of 95%4 ≈ 81%. The total accuracy can affect the final result, and is discussed in Chapter 5.
Figure 3.6.5: Schematic view of the force sensor positioning, where four force sensors are included in the test fixture.
3.6.3 Test Method As described and discussed in Section 3.5.2, the disadvantage of choosing the impact method of a pendulum sledgehammer is the need for many trial-and-error iterations. The test method of the recoil emulator is, thus, an iterative process of finding an optimal configuration that meets the constraints presented in Section 3.4.2. The iterative process is governed by the constraints that have been set up, which means that the process consists of three iterations. The iterative process is, thus, divided into three iterations, where each of the constraints presented in Section 3.4.2 is taken into account.
49 CHAPTER 3. METHOD
Since the recoil measures of the peak recoil, the impulse, and the peak-width depend on each other, there might be sub-iterations within each iteration that may not meet the chosen constraint of the iteration. As all sub-iterations are not of importance in the presentation of the results is only the optimal sub-iteration of each iteration be presented in the results of Chapter 4. By optimal means that the error is minimized, thus, the lowest e-value obtained from the iteration. The rest of the sub-iterations are presented in Appendix A.
Test Set up of Test Method
The test set up of the recoil emulator is described below in Figure 3.6.6. In the figure, the emulator is presented in a position where the sledgehammer is pulled upwards to a selected drop angle and released to hit the mockup flare. Each iteration will have the same configuration of holding and dropping the sledgehammer. The difference between each iteration is the configuration of the drop angle, the anvil material or damping material, and the weight of the sledgehammer.
Figure 3.6.6: Positioning of the sledgehammer before dropping it on the mockup flare.
50 CHAPTER 3. METHOD
Iteration 1 In the first iteration, the goal is to meet the first constraint of reaching a peak recoilthat fits the real test. In order to determine which drop angle is needed, the sledgehammer is released from different drop angles to measure the resulting peak recoil. In this iteration, there is only a change of one of the control parameters; thus, the impact speed, since the only parameter that is changed is the drop angle. The speed of the sledgehammer is affected by the drop height; hence, the drop angle.
The chosen number of drop angles that are tested in this iteration is arbitrarily chosen to be 45°, 60°, and 90°. Iteration 1, thus, has three sub-iterations. The optimal result of the first iteration is given when the peak recoil of the recoil emulator is within 5% of the peak recoil from the real test. The result from the first iteration is presented in Section 4.2, and the remaining sub-iterations, with non-optimal solutions, are presented in Appendix A.
Iteration 2 In the second iteration is the third constraint considered, the peak-width. As the first iteration resulted in the desired peak recoil described by Constraint 1, the drop angle from the first iteration will be fixed in Iteration 2. The peak-width can be controlledby attaching different kinds of damping materials on the anvil side of the hitting point, thus on the end cap of the flare. As described in Section 3.5.3, the hitting tip material should be harder than the anvil material. In order to minimize the number of parameters to change, the material of the sledgehammer head is fixed.
After consideration with experts within the area of countermeasure and pyrotechnic flares, the chosen damping materials to test are different types of rubber. The choice of only trying soft materials, in comparison to the previous related work described in Section 2.5 where a selection of the reports presented used different types of metals, is the ability to achieve a wide peak-width which can match the peak-width of the real test result. By using different types of rubber, the peak-width can be controlled by damping and smudging out the force-time curves of the impact from the sledgehammer.
The chosen rubber materials that are tested in Iteration 2 are:
• a thin rubber sheet, 1 mm - used for sealing and tolerances.
• silicone rubber, 2 mm - often used as a sealant in wet areas, from reference [4],
• Sorbothane, 5 mm - a damping material used in space components exposed to extensive shocks, from reference [49],
• silicone sponge rubber, 2 mm - used in various types of products for cushioning, from reference [50].
The damping materials are attached to the end cap of the mockup flare by using tape, as this makes it easy to attach and remove the different materials between each sub- iteration. As the second iteration tests four types of damping materials, there are four sub-iterations in Iteration 2. The optimal result compressed in Iteration 2 is presented in Section 4.3 and the rest of the sub-iterations are presented in Appendix A.
51 CHAPTER 3. METHOD
Iteration 3 In the last iteration, the goal is to meet the second constraint of obtaining an impulse within the chosen interval of the impulse provided by the real test. At the same time, the peak recoil and the peak-width should be kept at the desired levels described by the constraints. As these steps demand more and complex configurations in order to achieve an optimal solution, the third iteration will consist of various kinds of trial-and-error sub-iterations by changing more than one of the control parameters at the same time. An optimal solution is given when e is as close to zero as possible, which is described in Section 3.4.1, and where all three constraints set on the peak recoil, the impulse, and the peak-width are fulfilled. The parameters that are changed in this iteration are the weight if the sledgehammer, the drop angle, and the damping materials. The usage of damping materials is also changed by adding layers or combining different types of material. Which materials to be combined are determined from Iteration 2, where the results show which of the four damping materials that give the highest peak-width; thus, the optimal result from Iteration 2. The first parameter that is changed in the third iteration is the weight ofthe sledgehammer. The weight is changed by adding additional units of weights to it, by attaching heavy nuts with tape to the sledgehammer head. The second parameter that is changed is the damping material. The third iteration begins by using the same damping material that gave the highest peak-width in the second iteration; thus, this is used in the first sub-iterations of the third iteration. By adding layers of the damping materials, or combining the first damping material with the second-highest given peak-width from Iteration 2, the constraints on the peak recoil, the impulse, and the peak-width can be fulfilled. The optimal result obtained from Iteration 3 is presented in Section 4.4, and the rest of the sub-iterations in Appendix A.
Comparing the Results To be able to compare the results of each sub-iteration fairly, the force-time curves of each sub-iteration are presented in comparison to the desired result from the real test. As there might be a slight installation offset of the force sensors, as described in Section 3.6.2, the initial force measured by the sensors is subtracted during the whole test period. The force-time curve is also moved on the time-axis to match the curve of the real test. As described previously in the delimitations of Section 1.7, no values or numbers are presented on the axes. Thus, the time-axis and the force-axis can be moved around, as long as the shape of the force-time curve stays the same. In order to fairly compare the results of each sub-iteration, the tests are done with ten test samples of each sub-iteration with a satisfactory and sufficient data resolution of the sampling rate. The number of tests is arbitrarily selected based on the argument of providing a sufficiently good result, without causing too much data to be handledin the calculations. Since each test of each sub-iteration needs to be taken into account and handled in the data management, the number of tests required in each sub-iteration cannot be too large.
52 CHAPTER 3. METHOD
The ten test samples are then combined to give a mean value of the resulting force-time curve. The curves are added by each starting of the impact. By this means that each of the ten curves is summed up from the first value that there is an initial measurement of the force. The summation is to compensate for the delay in shock resonance or other factors. The following chapter presents the results compressed in the testing of the recoil emulator. The results are displayed by showing the difference from each iteration in comparison with the real test. Together with each result is the calculated error e displayed to present the error for each iteration, with a table presenting each of the data points used in the calculations of the error.
53 Chapter 4
Result
In this chapter are the compressed results from the iterative process from Section 3.6.3 presented in detail. The results are described according to each iteration performed in the process, presented with a resulting force-time curve in comparison with the real test and the corresponding value of the error e. All iterations from the iterative process are chosen not to be presented within the results as all are not of importance. Each iteration provides a number of sub-iterations, where the final result from each iteration is presented in this chapter. The rest of the sub-iterations within each iteration are presented in Appendix A, to provide transparency of the complete process.
4.1 Overview of Iterations
Based on the iterative process of the test method described in Section 3.6.3, the following set of iterations and sub-iterations, presented in Table 4.1.1, were performed in the testing of the recoil emulator. The table presents the corresponding test configuration of each sub-iteration, of the drop angle, used damping material, and added weights. The resulting force-time curves that correspond to the lowest error compressed in each iteration is presented in the following sections, Sections 4.2, 4.3 and 4.4. Iteration 1 and Iteration 2 are based on only changing either the drop angle or the damping material. Iteration 3 describes a combination of changing the weight, the drop angle, and the used damping material. The cause of the increase of the drop angle is described in Section 4.4 in this chapter. In Table 4.1.1, the added weight of the sledgehammer is abbreviated to weight units or w.u. In the resulting figures of the force-time curves presented below are the impulses for each graph calculated as the area between the curve and the x-axis; hence, the same calculation used to calculate the impulse of the real test, described in Section 3.3.2. In Table 4.1.2, a detailed description of the results compressed in the iterative process is presented. The table provides values for the difference between data points from the real test and the emulator, where ∆Bmax and ∆I is the peak recoil and the impulse, respectively, and dtemu is the peak-width from the recoil emulator. The value of the peak- width of the emulator is presented to determine which of the sub-iterations that gave the highest peak-width in Iteration 2. The reason why the values of ∆dt are not presented is for not being able to deduce what the real test value is, as this is confidential.
54 CHAPTER 4. RESULT
Table 4.1.1: Description of the iterations and sub-iterations performed in the testing of the recoil emulator with a description of the corresponding test configuration. The results are either presented in the this chapter or in Appendix A, where the lowest value on the error from each iteration is presented in the in this chapter.
Iteration Sub-iteration Test Configuration
1
1 45°
2 60°
3 90°
2
1 90°, thin rubber sheet
2 90°, silicone sponge
3 90°, Sorbothane
4 90°, silicone rubber
3
1 120°, +2 w.u., silicone rubber
2 120°, +2 w.u., 2 layers of silicone rubber
3 120°, +3 w.u., 2 layers of silicone rubber
4 120°, +3 w.u., silicone rubber + sponge
5 120°, +3 w.u. 2 layers of silicone rubber + sponge
55 CHAPTER 4. RESULT
Table 4.1.2: Results obtained for each sub-iteration with corresponding values for ∆Bmax, ∆I , dtemu and the error e. The values of dtemu represent the peak-width of the recoil emulator, which represents the time that the recoil force is above 70% of the peak recoil from the emulator. An optimal solution of the peak-width is then the value is between 1-2 time units. The resulting force-time curves from each iteration with the lowest error is presented in Chapter 4, while the remaining corresponding curves are presented in Appendix A. The lowest e-value is texted bold to easier see the values.
Iteration Sub-iteration ∆Bmax ∆I dtemu e
1
1 0, 9825 815, 6 0, 01 3, 6186
2 0, 9749 625, 9 0, 01 3, 3304
3 0, 2246 40, 89 0, 78 0,6921
2
1 0, 2785 11, 51 0, 70 0, 7833
2 0, 3697 77, 78 0, 77 0, 5523
3 0, 1121 5, 503 0, 76 0, 6055
4 0, 2881 0, 585 0, 78 0,3639
3
1 0, 0420 22, 66 1, 00 0, 2832
2 0, 1312 2, 072 0, 87 0, 2381
3 0, 1013 55, 55 0, 90 0, 2633
4 0, 0215 12, 71 1, 05 0,1687
5 0, 5181 558, 9 0, 60 0, 6348
56 CHAPTER 4. RESULT
4.2 Iteration 1
The first iteration of the test method, the goal was to fulfill the first constraint, presented in Section 3.4.2, as in reaching an accepted interval of the peak recoil of the real test. The first iteration provided three sub-iterations at three arbitrarily chosen drop angles of 45°, 60°, and 90°. The results show that the anticipated peak recoil was reached within the interval of the constraint at an angle of 90°, as illustrated in Figure 4.2.1. The corresponding error value of the first iteration resulted in the lowest value of e = 0, 6921.
Figure 4.2.1: The final result compressed in the first iteration of the iterative processof the test method. The result showed that the interval of the constraint of the peak recoil was reached at 90° with a resulting error value is e = 0, 6921.
57 CHAPTER 4. RESULT
4.3 Iteration 2
In the second iteration, the drop angle was fixed from the first iteration, while four types of damping materials were attached to the end cap of the mockup of the pyrotechnic flare. The four materials tested during the second iteration were: • a thin rubber sheet, • silicone rubber, • Sorbothane, and • silicone sponge rubber. The results showed that both the silicone rubber and the silicone sponge rubber gave the highest peak-width, which is almost within the anticipated interval of [1,2] time units, as described in by the third constraint. The corresponding error values of the silicone rubber and silicone sponge rubber are e = 0, 3639 and e = 0, 5523, respectively. Due to the lower value on the error value e with the silicone rubber, the material was chosen as the material to continue with into the third iteration. Figure 4.3.1 presents the result of the force-time curve of damping the shock with silicone rubber, at a 90° the drop angle.
Figure 4.3.1: The resulting force-time curve from the second iteration, where four types of damping materials were used in the testing of the recoil emulator. The result showed that the lowest error was given by using the silicone rubber, which resulted in an e = 0, 3639.
58 CHAPTER 4. RESULT
4.4 Iteration 3
The final iteration of the test method had the goal of reaching an optimal solution ofthe configuration of the recoil emulator by fulfilling all three constraints. Iteration 1provided a satisfactory drop angle of the sledgehammer, while Iteration 2 smudged the peak recoil of the force-time curve by damping the curve and providing a sufficient peak-width, the drop angle of the third iteration had to be increased to reach the anticipated peak recoil and impulse. The new drop angle was tested and resulted in a drop angle of 120°, which resulted in a sufficient peak recoil and impulse. The result from Iteration 3 is presented in Figure 4.4.1.
Figure 4.4.1: The optimal solution for the recoil emulator, where all three constraints set on the peak recoil, the impulse and the peak-width are fulfilled. The result of the error value is e = 0, 1687, which is the lowest value for all iterations.
In the third iteration, the performed sub-iterations depended on previous sub-iterations, as in if the resulting error value e was lowered or not for each sub-iteration. During Iteration 3, the drop angle was increased to 120° to meet the impulse and peak recoil constraints, as Iteration 2 resulted in a decreased peak recoil as the resulting curves showed a peak-width wider than without the damping effect. The third iteration was performed by changing the weight of the sledgehammer by increasing the weight. As Iteration 2 provided a damping material that fit the third constraint, the same material was used in sub-iterations 3.1, 3.2, and 3.3.
59 CHAPTER 4. RESULT
The result of sub-iterations 3.1-3.3 showed that the e-value did not decrease as much as needed to meet the constraints of the desired impulse, peak-width, and peak recoil. After sub-iteration 3.3, the added weight units were fixed while the damping material was changed by combining the two materials that gave the highest peak-width from Iteration 2. The combination of one layer each of the silicone rubber and the silicone sponge rubber resulted in the fulfillment of all three constraints, with the lowest error value of e = 0, 1687. The next sub-iteration investigated if the e-value could be decreased even more by adding another layer of silicone rubber. Sub-iteration 3.5 resulted in a higher e-value than was obtained in sub-iteration 3.4. Thus, the iterative process was finished as an optimal solution was found in sub-iteration 3.4.
4.4.1 Optimal Solution Hence, the optimal solution of the recoil emulator is given by sub-iteration 3.4, where all of the three constraints are fulfilled, with the lowest error. The optimal result is presented in Figure 4.4.1, where the peak recoil obtained from the recoil emulator is within 5% of the real test, the impulse is within 10% of impulse from the real test, and the peak-width of the emulator is within the 1-2 time unit range for ≥ 70% of the peak recoil of the real rest.
60 Chapter 5
Discussion
In the final chapter of this thesis, the project is discussed in relation to the statedaim, purpose, and goal presented in Chapter 1. The results compressed and presented in Chapter 4 are discussed based on the obtained optimal solution of the configuration of the recoil emulator, produced in the iterative test process presented in Section 3.6.3. The results are discussed based on the relation to the goal, its reliability, and validation. Within the chapter are drawn conclusions presented, together with improvements of the test method, used test equipment, the construction and design of the recoil emulator, and the scope of the project. The chapter ends by presenting future work related to the project and a suggestion of an implementation plan to make use of the recoil emulator in the production and validation of CMDS products.
5.1 Analysis of Result
In this section, the result obtained on the configuration of the recoil emulator is discussed in relation to the goal of this thesis, if the result is reliable, and the validation of the obtained result.
5.1.1 Recapitulation of Aim, Purpose, and Goal In order to discuss and give feedback on the results presented in the previous chapter, the aim, purpose, and goal of this dissertation are repeated. As described in Chapter 1, the thesis aims to model, construct and optimize a recoil emulator, to increase the knowledge of pyrotechnic flares, recoil motion behavior and force inducing mechanisms in pyroshock simulations. Thus, the goal of this thesis is to present a recoil emulator together with an analysis of how well it emulates the force-time curve of a pyrotechnic flare. The project has provided a recoil emulator, which has successfully emulated the recoil motion of an arbitrarily chosen pyrotechnic flare, typical for usage in CMDS products world-wide. The recoil emulator provided a result that is within the three stated constraints, given the optimization problem to be minimized. The corresponding error value is not zero. Thus, the final solution is not the most optimal, but ”good enough” as it fulfills all constraints set on the peak recoil, the impulse, and the peak-width. The goal of this thesis is, thus, reached.
61 CHAPTER 5. DISCUSSION
5.1.2 Reliability of Result The results obtained in the project can be discussed in terms of its reliability based on many aspects. When it comes to the measurements of the peak recoil, the impulse, and the peak-width, the calculations were performed in the same way as the real test was made. Hence, the comparison with the result from the recoil emulator and the real test can be seen as valid in the sense of calculation. When it comes to the measurement of the force, there are several aspects that need to be addressed. The first is the reliability of the force sensors. As stated in Section 3.6.2, the sensors combined have an accuracy of approximately 81%, meaning that the results can only be guaranteed at a level of 81%. This means that the force sensors might measure the wrong force, as in too much or too little, and give the wrong result in the end. The reliability of the test results is increased by the fact that the tests were repeated ten times. From each sub-iteration, the ten samples were summed up to make up a mean value from each testing, which increases the reliability of each sub-iteration as error measurements from one sample to another decreases with more samples. The result from the iterations can be questioned to some extent as there is an unknown force behavior when an additional ”hump” is recorded on each force-time curve. The hump has not been investigated or attempted to remove, which can be debated if it poses additional problems for the results. The hump may be due to measurement errors, or that the sledgehammer hits the mockup flare, which in this case needs some timeto transfer the movement and the energy to the breech plate. Another aspect that might interfere with the reliability of the results is the error of measuring the angle of the sledgehammer. The positioning of the sledgehammer is done manually by holding it to a chosen angle and dropping it. This might lead to the wrong drop angle, which in the case leads to the wrong desired potential energy to be transferred in the pendulum.
5.1.3 Validation of Result When it comes to the validity of results, only theory and previous work can prove whether or not justified results have been met. Comparing the real test and the obtained result with theory in recoil motion, as described in Section 2.2 and presented in Figure 2.2.2 and 2.2.3, the result, by looking at the graphs, in theory, are very similar in the shape, and the duration of the peak-width and the remaining slope before the recoil force is down back to zero. As the force-time curve representing the optimal solution of the recoil emulator fulfills the set constraints of the real test, the result is, in some sense, validated as a validation model has been applied and used in the development of the configuration of the emulator. The validation of the results from the perspective that the constraints would not be included in a valid interval is discussed in the following paragraph, where the validation of the result is rather in how the constraints are constructed. In the validation of the result, it can be found that not enough tests have been done to claim that the result is statistically valid, a few tests were performed in comparison with statistics. The same applies, however, to the real test case, where only an arbitrarily chosen case has been selected, and not a case consisting of multiple shoots that have been normalized or averaged.
62 CHAPTER 5. DISCUSSION
To be able to describe the result as a statistically substantiated result, more tests and samples need to be done. The number of repeated tests that must be performed to claim that the results of the recoil emulator and the real test are statistically valid has not been investigated in this thesis. The value should be investigated in further work in order to more easily determine how many tests are needed, since, in this case, there is an arbitrarily chosen number of tests performed. In the project, the number of samples was set to ten due to a time restriction, as more tests and more massive data sets requires more time and effort. Such improvement is, thus, discussed in the following section.
5.2 Improvements
Based on the discussion of the reliability and validation of the result stated above, suggestions for improvements are in this section discussed. Improvements to be made are stated within the test method, test equipment, design and construction of the recoil emulator, and the optimization model used in the validation of the configuration of the emulator.
5.2.1 Test Method
Since the test method provides the result of this thesis, this is of high importance to discuss and suggest improvements to be made for the repeatability of the project. The first improvement that needs to be addressed is the number of test samples performed in the method. In order to state that the results are statistically reliable, more samples need to be performed for each sub-iteration. The exact number of samples needed for each sub-iteration can be discussed in terms of statistical validity and is not determined within the thesis.
A needed improvement to be made within the test method is the choice of damping materials. In the thesis, these are arbitrarily chosen based on previous work were rubber was mentioned as used as the damping material. Previous work also presented the usage of different metals to increase the damping effect, which can be discussed in thefuture work of this thesis. Thus, to find an even more optimal solution to the configuration of the recoil emulator, there needs to be more extensive research made on damping materials and their damping factors. As this thesis is done within departments of mathematics and structural mechanics, the research of damping materials and their material properties is not applied. By doing more intensive research on materials, other materials with more suitable properties may be found to use in order to provide the anticipated peak-width of the recoil emulator with fewer iterations than stated in Iteration 2.
Another essential improvement to consider is what the sledgehammer hits on, by mean, whether it is credible to hit a mockup flare or if it can give better results from hitting directly on the test fixture. Hitting on a loose sitting mockup flare can cause some forces to be absorbed in unwanted places, and the measurement may be wrong. The improvement that can be made there is to try how results could be if a force transmission is reduced and the shock comes directly to the test fixture.
63 CHAPTER 5. DISCUSSION
5.2.2 Test Equipment Improvements to be made within the used test equipment, including the test fixture, are of various kind. The first improvement to discuss is the chosen test fixture. The fixture is based on a US Navy standard for testing the force obtained by CMDS products and various types of flares. The purpose of the test fixture is to testhow surrounding structures are affected by the forces that result from the firing offlares and countermeasures, such as aircraft bodies. If the purpose of the test firing of the flares is instead to test how different components react to the shocks, another fixture needs to be developed and used, where the sensors are placed at different positions. The positions of the sensors are set in such a way that the force of the aircraft is measured in a credible way, to reflect how the countermeasures system responds to the shock in reality. The improvement that can, thus, be made is to investigate and question what is important to test when firing pyrotechnic flares and adjust the fixture andthe configuration accordingly. Another improvement regarding the test equipment is the selection of sensors. The sensors used now are adapted for force measurements. Subsequently, there are other types of sensors that are more suitable for testing shocks and pyrotechnic shocks, with a higher sampling rate and resolution. An example of a possibly suitable sensor is a pyroshock sensor, model PCB350C24 manufactured by PCB Piezotronics, and described in reference [39].
5.2.3 Design and Construction When it comes to the design and construction of the emulator, some improvements need to be made. In part, the supporting framework needs to be strengthened to be able to hold many more tests. The current frame is possibly undersized, as it moves and swings at certain positions of the sledgehammer, even if it is fixed with multiple screws and angles. By increasing the thickness of the frame and the number of attachment points in the test fixture, the emulator can be strengthened, which can increase the durability. Another improvement that can be made about the design is the design of the sledgehammer. In part, the weight of the sledgehammer is not enough to since it requires quite high drop angles to reach the desired peak recoil. With an increased weight of the sledgehammer, the same force curve can possibly be obtained at lower drop angles. In part, the sledgehammer may need to be redesigned as the weight cannot currently be changed in a simple way as it is not part of the intentional design. In the weight change made in Iteration 3, extra weights were attached to the sledgehammer by tape. In a new design of the sledgehammer, easier ways of attaching extra weights may be considered to be able to change the weight and thus adapt the emulator to different types of flares. A further improvement that should be reviewed with the design of the recoil emulator is the use of an automatic mechanism to enable very repeatable testing. At present, the emulator is manually operated by holding up the sledgehammer and releasing it at the desired drop angle. The manual swing can cause unwanted measurement errors and makes it difficult to carry out large amounts of tests. With an automated control ofthe sledgehammer, several tests can be performed with greater precision. Preferably, a motor can be mounted to drive up the sledgehammer to the desired drop angle, then release and retract to repeat the test.
64 CHAPTER 5. DISCUSSION
5.2.4 Optimization Model and Constraints In the last section of improvements, there are reflections on the designed and used optimization model with related constraints. The model that was designed is based on essential measures in the field of recoil measurement of pyrotechnic flares, the recoil measures of peak recoil, impulse, and peak-width. These measures are developed by experts in the field of countermeasure, and, thus, credible to use in the model. When it comes to the conditions that are designed around the measures, some are taken from an arbitrary point of view, partly to be able to achieve a result that is ”good enough”, partly because it is almost impossible to achieve a perfect result. These conditions can be re-evaluated and redone to achieve an even more optimal result, meaning that the selected intervals of the constraints are narrowed or reduced. In order to validate the results that came with each iteration and sub-iteration of a selected configuration of the recoil emulator, a cost function was used, or an error model. The error was chosen to be minimized in order to understand whether a result may or may not converge to the optimal solution. The cost function is based on the squares of error, which is an arbitrarily chosen measure. To further improve the model, the error model can be remodeled to provide a further indication of whether a result is approaching an optimal solution or not.
5.3 Conclusions
By concluding the discussion of the achieved result of the recoil emulator, the research questions presented in Section 1.4 can be answered. This project proposes a recoil emulator that successfully emulates the recoil motion behavior of a pyrotechnic flare. The development of the recoil emulator is done based on theoretical background related to pyroshock testing, while previously made pyroshock testing machines inspire the design of the emulator. A selection of improvements can be applied to develop the project and the recoil emulator to make it even better. The result presented within the thesis states that it is possible to emulate the recoil motion behavior of pyrotechnic flares. The result shows that based on the constraints stated in the optimization problem, the solution to the recoil emulator is ”good enough”, and that the emulator can be used in the early testing of CMDS products if a selection of the improvements stated above is implemented and considered. Such an improvement is implementing a mechanism to provide the ability to repeat tests with automation as this would increase the repeatability of the emulator, and also provide the opportunity to perform tests at a particular frequency between shots to simulate how the product can react at repeated shoots with different intervals. With a recoil emulator, the company in question can increase the testing of pyroshock simulations and, thus, decrease the need for pyrotechnic tests. This might lead to less costly tests and that there might be more tests performed in an earlier stage than what is done at the moment. By testing products and possible design solutions at an earlier stage in a development process, the final product might not need as many tests inthe end, as some of the validation of the testings are done earlier in the project. This can lead to less expensive products, shorter development processes, and, most of all, an easier way of determining whether a design solution is suitable or not earlier in the process.
65 CHAPTER 5. DISCUSSION
5.4 Future Work
In order to be able to develop this work further, several steps are proposed below to proceed with the project. These are linked to different parts of the project, excluding the improvements discussed in previous sections.
5.4.1 Perform a SRS Analysis A clear proposal for future work within the project is to perform a SRS analysis on pyrotechnic flares. Performing the analysis means that the work changes its orientation from the force-time perspective to the acceleration-frequency perspective. The performance of the analysis on pyrotechnic flares can increase the understanding of its behavior and how a recoil emulator can be developed and further optimized. The increment in the understanding of the SRS properties of pyrotechnic flares can provide better precision and more understanding about the shocks induced by the pyroshocks that flares induce. The results obtained from the analysis can, for instance, be implemented in a simulation machine, a so-called Electrodynamic shaker, presented in Section 2.4.3, which may provide the ability to test how specific components are affected by pyroshock, rather than the complete structure. The SRS analysis is the most commonly used method to quantify pyroshock [29]. The analysis can, thus, provide a wider perspective in the development of a recoil emulator if it is performed.
5.4.2 Test Another Impact Method As described and argued in Section 3.5.2, there are different types of impact methods to apply in the design of a recoil emulator. In this work, the impact method that met most of the requirements set by the client in combination with the pros and cons of the method was chosen, the pendulum sledgehammer impact method. In the future development of the project, this impact method can be reviewed and tested again to investigate how the result would be with another chosen method. An example of a method discussed in the report is the use of a pneumatic actuator, where the piston strikes the surface to simulate the pyrotechnic shock. The advantage described in Section 3.5.2 with a compressed air cylinder is the ability to easily automate the construction and be able to apply a control unit that can be adapted to simulate certain specific firing sequences.
5.4.3 Structural Analysis of the Exposure to Pyroshocks When it comes to the science of shocks and mechanical shocks, as described in Section 2.5.2, many journals, articles, and books describe and analyze how structures and fixtures respond to repetitive shocks from a structural analytic perspective. In this work, a structural mechanical perceptive was chosen and limited to only looking at the force induced by pyrotechnic shocks from the pyrotechnic flares. Thus, a future focus onthe project may be to consider how countermeasure systems are affected by pyrotechnic shocks and mechanical shocks from long-term usage of a recoil emulator, with, for example, output samples and material property tests. With a combination of the SRS analysis advocated to perform in future work, described in Section 5.4.1, these can be combined to perform tests based on the specific frequencies that pyrotechnic flares induce, to be able to determine at which frequencies which selected components are critical.
66 CHAPTER 5. DISCUSSION
5.4.4 Determine damping Factors As described in the improvement section of this report, several tests should be done with the damping materials used in tests. For further future work, a further investigation of different types of cushioning material and their properties is proposed from a deep-dive material-technical perspective. Such an investigation, or research, may allow the selection of more suitable cushioning material for use in a recoil emulator, which may shorten the iteration process required in testing the optimal configuration of the emulator.
5.5 Implementation Plan for Recoil Emulator
Finally, to complete this project, an implementation plan is presented for companies that are interested in implementing such a type of test equipment as the recoil emulator is in existing verification and validation processes of products in countermeasure systems. This implementation plan is only a proposal on how this concept of recoil emulation can be adopted and implemented in pre-existing validation processes and tests. The implementation plan is based on the existing recoil emulator developed in this project, which means that the proposed improvements and future work described above may be repeated, and details are described in their entirety. Below is a preliminary proposal for the implementation plan, with steps that are proposed to begin with the development and implementation of the recoil emulator in existing processes. 1. Reinforce the current frame of the construction. The first thing that should be done with the emulator is to stabilize the existing structure, with thicker profiles and frames, and possible more stabilizing angle brackets and screws. The design should be more robust to be able to perform multiple tests and develop the emulator from a sustainable perspective. 2. Increase the weight of and redesign the sledgehammer. The second step that should be taken to implement the test method with a recoil emulator is to increase the weight and redesign the sledgehammer. As described in Section 5.2.3, where the improvements of the design and construction of the emulator are described, the current weight of the sledgehammer is too low, and the construction is not designed to change the weight. By redesigning to be able to change the weight, the recoil emulator can be used to possibly simulate other types of flares with other recoil force behavior, including higher peak recoil. 3. Automatizing the sledgehammer with a mechanical or electrical motor. The third step that should be made in the implementation plan is to apply the automation of the sledgehammer. It is suggested that a mechanical or electric motor should be coupled with the pivot shaft to enable better precision and a higher test rate of the tests. 4. Review the sensors. After reconstructions and improvements have been made around the design of the recoil emulator, the test fixture should be reviewed in a possible implementation. This includes
67 CHAPTER 5. DISCUSSION the sensors used in the test equipment. The sensors currently used are adapted primarily for force absorption and measurements. One suggestion mentioned in improvements is to replace these with other types of sensors that are more suited to shock absorption, described in Section 5.2.2. 5. Perform a SRS analysis for pyrotechnic flares. After a review of the existing test equipment, an SRS analysis of pyrotechnic flares should be proposed. As described in Section 5.4.1, SRS is the most common method for quantifying pyroshocks. The analysis can thus lead to a greater understanding of why the recoil emulator implementation is needed and how it can best be used in the verification and validation of products within CMDS. 6. Perform more tests and research on damping materials. The next to last step in this proposal for implementation is to perform several tests and investigate how different materials can cushion impacts. The analysis should be doneat a material property level to understand as much as possible why the material should be used, as well as whether it is appropriate in its use in CMDS products. In this work, only various types of rubber are tested in order to achieve the required peak-width. In further investigation, other materials with different properties can be found that can provide better damping. This is advocated as future work in previously presented paragraphs in Section 5.2.1. 7. Verify results of the recoil emulator with real tests. The final step in the plan is to verify, once again, the recoil emulator against realtests and the desired recoil force behavior. The verification should be done with intensive tests in a proposed iterative process if changes in the configuration are to be made. If all of these steps are taken from the current position of the recoil emulator, hopefully, successful implementation of the test method can be carried out. Further steps that should be taken may be further discussed with the company and the client as this depends on the direction and time perspective the project is being given.
68 References
[1] Alexander, J. The Shock Response Spectrum - A Primer. Society of Experimental Mechanics Inc., June 2009. [2] Baker, Matthew, Neal, Kelsey, Sweetland, Katrina, Stevens, Garrison, Harvey, Dustin, and Taylor, Stuart. Developing Conservative Mechanical Shock Specifications. Jan. 2016, pp. 43–51. isbn: 978-3-319-30086-3. doi: 10.1007/978- 3-319-30087-0_5. [3] Bhatnagar, R. M. Recoil Motion Theorem. Tech. rep. Ordnance Factiry Kanour, Kalpi Road, Kanpur India, 2005. [4] Biltema. Silikon. Retrieved May 21, 2020 from. url: https://www.biltema.se/ bygg/kemikalier/silikon/silikon-2000028562. [5] Binder, Joseph, McCarty, Matt, and Rasmussen, Chris. Development of a Pyrotechnics Shock Simulation Apparatus for Spacecraft Applications. California Polytechnic State University, San Luis Obispo, June 2012. [6] Boyd, Charles C. International Electronic Countermeasures Handbook. Horizon House Publications, Inc, 2004. isbn: 1091-9422. [7] California Institute of Technology. Jet Propulsion Laboratory. Retrieved Apr 4, 2020, from. url: https://www.caltech.edu/research/jpl. [8] Carlucci, Donald E. and Jacobsson, Sidney S. Ballistics: Theory and Design of Guns and Ammunition. Taylor and Francis Group, 2014, pp. 175–177. isbn: 978- 1-4665-6439-8. [9] Challeat, J. “Le Duc Empirical Model”. In: Rev. & Art. 65 (1904-1905), pp. 184– 185. [10] Chalmers, Richard H. “Shock Testing Machines”. In: Shock & Vibration Handbook. Ed. by Cyril M. Harris and Allan G. Piersol. 5th ed. McGraw-Hill New York, 2002. Chap. 26, pp. 1–14. [11] Davie, Neil T. and Bateman, Vesta I. Pyroshock Simulation for Satellite Components using a Tunable Resonant Fixture, Phase 1. Oct. 1992. [12] Davie, Neil T. and Bateman, Vesta I. “Pyroshock Testing”. In: Shock & Vibration Handbook. Ed. by Cyril M. Harris and Allan G. Piersol. 5th ed. McGraw-Hill New York, 2002. Chap. 26, pp. 15–33. [13] Edgren, John. “Så fungerar Saab-tekniken som ska skydda Eurofighter (This is how the Saab technology works to protect Eurofighter)”. In: NyTeknik (Apr. 2008).
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[29] Lee, So-Jeong, Hwang, Dae-Hyun, and Han, Jae-Hung. “Development of Pyroshock Simulator for Shock Propagation Test”. In: Shock and Vibration 2018 (Sept. 2018). [30] Li, Peng, Chen, Yongcai, and Yang, Yuliand. “Optimization Design of Structure Parameters of Dynamic Recoil device”. MA thesis. Army Engineering University, Hebei China, 2019. [31] LockHeed Martin. Electronic Warfare. Retrieved June 1, 2020 from. url: https: //www.lockheedmartin.com/en-us/capabilities/electronic-warfare.html. [32] Mittal, Vaibhav, Patnaik, M.N.M., Narayan, S. Shankar, and Sujata, Premkumar D. “Development of Pyroshock Simulation Device (PSSD) for Space Application”. In: Journal of Spacecraft Technology 29.1 (Jan. 2018), pp. 11–18. [33] NASA and Jet Propulsion Laboratory. “Pyrotechnic Shock Testing”. In: NASA (May 1996). Practice No. PT-TE-1408A. [34] Navy BMR Institute. Pyrotechnic, Screening, Marking, and Countermeasure Devices - Chapter 4: Pyrotechnics. Retrieved Feb 20, 2020, from. url: http : //navybmr.com/study%20material/NAVEDTRA%2014313B/14313B_ch04.pdf. [35] Newell, T. Mechanical Impulse Pyro Shock (MIPS) Simulation. NASA National Aeronautics and Space Administration, Oct. 1999. url: http://hdl.handle. net/2014/18340. [36] Newell, T. Mechanical Impulse Pyro Shock (MIPS) Simulation. Tech. rep. IEEE, Workshop on Accelerated Stress Testing; Boston, MA; United States: NASA and Jet Propulsion Lab., California Inst. of Tech.; Pasadena, CA, United States, Oct. 1999. [37] Ožbolt, Joško, Bošnjak, Josipa, and Sola, Emiliano. “Dynamic Fracture of Concrete Compact Tension Specimen: Experimental and Numerical Study”. In: International Journal of Solids and Structures 50 (25-26 2013), pp. 4270–4278. [38] PCB Piezotronics. PCB ICP Force Sensor - Model 208C05. Retrieved May 5, 2020, from. url: https://www.pcb.com/products?model=208c05. [39] PCB Piezotronics. PCB ICP Shock Sensor - Model 350C24. Retrieved May 24, 2020, from. url: https://www.pcb.com/products?m=350C24. [40] Pessen, D.W. Pendulum Impact Tester. US Patent 3,285,060. Nov. 1966. [41] Raoand, K.S. Bhaskara. and Sharma, K. C. “Art in Internal Ballistics”. In: Defense Science Journal 132.2 (Apr. 1981), pp. 157–174. [42] Saab AB. Air Force Solutions - CMDS. Retrieved Feb 15, 2020, from. url: https: //saab.com/air/#electronic-warfare%7C24992. [43] Saab AB. BOP-L: Smart and Lightweight Countermeasures Dispenser. Retrieved May 10, 2020, from. url: https : / / saab . com / globalassets / commercial / air / electronic - warfare / self - protection - systems / cidas - compact - integrated-das/cidas-pdf/bop-l-product-sheet.pdf. [44] Saab AB. IDAS - Integrated Defensive Aids Suite. Self-Defence For Airborne Platforms. Retrieved Mars 15, 2020, from. url: https : / / saab . com / air / electronic-warfare/self-protection-systems/idas/. [45] Saab AB. Saabs Historia (History of Saab). Retrieved Feb 15, 2020, from. url: https://history.saab.com/.
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72 Appendix A
Iterative Process of Test Method
In this chapter are all additional resulting force-time curves obtained in the iterative process of testing the recoil emulator presented. The graphs are listed in the order stated in Table 4.1.1, were the iterations and sub-iterations are presented. Iteration 1 has three sub-iterations, named 1.1, 1.2 and 1.3, were sub-iteration 1.3 is presented in the Chapter 4. Iteration 2 has four sub-iterations, named 2.1, 2.2, 2.3, 2.4, and were sub-iteration 2.4 is presented in Chapter 4. The final iteration, Iteration 3 has five sub-iteration, weresub- iteration 3.4 is presented in Chapter 4, and sub-iterations 3.1-3.3 and 3.5 are presented in this chapter. With each force-time curve presented in the following chapters is the corresponding error value e listed, together with a description of whether the sub-iteration fulfills the constraints or not.
A.1 Iteration 1
Figure A.1.1: Iteration 1.1: 45° drop angle of the sledgehammer. Resulted in an error value of e = 3, 6186, where non of the three constraints are fulfilled.
73 APPENDIX A. ITERATIVE PROCESS OF TEST METHOD
Figure A.1.2: Iteration 1.2: 60° drop angle of the sledgehammer. Resulted in an error value of e = 3, 3304, where non of the three constraints are fulfilled.
A.2 Iteration 2
Figure A.2.1: Iteration 2.1: 90° drop angle of the sledgehammer and damping material of a thin rubber sheet. Resulted in an error value of e = 0, 7833, where non of the three constraints are fulfilled. The peak recoil is almost reached, but is over the interval of +5% of the peak recoil from the real test.
74 APPENDIX A. ITERATIVE PROCESS OF TEST METHOD
Figure A.2.2: Iteration 2.2: 90° drop angle of the sledgehammer with damping material of silicone sponge rubber. Resulted in an error value of e = 0, 5523, and where non of the constraints are fulfilled. The peak-width of the curve is looking promising, butin relation to the peak-width constraint, it is not good enough to be fulfilled.
Figure A.2.3: Iteration 2.3: 90° drop angle of the sledgehammer with damping material of Sorbothane. Resulted in an error value of e = 0, 6055, where the constraint on the peak recoil is reached but not the constraint on the peak-width. The impulse constraint is almost fulfilled, hence the low value of the error value.
A.3 Iteration 3
75 APPENDIX A. ITERATIVE PROCESS OF TEST METHOD
Figure A.3.1: Iteration 3.1: 120° drop angle of the sledgehammer with an additional weight of 2 weight units added to the sledgehammer. The damping materials used in this sub-iteration is the silicone rubber, with one layer of it. Resulted in en error value of e = 0, 2832, where the constraints on the peak recoil, impulse and peak-width are almost fulfilled.
Figure A.3.2: Iteration 3.2: 120° drop angle of the sledgehammer with an additional weight of 2 weight units added to the sledgehammer. The damping material used in this sub-iteration is the silicone rubber, with two layers of it. Resulted in an error value of e = 0, 2381, where the constraints on the peak recoil and impulse are fulfilled but not the constraint on the peak-width.
76 APPENDIX A. ITERATIVE PROCESS OF TEST METHOD
Figure A.3.3: Iteration 3.3: 120° drop angle of the sledgehammer with an additional weight of 3 weight units added to the sledgehammer. The damping material used in this sub-iteration is the silicone rubber, with two layers of it. Resulted in an error value of e = 0, 2633, where the constraint on the impulse is reached, but not the constraints on the peak recoil and the peak-width.
Figure A.3.4: Iteration 3.5: 120° drop angle of the sledgehammer with an additional weight of 3 weight units added to the sledgehammer. The damping materials used in this sub-iteration are a combination of two layers of the silicone rubber and one layer of the silicone sponge rubber. Resulted in an error value of e = 0, 6348, where the constraint on the peak-width is reached, but not the constraints on the peak recoil and the impulse.
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TRITA -SCI-GRU 2020:234
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