Design and Construction of a Hybrid Rocket Propulsion System
Project Lead: Dalko H. Jeri
Co-leads: Grant Neville, Thomas Martinez
Senior Members: Thierry Lem, Harrison Rhein, Phung Chinh, Hayden Smith, Yuanrui Wang.
Junior Members: Zach Meinert, Marcos Lipic, Will Lindsay.
MECH 4045-002: Senior Design II
Professor Doug Gallagher
May 06, 2020 2
Abstract
The main objective of RocketLynx is to design, build, and test a lightweight hybrid rocket propulsion system. This hybrid rocket engine was designed to be capable of producing a peak thrust 250 lbs. in a 10-second static fire. This rocket engine is designed around Spaceport America Cup requirements, since one of our long-term objectives is to eventually participate in this competition with a similar engine. Spaceport America Cup is one of the largest intercollegiate and international rocket competitions in the world happening every summer in White Sands, NM. The hybrid rocket engine consisted of an oxidizer based on Nitrous Oxide, also known as NOS, and hydroxyl-terminated polybutadiene as a solid fuel, also known as HTPB. The project was divided into different sub-teams according to the different components of the engine: the oxidizer, combustion, nozzle, manufacturing, and data acquisition. RocketLynx also built its own test stand apparatus consisting of a steel frame, a load cell, and linear bearings which was responsible for measuring the thrust produced by the engine. A monitoring system was also developed and integrated in order to keep record of the temperatures and heat transfer from the combustion process to the combustion housing. This system was integrated in order to protect the structural integrity of the housing from thermal shocks that resulted in creep. The short-term objective was to conduct a static fire test by the end of March 2020. Since the pandemic shut down campus and other facilities necessary for the completion of our static fire test, the project was halted in the middle of the manufacturing phase. This report reflects only the work of our team done from January to early March of 2020. 2
Acknowledgements
Thanks to our advisors: John from TRIPOLI, Juan Rodelo, Eric Hernandez, Rudy for the website, Nicholas T, and Hans, Terry, and James from the Machine and Elec Shops at JILA for their endless support and resources. This work is dedicated to the countless hours each of us spent away from our loved ones, to sleepless nights, and to the red vines with pretzels dinners. The work is also dedicated to our junior members, especially Marcos, Zach, and Will, who will continue the legacy of RocketLynx, paving the road to the Spaceport America Cup. 2
Table of Contents
Abstract ...... 1
List of Figures ...... 4
List of Tables ...... 5
Introduction ...... 8
Chapter 1 Initial Considerations ...... 9
Chapter 2: Oxidizer Subsystem ...... 12
Chapter 3: Combustion Chamber Housing Subsystem ...... 14
Chapter 4: Fuel Grain Subsystem ...... 25
Chapter 5: Supersonic Nozzle ...... 31
Chapter 6: Test Stand and Data Acquisition ...... 36
Chapter 7: Manufacturing ...... 39
Chapter 8: Test Plans ...... 48
Chapter 9: Path Forward ...... 49
References ...... 50
Appendices ...... 51
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List of Figures
Figure 1 ...... 15
Figure 2 ...... 18
Figure 3 ...... 18
Figure 4 ...... 20
Figure 5 ...... 22
Figure 6 ...... 23
Figure 7 ...... 24
Figure 8 ...... 26
Figure 9 ...... 27
Figure 10 ...... 27
Figure 11 ...... 29
Figure 12 ...... 30
Figure 13 ...... 32
Figure 14 ...... 32
Figure 15 ...... 33
Figure 16 ...... 34
Figure 17 ...... 35
Figure 18 ...... 35
Figure 19 ...... 37
Figure 20 ...... 37
Figure 21 ...... 39 2
Figure 22 ...... 40
Figure 23 ...... 41
Figure 24 ...... 42
Figure 25 ...... 43
Figure 26 ...... 44
Figure 27 ...... 45
Figure 28 ...... 46
Figure 29 ...... 47
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List of Tables
Table 1 ...... 25
Table 2 ...... 30
Table 3 ...... 46
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Introduction
Oxidizer Subsystem
The oxidizer subsystem has moved to the manufacturing phase. The team has almost finished with the manufacturing of the injector. The only machining processes left are to CNC the tabs that will interlock with the lid and to weld the lid of the aluminum injector to the body of the annulus. Once welded, we can begin testing mass flow rate using water. This way we can observe atomization and easily adjust pintle height to achieve optimal mass flow rate.
Combustion Chamber Subsystem
The combustion subsystem is almost done with the work needed for a full test. The thermocouple assembly system has been tested using Kapton tape and aluminum foil. This creates a reliable way for the thermocouples to be attached to the combustion chamber housing. Once these thermocouples are attached a circuit control system has been built to average the temperature of the housing across the two thermocouples. One temperature reading will be taken near the beginning of the fuel grain, and the other will be taken near the nozzle.
The design for the lid and injector assembly has been finalized and manufacturing of the lid is on hold for now. The lid will be constructed out of aluminum 6061 and will have to be machined on the CNC. There will be a locking slot that the injector will twist and lock into.
There will also be two glands for O-rings that will need to be manufactured. There will be one gland that is machined directly into the injector itself. The other gland will be a 45° triangle that will be taken off of the combustion chamber housing. The 45° triangle will be 2 the gland for the static crush fit. The last thing that needs to be machined is the threads that will tighten and hold the lid onto the combustion chamber housing. The threads will be machined onto the outside of the combustion chamber housing itself. This will provide a strong attachment that will withstand the pressures created by combustion as well as fixing small tolerance issues. The threading will allow the entire assembly to be compressed or given more space depending on the amount that the lid is tightened.
Nozzle Subsystem
The nozzle has been redesigned to incorporate an insulation in order to protect the sealing O-rings that are in great risk of failure due to the temperatures created by the internal combustion gases. A CFD simulation was done to the redesigned nozzle to provide proof of concept. The simulation deviated 5-10% from the values the nozzle was designed to output.
The exit pressure and Mach number in the simulation gave proof of supersonic behavior, which gave enough validation to move on to the machining phase. A manual lathe with low
RPM and gentle moves was used in the machining of the graphite nozzle. Proper safety precautions were used in the machining process, and the final machined product was successfully implemented in the assembly.
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Chapter 1: Initial Considerations
1.1 Manufacturing Overview
The main objective of this semester was to manufacture a working hybrid propulsion system. There were four main components that needed to be manufactured in order to have a working propulsion system. The oxidizer subsystem needed to manufacture the pintle injector along with the lid that will house the injector. Once the lid and injector were manufactured, the combustion chamber housing needed to have threads machined onto the outside surface of the housing in order to attach to the injector assembly. The fuel grain team needed to mix and manufacture the full-size fuel grain that would be used in the rocket. The last piece that needed to be manufactured was the supersonic nozzle.
1.2 Areas of Concern
All of these four separate assemblies must be created with high tolerances in order to seal off the hot exhaust gases from escaping the actual combustion chamber. This would cause catastrophic failure, most likely in the form of a rupture of the combustion chamber housing. Last year’s team was led by Juan Rodelo. The reason why the hybrid engine failed last year was a supposed lack of proper seals. This allowed hot exhaust gas to flow where it was not designed to flow. The result was an aluminum combustion chamber housing that ruptured, effectively ending the project.
To avoid this problem, research has been done on O-ring groove design and the proper seal that is needed to keep the hot gases in the combustion chamber housing itself.
There will be two seals that will be used at the injector side of the assembly. There will also 2 be two seals used on the nozzle side of the combustion chamber housing. With proper groove design and O-ring selection, the gases should be sealed off should be directed to the appropriate areas.
1.3 Budget
The total budget for this project was $2400. The total expenditures so far are at
$1047.62. The remaining balance to date is $1352.38.
Item Product Price (USD$)
Oxidizer Nitrous Oxide $83.43
Fill System 2 x ¼ NPT adapter Re-used
Oxidizer Control Valve 2 x Ball Valve Re-used Subsystem Injector Hybrid Motor Injector Free
Ceramic Fiber Owens Corning Not Necessary
Control Unit Arduino Uno R3 $35.00 Bread Board 2xMAX6675 Cables 2xLED
Solid Fuel Hydroxyl-terminated $130.00 Polybutadiene
Modified MDI Isocyanate $30.00
Combustion Housing Aluminum 6061 T6 $247.62 Chamber Chamber Subsystem Phenolic liner Phenolic Motor Liner, RMS- $38.00 75/3840
Casting tube 75mm Casting Tube, 2.557" $8.00 O.D. 2
Igniter 12V Pyrotechnic Initiator (E- $25.00 match)
Solid Propellant Pyrodex Pellets $35.00
Thermocouple 6 x Temperature Sensors $120.00 (Type K)
Thermocouple Type K Thermocouple Tester $13.00 Tester
Circuit Box Polycase circuit box $7.20
Nozzle Graphite Fine Extruded $164.00 Supersonic Nozzle Retaining Ring 1060-1090 Spring Steel $9.02
Pliers Fixed-Tip Retaining Ring Pliers $25.16
Thrust Test U-bolts Alloy Steel U-Bolts(x3) $7.06 Stand M6 Bolts Alloy Steel M6 Bolts $9.93
Load Cell 500kg Load Cell $60.00
TOTAL = $1047.42
1.4 Timeline
There are 16 weeks in this semester starting on 22 Jan 2020 and finishing up on 30
April 2020. Attached in Appendix C is a general timeline, known as a Gantt Chart. There are a few important events throughout the second portion of our senior design timeline:
Week 8: Midterm Presentations. Week 11: Static Fire Test. Week 12-15: Data Analysis and Validation. Week 16: Finals Exams and Presentations. Week 17: Graduation.
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Chapter 2: Oxidizer Subsystem
2.1 Oxidizer Feed System
The feed system will need to transport .15 kg/sec of nitrous oxide over about 10 seconds at 650 psi for a total of 1.5kg of nitrous being used. The feed system consists of two ball valves with rated pressure of 1000 psi, we also need a male to male ¼ NPT adapter since we are using the same valves as last year. Our design changed requiring that both sides of the valves be male, since it was cheaper to buy an adapter rather than another valve with the proper exit. The valves can be remotely opened and closed with an Arduino in 15-degree intervals to change flow rate mid-burn. During testing we can refine the exact degree that the valves are open to but since we have multiple ways to limit flow, it is easier decided through testing than through the numbers. The Arduino powered servos will be attached to the valves by way of a 3D printed housing. The housing is made out of PLA and will keep the gears in full contact and completely clean. There were a few concerns about using PLA since it has a lower melting point than PETG, but the temperatures should not exceed 200 °F. After testing, it will be easy to see if the PLA holds up, but it’s expected that there won’t be any problems with melting due to its location.
2.2 Injector
The injector will be machined out of 6061 aluminum. The main role of the injector is to atomize the liquid NOS for easier combustion. Last year's team had an issue with the injector and the combustion chamber separating during a test, causing gas to leak out of the combustion chamber and into the combustion housing, superheating the aluminum body 2 and causing it to burst open. To avoid this from happening again, the injector will be integrated into combustion housing by interlocking the injector into an aluminum cap and screwing the cap directly onto the housing. There will be two O-ring seals to further prevent any damage. We’ll be wrapping the phenolic tube with heat resistant tape to help further seal any vapors from leaking into the combustion housing. Testing will provide the team answers about if our adjustments have worked or not. The injector must maintain at least 0.15kg/s of mass flow. This will be done by opening the pintle by at least 1.5mm, a larger value will decrease velocity, but the mass flow rate will remain relatively unchanged with the larger exit area.
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Chapter 3: Combustion Chamber Housing
3.1 Combustion Chamber Insulation
Insulating the walls of the combustion chamber housing is going to be a major concern for our group. The HTPB will burn at a high enough temperature to decompose the nitrous oxide into nitrogen and oxygen. This creates an environment where pure oxygen is being combusted with the solid HTPB resin. The insulation that will be used to isolate the internal combustion from the walls of the combustion chamber housing will be phenolic tubing. This tubing was purchased from an online retailer and has the specific purpose of providing insulation to protect the combustion chamber housing.
The phenolic tubing that was bought has a slightly smaller outer diameter than the inner diameter of the combustion chamber housing. In order to fix this slight tolerance discrepancy, heat resistant aluminum tape has been bought and will be taped onto the ends of the phenolic tubing. This will provide a larger diameter that will tighten the fit of the phenolic tubing inside the combustion chamber housing.
3.2 Machining the Chamber Housing
Machining the housing should come with relative ease. The initial design was created using SolidWorks and was drawn up using proper requirements for a manufacturing plant in China. By sending the initial design overseas and shipping it back to the states, the overall cost for the part was lower than what it would have been if all of the machining would have been done here, with cost of time, labor, and materials kept in mind. The manufacturing plant in China was able to make the part in only a week and done within an acceptable tolerance. 2
This part is currently longer than what is needed, as this creates a small room for error when machining the part down to its final dimensions. In order to attach the lid to the chamber, a threading will need to be machined onto the top of the chamber. As it currently stands, the length of the chamber housing will not fit into any machines, so it will need to be cut quite a bit before the process can be moved forward. Total machining time for this part should not take over a week’s worth of time, as this is one of the easier parts to finish manufacturing.
Time is crucial towards the end of the building process, and this part needs to be finished in a reasonable time. Per group discussion, this will be the last part to be machined. Once it is finished, the solid fuel will be packed in the housing, along with the nozzle, and then the cap with the injector will be screwed into place and the system will be ready for testing.
3.3 Attaching the Thermocouples to the Housing
Figure 1: Aluminum Foil with Kapton Tape (ECD).
Attaching the thermocouple to the chamber is not a difficult task. There are about 8 industry standard ways of attaching thermocouples to materials. The method that provides the most solid contact between the thermocouples and the material is a UV Cure Epoxy ®.
This method cures in only 10 seconds and is universal among plastics and metal. The UV 2
Cure Epoxy ® route is highly expensive ($1,500-$2,000), so it is not an ideal method for this iteration, but it would be ideal if the budget allowed for it. That leads into a cheaper option,
Kapton tape. Kapton tape is used across many industries and is the most used method on the planet. One of the possible concerns using Kapton is the adhesive melting during testing, but
Kapton tape can withstand temperatures of up to 500 F. Another concern would be that
Kapton tape may not create the most solid contact between the thermocouple and the chamber housing. This will be handled by using a small square of aluminum foil and placing it over the thermocouple, flattening the square piece against the housing, and then attaching the tape. An example of this can be found in Figure 1 above. By adding the layer of aluminum foil, the heat resistance between the housing and the Kapton tape also increases slightly. This method also allows for the thermocouples to be easily removed after testing and used for later tests. Keeping the budget in mind, this is an ideal side effect of using this method.
This method will have to be tested before it is chosen as the way to create the attachment. A small section of spare aluminum will be used, and the thermocouples will be attached using the Kapton and aluminum foil method. The small section will be heated, and the readings will be recorded to make sure that the readings are proper. This is the method that will be used in the final test of the propulsion system itself if successful.
3.4 O-ring Seals for the Combustion Chamber Housing
The hybrid system is reliant on O-ring seals to keep the heated gas inside of the combustion chamber. The groove design that will actually seat the O-ring is critical. The designs and dimensions came from Parker O-Rings. This manufacturer has detailed documents on how to design and construct the groove for the O-rings. 2
There are 4 main problems that can damage O-rings and lead to premature failure and a broken seal. The first problem that can occur is rolling the O-ring while installation is occurring. This rolled and twisted O-ring will lead to spiral fractures along the O-ring. The spiral fractures will lead to a failed seal. The second problem is over-elongation of the O- ring. During installation an O-ring can be stretched no more than 50% of their inner diameter. Over-stretching of an O-ring can lead to permanent deformation and a failed seal.
The third problem is damage to the O-ring due to sharp edges or burs in the manufactured surface. Sharp edges and burs can damage the integrity of the O-ring and lead to premature failure. The fourth possible problem is a failure to properly clean surfaces or improper use of solvents. This could once again lead to an improper seal or O-ring damage.
These four various issues could lead to a failure for the entire hybrid propulsion system. Therefore, the seal designs are necessary for the success of the project. The seal that will be used to seal off the lid and the combustion chamber itself will be a static crush seal. These types of seals require plastic deformation of the O-ring. The seal is ideal because we cannot cut any grooves into the housing chamber. Apple Rubber’s guide to groove design is the document that is guiding our construction.
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Figure 2: Apple Rubber Diagram on Static Crush Seal.
Using the Apple Rubber static crush seal below, the correct O-ring could be chosen.
Figure 3: Apple Rubber O-Ring Selection for Static Crush Seal.
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The gland depth that was chosen was the smallest gland depth of 2.41mm. We chose the smallest gland depth because the gland will be cut into the combustion chamber housing itself. There is not a lot of material to work with and the structural integrity of the combustion chamber housing cannot be compromised.
The next seal that had to be determined was the static axial seal on the injector. Once again Apple Rubber’s guide to groove design was used. The chosen O-ring for the axial seal will correspond to a groove width of 0.125in and a groove depth of 0.049in. The O-ring that was selected for this seal was a size 33 O-ring.
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Figure 4: Apple Rubber O-Ring Selection for Static Axial Seal.
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3.5 Ignition system
The purpose of the igniter is to initiate the combustion of the propellants. When the oxidizer is ignited inside the combustion chamber, it starts to heat up and burn the solid fuel.
As this solid fuel burns, the chamber’s overall temperature and pressure begin to increase at nonlinear rates. In a hybrid engine, the combustion takes place just off the surface of the solid fuel. The combustion will take place between fuel that has just been vaporized by the heat of combustion, and atomized oxidizer injected into the head of the combustion pressure. The success of ignition of the propellants depends on the combustion pressure. When starting the ignition process, the pressure generated by hot gas must be higher than the critical pressure (P), which is the minimum pressure to ignite the solid propellant. There are different types of igniters used to initiate combustion namely; pyrotechnic, augmented spark, plasma torch, and catalyst bed. We will use a pyrotechnic igniter to ignite our hybrid rocket. A pyrotechnic igniter is a simple device which permits you to launch rockets by remote control; we will use an Arduino to control the ignition remotely. We will use a power supply of 12V and 22-gauge copper wire because of its electrical conducting capabilities. The launch system will have a safety interlock key in series with the launch switch and will use a launch switch that returns to the “off” position when released
For our ignition system, we used 12V power supply to test for our igniter which worked successfully. One of the main concerns we had with our ignition system was how to increase the burning rate and the burning surface of the igniter. The first test we did with our igniter we noticed that the igniter had small burning rate (less than 3 seconds) and didn’t produce enough heat which can heat up HTPB fluid (Hydroxyl-terminated Polybutadiene). 2
In order to increase the burning surface of our igniter, we used pyrodex in the form of pellets. Pyrodex pellets are similar in composition to black powder, consisting of primarily charcoal, sulfur, graphite, and potassium perchlorate, but it’s less sensitive to ignition and less dense than black powder. Pyrodex is more energetic, and when ignited can produce a high enough temperature (approximately 740 °F) to heat up the HTPB fuel. and can provide a larger burning rate. The pyrodex pellets were purchased at Bass Pro Shop.
We will feed the igniter wire through the pellet hole and sit the igniter tip right next to it in order to produce better ignition. The Figure 5 shows how we will connect the igniter with the pyrodex pellets.
Figure 5: How to connect igniter with pyrodex pellets. 2
Figure 6: Electric schematic of the ignition system.
The figure above shows a complete circuit for our ignition system to work. 12V power supply will be powered up by the external battery (Adapter) and connected to the first switch (C). Once we flip the switch, the LED#1 (white) light will come on which signals that the actual ignition system is being powered. Once we flip the switch, we have a different system that will tell us our system is ready to be ignited (LED#2). Once the first switch is on, it will connect us to the second switch (D). The second switch is a push button, once pushed it will light the pyrotechnic load, activating the ignition system. As soon as this button is pushed, a third LED (LED#3) will confirm the status of the electric connection after the ignition. The current goes through a 2N222 NPN transistor (A) and the main purpose of the transistor is to charged up the capacitor, and work as an inverter so we can have a blinking 2 signal on LED#3, and the capacitor is acting as a charger. That's how we are able to determine the rate at which the LED is blinking. Then we have a second section (B). As soon as we flip the switch (D) the piezo buzzer will alert us that the system is ready to go off. Figure 7 below shows the picture of our ignition system.
Figure 7: Ignition control system
We encountered some connectivity issues with our ignition system. We used a low resistor; as a result, the transistor in our circuit was supplied with too much current and burned out.
Our next task was to replace the transistor, the new one must be able to handle the higher current the circuit is producing.
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Chapter 4: Fuel Grain
4.1 Fuel Sample Construction
Before we could manufacture our full-size fuel for the rocket, a sample of HTPB was cast and tested for combustibility. The dimensions and mass of the sample are listed in Table
1. The first step in manufacturing the sample was calculating the volume of the mold and then converting it into the total mass of the fuel since the mixing ratio of the HTPB is by weight. The total mass for this testing sample was 16 grams, but we had to make a little more than 16 grams because the mixture had a very high viscosity, some of it would stick to the mixing container and it was impossible to get exactly the same amount that we put in. The required mass of modified MDI isocyanate curative and R-45 HTLO resin were determined from the total mass and mixing ratio which is 12.9% curative and 87.1% resin.
Fuel Grain Length (mm) Inner Diameter Outer Diameter Initial Mass Configuration (mm) (mm) (gram)
Cylindrical 50 10 22.50 16
Table 1: Sample dimensions and mass
The next step was to weigh the required amount of the curative and resin using a digital balance with at least one decimal place display for accuracy. The mixture was mixed for about five minutes and then placed in the vacuum chamber. In order to remove air bubbles from the mixture, the vacuum pump is required to pull 29 inches of Hg at sea level or 24-Hg at the elevation of Denver, CO. Figure 8 shows a pressure of 26.5-Hg during the degassing process which meets the requirement for the pump. It took about five minutes for 2 the mixture to stop rising. This indicated that all the bubbles had been removed and the mixture was ready to be poured into the casting tube.
Figure 8: Pressure of the vacuum chamber during degassing process
The casting tube was sprayed with mold released twice and waited for a few minutes before the mixture was poured into it. It took about five days for the sample to be cured at room temperature. The average ambient temperature and pressure during those five days was about 21 degrees Celsius and 81.5 kPa. Figure 9 shows the fully cured HTPB sample.
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Figure 9: Fully cured HTPB sample
After the HTPB sample was cured, it was burned by using a lighter to make sure the mixing ratio and mixing process were correct. Figure 10 shows the fuel during and after burning. After confirming that the sample could catch on fire, we moved forward with testing it with our ignition system.
Figure 10: HTPB sample during and after burning
4.3 Fuel Testing
Test equipment:
● Test stand 2
● Nitrous oxide
● Ignition system
● Fire extinguisher
Rocket safety:
● The testing area must be well-ventilated and cleared of flammable materials (e.g.
dry grass).
● Weather conditions must be safe with wind speeds less than 20 mph.
● The test stand should be on a flat and level surface.
● Check the oxidizer hoses to see if they are connected properly and free of leaks.
● Use a countdown before igniting to make sure everyone is in a safe distance from
the test stand.
● Always wear safety glasses.
● Have fire extinguishers ready.
Procedure:
● Insert the electric match into the Pyrodex pellet.
● Insert as many Pyrodex pellets as needed into the HTPB.
● Connect the electric match to the igniter circuit.
● Secure the HTPB firmly to the test stand.
● Start to release a small amount of nitrous oxide into the HTPB.
● Use a countdown before turning on the switch on the igniter circuit.
● When the electric match fires, it will ignite the Pyrodex pellet. In order to ignite the
oxidizer and fuel, the Pyrodex pellet must be able to burn long enough to heat the
HTPB, so that it can get to the ignition temperature. 2
● Increase the amount of nitrous oxide until it reaches a consistent burn.
Figure 11 shows the HTPB sample after the test. The fuel did not ignite since the ignition system did not provide enough energy for the HTPB to reach its ignition temperature. Also, the weather conditions were not ideal on the day we tested the fuel. The temperature was about 30 degrees Fahrenheit and the wind speed was about 17 mph which is very close to 20mph. Once the HTPB sample is successfully ignited, we will cast the full- size fuel for our rocket.
Figure 11: HTPB after the test
4.4 Fuel Grain Construction
The casting process is very similar to the one for the fuel sample construction. The dimensions and mass of the fuel is listed in Table 2. The mold for our full-size fuel was 3D printed so that it could be more accurate. Figure 12 shows the casting tube and mold release spray for the fuel. The vacuum chamber also has a larger capacity which is five gallons with the dimensions of 30 x 30 cm. The vacuum chamber must be able to fit a mixing container holding five times the amount of the fuel because the mixture will rise very high during the degassing process. With a large vacuum chamber, the entire fuel can be cast at once since it 2 is time consuming to cast smaller blocks and then connect them together. It will also take about five days for the fuel to be cured.
Fuel Grain Length (mm) Inner Diameter Outer Diameter Initial Mass Configuration (mm) (mm) (gram)
Cylindrical 300 40 63.32 530
Table 2: Full size fuel dimensions and mass
Figure 12: Casting tube and mold release spray for full size fuel
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Chapter 5: Supersonic Nozzle Modifications
5.1 Sealing Method
The nozzle went under a few changes in design, mainly due to the concern of heat transfer, proper sealing, and securing the internal components of the combustion housing. It was thought at the beginning that one O-ring would efficiently work towards sealing the nozzle and prevent any leaks in pressure, but, to be certain of a proper sealing, two O-rings will be included with a 5mm distance apart. The expectation is that in the case that the first
O-ring fails due to high temperatures through gases, the second O-ring will prevent any further pressure leakage. The material of the O-rings also changed. Buna-N O-rings were first considered because of their maximum temperature of 400 °F. This O-ring has now been redesigned and changed for Viton O-rings. This change of O-ring will now give a maximum working temperature up to 450 °F, which is one of the highest temperature O-rings that is still around the same price.
The first nozzle design did not consider the possibility of an inner insulation to protect the nozzle from the combustion housing walls and safeguard the O-rings from the gases produced by the inner combustion to reduce the possibilities for O-ring failure. For this reason, a section at the beginning of the nozzle was modified. This area of 15 mm in length is dedicated to connecting the nozzle directly into the phenolic tube as shown in Figure 13.
As part of this modification a chamfer of 2.55 mm was also added to the edge of the beginning of the nozzle easier attachment. These modifications are highlighted in blue in the same figure. This area also pushed back the O-rings to the end of the nozzle. 2
Figure 13: Nozzle Redesign.
5.2 Retaining Ring
The retaining ring had the main purpose of securing the nozzle, HTPB, and phenolic tube inside the combustion housing and prevent these components from being expelled to the outside due to the combustion in the chamber and choked flow through the nozzle. The internal ring needed a proper groove, which is expected to be added to the combustion housing, in order to maintain the structural integrity of the groove itself and the O-ring and prevent any malfunctions within the assembly. Figure 14 shows the potential damage the retaining ring groove can encounter if not properly designed.
Figure 14: Potential groove damage. 2
In the retaining groove design, the distance between the start of the groove and the outside wall is known as edge and is depicted as n in Figure 15, and the depth of the groove is depicted as d. In order preserve the structural integrity and provide a safe groove design n/d must be greater than or equal to 3a.
Figure 15: Retaining ring groove design.
The current groove design has the following measurements: n = 8 and d = 2.125.
These dimensions give a ratio of approximately 3.77 which is greater than 3 as prescribed for a safe design. A mechanical drawing of this groove design is included in Appendix B.
5.3 Nozzle CFD
Flow Simulation, a feature in the SolidWorks 2019 Student package, was used in order to get a Computational Fluid Dynamics simulation. Carbon dioxide was used as the working fluid since this is the closest gas that is similar to the gas that is created in the combustion of nitrous oxide and HTPB. An adiabatic wall with roughness equal to zero was also assumed.
The mesh was set to automatic level 5 with rectangular elements in 2D domain. The boundary conditions were set at inlet with a mass flow rate of 0.115529 kg/sec with stagnation properties of 1275000 Pa and 2850 °K, and at the outlet as atmospheric properties of 99898.65 Pa and 290 °K. 2
The results of this simulation showed a supersonic flow behavior inside the nozzle with a maximum exit Mach number of 1.87 while the design was meant to produce an exit
Mach number of 2.0. Figure 16 shows the Mach number cut plots inside the nozzle.
Figure 16: Mach numbers along nozzle.
Similarly, the pressure distribution inside the nozzle was sound and consistent with what is expected of supersonic nozzles. The exit pressure in the simulation was of 131887.03
Pa while the designed exit pressure was of 148105.69 Pa. This pressure difference is within
10%, and it approves of the initial supersonic nozzle design. In the following page, Figure 17 shows the pressure cut plots and Figure 18 the final nozzle assembly. 2
Figure 17: Mach numbers along nozzle.
Figure 18: Final Nozzle Assembly.
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Chapter 6: Test Stand and Data Acquisition
6.1 Test Stand Design and Construction
A group of dedicated underclassmen have been in charge of designing and constructing a test stand that will be able to measure the thrust that the hybrid propulsion system will generate. The design will utilize linear bearings that will create a frictionless contact between a combustion chamber mount and the tracks that the linear bearings are seated on. This will allow the hybrid propulsion system to transmit all of its thrust to the load cell with minimal friction. The thrust tester is expected to encounter a force of 250 lbs. of thrust and designed to withstand up to 1500 lbs., giving it a safety factor of 6.
Three linear bearings, that are bolted to a base, will form a tripod upon which the combustion chamber mount will be placed. The combustion chamber itself will sit in the mount, being secured by two U-bolts along its length and by tube steel at its front. Those tube steel pieces are two of the vertices of a rectangular piece that separates the front of the combustion chamber to the load cell. This buffer provides room for the tube that connects the combustion chamber to the oxidizer. The base, the mount, and one of the linear rails are all made of mild steel and were constructed by welding. Two views of the thrust tester design with the combustion chamber are provided in Figures A and B.
In the interest of obtaining the most accurate data, the thrust tester will be placed on a perfectly level surface when being used. Additionally, holes will be drilled into the base so that stakes can be inserted, attaching to apparatus to the ground which provides further stability to the thrust tester. The apparatus will be placed on a dirt surface when used, 2 allowing small holes to be dug in the ground so the screws that protrude through the bottom of the base do not alter the testing stand’s orientation.
Figure 19: Exploded CAD design of thrust tester with combustion chamber.
Figure 20: CAD assembly of thrust tester with combustion chamber.
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6.2 Load Cell and Data Acquisition
The load cell will be bolted to the base of the thrust tester with high grade screws, allowing it to stand upright and measure the shear force placed on it from the combustion chamber mount. The load cell that is being used was ordered from China and will be calibrated using Drew Hanzon’s help. The load cell is a 500 kg single-point load cell designed to measure shear forces from a single direction. It requires a 12-volt supply, an analog to digital converter, and signal amplifier to acquire useful data. The actual range of the load cell is from 60kg to 500kg which means the minimal force to get accurate data is 132lbs. This is acceptable for our application because the force should create a square wave at 150lbs.
There will be almost no inaccurate data points between 0 lbs. and 132 lbs. due to the way the combustion chamber was designed.
The load cell must be connected to an amplifier and analog to digital converter. In the case of this project, those two components were combined onto a single chip for portability and simplification. The HX711 is a 24-bit ADC precise converter made specifically for load cells The ADC will measure the analog signal too small for the MCU to identify. The load cell outputs (±40mV). The ADC outputs data in the form of serial bits that an MCU can understand. It also outputs clock data from the internal oscillator on the chip. The point of failure in this chip is the output data rate, which is at 80 Hz. At that point the chip cannot record the small vibrations the rocket engine might output.
To translate the output of the ADC an Arduino UNO will be used to calibrate and compile the serial data, which will either store the data on an SD card or Excel file to be analyzed after the test.
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Chapter 7: Manufacturing
7.1 Injector Pintle
The first part was the pintle of the injector. This was created using aluminum 6061 round stock. The shaft was turned on a lathe to design specifications, leaving enough material for the 45-degree flange of the pintle. Once the shaft was finished, 10 mm threading was cut into the shaft.
Figure 21: Manufacturing of the Pintle.
To cut the 45 degrees flange, we used a program on the lathe set to 45 degrees and made dozens of passes until we achieved the desired dimensions. 2
Figure 22: Pintle Before Lathing.
The excess material was cut off from the base.
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Figure 23: Finished Pintle.
7.2 Injector Annulus
The round stock material was originally anodized 6061 aluminum. It was much larger in diameter than was needed but the only material large enough in diameter for the body of the annulus. The round stock was cut into square stock and then turned on a lathe to get it back to round stock and closer to the dimensional size that was specified in the design.
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Figure 24: Rough Cuts on the Annulus.
Once turned to the correct dimension, an internal taper was cut using the same program that was used to cut the pintle angle. 2
Figure 25: Turning the Annulus.
Once the internal taper was done, the part was flipped and began taking material out of the annulus itself. Once down to size, a fillet program was used to cut out the correct radius of the annulus. 2
Figure 26: Annulus before tabs are cut out and lid welded.
The final machine processes to be done are to machine out the tabs that will interlock with the lid and to weld the lid to the annulus.
7.3 Graphite Nozzle
Graphite was very delicate since it is a ceramic and machining the nozzle out of this material required gentle turns and slow RPM. The nozzle was machined using a manual lathe.
Since this material can be toxic if pulverized, proper safety precautions must be taken under consideration. A vacuum or dust absorber was necessary in the machining process to absorb the graphite powder. Safety glasses were also a must for general machining safety. A mask 2 or respirator was another safety measurement that was taken in order to prevent breathing any graphite particles. Latex gloves were used in order to provide a cleaner machining environment; graphite powder tends to stick to almost anything.
The stock piece was of 90 mm in length and almost 80 mm in outer diameter. The length of the stock piece was chosen in order for the grips in the chuck to safely hold on tight to the graphite while spinning and to provide a clearance for the lathe cutting tools in each step of the machining process, specifically in the groove design and facing on the outer surface of the stock piece. Since graphite is brittle, an RPM of 450 was used at all times to prevent any chipping or cracks. This low RPM and gentle turns gave smooth surfaces inside the nozzle, where it most needed it. Figure 27 shows the set up. In the same figure, the graphite powder can be seen laying on a napkin.
Figure 27: Nozzle stock piece set up.
Table 3 shows the general machining procedures that were involved in the machining of the nozzle. The chamfer was done manually with an 800-grit sandpaper rather than a chamfer tool since the edges were prone to chip as shown in Figure 28. 2
Feature Tool Comments
Center hole Center Drill
Smaller OD Throat hole 11/32 drill bit Initiate the throat hole
Throat hole 35/64 drill bit This will create the nominal throat OD.
Inlet Taper at 45° ⅛ carbide tip
Outlet Taper at 15° ⅛ carbide tip Once at the edge of the throat. Zero out and change the compound angle to 15°.
Proper OD Facing tool Trim until designed OD
External O-rings size 230 Grooving Tool
Smooth inner surfaces Fine sandpaper Done manually.
Actual nozzle length Parting tool Cut stock piece to proper nozzle length
Table 3: Nozzle machining process.
Figure 28: Edges chips from a Left-to-right tool translation.
A left-to-right tool translation will most likely end in chipping off the edges as shown in the figure above. Therefore, it is recommended that a right-to-left or a direction towards the most stock area be used for manufacturing this type of ceramic. The taper regions were 2 done in a layer by layer fashion. No lubricant was used in the process. To smooth out the inner surfaces of the nozzle, fine sandpaper and special wax was used manually while the stock piece was rotating.
Figure 29: Final machined nozzle.
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Chapter 8: Test Plans
8.1 Testing Plans: Injector
To test out the injector, we will first do a hydro test and shoot water through the injector hooked up to a water hose. The purpose of this is to: 1) check for leaks under a load and 2) measure the range of mass flow that the injector will allow to flow through it under various conditions. We will do this by turning on the hose hooked up to the injector for a specified amount of time. The injector will then pour water into a bucket. We will then measure the amount of water that has flowed through. By measuring the weight of water over the amount of time that the injector had water flowing through it, we will be able to get the kg/sec mass flow of the injector. We chose to use water since we will be using, under ideal conditions, liquid Nos, which has a similar density to liquid water. This is where we will fine tune what height to set the pintle at to achieve the calculated mass flow to achieve theoretical oxidizer/fuel mix ratio mass flow.
8.2 Testing Plans: Oxidizer subsystem
Once we have experimental data of the injector, we will be able to set the pintle to desired height for the optimal mix ratio mass flow. Then we will attach the entire system
(mechatronics and valves) to an empty unpressurized bottle. The empty bottle will be on a scale with a camera. We will then measure the liquid Nos mass flow with our oxidizer system to see if we can achieve optimal flow that we achieved with the water mass flow test. The camera will record the change in mass in real time to reduce measuring errors.
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Chapter 9: Path Forward
Overall, the team is eager to finish manufacturing and begin testing. Our 1st location choice is on a plot of private land in Castle Rock. Once testing is completed, we will be able to observe how the system performs under testing conditions and how well it holds up post- test. After testing, we will be able to either redesign, or to prepare for a possible second test at a larger scale, time permitting.
The next team decision that needs to be made is what to do with the excess money in our budget. Discussions have taken place between the team leads, and the next major step will be to spend the excess money on materials that next year’s team will need. The next year’s team will need to come up with a list of expected materials that will be needed. Once this list has been completed by next year’s team, the remaining budget will be spent on that list.
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References
“Thermocouple Attachment Methods.” ECD, 9 May 2018, www.ecd.com/support
/learning-center/resources/thermocouple-attachment-methods.aspx
“Calculation for Retaining Ring Reference” Ochiai Products , Last accessed: 03 MAR 20 http://www.ochiai-if.net/products/pdf/001/001002.pdf
“The One Stop for All Your Sealing Needs.” Rubber Seals, Sealing Devices & O-Ring Seal Design | Apple Rubber Products, www.applerubber.com/.
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Appendix A
#define inputCLK 4 #define inputDT 5 #include
void setup() {
// put your setup code here, to run once: pinMode (inputCLK, INPUT); pinMode (inputDT, INPUT); pinMode (bt1, INPUT); pinMode (bt2, INPUT); pinMode (bt3, INPUT); pinMode (bt4, INPUT); pinMode(led1, OUTPUT); pinMode(led2, OUTPUT); pinMode(led3, OUTPUT); pinMode(led4, OUTPUT); 2
Serial.begin (9600); myservo.attach(9); lcd.init(); lcd.backlight(); lcd.home(); Serial.print("Position: "); Serial.print(tcounter); previousStateCLK = digitalRead(inputCLK);
} void loop() { // put your main code here, to run repeatedly:
b1State = digitalRead(bt1); b2State = digitalRead(bt2); b3State = digitalRead(bt3); b4State = digitalRead(bt4); currentStateCLK = digitalRead(inputCLK);
if (b1State == HIGH){ bcounter = 15; tcounter = bcounter; myservo.write(tcounter); Serial.println(tcounter); digitalWrite(led1, HIGH); delay(500); digitalWrite(led1, LOW);
} else if (b2State == HIGH){ bcounter = 30; tcounter = bcounter; myservo.write(tcounter); Serial.println(tcounter); digitalWrite(led2, HIGH); delay(500); digitalWrite(led2, LOW); } else if (b3State == HIGH){ bcounter = 45; tcounter = bcounter; myservo.write(tcounter); Serial.println(tcounter); digitalWrite(led3, HIGH); delay(500); 2
digitalWrite(led3, LOW); } else if (b4State == HIGH){ bcounter = 60; tcounter = bcounter; myservo.write(tcounter); Serial.println(tcounter); digitalWrite(led4, HIGH); delay(500); digitalWrite(led4, LOW); } else if (currentStateCLK != previousStateCLK){
if (digitalRead(inputDT) != currentStateCLK){
counter --; tcounter --;
if (tcounter<0){ tcounter=0;
} } else {
counter ++; tcounter ++; if (tcounter>180){ tcounter=180; }
} myservo.write(tcounter); Serial.print("Position: "); Serial.println(tcounter); } previousStateCLK = currentStateCLK; lcd.setCursor(0, 0); lcd.print("deg: "); lcd.print(tcounter); lcd.print(" "); } 2
Appendix B
Retaining Ring Groove:
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Retaining Ring by McMaster Carr:
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Appendix C
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