PRISM: Passive Inertial Stabilization Mechanism Final Technical Report MAE 4152W: Capstone Design Project 5 May 2019

GWU Rocket Team: Capstone Design Team

Noah H. Bakr Colin J. Pate Thomas Susi Jeremey Waldron

1 1. Abstract

Passive Ramjet Inertial Stabilization Mechanism, henceforth referred to as PRISM, is an aerodynamically driven gyroscope which aims to counter the natural pitching and yawing moments of a sounding rocket. The concept was originally developed for the Intercollegiate Rocket Engineering

Competition, and strict requirements had to be met in order for PRISM to be accepted. PRISM must: integrate into a standard six inch diameter rocket, be a passive system (i.e. not able to be controlled after launch), limit the pitch and yaw of a rocket to within ten degrees, not add significant drag to a standard rocket, and must withstand the physical forces of launch and landing. Based on these requirements, the following design was conceived. It is made up of a forward to direct the airflow through the to outlet holes, a turbine fan blade mounted on a steel shaft assembly, and an electronics bay located in a standard six inch rocket coupler. During flight, airflow is naturally forced through the hollow nose cone, the turbine blade extracts energy from that airflow, and the turbine blade-shaft assembly spins as a result. The spinning shaft provides the torque, or rotational force, to counter the pitch/yaw of a rocket through a phenomenon known as gyroscopic precession. A spin test was conducted to determine if the turbine blade would spin up using a compressed air line shot directly against the blade, and the shaft successfully spun up to about 3,000 revolutions per minute. A flight test was conducted with PRISM mounted on a standard six inch rocket; the rocket flew at about 305 miles per hour and PRISM spun up to 8,000 revolutions per minute, and the preliminary data shows that the rocket hovered at zero degrees in both the pitch and yaw directions. The results are promising for this pioneering system.

2 2. Team Member Roles

Jeremey Waldron (Team Lead) ​ ​ - Electronics Systems Design - Electronics Systems Manufacturing - Software Developer - Nose Cone Manufacturing - Outlet Cone Manufacturing

Noah Bakr - Gyroscopic precession analysis - System torque requirements calculations - Forward hardpoint manufacturing - Turbine blade and shaft manufacturing/assembly - Final Report Editor

Thomas Susi - Modeling and Analysis Lead - Pressure Abnormality CFD - Stability and Drag CFD - Structural Manufacturing and Assembly - Flight Test Integration

Colin Pate - Transonic Stability Analysis - Inlet Geometry Design - Internal Flow Geometry Design - Manufacturing Assistant

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3. PRISM Introduction

A. Project Scope

This capstone project is an exercise in subsonic aerodynamics and stability control. PRISM aims to create a system capable of passively stabilizing a rocket during ascent. This system is to be integrated into the GW Rocket Team Spaceport America 2019 to provide a straight and stable flight to

10,000 feet. This will be the culmination of previous projects and experiments within the GW Rocket organization to provide added aerodynamic stability to the airframe with different payloads or subsystems. It will be constrained to flights below the transonic range due to the complex nature of shocks formed in this region. A successful PRISM nose cone will be able to create stable flight passively and collect the necessary data to corroborate this.

The ram air stabilization technology that will be proven possible within PRISM is applicable not only to sounding rocket stability but also the aerospace industry as a whole. Next steps with the PRISM system include changing the hardpoint geometry to act as an aerospike. The change in geometry would allow PRISM to not only operate in transonic, but also hypersonic environments. The aerospike would prevent hypersonic air from flowing through the inlet once a sufficiently high velocity is achieved, therefore providing gyroscopic stabilization for a hypersonic vessel. The gyroscopic stabilization would also allow the hypersonic vessel to be finless, allowing further reductions in drag for the vehicle.

B. Technical Literature Review

1) Inertial Stabilization

The system will act as a control moment gyroscope (CMG) to control the pitch and yaw of the rocket mid-flight. Gyroscopic stabilization systems are well documented in rockets, with successful

4 rockets such as the Saturn V employing three gimbals to control the rocket’s attitude (roll, pitch, and yaw). These gimbals relayed the rocket’s attitude to a central flight computer, which adjusted the thrust of Saturn V to correct its attitude. Mathematically speaking, Gyroscopic Stabilization Systems can be simply modeled as any other rotating system:

흉 = I * 휶

Equation 1. Torque Definition where 흉 is the torque provided, I is the moment of inertia of the system, and 휶 is the rotational acceleration rate of the system; the torque produced by the rotation of the turbine blade and mass, provides a direct counter moment to the natural motion of the rocket in flight. This fundamental process is called precession and is the actual physical means by which a gyroscopic device produces a stabilizing torque opposite of the body’s motion [1]. The objective of a CMG is to rotate a mass, generally using an electric motor, to provide a direct torque on the system via the CMG support structure to orient the rocket in a desired direction. Douglas Havenhill patented a direct torque control moment gyroscope to be used in satellites, and research into his design heavily influenced the initial design of PRISM [2].

Keshtkar, Moreno, et al further demonstrated how this same gyroscopic system could be applied to a satellite to fully control its attitude, which is directly applicable to PRISM [3]. The physics behind

PRISM are well supported and well documented, and the system in theory should be physically effective.

2) Deviations from Regular Nosecone Geometries

In order to reach an altitude of 10,000 feet above ground level using solid rocket engines, a rocket must travel at transonic speeds. In doing so, the rocket must forego the instabilities associated with the varying aerodynamic conditions which occur while going transonic. The geometry of PRISM’s

5 inlet strays from traditional nosecone designs and therefore could jeopardize the stability of the rocket as it passes through the transonic region.

Any geometries which deviate from a smooth bodied cone or ogive shape can affect the performance of a rocket significantly, particularly in the transonic region of flight. At these speeds, the center of pressure tends to drift forward, leading to a proportional loss in static stability. Stability is based on the distance between the center of gravity and center of pressure measured in body diameters.

NASA has conducted ample research on the transonic aerodynamics of rockets with unusual rocket shapes[4, 5], finding that the presence of fairings and control auxiliary rockets leads to increases in axial loading and that shockwaves form near sudden changes in geometry due to progressive pressure build up. Therefore, it is likely that any sudden change in area or the presence of any shoulders on the nosecone of the rocket body will result in the formation of shocks. Furthermore, any sufficiently thin surfaces in contact with the transonic flow may experience fluttering[6], which may add to vibrations experienced by any payloads. These attached shocks are not likely to jeopardize the integrity of the rocket, however a detached (bow) shock could pose significant issues.

Bow shocks are essentially shock waves which have become detached from the body of the rocket and now lead ahead of it. The aerodynamics of detached shocks over blunt body objects is laid out in a Rand Corporation study[7]. The study found that while a leading nose bow shock decreases heating and friction effects along the length of the rocket body, it generates a large amount of pressure drag and therefore may have a negative effect on stability. This mainly applies for only slightly blunt objects, where the strength of the shock just near the corner is nearly equal to the strength of a normal shock. As long as the nosecone has a sweep back angle which is sufficiently high, the shock is unlikely to become detached [8], and therefore can be avoided with an increase in inlet cone length. Transonic ​ flight, and any associated changes in aerodynamics will also likely effect inlet dynamics and therefore

6 flow fan performance. As the inlet approaches high pressure ratios between the high internal pressure and low external pressure, and the flow capacity, the energy being transferred to the turbine may decrease [9].

The addition of the passive stabilization mechanism and its unusual geometry of the nose cone may cause some detrimental aerodynamic effects at transonic speeds, but a sufficient safety factor in stability - greater than 15% - should mitigate any changes in the center of pressure. Additionally, the improved altitude performance from inertial stabilization should outweigh the marginal losses in stability.

3) Aerodynamics

In order for PRISM to be effective, the inlet geometry of the nosecone must be designed to maximize flow over the fan blades without destabilizing the rocket altogether. This calls for preliminary modeling of the known airflow specifications during flight, including: changes in velocity, elevation, air pressure, and mass flow rates.

Inlet geometries for typical turbine engines vary greatly depending on the air speeds involved.

There are three main scenarios (based on the Mach number of the airflow) that are widely studied and understood: subsonic speeds, supersonic speeds and hypersonic speeds. Subsonic is generally between

Mach 0 and 0.8, supersonic is 1.2 to 5 and hypersonic is above Mach 5. The subsonic region is the most well understood as air compressibility is not yet a key factor. These type of inlet geometries have relatively simple shapes and a smooth lip. Supersonic inlets have sharp lips and internal geometries to slow down the flow to subsonic speeds before the turbine [10]. A combination of these designs must be considered for an inlet area that works within a range of speeds, especially around Mach 1.

Transonic air speeds range from Mach 0.8 to 1.2, which is dependent on the air speed itself and the air temperature. The shocks that begin to form can sum up to form larger shocks on the airframe,

7 causing massive amounts of drag and instability on the system. These shocks can also be prevalent at the turbines rotor blade tips as they spin at high speed. The shocks formed off the blade tips can cause instabilities and must be avoided [11].

The speed regimes involved are only one factor, as mass flow rate is the key characteristic that must be considered. This is reliant on the density and velocity of the air in question, as well as the inlet area itself. As the density and velocity are not constant, the mass flow rate is not constant for such a system and could lead to problems with choked flow in the system, which occurs as the flow encounters a constriction of some sort. The flow’s velocity is then limited or stopped completely as the mass flow rate fluctuates. Considering these factors, the mass flow rate through an internal system can be converted directly into a torque. This must, however, take into account inefficiencies. The first would be the inefficiencies seen on the turbine itself, but there are also airflow inefficiencies to consider.

To determine how efficient an inlet is, a series of inlet distortion indices are used [10]. One of these inlet indices is the pressure recovery ratio. This characterizes the losses seen within the airflow - the higher the number, the better. These inefficiencies in airflow often result in excess drag as well. Within the transonic region, as shocks begin to stack up, wave drag can occur with the shocks energy being dissipated on the airframe [11]. Another form is seen in spillage drag as airflow avoids the turbine blades and spills over the lip of the inlet [10].

4) Rotation Rate Measurement

When conducting experiments with new rocket technologies, the most vital part of the test is the post flight analysis to determine if the experiment was successful. For PRISM, the most important data to record is the rotation rate of the turbine within the nose cone with respect to time. This data will determine how quickly the fan gets up to speed, and allow for calculations to determine the kinetic

8 energy that can be extracted from the ram air. The sensor to read and determine this data is a laser tachometer.

A tachometer is a sensor which is used to measure the rotation rate of an object such as the rotation rate of a shaft. The unit for rotation rate that is frequently used is RPM, or rotations per minute.

Tachometers can be categorized as analog, digital, contact, and non-contact. A digital, contact tachometer uses an on/off digital pulse which is sent to a computer or micro-controller. The digital signal is able to be sent to a liquid crystal display or LCD screen so that a person can observe the rotation rate measurement in real time. A digital signal is also unique due to its ability to be stored on a computer or micro controller’s memory so that it can be observed at a later time [12, 13, 15].

Analog Tachometers use a needle and dial type interface similar to a tachometer that may be observed within the dashboard of a car. Analog tachometers use the increase in rotation rates to create a voltage change in a dial. As the rotation rate of the object increases, the dial increases proportionally.

Analog tachometers are not able to store data themselves, but analog tachometers can make use of simple analog computers and record data on to tapes that can be replayed to observe previous data.

Analog tachometers work by converting rotation speed into a voltage by using a external frequency to voltage converter. Analog tachometers frequently use contact type sensors due to their reliability and robustness.

Contact sensors, as the name implies, make physical contact with the rotating body. The most common form of contact sensor utilizes an optical encoder or a magnetic sensor which physically connects to the rotating shaft. An optical encoder uses a perforated disk which mounts onto the rotating shaft, each line on the disk indicates a specific amount of rotation of the shaft [14]. Optical encoders have the ability to provide extremely high resolution at the cost of processing power necessary to observe the increase in data. Magnetic sensor type tachometers use hall effect sensors and multi pole

9 magnets that are fixed on the rotating shaft [16]. Similar to the optical encoders, each time the magnetic field changes as the multipole magnet rotates, one rotation is calculated. Magnetic tachometers do not offer as high as resolution as optical encoders, and they are more difficult to install.

For extremely high rotation rates, like PRISM, non-contact laser or LED (light emitting diode) type tachometers are necessary. Laser tachometers are frequently digital tachometers which use a laser pulse, and a reflective coating on the rotating shaft, to determine when the shaft rotates [17]. PRISM will be using this type of sensor because the reflective tape will not interfere with the balance of the high speed shaft. LED tachometers are also rated for the G forces that the system will experience during the boost phase of the rocket’s flight.

C. Design Requirements

The functional requirements of PRISM were derived from three separate entities: the

Intercollegiate Rocket Engineering Competition’s requirements, the course requirements for MAE 4152

(the Mechanical Engineering Capstone Design Course at The George Washington University), and the team’s own personal ambitions for the project. Further compliance and functionality must have been met as well in order to comply with federal weapons/missile laws. Merging the customer’s requirements with the team’s course requirements and personal desires, the following list of design requirements was devised:

● Must be able to provide a counter torque to dampen the natural pitch/yaw of the rocket during

flight. (Limit to 10º Pitch/Yaw Range)

● Must be able to log critical data such as fan blade rpm up to 88,000 rpm

● Must not add significant amounts of drag to the rocket (rocket maintains around 2 calipers in

10 stability) [9]

● Must be able to integrate into the main rocket frame with 6” OD without any major frame

modifications

● Airflow must not choke within the system (No Backflow)

● Must be a passive system; i.e. it can not be controlled except by natural airflow during flight

● Must be able to handle aerodynamic and mechanical stresses during lift off (20G acceleration

and delta V from 0 mph to near 700 mph) and absolute max RPM of 88,000 rpm

4. PRISM Design Description

A. Design Summary

Figure 1. PRISM Rendering

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Figure 2. PRISM Rendering

Pictured above (in Figures 1 and 2) are SolidWorks renderings of the full PRISM assembly.

PRISM is designed to: allow an optimal amount of airflow through the nose cone, extract as much kinetic energy as possible from the airflow, minimize frictional losses in the shaft, eject the air out of the system, cause minimal extra drag on a six inch rocket, and integrate seamlessly into the rocket. In terms of parts and fabrication, PRISM was relatively easy to construct and assemble (with some minor hiccups along the way of course). The brunt of the work fell into the design stage, and careful planning was made to ensure PRISM’s ease of assembly and functionality. PRISM, in its current state and final design iteration consists of 5 subsystems/parts: the forward hardpoint, the aerostructure housing, the turbine assembly, the outlet cone, and the electronics/data logging system. Each subsystem/part went through several iterations, prototypes and testing to get into their current forms, and PRISM as a whole underwent at least four iterations before the current state. Each system’s design and fabrication was spearheaded by one team member, with all team members assisting in almost all aspects of the design and manufacturing processes. Each sub-system/part’s detailed design and manufacturing process is detailed below.

12 B. Subsystem Designs

1) Forward Hardpoint

The forward hardpoint is one of the most important parts of PRISM as it holds the rotating shaft and turbine blade and allows all of the air to actually flow into the system. The configuration shown in

Figure 3 below shows the support and hollow air flow pockets. One can also clearly see the inlet cone and cowling/ in Figure 4. The cowling is properly sized to sit on top of a pre-cut lip of the stock rocket nose cone, and the hollow areas have been sized to allow for a safety factor for the inlet/outlet area ratio. The material selected for the hard point was 6061-T6 aluminum, an grade aluminum alloy. It was chosen due to its strength, light weight, ease of machinability, and low cost.

Figure 3. Forward Hardpoint Isometric View

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Figure 4. Side View of the Forward Hardpoint

Since the forward hard point also acts as a new nose cone for the rocket, the structural dynamics and aerodynamics of the part are crucial. Based on data collected from test launches/research, the hollow inlet area pictured above in Figure 3 was calculated to allow a high enough air flow rate air into the turbine so that it can produce enough torque to counter the pitch/yaw of the rocket. The interior support struts must be robust enough to hold the entire piece together, but also can’t obstruct the airflow in any significant way. The struts were calculated to only be 1/8-inch-thick with 1/4-inch-radius fillets connecting them to the center bearing mount. To ensure its structural integrity against oncoming winds at a speed of Mach 0.75, the part was machined out of a solid aluminum cylinder, with no extra parts

(with potential failure points) added on.

Aerodynamically speaking, the two most important geometric aspects of the hardpoint are the inlet cone and cowling, as seen above in Figure 4. In the transonic speed regime (0.8 Mach will be the maximum speed that the rocket will fly at), air compressibility begins to take an effect and shock waves begin to form in certain spots on the rocket body. Due to this concern, the inlet cone was chosen to have a shallow angle of 22 degrees. Should any shock waves form, the shocks should be attached to the

14 conical body, which will cause much less drag than if any bow shocks formed on a blunt nose cone.

Furthermore, the cowling (rounded edge on the outside of the hard point) has been manufactured to match the same curvature of the cut nose cone that the hard point rests on. This allows for a seamless integration of the two parts, and will minimize any drag caused by the joint since the air will be following the same curvature the whole way.

The complexity of the hard point required CNC machining methods to ensure proper manufacturing. The machining process was linear, since it was all machined from one solid stock of aluminum (4.5” in diameter by 6” high). The stock was cut down in a drop saw to a shorter height so that the round would fit in the machines more readily. The round was then turned down using a CNC program to the final outer dimensions of the hard point (previously seen in Figure 4). The piece was then inserted in a mill where a CNC program cut out the pocket on the wide end to hold the thrust bearing

(which will hold the rotary shaft at one end), as seen in Figure 5 below. Further CNC programs were written to cut the hollow pockets out of the interior using the same mill, and great care was taken to ensure a smooth surface finish for the airflow. The piece was mounted into a CNC lathe, where the other half of the wide end was hollowed out manually using a long boring bar, so that the entire interior pocket became hollow. On the same machine, two more CNC codes were written to create the inlet cone

th and cowling ring by simply tapering each down to as fine of an edge as possible (1/100 ​ of an inch). ​ A test fit into the cut nose cone, in Figure 6, showed that the forward hardpoint lip sat on top of the nose cone as intended. Simulations were also completed in SolidWorks (Appendix A) using a 150 pounds per square inch pressure (the pressure from the machine shop compressed air) on the top of the hard point. These simulations were conducted to insure the structural integrity of the hard point during flight. The simulations show that the forward hard point will withstand the force it will be up against.

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Figure 5. Hard Point Bottom View Figure 6. Completed Forward Hardpoint

2) Aerostructures

The internal geometry of PRISM is designed to maximize the amount of work which can be extracted from the inlet air. Once the air passes through the inlet hardpoint, it enters the main internal cone which follows the same external geometry as the nosecone. The nosecone provides the main fiberglass frame of PRISM and is a slight ogive or parabolic cone shape, which expands from four to six inches in diameter over its length. This expansion will slow the flow and assist in managing the varying aerodynamics conditions at transonic speeds. The inlet is located at the top of the nosecone and is three inches in diameter. Four separate outlets are cut out of the bottom of the nosecone and have a combined area which is 1.5 times larger than the inlet area to avoid choked flow. Achieving choked flow in flight would result in “spillage” of flow from the top of the nosecone, thus greatly increasing drag and reducing stability. To ensure there is no chance of choked flow developing during flight, an area ratio of

16 1.5 was selected (assuming a ratio of specific heats of 1.4), preventing choked flow up to roughly Mach

1.8 according to Equation 2. Given the inlet area from the hardpoint cross-section the outlet area is defined by the 1.5 times relation as shown below in Equation 3.

Figure 5. Full PRISM assembly

Equation 2. Area Ratio Equation for Choked Sonic Flow

Equation 3. Inlet/Outlet Area Ratio and Dimensions calculations

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Figure 6. Nosecone geometry and outlet side view with dimensions in inches

However, as the rocket approaches its maximum velocity, transonic aerodynamic variations may reduce the efficiency of the assembly as wave drag forms at the tip of the turbine. Wave drag forms when the tips of the turbine blades begin to approach the speed of sound and generate shockwaves.

There is also sufficient bypass surrounding the turbine assembly in order to minimize the disturbance to the overall flow passing through PRISM. As the turbine spins, a central column of air will transfer its kinetic energy to the axle assembly and then mix and exit with the bypass flow. Additionally, this prevents any chance of the turbine colliding with the outside walls of the nosecone as it spins at high velocity.

Once the air passes the turbine, it is then guided towards the outlets by the outlet cone. The outlet cone was 3-D printed to accommodate its unique geometry. Two designs were considered, the

“carousel” top and smooth cone. The outlet cone is designed to condition the air flow to reduce drag and ​ turbulence in the air generated by PRISM. The “carousel” top design, as seen in Figure 7, was chosen as it will best limit the turbulence in the air towards the outlets. However, due to the angularity of the flow induced by the turbine according to Euler’s turbine law, the design was modified. This angular flow could collide with the carousel tops and generate an equal and opposite spin of the rocket, thus reducing the effectiveness of the overall system. Instead, the smooth cone allows the angular flow to simply spiral

18 out from the outlets without exerting any counteracting forces on the rocket body. The final design as seen in Figure 7, is a compromise between the smooth cone and minor carousel tops to minimize disturbance at the outlets.

Figure 7. Large carousel tops on outlet cone versus smooth cone

3) Turbine/Shaft Assembly

The main turbine assembly contains the turbine blade, shaft collars, axel, and bearings - this assembly is the actual mechanism by which the gyroscopic control moment is produced. The turbine blade itself was extracted from an EDF (a small engine made for model jet planes). The blade is held to the shaft be being sandwiched between two shaft collars, which are screwed together and tightened to the shaft. The turbine assembly is mounted by a thrust bearing near the inlet hardpoint, and a rotary bearing beneath the outlet cone. Sensors mounted in the coupler record flight performance and turbine revolutions per minute. As the turbine is spun by the incoming air, it generates a strong moment perpendicular to the direction of flight. This exerts a gyroscopic force via precession, counteracting changes in the pitch and yaw of the rocket caused by gravity turn. Gravity turn occurs due to the introduction of offset to the launch angle from wind cocking, launch rail angle, or aerodynamic

19 disturbances. Gravity then progressively pulls on the center of gravity of the rocket causing it to slowly pitch and yaw in the direction of the offset. This can have a major effect on achieved apogee and therefore reduce the performance of the rocket. Most of the inertia force comes from the mass of the steel axel and the high angular velocity of the turbine, which is rated to 80,000 rpm.

Figure 8. Turbine Assembly Rendering

Equation 4: Turbine Assembly Torque Calculations

20 4) Electronics and Data Logging

The most important part for proving PRISM’s functionality is the data logging circuit. Over the past few months, the development of the data logging technology has persisted without any major issues. The system is currently constructed within a fully soldered breadboard and is able to log inertial motion, temperature, altitude, and propeller speed at 9 Hz. In Figure 9 below, the circuit can be observed in a prototyping breadboard.

Figure 9: Data logging circuit on breadboard ​ ​ The tachometer, which is used in the circuit, consists of an infrared LED (Light Emitting Diode), an infrared sensor, and reflective tape on the spinning surface. The program for sensing rotation is written such that when the function is called, the arduino will count the number of down pulses from the LED sensor in a set period of time, indicating that the IR sensor has observed a pulse, and the pulse has passed. Using this method, precise rotation measurement can occur without a heavily impacting the operation of the other sensors. The operation time for this function is currently 250 milliseconds and has been tested at rotation rates up to 36,000 rotations per minute.

21 The most difficult item to operate was the SD card reader. The SD card reader is one of the most crucial parts of the circuit because it is where all of the post flight data will be stored. While checking the communication voltages, it was discovered that the Arduino Nano operates at a communication voltage of 5V, and the SD card operates at a communication voltage of 3.3V. The voltage difference caused a need for a logic converter and that converter was used to convert the serial communication from 5V to 3.3V to communicate from the arduino to the SD reader. After experimentation with the SD reader, this was not successful. A bi-directional logic converter was sourced and converted the 5V arduino signal to 3.3V for the SD card, and allowed the 3.3V signal from the SD card to be boosted to

5V so that it could communicate to the arduino. The bidirectional converter solved the communication issues between the SD reader and the Arduino, allowing the circuit to operate successfully and record attitude, altitude, temperature, and RPM data onto the SD card as a CSV, or comma separated value file.

5. Evaluation and Testing

A. Evaluation Summary

In terms of evaluating and testing the effectiveness of PRISM, two main tests were performed: a static spin test and a full test launch. Previous test were also conducted along the way such as testing the electronics with a known rpm to verify the output, test fitting everything before the final assembly, and several computer simulations to model PRISM before it was fully assembled and ready to be tested as an integrated system. The static spin test was conducted simply by mounting PRISM to a milling bed with ratchet straps, and then blasting pressurized air through a nozzle to test how fast the shaft will spin under a certain airflow. Once PRISM showed that it was capable of spinning under airflow, the system was ready to be launched in an actual rocket. The test launch occurred in a standard six-inch diameter

22 sounding rocket under slight rain, and the system performed very well. More details about each of these tests will be discussed further on in the report.

B. Subsystem Testing and Evaluations

1) Integrated CFD Analysis

PRISM’S settled-upon design needed to be validated before manufacturing could begin to insure the system behaved in the manner intended. PRISM has flow exiting the base of the nosecone that could have interfered with pressure ports along the side of the rocket. These static ports included data collection for the PRISM system itself as well as the necessary for firing the ejection charges in the recovery system. Tripping these altimeters early could have catastrophic consequences on the airframe. To determine the new pressure field caused by PRISM, SolidWorks Flow Simulation was carried out to analyze the pressure discrepancies around the rocket not seen with typical ogive nose cones. Pulling from the simulations, Figure 10 shows the cut plot of the pressure expected along the surface of the airframe with an ogive nosecone. This acts as a control in this testing. Alongside this,

Figure 11 shows the pressure seen with PRISM engaged. The red circles in Figures 10 and 11 show where the static ports for the rocket’s altimeters are located. Comparing the two models, there is no difference in pressure that could cause the ejection charges to fire before apogee. The static ports within

PRISM are circled in orange in Figure 11. There is a slight drop in pressure seen here due to the outflow from the nosecone. This could cause some discrepancies in the data collected in this package. However, the rocket has redundancy in data collection packages to combat this. This testing had validated the system design so that no pressure discrepancy will result in flight failures.

The system’s stability then had to be validated to insure steady flight, which was typically determined with OpenRocket. This software could not analyze the complex geometry of PRISM, so

23 analysis needed to be done with SolidWorks Flow. The CFD was used to determine the center of pressure in the PRISM nosecone to then compare to the center or pressure seen in a normal, ogive nosecone. This COP position relative to the nosecone base was determined with Equation 5.

COP (X) [meters] = GG T orque(Y )/GG F orce (Z)

Equation 5: Center of Pressure Location

It resulted in a distance along the x-axis from the origin that shows the center of pressure position. This distance was determined a second time by flipping the torque and force axes. An average was then taken between the two distance values to find the center of pressure. Figure 10 shows the resulting data for

PRISM, including drag, torque, and center of pressure locations. This technique for finding the center of pressure was validated using the ogive nosecone. Because a known value for the center of pressure in an ogive nosecone could be found with the OpenRocket, a comparison was made between that location and the location found in SolidWorks seen in Figure 11. These values aligned, validating the technique used on PRISM. The findings showed that the center of pressure for PRISM had shifted towards the leading tip by 2 inches. This COP shift was beneficial for the overall design because it made the rocket slightly more stable, but it was effectively negligible when integrated with the full airframe’s COP.

Completing these sets of analysis validated the design as it stands to then be manufactured.

These simulations were necessary to confirm the integrity of PRISM and the data collection systems of the overall rocket. This testing proved that the design implemented will not put the whole rocket in jeopardy as it releases from the launch pad. The data collection systems, ability to fire the ejection charges, and stability of the rocket in flight were found to be similar to those seen with an ogive nosecone, which is desirable in the end.

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Figure 10: Pressure Ogive Nosecone Figure 11: Pressure PRISM Nosecone

25 2) Electronics Testing

Figure 12: Testing the operation of the IR tachometer using a Dremel

The testing for the rotation sensor used a of a piece of reflective tape, the IR emitter, the IR sensor, a dremel with a cutting wheel attachment, and a commercially available laser tachometer pictured in Figure 12 above. The reflective tape was placed on the cutting wheel of the Dremel rotary tool, which has operational speeds from 0 to 35,000 rotations per minute. When the dremel was activated to a specific RPM, such as 10,000, the speed is confirmed with the commercially available laser tachometer, and then also checked with the IR tachometer that was designed for PRISM. Further increasing the rotation rate to 35,000 RPM, as well as verifying the rotation rate with both the commercially available laser tachometer and the IR tachometer designed for PRISM, has confirmed the operation of PRISM’s tachometer circuit to 35,000 RPM. Based off of the CFD calculations done for

PRISM, the fan will not achieve higher RPMs than 35,000 RPM. Based on this CFD data, the tachometer will be able to operate within the boundaries of it’s tested limits and will be able to provide accurate data for the team to analyze.

26 3) Forward Hardpoint Testing

Figure 13: SolidWorks FEM Simulation on the Forward Hardpoint

In order to verify the structural integrity of the forward hardpoint against the aerodynamic forces during flight, finite element analysis was conducted in SolidWorks to asses the deformation and yield point of the part under certain conditions. A 150 psi pressure (the pressure from the compressed air lines available to the team, and roughly 15 times the dynamic pressure during flight) was placed on the part in a SolidWorks FEM simulation. The part was determined to not deform and the stresses experienced by the part were not high enough to cause yielding. The forward hardpoint was manufactured out of one solid piece of aluminum to insure its structural integrity, and the SolidWorks simulations verify the team’s decision on this, as well as the airworthiness of the forward hardpoint.

27 4) Turbine Assembly Testing ​

The functionality of the turbine assembly is essential to the overall functionality of PRISM.

There was not much to go wrong during testing of the assembly, but some issues arose nevertheless.

During manufacturing, two holes in the blade itself were very slightly off tolerance, so the assembly was not spinning properly and was off balance; this was quickly fixed by re-drilling a new set of holes and offsetting the shaft collars onto these new holes. Another issue arose with the bearings that the team had initially chosen - sealed bearings do not spin as well as unsealed ones (the team had initially chosen sealed bearings). There was also another tolerancing issue while press fitting the bearing into the bulkplate; both of these issues combined such that PRISM was not spinning to the teams’ liking. New bearings were ordered and the bulk plate was further milled down such that the press fit of the bearing was not so tight. The new unsealed bearing and new hole solved the problems - a simple spin test by hand of the turbine assembly showed much less friction in the bearings, and PRISM spun much easier after the team made these fixes.

5) Static Spin Test

The static spin test was intended to prove the system’s rigidity and ability to spin once manufactured. With PRISM completely assembled, the system was securely mounted horizontally to a workstation with straps. The first static test was done without any electronics mounted to solely test the mechanical parts. Machine shop compressed air was directly blown onto the fan blades with a nozzle to begin the spin up. The sound of the turbine spinning allowed the team to recognize when the blade had reached top speed as it became a constant high-pitch. The air was then cut with no failures to any mechanisms in the system. This successful test was followed by the same test but with the data logging system implemented. The system fed the data directly to a laptop for real time verification. Again, the

28 blade was spun up to top speed. The data logging system showed the spin maxing out around 4,000

RPM. This was also verified with a commercial handheld tachometer.

Figure 14. Static Spin Test Setup

6) Test Launch

A full scale test launch of PRISM was required to confirm or deny the most significant functional requirements set out for this project. These included the datalogging, flight worthiness, and proof of concept factors involved with PRISM. The test was performed on April 13th, 2019 in Culpeper,

Virginia. PRISM was successfully mounted onto a six inch rocket body via four shear pins. The airframe had been flown previously by the team and would provide a low-risk platform for PRISM’s maiden flight. An M1500 Aerotech commercial motor was chosen for propulsion to provide the necessary speed and altitude to test the system while also being low-risk when compared to student-manufactured motors. A dual-deploy recovery system was implemented to reduce drift on descent. This consisted of a

29 drogue chute being deployed at apogee and a main chute being deployed at 700 feet. The altimeters and black powder charges necessary for this system were made redundant to lower the risk of catastrophically damaging PRISM on descent. PRISM was launched and recovered successfully with this airframe setup. All necessary data was logged by the electronics system, and some post processing of that data appears below.

Figure 15. PRISM RPM Data

Above appears a plot of PRISM’s spin rate vs time. One can see that PRISM rotated for roughly nine seconds during ascent, and reached a peak revolution rate of about 8,000 RPM at around three seconds into flight. Based on this RPM data, a MATLAB code was written (Appendix C) to calculate the counter moment that PRISM was producing, and a plot of PRISM’s instantaneous torque and average torque appears below in Figure 16. PRISM’s average torque during this flight was figured to be

142 Nm, with the maximum torque produced being about 1500 Nm just after launch. Preliminary data also showed the the Rocket’s roll rate hovered around 0 degrees/s in both the x and z directions (pitch

30 and yaw). As a control method, the same airframe will be launched without PRISM in it to assess the roll rate data in an uncontrolled flight. The data from this flight will then be compared to the PRISM test flight data to determine how effective PRISM is. However, the preliminary data of 0 degrees/s roll is very promising even without comparing it to a control flight.

Figure 16. PRISM Torque Data

6. Summary and Recommendations

Overall, the first iteration of the Passive Ramjet Inertial Stabilization Mechanism accomplished all of its functional requirements and initial test results look promising for future use. The first test flight with PRISM achieved a stable, straight low altitude flight. Upon further inspection, some strengths and weaknesses appear obvious. PRISM was easy to manufacture, was not expensive to produce, is easily

31 integrated into any six inch rocket (and can be modified to fit into any rocket), and is a mechanically simple system. On the other hand, pressure drag due to the turbulent air re-entering the airstream led to losses in final achieved altitude. Additionally, there were preliminary concerns that the addition of

PRISM to the rocket could upset that stability of the rocket overall. However, both simulations using computational fluid dynamics and flight tests have shown that PRISM improves the static stability of the rocket and has little effect on the overall drag of the flight vehicle.

This system would prove particularly useful for finless highspeed rockets, where static stabilization effects (i.e. fins) are minimal. Minor improvements to the inlet and outlet sections could allow the system to operate at high speeds and minimize the shock effects encountered. Using internal gyroscopic stabilization, the aerostructure would only require minimal control surfaces to maintain stability. Coupled with an aerospike, high-speed flow would be sonically decelerated and then enter a turbine PRISM system. Similar flow turbine designs could be used to power aerodynamic driven pump systems on hybrid and liquid rockets.

By mitigating the need for rocket spin stabilization, PRISM allows rockets to act more flexibly for hosting payloads with specific observation or orientation-keeping requirements. Traditionally low altitude sounding rockets use spin stabilization to maintain a straight flight. However, spin is not always compatible with sensitive or optical payloads which require steady flight and or a specific flight orientation. Additionally, removing the spin of the rocket in flight ensures that at apogee recovery harnesses won’t get tangled at deployment.

Testing showed the need for some minor design changes, the first being an increase in the tolerance of the turbine axle and 3-D printed outlet cone. Initial spin tests also revealed that the first turbine assembly did not spin as easily as desired, likely due to friction between the rotary bearings and bearing plates. By grinding the plates back and reassembly the rotary bearing attachment, spin up time

32 was reduced. Finally, on landing a significant amount dirt entered the inlet cone; adding spring loaded or recovery deployment activated panels to the hardpoint, could mitigate this.

Overall, this iteration of PRISM ended up being much more successful than the team had ever imagined. Below, one can find a final comparison of PRISM’s design requirements as well as the status of those requirements (all were met). As previously discussed, this mission set out as a proof of concept for aerodynamically driven gyroscopic stabilization, but with the success of PRISM, the team sees this project going much further and into many more applications, especially with high speed and hypersonic rockets and missiles. The team would like to extend a special thanks to Dr. Murray Snyder (our mentor and team sponsor), Dr. Andrew Cutler (our primary mentor), Dr Mehdi Naderi Abadi, Mr. Bill

Rutkowski, Mr. Tom Punte, and Dean Rumana Riffat of The School of Engineering and Applied

Science at The George Washington University for making this project possible.

Functional PRISM Functional Status Requirement

1 (Pitch/Yaw) Test flight shows that the rocket hovered at 0 degrees pitch/yaw for most of the flight, but the team is waiting for the next test flight on May 12th sans PRISM as a control flight to verify the data

2 (Data Collection) Electronics system (tachometer, circuit, etc) successfully logs data at a rate above 10 Hz and has been tested up to a speed of 35,000 rpm, which is much faster than PRISM is expected to speed

3 (Stability/Drag) CFD shows that minimal change in stability and drag - the center of pressure for the PRISM nosecone moves 2in to the rear, which is negligible when integrated into the full rocket.

4 (Integration) PRISM fits entirely within a stock nosecone and coupler, both of which integrate seamlessly into our rocket (and will fit into any 6 inch diameter rocket)

33 5 (Choked Flow) CFD and testing evaluations have shown no choked flow through the system - the data showing the system actively spinning during boost confirms this airflow.

6 (Passive) Turbine does not spin unless air is flowing against it - therefore it is a passive system and is only activated by natural airflow during flight

7 (Launch PRISM was successfully launched and recovered all in one piece Survivability) with no damage or parts displacement

7. References

[1] Keshtkar, Sajjad; Moreno, Jaime A; Kojima, Hirohisa; Uchiyama, Kenji; Nohmi, Masahiro; Takaya, Kesiuke. “Spherical Gyroscopic Moment for Attitude Control of Microsatellites”. Acta ​ Austronautica Volume 143. Feb 2018: 9-15. ScienceDirect, 5 Oct. 2018, ​ ​ ​ https://www.sciencedirect.com/science/article/pii/S0094576517307038#bbib1 [2] VEEM Ltd. “How Gyrostabilizers Work.” VEEM Gyro, 20 Jan. 2019, ​ ​ veemgyro.com/how-gyrostabilizers-work/. [3] Havenhill, Douglas D. Direct Torque Control Moment Gyroscope. US Patent Number: ​ ​ US5386738A. 7 Feb. 1995. [4] Suttles, J. T., “Aerodynamic Characteristics from Mach 0.22 to 4.65 of a Two-Stage Rocket Vehicle Having an Unusual Nose Shape,” TN D-2163, 1964 [5] Williams, K. H, and James Jr., R. L., “Flight Demonstration of Nose Mounted Rotating Solid Propellant Rocket Control System and a Comparison with Analog Studies,” TN D-3335, 1966 [6] Olsen, J. J., “Local Aerodynamic Parameters for Supersonic and Hypersonic Flutter Analysis,” AFFDL-TR-65-183, 1965 [7] Krasnov, N. F., “Aerodynamics of Bodies of Revolution,” R455 Part II, Chapter VII, 2006 [8] Nagamatsu, H. T., “Theoretical Investigation of Detached Shock Waves,” 1949 [9] Fangyuan, L., and Fabian, J. C., and Key, N. L., “Interpreting Aerodynamics of a Transonic Impeller from Static Pressure Measurements,” 2018 [10] Hall, N. (Ed.). (n.d.). Inlets. Retrieved November 18, 2018, from ​ https://www.grc.nasa.gov/www/k-12/airplane/inlet.html [11] El-Sayed, A. F., & Emeara, M. E. (2016, January 28). Intake of Aero-Engines: A Case ​ ​ Study[Scholarly project]. In ResearchGate. Retrieved November 18, 2018, from ​ ​ ​ https://www.researchgate.net/publication/295011091_INTAKE_OF_AERO-ENGINES_A_CASE_ STUDY

34 [12] SKYbrary Wiki. (2017, July 29). Retrieved November 18, 2018, from ​ https://www.skybrary.aero/index.php/Transonic_Flight [13] InvenSense Inc, “MPU-6000/MPU-6050 Product Specification”, InvenSense Inc, August 19 2013, URL: https://store.invensense.com/datasheets/invensense/MPU-6050_DataSheet_V3%204.pdf ​ [14] M. Ehikhamenle, B.O. Omijeh “Design And Development of A Smart Digital Tachometer Using At89c52 Microcontroller” American Journal of Electrical and Electronic Engineering, vol 5, No-1,2017. [15] Lady, Ada, “Adafruit Micro SD Breakout Board Card Tutorial” Jul 31, 2013, URL: https://learn.adafruit.com/adafruit-micro-sd-breakout-board-card-tutorial?view=all [16] Bonert, Richard, "Design of a high performance digital tachometer with a microcontroller," Instrumentation and Measurement, IEEE Transactions on , vol.38, no.6, pp.1104,1108, Dec 1989 [17] Steve Maas, “Austin-Healey Sprite Electronic Tachometer Conversion”, Long Beach, California, March 22, 2009, URL: http://www.nonlintec.com/sprite/Sprite_Electronic_Tach.pdf ​

8. Appendices

Appendix A: CAD Models and Drawings

Figure 11. PRISM Full CAD Model

35

Figure 12. Inlet Hardpoint cross-section with dimensions in inches

Figure 13. Turbine Blade

36

Figure 14. Transparent View of Nosecone

Appendix B: CAD Simulations

Figure 15: Stability PRISM Nosecone

37

Figure 16: Stability Ogive Nosecone

38 Appendix C: MATLAB Codes

A) RPM Data and Torque Calculations