PRISM: Passive Ramjet 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 hardpoint to direct the airflow through the nose cone 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 3 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 airframe 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.