Justin Self

DESIGN REVIEW For the 2018 ESRA Intercollegiate Rocket Engineering Competition DESIGN REVIEW OF THE PASSIVE ANTI-ROLL STABILIZATION SYSTEM “ROLLERONS” Lead Mechanical and Aeronautics Engineer: Justin Self Mechanical and Aeronautics Engineer: ​ [name omitted for privacy] Clark Aerospace Engineering INTRODUCTION The ESRA 2018 rocket will be employing an idea borrowed from the US Navy, NACA (NASA circa 1958), and the famous AIM-9 Sidewinder missile: passive roll stabilizers mounted to the rocket fins known as ​rollerons. ​The focus of this critical design review is to provide clear answers to the inquiring reader and to provide perspective on the story, journey, research, testing, and future plans of the rolleron design team’s part of the Clark Aerospace ESRA 2018 rocket team. HISTORY OF THE ROLLERON FIGURE 1​ - Rollerons on the Sidewinder Missile - image courtesy of Muraer - WikiMedia Commons The early Sidewinder missiles, designed by NACA, used a “spinning seeker” that calculated the angle to the target through timing based on the known rotational rate of the seeker. If the airframe was also rolling, the timing was thrown off and the missile would not guide properly. A similar design, the AIM-4 Falcon, included a gyroscopic system linked to the controls to address this, but this design added considerable complexity. The Rolleron’s passive, air-powered design and simple mechanism turned out to be more desirable while achieving the same results. Each of the four fins were outfitted with the rolleron: a small control surface similar to an aileron or vertical stabilizer in an airplane, with a metal wheel (disk) inside. The wheel is partially exposed to the wind and is allowed to spin freely inside the control surface. The wheel, having 24 teeth and being exposed to the air, spools up to incredible speeds and develops considerable gyroscopic force. Gyroscopic precession results, thus moving the small control surface ​into​ the wind, and, as is proven by the success of some early Sidewinder missile tests, and the fact that the rolleron saw mainstream production for ---- years on the missile, proves they fulfilled their purpose: correcting any undesirable roll of the craft (Muraer). The brilliance of the rolleron is that it is ​totally passive​-- the only thing required to spool up the gyro wheel is airflow. The damping of the rolleron control surface has, according to the NACA document, a significant effect on the rolleron’s ability to eliminate dynamic roll instability during flight. NACA used a pressurized hydraulic hinge (a clever design), but the ESRA 2018 rocket’s rolleron will attempt to dampen the control surface using a more basic mechanism. STUDENT CHOICE During the early days of brainstorming the Clark “Vulcan” 2018 Competition launch vehicle, someone suggested, “What about rollerons?” The common response was, “What’s a rolleron?” Since then a small team of aspiring rocket engineers have accepted the challenge of researching, reviewing, and reverse-engineering NACA’s rolleron for use on the competition rocket. The initial purpose of the rolleron was to stabilize the rolling motion of the missile for the purpose of increasing accuracy for the guidance system, but the ESRA 2018 team, of course, is only interested in the stabilization properties in order to keep the launch vehicle stable and to surmount a formidable engineering obstacle. DESIGN The rolleron assemblies that the engineering team is employing were completely student-designed after many hours of poring over an old “declassified” NACA research paper [by Brown Jr., Nason] someone found from an online research archive. The design has been through several iterations, 3D printed test models, several rounds of testing, and more research and development. As of this writing, a full-scale metal prototype has not yet been manufactured. The gyro wheels will be manufactured (by students at Clark College’s own machine shop) out of steel or brass to ensure an adequate mass necessary to develop the gyroscopic force that will provide mechanical advantage to the system. Currently, the team is actively fine-tuning equations that will provide optimal mass, density, and operational angular speeds necessary for superb function of the disks, in order to cut production costs and, again, summit the mountain of an engineering feat deemed “very complicated” by a local PhD in physics. The engineering team is up for the challenge. FIGURE 2:​ The gyro wheel; the heart of the rolleron. The rocket fins will be made of carbon fiber, custom built by a local carbon fiber manufacturing facility, with whom the Aerospace program has spent several years developing rapport and professional relationship. The rollerons will attach to the carbon fiber fins via an embedded student-designed aluminum mounting system, and will swing on a “keyhole” type hinge manufactured from aluminum by the machine shop. The degree of range is 30 degrees from center on each side, thus, the maximum deflection of the rolleron control surface will be 30 degrees in either direction (See ​FIGURE 3 ​below) FIGURE 3:​ ​Showing maximum deflection (30 degrees) of rolleron The damping force of the rolleron hinge will be modified by using different materials for the “thrust bearing” designed for this purpose. The plan is to use materials with different coefficients of friction against a steel plate in order to make the rolleron “sticky” or “loose”. Rigorous future testing of this damping mechanism will verify the effects of the design. FIGURE 4:​ Showing the custom thrust bearing and pivot point FIGURE 5: ​Assumed positive directions of moments and angles, courtesy of NACA The particularly useful NACA document included a simple sketch of the assumed positive directions of moments and angles with rollerons on the rocket fins, shown in ​FIGURE 5. TESTING AND RESULTS The team believes in rigorous design work, computer model testing, building prototypes, breaking the prototypes, building again, and testing until the team is satisfied with a solid product. As of this writing, the team is currently in the “breaking and rebuilding” stage of 3D printed models of the system. The final rolleron system has undergone no less than five design revisions, several 3D printed models, a few confusing equations, and many hours on the computer drafting program-- back to the drawing board! Below is the current record of computer model testing and real-world prototype testing. Brackets [] indicate a planned test. ROLLERON TESTS TO DATE DATE TEST NO. TEST PERFORMED OBJECTIVE OF TEST DEC 8, 2017 1 COMPUTER FLOW TEST; MULTIPLE TO DETERMINE THE AERODYNAMIC EFFECT, IF ANY, OF CURRENT DESIGN, NOT TAKING INTO CONSIDERATION THE MECHANICS OF THE ROLLERON. THE TEST WAS PERFORMED USING A STATIC, NON PERFORMING MODEL OF THE ROLLERON. JAN 24, 2018 2 FIRST GYRO WHEEL TEST JAN 25, 2018 3 JAN 25, 2018 4 MODEL USED FOR TESTING RESULT OF TEST(S) OBJECTIVE (S) OF NEXT TEST FIRST ITERATION DESIGN DESIGN HAD LITTLE TO NO ADVERSE EFFECT ON OVERALL LAUNCH VEHICLE AERODYNA MIC BUILD A PROTOTYPE BASED ON THE COMPUTER DESIGN AND BEGIN AIRFLOW TESTING TEST THE STABILITY OF THE GYRO WHEEL WITH HIGH SPEED BEARING INSTALLED FIRST 3D PRINT WITH HIGH SPEED BEARING STABLE ROTATION AT LOW SPEED INCREASE AIRFLOW SPEED, DETERMINE RPM OF GYRO WHEEL GYRO WHEEL RPM SPEED TEST DETERMINE RPM OF SPINNING WHEEL USING LASER TACHOMETER AT HIGHEST AIRFLOW POSSIBLE 3D PRINT WITH HIGH SPEED BEARING 15,174 RPM MEASURED AT 63.56 M/S AIR SPEED. DETERMINE MAX RPM FOR FLIGHT; BUILD FULL 3D ASSEMBLY, RETEST THEORETICAL FLIGHT AT HIGHEST VELOCITY DETERMINE MAX RPM OF GYRO WHEEL DURING FLIGHT OF ROCKET AT CURRENT COMPUTER MODEL DATA AT 1000 F/S; ω = 7621 RAD/S = TEST HIGHER RPM BEARING IN FULL HIGHEST PROJECTED VELOCITY AND TEST DATA FROM TEST 2 71,826.20 RPM MAX ASSEMBLY FULL ASSEMBLY AIRFLOW TEST (FAAT) DETERMINE STABILITY OF GYRO WHEEL INSIDE HOUSING; TEST FOR FITMENT OF BEARING AND SPACERS NEWEST ITERATION 3D PRINT; FULL ASSEMBLY; LARGER BEARING BEARING WAS NOT SEATED PROPERLY; THE GYRO WHEEL CAUGHT ON EDGE FIT BEARING SECURELY; ADJUST INNER WIDTH OF HOUSING, IF NECESSARY MAR 6, 2018 5 MARCH 2018 [6] FAAT DETERMINE STABILITY OF GYRO AT >= 15K RPM 3D PRINT FULL ASSY. [TBD] [TBD] MARCH [7] FAAT TEST DEFLECTION OF CONTROL SURFACE FULL FIN ASSY. PROTO [TBD] FLIGHT TEST FUNCTIONALITY AND USABILITY At the end of the day, the question remains: ​Will these actually work? ​NACA flight investigations using a rocket-powered Sidewinder test model produced positive results, and answered the question with a resounding “yes”, as proven by the presence of rollerons on the Sidewinder missiles for more than forty years. The NACA flight investigation indicated that [the] dynamic roll-rate stabilization system roll-rate stabilized the missile within +/- 20 degrees per second through the proposed operating Mach number range of 0.9 to 2.3 (Brown Jr., Nason). This information from NACA is particularly useful, as the ESRA 2018 rocket is expected to reach ~1000 feet per second, or about Mach 0.9. Furthermore, the model used in the actual Sidewinder flight investigation noted had an empty (without the additional sustainer motor) weight of 105 lb, while the ESRA 2018 rocket ​weighs in a 92 lb empty. It is for these reasons that the NACA document flight data was taken into careful consideration during the research and development processes. Notable data from the document include the following. 1) Prior to the flight test of the model, the gyro wheels of the rollerons were given an initial rotational speed by means of applying a source of air to each of the rollerons while the model was on the launch pad. Since the Sidewinder missile was to be launched mid-flight by the aircraft, giving the gyro wheels an initial speed provided a test in which the speed of the gyro wheels is closer to the actual operational launching rotational speed than if no initial rotational speed had been applied. 2) The equations and graphs from the model telemetry provide sufficient data on the operation of the rollerons at or below Mach 1, which falls into the expected velocity of the ESRA 2018 rocket. Notably, the NACA flight investigation proved that rollerons did in fact stabilize the model within +/- 20 degrees per second through Mach numbers 0.9 - 2.3. Calculating the angular speed of the rocket using gyro wheel speeds of 10,000 rpm, 60,000 rpm, and 72,000 rpm (the expected maximum angular speed of the gyro wheel for the ESRA 2018 rocket) produced the following results: SYMBOLS ω = angular speed of rocket (rad/sec) I = moment of inertia L = angular momentum EQUATION FOR ANGULAR MOMENTUM L=Ixω BY THE PRINCIPLE OF CONSERVATION OF ANGULAR MOMENTUM, L​rocket = ​L​gyro Solving for ​ω​rocket produces: ω​rocket​ = I​rocket /​ L​gyro RESULTS Gyro speed: 10,000 rpm ω​rocket = 3​.712 rad/sec Gyro speed: 60,000 rpm ω​rocket = 0.618​ rad/sec Gyro speed: 72,000 rpm ω​rocket = 0.515​ rad/sec As the gyro wheel speed increases, the angular speed (undesirable roll) of the rocket indeed “stabilizes”. As a result of these calculations, it is concluded that for the ESRA 2018 rocket, a supply of air channelled to the gyros ​prior to launch is preferable​ in order to produce an initial rotational speed, giving the gyro wheels a “head start” towards optimal operational speeds. SAFETY The ultimate goal of the ESRA 2018 rocket is to take the launch vehicle and payload to 10,000+ feet safely, keeping the rocket stable on the way up, but not at the cost of human life. The utmost concern of the team, however, is the safety of the team members and those nearby the rocket during flight. As such, the team is taking the time to perform rigorous testing during the development of these rollerons in controlled environments, attempting to “break” them and test the materials to their maximum stress levels. Spinning gyro wheels of steel pose an obvious safety concern. The designers have planned for the safest possible operation of the rolleron assembly, and built several safeguards into the system. Firstly, the control surface is designed to fit inside the female embedded hinge bracket (see ​FIGURE 6​) from only one direction: the top. Unless there is a rapid unplanned disassembly of the launch vehicle that destroys the rolleron assembly, the gyro wheel assembly will remain safely embedded in its place. FIGURE 6: ​ Showing angle of hinge and studs into fin to support rolleron Second, there are three cap screws in a secure cap ensuring the control surface to stays tightly attached to the fin. The embedded internal threads for the cap are securely positioned by the carbon fiber fins, being assembled prior to the carbon fiber wrapping. These embedded brackets holding the hinge and threads for the cap are going to be installed by professionals at the carbon fiber manufacturing facility. FIGURE 7:​ ​Showing the cap holding the rolleron in place Thirdly, the designers have come up with a clever solution in the unlikely case the rocket is unable to fly with the rollerons. A second set of ‘dummy inserts” have been developed that will slide into the rollerons’ place when removed, rendering the fins a single, solid airfoil (see FIGURE 8). FIGURE 8: ​Showing the optional “dummy fin” insert End of document, QED. APPENDIX B WORKS CITED Muraer, ​File:Sidewinder Fin.JPG, ​WikiMedia Commons, 12 November 2014, https://commons.wikimedia.org/wiki/File:Sidewinder_Fin.JPG . Accessed 21 March 2018. Brown Jr., Nason; Flight Investigation to Evaluate the Roll-Rate Stabilization System of the Naval Ordinance Test Station Sidewinder Missile at Mach Numbers 0.9 to 2.3; University of North Texas Digital Library; https://digital.library.unt.edu/ark:/67531/metadc53266/ Lerner, ​Air & Space Smithsonian, November 2010, Volume 25, Number 5.​ "Sidewinder: The Missile That Has Rattled Enemy Pilots Since 1958", p54-61. All computer modeling and flow testing was performed with student-edition versions of Solidworks.www.Solidworks.com