An outcomes-based educational/instructional experience for teaching solid rocket propulsion concepts

The Aeronautics Department at the United States Air Force Academy (USAFA) introduces solid rocket propulsion fundamentals to aeronautical engineering cadets in Aeronautical Engineering 361 (AE361), Propulsion I. The Air Force uses solid rockets in a wide range of air-to-air, air-to-ground, and ground-tospace applications, and the graduates of our program primarily serve as pilots, researchers, or acquisition professionals. AE361 educational outcomes require students to calculate rocket thrust (Fi) and specific impulse (Isp) and discuss the effect of altitude on rocket nozzle performance. In learning this material, the cadets design convergent-divergent (CD) nozzles that are under, over, or optimally expanded. They test the performance of each nozzle and compare measured data with their calculations. This paper discusses the design, commissioning, and practical use of a new solid rocket motor demonstrator rig used in the USAFA Aeronautics Laboratory to better meet the desired AE361 course outcomes. INTRODUCTION The purpose of the AE361 junior-level propulsion course rocket laboratory is to introduce solid rocket propulsion and nozzle design. Desired educational outcomes include calculating FI and Isp based on a given combustion pressure (Pc), temperature (Tc), throat area (At), and propellant chemical composition. Another key outcome is determining the effect of altitude on rocket nozzle performance by evaluating nozzles at * Assistant Professor, AIAA Member * Instructor of Aeronautics, AIAA Member This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. altitudes that create over, under, and perfect expansion conditions; cadets are required to understand the associated effects on Fj. An added benefit of designing a nozzle on paper followed by performing an experimental analysis is a better understanding of the many trade-offs required to predict, design, build, and fire an optimal propellant-nozzle combination. Cadets consider the chemical makeup of the propellant, the effects of total and ambient pressure and temperature on thrust production, and the nozzle's material properties. They also quantify the validity of the adiabatic nozzle assumption and compare solid rocket performance to other available propulsion systems. BACKGROUND Previously, introducing solid rocket propulsion to cadets involved statically firing Army 2.75-inch foldedfin rocket motors in the USAFA Aeronautics Laboratory Rocket Test Cell (RTC), located northwest of Colorado Springs, Colorado. Rocket firing provided FI levels via a load cell and either measures or estimates of Pc, leading to general comments on the estimated rocket performance. However, the lab included no nozzle design considerations. The 2.75-in solid rockets are core burners. Rockets previously had their core tapped to provide direct Pc data during firing; however, this practice was discontinued due to high cost and risk associated with drilling and tapping the propellant. A further safety hazard was simply firing a rocket motor that generates these relatively high values of Fi and Pc; a typical motor produced «1600 lbf of thrust an a combustion pressure of over 1400 psig during its approximately one-second burn. Additional problems included convoluted storage, transportation, and test procedures. The rockets, classified as a Class C weapons-quality explosive, had to be stored off-site, transported under armed guard, and fired within hours of delivery since the RTC is not an approved explosives storage facility. Finally, the firings generated a significant noise hazard for the numerous personnel who work in and around the RTC facility. All these