Design And Simulation of a Static Fire Test Stand for Solid Rocket Motor Testing

This paper details the design and simulations of a vertical static fire test stand for solid rocket motors, developed to support the Cougar Rockets team at Southern Illinois University Edwardsville (SIUE), in motor testing and development. A key function of the test stand is the ability to record rocket motor thrust curves, allowing us to verify the repeatability of rocket motors, as well as developing experimental motors. The design and simulation are integral due to the lack of commercially available test stands, resulting in minimal alternatives to compare results to. The test stand is engineered to accommodate solid rocket motors up to the "L" impulse class, designated up to 5120 Newton-Second impulse. This classification aligns with the upper limit of "Level 2" high-power certification in amateur rocketry, making it suitable for university-level research and competition teams. The design consists of a structural frame made of an aluminum t-slot extrusion, allowing for an adaptable, modular, and easily transportable assembly. To ensure compatibility with a variety of solid rocket motors, the stand features a machined aluminum stepped motor mount, allowing for motors with different diameters. Of the same material, we use a load-cell-to-frame mount, forming a cohesive load cell assembly, allowing us to get accurate data without risk of external forces interference. Further motor securement includes ground anchor tensioning and a stainless-steel sliding and locking system, adjustable for different motor heights and widths. The orientation of the test stand follows a vertical configuration, where the motor's nozzle is directed upward, thrusting into the ground. This approach, while less common in the aerospace industry, is particularly suited for university-level testing due to its simplicity in securing the test stand and mitigating external interference from horizontal thrust forces. To verify the structural integrity and safety of the design, structural and fluidic simulations were conducted. For structural analysis, we simulated static loads, modal (vibration) analysis, and transient load analysis. For fluid dynamics analysis, we used computational fluid dynamics (CFD) to simulate the exhaust in terms of velocity, pressure and temperature distributions. For simulations, we used ANSYS Workbench and ANSYS Fluent. The highest max-thrust motor within the intended testing range was used as a benchmark for these simulations, ensuring that the test stand is designed to withstand worst-case scenarios. For structural analysis, we used material properties of aluminum 6063 (for T-slot extrusion parts) and 6061 (for machining parts), with yield stress of 172 MPa and 241 MPa, respectively. The Young’s modulus used in the simulations of both alloys is 69 GPa. Static and transient FEA simulations confirm that the test stand can endure regular use as well as catastrophic motor failure (CATO) without compromising component integrity. A safety factor of 2.0 was achieved for critical components, particularly the motor mounts, retainers, and tensioning points. Vibration analysis shows that the first three natural frequencies are within 43-69 Hz range. Available NASA data shows rockets typically generate a 5-200 Hz range due to acoustic noise reflecting off the ground during takeoff. However, given our orientation and short test duration, we conclude that harmonic excitation force will not be high enough to induce resonance. Thermal-fluid interactions were also analyzed using CFD simulations to evaluate the effects of motor exhaust on the frame. When testing shorter rocket motors, portions of the frame may be exposed to high-temperature exhaust gases. The CFD results indicate that exhaust temperatures can reach up to 2700 Kelvin right at the nozzle exit and 1000 Kelvin in the outer exhaust plume, which is verified with manufacturer data. We conducted heat transfer analysis to determine the temperature distribution within the working space of the system to ensure thermal stresses do not compound the structural stresses applied. The transient heat transfer simulations were conducted under the assumption of forced convection between the exhaust gases and the frame. The results show that at such short rocket motor burn times on high-thrust motors (1-3 sec), the temperature of the frame is within acceptable ranges, below 150 C. Through careful designs and verified structural and fluid-thermal analysis, we have developed a test stand for Amateur and University-Level Rocketry. Proper material selection gives us a safety factor of 2, ensuring that the stand is operable even during catastrophic failure. The next stage of development of this project includes manufacturing of parts, assembly, and testing. Although it is not within the scope of this paper, it will allow us to verify our results in the future with experimental data.