Mach, Reynolds, and Strouhal Number Effects on Deployable Aeroshell Pitch Dynamics

This paper presents a computational study exploring the dependence of oscillation rates and Mach, Reynolds, and Strouhal numbers on the dynamic stability of a mechanically deployable aeroshell. Computational fluid dynamics free-oscillation simulations are performed to compute the pitch damping coefficient with dependence on pitch rate. The pitch damping coefficient is split in its fore- and aftbody components. The aftbody loads are shown to excite the oscillations at low angles of attack. High values of Mach number tend to decrease the aftbody damping coefficient, supporting the theory that the instability stems from a delayed adjustment of the recompression shocks with motion. The wake flow structure changes considerably at low Reynolds numbers, leading to significant and unpredictable changes in the aftbody damping coefficient, with relevance for Mars entry and ground testing. Conversely, the forebody loads always damp the oscillations, with a forebody damping coefficient that results broadly independent of Mach and Reynolds numbers. It is further shown how the forebody damping coefficient increases considerably with Strouhal number. Previous studies rarely considered this influence and assumed its effects to be caused by changes in Mach number, potentially leading to inaccuracies when flying through different atmospheres or along different trajectories.