Nonlinear, Rate-Dependent Evaluation of Flight Damping Derivatives of the Orion Crew Module

In the continued development of the Orion crew module, NASA is challenged with characterizing and predicting static and dynamic stability during the atmospheric reentry of the capsule. A thorough understanding of the aerodynamic behavior of the crew module in the subsonic regime is essential for safe return, as this flight regime presents the largest dynamic instability recently demonstrated by the Ascent Abort-2 (AA2) flight test. Forced oscillation wind-tunnel tests were conducted with varying frequency, amplitude, and offset angles to determine the rate-dependent static and dynamic derivatives. A novel data reduction technique was employed to obtain a nonlinear representation of the damping derivatives of the capsule as a function of both its angular position and rate from the aerodynamic force/moment measurements. Finally, a nonlinear, second-order model, two-degree-of-freedom ordinary differential equation was formulated to predict the response of the crew module based on initial angular positions, alpha and beta, and angular rates, alpha and beta. The accuracy of the model was assessed through direct comparisons with the NASA Ascent Abort-2 flight test. Through benchmark comparisons against the AA2 flight-test data and rate-invariant models, the utility of the second-order model was proven, establishing a low-cost, computationally efficient manner of predicting crew module motion and stability that could be leveraged in flight to improve guidance and navigation control algorithms.