The aerodynamics of axisymmetric blunt bodies flying at angle of attack

The Mars Science Laboratory entry capsule is used as an example to demonstrate how a blunt body of revolution must be treated as asymmetric in some respects when flying at a non-zero trim angle of attack. A brief description of the axisymmetric moment equations are provided before solving a system of equations describing the lateral-directional moment equations for a blunt body trimming at an angle of attack. Simplifying assumptions are made which allow the solution to the equations to be rearranged to relate the roll and yaw stability with sideslip angle to the frequency of oscillation of the vehicle body rates. The equations show that for a blunt body the roll and yaw rates are in phase and proportional to each other. The ratio of the rates is determined by the static stability coefficients and mass properties about those axes. A trajectory simulation is used to validate the static yaw stability parameter identification equation and a simple method of identifying the oscillation frequency from the body rates. The approach is shown to successfully extract the modeled yaw stability coefficient along a simulated Mars entry. Mars Science Laboratory flight data results are presented from earlier work which indicate that results from both the validation case and flight data are in agreement with preflight predictions. A brief discussion of the dynamic stability is also provided. Trimming at a nonzero angle suggests that the typical axisymmetric models of the dynamic stability coefficients should be modified. However, further experimental or computational work must be done to separate damping due to body rates and wind relative rates before the correct lifting formulation would affect simulation results.