Model-based aerodynamic-angle attitude control of an atmospheric entry capsule

The paper describes the attitude control system of a low lift-to-drag biconic atmospheric entry capsule based on the Embedded Model Control methodology. The control structure derives from the development of the attitude dynamics and kinematics written in terms of aerodynamic angles, instead of Euler/quaternion kinematics. A detailed development of the simplified set of equations linking the torques generated by the reaction control system with the time evolution of the aerodynamic angles is provided. The simplified set of equations becomes the core of the control algorithm. The bank angle dynamics is shown to be fourthorder and forced by yaw and roll torques. A dynamic dispatching technique is proposed for converting fourth-order dynamics into a pair of second order systems. Nonlinear dynamic inversion and active disturbance rejection are employed to handle gyroscopic torques, parametric errors and to compensate for angular variation of translational velocity. A bank reversal logic is designed to reduce the effect of bank reversals on the translational motion. The performance of the attitude control algorithm has been tested on a high fidelity simulator and relevant results are presented.