Attitude Control for Low Lift/Drag Reentry Vehicles

We propose an attitude control system for a re-entry capsule with a low lift/drag ratio (0.3). The control law is based on quaternions and on a sliding manifold approach. By using the singular perturbation theory, a feedback controller is designed with robustness properties with respect to structural parametric uncertainties and environmental disturbances. The system state remains in a neighborhood of a reference attitude and the control signal is close to the well-defined equivalent control. The reference attitude is commanded by a trajectory controller that steers the vehicle on a reference precalculated path. This controller is based on a time-varying linear quadratic and a variable structure system strategy to meet landing accuracy requirements. Moreover, a new modulator scheme is proposed to modulate the thrust torque commanded by the attitude controller. The overall control law is tested by using a six-degree-of-freedom model of the capsule, taking into account an oblate rotating Earth, and gravitational field, aerodynamics, propulsive forces, and moments. Simulation results show the effectiveness of the proposed control strategy in meeting the requirements on landing accuracy and on heating and load factor limits.