FAST ATTITUDE MANEUVERS AND ACCURATE POINTING FOR AN ULTRA AGILE SPACECRAFT WITH FLEXIBLE APPENDAGES ACTUATED BY THRUSTERS AND REACTION WHEELS

This paper faces with the problem of the attitude control of a flexible spacecraft with large solar panels, actuated by thrusters and reaction wheels, with demanding requirements on pointing error and fast large attitude maneuvers. To accomplish these two tasks, a functional separation has been performed: the hydrazine thrusters are used for the attitude maneuvers with a feed forward approach, while the reaction wheels are used in closed loop to refine the maneuver and to have small pointing errors. The reaction wheels are in the classical pyramidal configuration, while the thrusters are mounted to have a fast re-orientation around the required directions. The thruster allocation is also optimized to increase the resulting torque in order to reduce the fuel consumption for the maneuvers. Since the attitude dynamics is affected by the flexibility of the solar panels, this effect is taken into account in the maneuvers and the accurate pointing control. The attitude dynamics equations are found using a Lagrangian approach to include the flexibility of two symmetric solar panels mounted on the spacecraft. Adaptive and robust control techniques are compared in the designing of the suitable attitude control laws able to compensate the flexible dynamics. Numerical simulations are performed to evaluate the AOCS behavior during the fast large attitude maneuvers and the accurate pointing.