Contingency Mixed Actuator Controller Implementation for the Dawn Asteroid Rendezvous Spacecraft

In June of 2010, on its way to the asteroid Vesta, the first of its two asteroid destinations, the Dawn spacecraft experienced a high friction anomaly on one of its four reaction wheels. The wheel was taken out of service, and some limited testing indicated that it was likely unusable for the approaching Vesta campaign. As a contingency against an additional reaction wheel failure in the now non-redundant complement, a development effort was initiated to implement a control mode that would use only two reaction wheels in a mixed actuator mode with thrusters for three-axis control. Compared to the existing backup allthruster controller, this mixed mode would provide better pointing with less propellant expenditure during science operations. The mixed actuator mode also needed to have the capability of performing all planned science operations with activation at any time during the mission. Implementing this new control mode on an operational spacecraft without affecting existing capability and normal operational procedures presented its own set of challenges. After relatively short development and testing phases the new version of the flight software containing the mixed actuator controller was uploaded to the spacecraft in early 2011. The implementation of the new controller is described herein. Simulation results are presented comparing its performance with the backup all-thruster controller, in terms of both pointing and propellant efficiency. Operational strategies for overcoming certain design limitations are also discussed.