Robust optimal solution to the attitude/force control problem

Many spacecraft have an orbit control subsystem (OCS) which applies forces in order to maintain the orbital parameters within certain bounds. On the o ther hand, the attitude control subsystem (ACS) applies torques so that the spacecraft keeps its desired orientation. We explore the interaction of an orbital control maneuver with the attitude of a spacecraft. We propose to use the thrusters that provide the force for the orbit control as actuators to control the attitude as well. We obtain the conditions on the thruster configuration which guarantee that any perturbation torque can be compensat ed while performing the orbit control maneuver, We prove that the minimum n umber of thrusters which solve this problem is 4. Furthermore, we prove tha t the same problem can be solved with a minimum of 6 thusters if a single f ailure in any thruster is accepted. We compute a robustness index which mea sures the maximum allowable uncertainty in the thruster configuration while still solving the problem stated previously. in the case of 4 thrusters wi th bounded thrusts, we solve the problem of maximizing the resulting force in a certain direction.