An Axis Sequential Reaction Control System Design for Spacecraft and Launch Vehicle Attitude Control

This paper describes an axis sequential control strategy for a thruster-based Reaction Control System (RCS) using a phase plane controller. For a four thruster RCS, some RCS commands for three axes attitude control cannot be achieved by firing any combination of thrusters without introducing unwanted perturbation torques on other axes. Assuming weak dynamic couplings among axes, the attitude of each axis can be controlled independently. The individual command torques for each axis can be met by firing a combination of thrusters one axis at a time, instead of torque commands for all axes at the same time. Thus, an axis sequential RCS control is proposed, where the roll, pitch, and yaw axis are controlled in sequence: one axis is controlled by RCS at each controller update instance in a prescribed order. In this way, there is no control interference among axes and the torque commands are always achievable. Simulation results, of this sequential RCS control on a launch vehicle model, are satisfactory provided a fast enough controller update frequency. Stability analysis of the sequential RCS, controller update frequency effects on stability and performance, attitude control performance, and fuel efficiency for the proposed axis sequential RCS are investigated using simulation. This study provides an alternate RCS design when the number of thrusters of RCS is limited due to other vehicle design constraints such as payload.