Minimum control authority plot - A tool for designing thruster systems

A new performance measure for thruster systems called the minimum control authority is defined. It is one of the most important performance measures since it corresponds to the worst case output of the thruster system. Various thruster system designs can easily be evaluated based on this performance measure. Several techniques are derived to calculate the minimum control authority as a function of the thruster configuration, the way in which the thrusters are controlled, and the physical mechanism that limits the thrusters outputs. A technique to generate a plot of the worst case moment vs force-generating capability of a thruster system is also derived. This so-called minimum control authority plot can be used to evaluate the worst case output of various thruster system designs against each other and against the worst case disturbance forces they have to overcome. To illustrate the concept and demonstrate its usefulness, the minimum control authority plot is applied to evaluate various thruster system designs for the Gravity Probe B (GP-B) spacecraft. The baseline thruster for GP-B was selected based on the minimum control authority plot. Although the focus of this article is on thruster systems, the techniques developed are applicable in general to any system of actuators of any type, for example, the primary flight control effectors of highly maneuverable aircraft.