Use of control moment gyros for the stabilization of a spinning satellite

Abstract

SET of control moment gyros (CMGs) can be used as the actuator in an attitude control system by being caused to exchange angular momentum with the spacecraft. The advantages of using CMGs for the stabilization of inertially referenced satellites have become well known1'2 during the development of the attitude control system for the Apollo Telescope Mount of NASA's Skylab Program. This paper is concerned with the feasibility of using CMGs to control a satellite which is spinning relative to a set of inertial coordinates. It is shown that the spin introduces a fundamental difference in the mechanism of opposing applied torques on the spacecraft with the CMGs of an attitude control system. This is related to the general result that the CMGs in the attitude control system of a satellite which is spinning relative to an inertial coordinate system can accumulate bias momentum only about the spin axis of the spacecraft as a result of an applied torque which is constant in spacecraft coordinates. A constant torque applied about a spacecraft axis which is perpendicular to the spin axis results in a periodic momentum requirement on the CMGs. In addition, an example is presented in which the CMG momentum requirement for a spinning satellite is less than that of a similar, nonspinning satellite. The presentation of these concepts is organized in the following way: Sec. II includes the development of the equations which describe the form of the momentum variation of the CMGs in response to an arbitrary set of external torques; in Sec. Ill an example of the use of CMGs for stabilizing the attitude of an Earth-pointing satellite is presented; and Sec. IV contains some concluding remarks.