Gyroscopic Coupling in Space Vehicle Attitude Control Systems

Abstract

The problem of controlling the attitude of a space vehicle is unusual in several respects. While the required precision may be extreme—less than 0.1 second of arc in certain cases—the required response time may often be very slow, measured in minutes or hours. Vehicles weighing tons may have to be controlled by inch-ounces of torque, and control energy is at an extreme premium. The present paper discusses the effects, on performance, of inter-axis coupling due to internal spinning parts. A de-coupling computer to nullify gyroscopic torque is described, and its utility is evaluated. The computer is found to improve precision, but to reduce energy consumption only in certain cases. It is shown that by postulating such a computer the performance of a given system may be accurately evaluated on the basis of much simpler single-axis relations, even though strong coupling is present. Specifically: (1) It is shown that the best available performance is established by postulating de-coupling control; and (2) a method is given for determining the amount by which a conventional system will fail to achieve that performance.