2.1.2 Stabilization and Control
The purpose of a stabilization and control (S&C) system is to orient and maintain the space vehicle with a specified attitude and/or attitude rate. Conventionally, analog circuitry has been employed to provide the required control signals to the appropriate effectors (e.g., engine gimbal actuator, reaction jets, and control moment gyros). During the development of the Apollo spacecraft, however, it was decided to switch from an analog to a digital mechanization of the autopilot in the onboard computer (refs. 8 and 9). Digital S&C systems have since been designed and flown on a number of space vehicles, including both the Apollo command and lunar modules, Titan 3-C (ref. 10) and the improved Centaur (ref. 11).
Since the dominant natural frequencies of an S&C system are much higher than those of the G&N system, a digital S&C system has a significant effect on the speed requirements of the onboard computer. An example of the computer requirements for a reaction jet S&C system during coasting flight is obtained from experience with the Apollo program; about 2,000 memory words and 15,000 operations per second are necessary to support the control system itself, and another 2,500 words provide associated functions such as star tracker calculations (ref. 7). The requirements for S&C during powered flight are even more demanding. For example, the Apollo lunar module digital autopilot uses about 3,500 memory words and on the order of 25,000 operations per second (ref. 12).
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