The control of a space vehicle can be divided into two parts. The most obvious part includes the rockets and airfoils that directly steer the vehicle and control its speed. Less apparent are the computer systems that control the rockets and airfoils. These systems rely on measurements from various instruments, as well as knowledge of the vehicle's planned route, to determine how the rockets and airfoils should be used.
In a way, these computer systems act collectively like a car driver who relies on what she knows and senses to make decisions about car speed and direction. The driver then uses the steering wheel, gas pedal or brakes to act on these decisions, just as the computer systems would use the rockets or airfoils of a space vehicle. Similarly, while a car driver can refer to landmarks and street signs to determine if the car is off course, the flight control computer systems depend mainly on inertial guidance to make this determination.
The inertial guidance system calculates the vehicle's speed, direction and location, and issues control commands. "Inertial" means that it is based on measurements of acceleration. The system consists of a computer and an inertial measurement unit (IMU) comprising three accelerometers mounted on a gyroscopically stabilized platform. Accelerometers are mechanical devices that respond to acceleration. Acceleration can be felt when a car changes speed or makes a turn. If these accelerations could be monitored and split into northward, eastward and upward directions, then the location and speed of the car at any given moment could be determined. In a similar way, a rocket's inertial guidance system measures acceleration along three principal directions. To keep the accelerometers always pointing in these same principal directions, gyroscopic devices sense changes in direction and move the IMU platform to counter them.
Once the inertial guidance computer decides a course change is needed, it issues control commands to the space vehicle's rockets and airfoils. Airfoils are useful only when the vehicle travels through air, during launch, for example. Typically, moveable flaps on fins serve as airfoils. Just as a rudder steers a boat by diverting water flow, an airfoil steers the vehicle by diverting the flow of air.
Rocket-based control can be used during both launch and in space. It relies mainly on diverting the direction of the rocket's exhaust, and on controlling the amount of exhaust. The most direct form of rocket-based control swivels the rocket motor or its nozzle to steer the vehicle's direction. This is one of the methods used to control the space shuttle during launch.
Another method employs moveable flaps in the rocket motor to divert the exhaust flow direction. A variation of this uses a stream of gas or liquid in the rocket nozzle to divert the exhaust flow. Auxiliary engines and gas can provide the delicate control sometimes needed in space because valves can slow and even stop the exhaust of the liquid fuels or gas propellants whenever it is needed. In contrast, solid fuel, like that used to launch the shuttle, must burn until used up.
see also Guidance and Control System (volume 3); Gyroscopes (volume 3); Inertial Measurement Units (volume 3); Space Shuttle (volume 3).
Richard G. Adair
In Space. How Things Work. Alexandria, VA: Time-Life Books, 1991.
Mallove, Eugene F., and Gregory L. Matloff. The Starflight Handbook: a Pioneer's Guide to Interstellar Travel. New York: John Wiley & Sons, 1989.
The Guidance of Space Vehicles. <http://www.hq.nasa.gov/office/pao/History/conghand/guidance.htm>.