A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels are used for attitude control. Use of diverted engine thrust to provide stable attitude control of a short-or-vertical takeoff and landing aircraft below conventional winged flight speeds, such as with the Harrier "jump jet", may also be referred to as a reaction control system.
Two of four Reaction Control System thruster quads on the Apollo Lunar Module
Gemini Orbit Attitude and Maneuvering System, and Reentry (mislabeled "Reaction")[clarification needed][citation needed] Control System
RCS thrusters on the nose of Discovery, a Space Shuttle orbiter.
A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.
A small reaction wheel viewed in profile
A momentum/reaction wheel comprising part of a high-accuracy Conical Earth Sensor to maintain a satellite's precise attitude