A reaction control system (RCS) is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw). This is in contrast to a spacecraft's main engine, which is only capable of providing thrust in one direction, but is much more powerful.
RCS systems often use combinations of large and smaller (vernier) thrusters, to allow different levels of response from the combination.
Reaction control systems are used:
- for attitude control during re-entry;
- for stationkeeping in orbit;
- for close maneuvering during docking procedures;
- for control of orientation, or 'pointing the nose' of the craft;
- as a backup means of deorbiting;
- for Harrier "jump jet" stabilisation during operations below stall speed.
Because spacecraft only contain a finite amount of fuel and there is little chance to refill them, some alternative reaction control systems have been developed so that fuel can be conserved. For stationkeeping, some spacecraft (particularly those in geosynchronous orbit) use high-specific impulse engines such as arcjets, ion thrusters, or Hall effect thrusters. To control orientation, a few spacecraft use momentum wheels which spin to control rotational rates on the vehicle.
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