A reaction control system (abbreviated RCS) is a component of a spacecraft. Its purpose is guidance and steering. 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 (pitch, yaw, and roll). This is contrast to a spacecraft's main engine , which is only capable of providing thrust in one direction, but is much more powerful.

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 de-orbiting .

Because spacecraft only contain a finite amount of fuel and there is little chance to refill them (aside from visits by a Space Shuttle, which are very rare), 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 ion thrusters. To control orientation, a few spacecraft have begun to use momentum wheels which spin to create torque on the vehicle.

Location of Thrusters on Space Capsules

The placing of the approach or translation engines (which cause the spacecraft to move) on the surface of a spacecraft has one important requirement that the placing of the orientation thrusters (which cause the spacecraft to turn) does not. If the direction of thrust of the former does not pass through the center of mass of the spacecraft, when tracked backward from the nozzle, the spacecraft will turn as an unwanted side effect. Sometimes this is unavoidable, but spacecraft are not operated by automatically firing the orientation thrusters to counteract this because such a system might fail. So a separate step of re-orientation is required afterward.

Translation thrusters thus have less of a variety of permissible locations than do orientation thrusters. In the Apollo spacecraft, both the Service Module and the Lunar Module, as well as the Chinese Shenzhou spacecraft, they are grouped in blocks of four, which are themselves attached to the outside of the spacecraft at each end of the two axes of a cross-section of the spacecraft through the long axis. Used in a variety of combinations, these thrusters are sufficient for both approach and orientation. Other designs use separate sets of thrusters.

A similar pattern is seen in the forward compartments of the Mercury and Gemini spacecraft. This is the equivalent of removing the two nozzles from each of the blocks of four which point in the longitudinal directions, then pushing the blocks inward, and cutting slots for the exhaust to escape. (This grouping is then rotated by 45 degrees.) These thrusters, however, are only used after the re-entry rockets or other modules have been jettisoned; any translation of the spacecraft that they would provide is a mere by-product. Indeed, the Mercury spacecraft has no separate capacity for translation at all. The re-entry modules of both Apollo and Soyuz have their thrusters ungrouped.

A pair of translation thrusters to go forward are located at the rear of both the Gemini and Soyuz spacecraft; the counter-acting thrusters are similarly paired in the middle of each spacecraft, pointing a bit outward besides forward. These act in pairs to prevent the spacecraft turning. The thrusters for the lateral directions are mounted as close to the center of mass of each of these spacecraft as well, but Gemini has only one engine for each of the directions while Soyuz continues with pairs.

None of these engines is intended for orientation. For that purpose, both Gemini and Soyuz have engines at the extreme rear of the spacecraft. Here Soyuz uses engines only one-tenth the power of the others, arranged in a unique pattern, while Gemini has engines arranged in the same pattern of eight as it uses for re-entry.

Gemini has no main orbit maneuvering engine as do the Apollo Service Module or Soyuz. It was light enough to change orbit without a separate engine.

Finally, Soyuz has a thruster at the rear of the spacecraft that points parallel to each solar panel, but which is not used for rendezvous at all. Instead, when the solar panels are pointing to the sun, the option exists to use this motor to spin the spacecraft to keep it pointing to the sun by gyroscopic action. Otherwise, a computer system would be kept running to automatically keep the panels so pointed, wasting electricity and propellant. The spin is stopped by the counterpart engine on the other side.

Location of Thrusters on Spaceplanes

The suborbital X-15 and a companion training aerospacecraft, the NF-104 AST, which would zoom to an altitude that rendered their aircraft controls unusable, established the basic locations for thrusters on winged vehicles not intended to rendezvous in space; that is, those that only have orientation thrusters. Those for pitch and yaw are located in the nose, forward of the cockpit, and replace a standard radar system. Those for roll are located at the wingtips. The X-20, which would have gone into orbit, continued this pattern.

Unlike these, the Space Shuttle has many more thrusters, for it does rendezvous in orbit. No nozzles are on the underside of the craft, which would have pierced the heat shield. And the rearward-facing thrusters are located in the tail.