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Areaction control system (RCS) is a spacecraft system that usesthrusters to provideattitude control andtranslation. Alternatively,reaction wheels can be used for attitude control, rather than RCS. Use of diverted engine thrust to provide stable attitude control of ashort-or-vertical takeoff and landing aircraft below conventional winged flight speeds, such as with theHarrier "jump jet", may also be referred to as a reaction control system.[1]
Reaction control systems are capable of providing small amounts ofthrust in any desired direction or combination of directions. An RCS is also capable of providingtorque to allow control ofrotation (roll, pitch, and yaw).[2]
Reaction control systems often use combinations of large and small (vernier) thrusters, to allow different levels of response.
Spacecraft reaction control systems are used for:
Because spacecraft only contain a finite amount of fuel and there is little chance to refill them, alternative reaction control systems have been developed so that fuel can be conserved. For stationkeeping, some spacecraft (particularly those ingeosynchronous orbit) use high-specific impulse engines such asarcjets,ion thrusters, orHall effect thrusters. To control orientation, a few spacecraft, including theISS, usemomentum wheels which spin to control rotational rates on the vehicle.
TheMercuryspace capsule andGemini reentry module both used groupings of nozzles to provideattitude control. The thrusters were located off theircenter of mass, thus providing atorque to rotate the capsule. The Gemini capsule was also capable of adjusting its reentry course by rolling, which directed its off-center lifting force.[clarification needed] The Mercury thrusters used ahydrogen peroxide monopropellant which turned to steam when forced through atungsten screen, and the Gemini thrusters usedhypergolicmono-methyl hydrazine fuel oxidized withnitrogen tetroxide.
The Gemini spacecraft was also equipped with a hypergolicOrbit Attitude and Maneuvering System, which made it the first crewed spacecraft withtranslation as well as rotation capability. In-orbit attitude control was achieved by firing pairs of eight 25-pound-force (110 N) thrusters located around the circumference of its adapter module at the extreme aft end. Lateral translation control was provided by four 100-pound-force (440 N) thrusters around the circumference at the forward end of the adaptor module (close to the spacecraft's center of mass). Two forward-pointing 85-pound-force (380 N) thrusters at the same location, provided aft translation, and two 100-pound-force (440 N) thrusters located in the aft end of the adapter module provided forward thrust, which could be used to change the craft's orbit. The Gemini reentry module also had a separate Reentry Control System of sixteen thrusters located at the base of its nose, to provide rotational control during reentry.
TheApollo Command Module had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini.
The ApolloService Module andLunar Module each had a set of sixteenR-4D hypergolic thrusters, grouped into external clusters of four, to provide both translation and attitude control. The clusters were located near the craft's average centers of mass, and were fired in pairs in opposite directions for attitude control.
A pair of translation thrusters are located at the rear of the Soyuz spacecraft; the counter-acting thrusters are similarly paired in the middle of the spacecraft (near the center of mass) pointing outwards and forward. These act in pairs to prevent the spacecraft from rotating. The thrusters for the lateral directions are mounted close to the center of mass of the spacecraft, in pairs as well.[citation needed]
The suborbitalX-15 and a companion training aero-spacecraft, theNF-104 AST, both intended to travel to an altitude that rendered their aerodynamic control surfaces unusable, established a convention for locations for thrusters on winged vehicles not intended to dock in space; that is, those that only have attitude control 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. TheX-20, which would have gone into orbit, continued this pattern.
Unlike these, theSpace Shuttle Orbiter had many more thrusters, which were required to control vehicle attitude in both orbital flight and during the early part of atmospheric entry, as well as carry out rendezvous and docking maneuvers in orbit. Shuttle thrusters were grouped in the nose of the vehicle and on each of the two aftOrbital Maneuvering System pods. No nozzles interrupted the heat shield on the underside of the craft; instead, the nose RCS nozzles which control positive pitch were mounted on the side of the vehicle, and were canted downward. The downward-facing negative pitch thrusters were located in theOMS pods mounted in the tail/afterbody.
TheInternational Space Station uses electrically poweredcontrol moment gyroscopes (CMG) for primary attitude control, with RCS thruster systems as backup and augmentation systems.[5][unreliable source?]
