RCS thrusters on the nose of Discovery, a Space Shuttle orbiter. The thrusters for the lateral directions are mounted close to the center of mass of the spacecraft, in pairs as well. These act in pairs to prevent the spacecraft from rotating. The clusters were located near the craft's average centers of mass, and were fired in pairs in opposite directions for attitude control.Ī 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. The Apollo Service Module and Lunar Module each had a set of sixteen R-4D hypergolic thrusters, grouped into external clusters of four, to provide both translation and attitude control. The Apollo Command Module had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini. 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. 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. 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). 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. The Gemini spacecraft was also equipped with a hypergolic Orbit Attitude and Maneuvering System, which made it the first crewed spacecraft with translation as well as rotation capability. The Mercury thrusters used a hydrogen peroxide monopropellant which turned to steam when forced through a tungsten screen, and the Gemini thrusters used hypergolic mono-methyl hydrazine fuel oxidized with nitrogen tetroxide. The Gemini capsule was also capable of adjusting its reentry course by rolling, which directed its off-center lifting force. The thrusters were located off their center of mass, thus providing a torque to rotate the capsule. The Mercury space capsule and Gemini reentry module both used groupings of nozzles to provide attitude control. Gemini Orbit Attitude and Maneuvering System, and Reentry (mislabeled "Reaction") Control System To control orientation, a few spacecraft, including the ISS, use momentum wheels which spin to control rotational rates on the vehicle. For stationkeeping, some spacecraft (particularly those in geosynchronous orbit) use high- specific impulse engines such as arcjets, ion thrusters, or Hall effect thrusters.
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