A reaction control system, abbreviated RCS, is a subsystem of a spacecraft. Its purpose is attitude control 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.
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:
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.
Location of thrusters on space capsules
Two Apollo spacecraft (the Service Module and the Lunar Module) had translation thrusters grouped into external blocks of four, which served to translate and orient the spacecraft. Other designs used separate sets of thrusters for these two tasks. The Apollo thrusters were configured to allow "coupled" RCS firings (where thrusters on opposite sides of the spacecraft fired together), which allowed adjusting the vehicle attitude without affecting the critical accuracy of their orbital, trans-lunar and trans-earth trajectories.
The Mercury and Gemini spacecraft each had groupings of two nozzles inserted into their forward compartments, with slots cut out from which the exhaust could escape. These thrusters were used for orientation, not translation. (Indeed, the Mercury spacecraft had no separate capacity for translation at all.) Similarly, the command modules of both the Apollo and Soyuz spacecraft have their re-entry RCS thrusters ungrouped.
Gemini, due to its relatively low mass, was able to change its orbit using its thrusters, and did not require an engine (unlike its heavier descendants).
A pair of translation thrusters 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 (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 each of these spacecraft as well, but Gemini has only one engine for each of the directions while Soyuz again uses a pair.
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.
The placement of the translation thrusters (which are used to alter the spacecraft's velocity) has one important requirement that the placement of the orientation thrusters (which are used to rotate and orient the spacecraft) does not: if the direction of thrust of the translation thrusters does not pass through the center of mass of the spacecraft (when tracked backward from the nozzle) the spacecraft will rotate--an unwanted side effect. Current and past spacecraft are not operated by automatically firing the orientation thrusters to counteract this rotation because such a system might fail, so manual re-orientation is required afterward. Because of these constraints, translation thrusters can generally be placed in fewer locations than orientation thrusters.
Finally, Soyuz has a thruster at the rear of the spacecraft that points parallel to each solar panel. This thruster is used for orientation, but has the unique application of keeping the spacecraft's solar panels pointing towards the sun. Without this thruster, a computer system would have to keep the panels properly aligned, wasting electricity. The spin is dampened by a counterpart thruster on the other side.
Location of thrusters on spaceplanes
The suborbital X-15 and a companion training aerospacecraft, the NF-104 AST, which would travel 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. Shuttle thrusters are grouped in the nose of the vehicle and on each of the two aft Orbital Maneuvering System pods. No nozzles pierced the heat shield on the underside of the craft. Instead, the nose RCS nozzles which control positive pitch are mounted on the side of the vehicle, and are canted downward. The downward-facing negative pitch thrusters are located in the OMS pods mounted at the tail.
Space Station Reaction Control Systems
Published in July 2009.
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