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<p>A <b>reaction control system</b> (<b>RCS</b>) is a spacecraft system that uses <a href="Thrusters_(spacecraft)" title="Thrusters (spacecraft)">thrusters</a> to provide <a href="Spacecraft_attitude_control" class="mw-redirect" title="Spacecraft attitude control">attitude control</a> and <a href="Translation_(physics)" class="mw-redirect" title="Translation (physics)">translation</a>. Alternatively, <a href="Reaction_wheels" class="mw-redirect" title="Reaction wheels">reaction wheels</a> can be used for attitude control, rather than RCS. Use of diverted engine thrust to provide stable attitude control of a <a href="V/STOL" title="V/STOL">short-or-vertical takeoff and landing aircraft</a> below conventional winged flight speeds, such as with the <a href="Hawker_Siddeley_Harrier#Controls_and_handling" title="Hawker Siddeley Harrier">Harrier "jump jet"</a>, may also be referred to as a reaction control system.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

</p><p>Reaction control systems are capable of providing small amounts of <a href="Thrust" title="Thrust">thrust</a> in any desired direction or combination of directions. An RCS is also capable of providing <a href="Torque" title="Torque">torque</a> to allow control of <a href="Rotation" title="Rotation">rotation</a> (<a href="Aircraft_principal_axes" title="Aircraft principal axes">roll, pitch, and yaw</a>).<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

</p><p>Reaction control systems often use combinations of large and small (<a href="Vernier_thruster" title="Vernier thruster">vernier</a>) thrusters, to allow different levels of response.

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<p>Spacecraft reaction control systems are used for:

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<ul><li><a href="Spacecraft_attitude_control" class="mw-redirect" title="Spacecraft attitude control">attitude control</a> during different stages of a mission;<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></li>

<li><a href="Orbital_station-keeping" title="Orbital station-keeping">station keeping</a> in <a href="Orbit" title="Orbit">orbit</a>;</li>

<li>close <a href="Space_rendezvous" title="Space rendezvous">maneuvering</a> during <a href="Space_rendezvous" title="Space rendezvous">docking</a> procedures;</li>

<li>control of <a href="Orientation_(geometry)" title="Orientation (geometry)">orientation</a>, or "pointing the nose" of the craft;</li>

<li>a backup means of <a href="Atmospheric_reentry" class="mw-redirect" title="Atmospheric reentry">deorbiting</a>;</li>

<li><a href="Ullage_motor" title="Ullage motor">ullage motors</a> to prime the fuel system for a main engine burn.</li></ul>

<p>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 in <a href="Geosynchronous_orbit" title="Geosynchronous orbit">geosynchronous orbit</a>) use high-<a href="Specific_impulse" title="Specific impulse">specific impulse</a> engines such as <a href="Arcjet" class="mw-redirect" title="Arcjet">arcjets</a>, <a href="Ion_thruster" title="Ion thruster">ion thrusters</a>, or <a href="Hall_effect_thruster" class="mw-redirect" title="Hall effect thruster">Hall effect thrusters</a>. To control orientation, a few spacecraft, including the <a href="International_Space_Station" title="International Space Station">ISS</a>, use <a href="Momentum_wheel" class="mw-redirect" title="Momentum wheel">momentum wheels</a> which spin to control rotational rates on the vehicle.

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<div class="mw-heading mw-heading2"><h2 id="Location_of_thrusters_on_spacecraft">Location of thrusters on spacecraft</h2></div>

<p>The <a href="Project_Mercury" title="Project Mercury">Mercury</a> <a href="Space_capsule" title="Space capsule">space capsule</a> and <a href="Project_Gemini" title="Project Gemini">Gemini</a> reentry module both used groupings of nozzles to provide <a href="Spacecraft_attitude_control" class="mw-redirect" title="Spacecraft attitude control">attitude control</a>. The thrusters were located off their <a href="Center_of_mass" title="Center of mass">center of mass</a>, thus providing a <a href="Torque" title="Torque">torque</a> to rotate the capsule. The Gemini capsule was also capable of adjusting its reentry course by rolling, which directed its off-center lifting force. The Mercury thrusters used a <a href="Hydrogen_peroxide" title="Hydrogen peroxide">hydrogen peroxide</a> monopropellant which turned to steam when forced through a <a href="Tungsten" title="Tungsten">tungsten</a> screen, and the Gemini thrusters used <a href="Hypergolic" class="mw-redirect" title="Hypergolic">hypergolic</a> <a href="Mono-methyl_hydrazine" class="mw-redirect" title="Mono-methyl hydrazine">mono-methyl hydrazine</a> fuel oxidized with <a href="Nitrogen_tetroxide" class="mw-redirect" title="Nitrogen tetroxide">nitrogen tetroxide</a>.

</p><p>The Gemini spacecraft was also equipped with a hypergolic <a href="Orbit_Attitude_and_Maneuvering_System" title="Orbit Attitude and Maneuvering System">Orbit Attitude and Maneuvering System</a>, which made it the first crewed spacecraft with <a href="Translation_(physics)" class="mw-redirect" title="Translation (physics)">translation</a> 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.

</p><p>The <a href="Apollo_command_and_service_module" title="Apollo command and service module">Apollo Command Module</a> had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini.

</p><p>The Apollo <a href="Apollo_command_and_service_module" title="Apollo command and service module">Service Module</a> and <a href="Apollo_Lunar_Module" title="Apollo Lunar Module">Lunar Module</a> each had a set of sixteen <a href="R-4D" title="R-4D">R-4D</a> 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.

</p><p>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.

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<div class="mw-heading mw-heading3"><h3 id="Location_of_thrusters_on_spaceplanes">Location of thrusters on spaceplanes</h3></div>

<p>The suborbital <a href="North_American_X-15" title="North American X-15">X-15</a> and a companion training aero-spacecraft, the <a href="Lockheed_NF-104A" title="Lockheed NF-104A">NF-104 AST</a>, 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. The <a href="Boeing_X-20_Dyna-Soar" title="Boeing X-20 Dyna-Soar">X-20</a>, which would have gone into orbit, continued this pattern.

</p><p>Unlike these, the <a href="Space_Shuttle_orbiter" title="Space Shuttle orbiter">Space Shuttle Orbiter</a> 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 aft <a href="Space_Shuttle_Orbital_Maneuvering_System" class="mw-redirect" title="Space Shuttle Orbital Maneuvering System">Orbital Maneuvering System</a> 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 the <a href="Space_Shuttle_Orbital_Maneuvering_System" class="mw-redirect" title="Space Shuttle Orbital Maneuvering System">OMS</a> pods mounted in the tail/afterbody.

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<div class="mw-heading mw-heading2"><h2 id="International_Space_Station_systems">International Space Station systems</h2></div>

<p>The <a href="International_Space_Station" title="International Space Station">International Space Station</a> uses electrically powered <a href="Control_moment_gyroscope" title="Control moment gyroscope">control moment gyroscopes (CMG)</a> for primary attitude control, with RCS thruster systems as backup and augmentation systems.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://ntrs.nasa.gov/api/citations/20130010548/downloads/20130010548.pdf">""What is RCS?" by NASA in a PDF file"</a> <span class="cs1-format">(PDF)</span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html#sts-rcs">"REACTION CONTROL SYSTEM"</a>. <i>science.ksc.nasa.gov</i>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFColasValenzuela2020" class="citation cs2">Colas, Armand L.; Valenzuela, Juan G. (2020-08-17), <span class="id-lock-subscription" title="Paid subscription required"><a rel="nofollow" class="external text" href="https://arc.aiaa.org/doi/10.2514/6.2020-3526">"Reaction Control System Performance Characterization using Vacuum Chamber Thrust Stand"</a></span>, <i>AIAA Propulsion and Energy 2020 Forum</i>, AIAA Propulsion and Energy Forum, American Institute of Aeronautics and Astronautics, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.2514%2F6.2020-3526">10.2514/6.2020-3526</a>, <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-62410-602-6</bdi>, <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:225270552">225270552</a><span class="reference-accessdate">, retrieved <span class="nowrap">2022-09-27</span></span></cite></span>

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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.nasa.gov/history/SP-4002/p1b.htm">"Project Gemini - A Chronology. Part 1 (B)"</a>. <i>www.nasa.gov</i><span class="reference-accessdate">. Retrieved <span class="nowrap">2024-08-27</span></span>.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external free" href="http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=34777.0;attach=586775">http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=34777.0;attach=586775</a> </span>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

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<div class="side-box-text plainlist">Wikimedia Commons has media related to <span style="font-weight: bold; font-style: italic;"><a href="https://commons.wikimedia.org/wiki/Category:Reaction_control_systems" class="extiw external" title="commons:Category:Reaction control systems">Reaction control systems</a></span>.</div></div>

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<li><a rel="nofollow" class="external text" href="https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html">Space Shuttle RCS</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20090524033739/http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html">Archived</a> 2009-05-24 at the <a href="Wayback_Machine" title="Wayback Machine">Wayback Machine</a></li></ul></div><div><div class="zim-footer">

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