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Reaction Control System Thrusters, or RCS Thrusters are small engines that thrust on three vectors in-game, and used on rockets to control rotation, translation and/or retrorockets.

The exhaust from the RCS unit

Similar to the Ion Engine, it does not need to be attached directly to a fuel tank and will always sip and drain the fuel to all fuel tanks equally. By tapping on any RCS thruster on the rocket, all RCS attached to the rocket will be activated and can only be used simultaneously.

The engine is commonly used for precise movements of rocket, either for docking or tiny orbital adjustments. It can also be used on rockets trying to slow down, because they act like real life retrorockets.

If the RCS is enabled, six buttons will appear on the bottom of the screen. The number of buttons depends on the version of the game. On old versions, a D-pad with two rotational button RCS control panel is shown; in new versions, they are placed in a 3×2 table. The functionality of these two are completely identical.

Pre-

1.35

Post-

1.35

The shaded left/right buttons rotate the rocket; the unshaded directional arrows translate the rocket. The translation of the rocket changes as the direction of the controlling part's orientation changes. 

To use RCS for rocket translation, it is recommended their centre-of-thrust in each direction is perpendicularly near to the rocket's centre-of-mass. Slightly uneven RCS thrust will be cancelled out by the rocket's control vehicle's (Capsule or Probe) torque.

To use RCS efficiently for rocket rotation, it is recommended each one is as far from the centre-of-mass as possible, to maximise distance from the pivot and consequent moment force, and that they're offset orthogonally, not diagonally, to minimise cosine decay.

Efficiency

A single RCS burns 5t of fuel in 3 minutes and 20 seconds. It has 50% the efficiency of a Hawk Engine, 42% the efficiency of a Valiant Engine and 46% the efficiency of a Kolibri Engine. It is the least efficient engine. On Earth, it can lift 1450 kilograms of weight.

Bugs

Main article:
  1. Until version 1.5, if all fuel was consumed, RCS switched to using electricity, and could run without propellant. With power generators like Solar Panel, and the RTG, this behavior could be abused to make an infinite-propulsion rocket.
  2. Sometimes, If you used an engine from the fuel from a tank that was used by RCS, all the fuel would drain from the vehicle, say if you activated RCS and drained any fuel from a tank, then transferred fuel from that tank into another for the engines, all the fuel would suddenly be gone.

Retrorockets

RCS is your best opt to slow down your rocket in longer terms. Use the up button on the pad and they can slow down a rocket efficiently.

Notes

RCS can be only used as retrorockets when below the timewarpHeight to stabilize the speed.

Trivia

  • An RCS Thruster has a thrust of 1.5t, the same thrust as the Ion Engine has.
  • If you attempt to fire RCS diagonally (in a direction between 22.5º and 67.5º off the directions of two nozzles on the same thruster), both nozzles will fire simultaneously, increasing the thrust to 2.12132034356 tons but decreasing the efficiency (through cosine decay) to 84.85s.
  • An RCS Thruster can be used as a launch vehicle engine in objects that have very very low gravity (like Phobos and Deimos).
  • When it's facing on the other side and clipped with a part, one nozzle is gone. This can be used as a decorative purpose however this can be done with BP-Editing.
Parts
Basics ParachuteCapsuleHeat ShieldSeparatorFuel TanksHawk EngineValiant EngineKolibri EnginePeregrine EngineRCS ThrustersSide SeparatorSolid Rocket BoostersAerodynamic Nose ConeLanding Leg
6 Wide 6 Wide Aerodynamic Nose ConeFuel TanksSeparatorHeat ShieldEngine Base
8 Wide 8 Wide Aerodynamic Nose ConeFuel TanksSeparatorHeat ShieldEngine Base
10 Wide Fuel TanksSeparatorHeat ShieldEngine Base
12 Wide Fuel TanksSeparatorHeat ShieldEngine Base
Engines Titan EngineFrontier EngineHawk EnginePeregrine EngineValiant EngineKolibri EngineRCS ThrustersIon Engine
Aerodynamic Parts ParachuteSide ParachuteAerodynamic Nose ConeAerodynamic Fuselage
Fairings Fairing
Structural Structural Part
Other Docking PortLanding LegsIon EngineProbeRCS ThrustersWheelsSolar PanelFuel Pipe
Currently removed in Version 1.5
BatteryRTG
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