Reaction Control System

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The Reaction Control System, or RCS, is a set of monopropellant-fuelled (with the exception of the Vernor Engine) thrusters primarily intended for vessel orientation in a vacuum. It is toggled on and off by pressing the R key (default key binding). Kerbals on EVA have EVA Jetpacks that automagically refill when re-boarding a craft, though the controls for an EVA jetpack are completely different than for a spacecraft. This article does not discuss EVA jetpacks.

In order to use RCS, a monopropellant tank and at least two RCS thrusters are needed. The fuel tanks for monopropellant are different from solid or liquid fuels tank. There is one tank for each radius, and two radial-mounted tanks. Apart from the dedicated tanks, some monopropellant is stored in command pods but not in probe cores.

It should also be noted that the SAS system will make use of the thrusters if RCS is enabled at the same time as SAS. To avoid a constant drain of RCS fuel, it is recommended to not have both the RCS and the SAS systems enabled at the same time, unless you need to keep your ship dead level and/or have RCS fuel to spare. It also important to note that enabling both systems at once may cause multiple command-and-control subsystems (i.e. reaction wheels and RCS thrusters) to fight against each other, burning excess resources (electric charge and monopropellant in this case.)

In real life, RCS is primarily used to control the attitude of a spacecraft rather than its orbital velocity.


A size comparison of the available RCS thruster parts

The thrusters can be located anywhere on a ship. Unlike jet and liquid fuel engines, there is no need to place RCS thrusters on RCS fuel tanks or manually run fuel lines to them. The thruster block can provide full rotation control, whereas the linear port can only provide thrust in one direction. For rotational purposes, it is advisable to place the thrusters as far away from your center of mass as possible, thus maximizing the torque they generate. For the most effective control, it is typically most practical to place RCS thrusters with four way symmetry. In theory, you can provide suitable roll, pitch, and yaw control with just one set of four way thruster blocks. Translation maneuvers are easier when the thrusters for a specific direction are evenly distributed on both sides of the center of mass so they don't apply torque. This is usually accomplished with one set at the top and another at the bottom of your rocket. SAS may dampen unintentional changes of heading during translation movement.

If you are in space and enable RCS and notice suddenly that the axes of control seem to be all wrong, it's probably because your craft's attitude (not to be confused with a crew with a bad attitude) has it positioned in such a way that its control axes do not align with the navball. For this reason, you can set the camera to LOCKED mode such that it always aligns with the spacecraft (i.e. up on camera is up on the spacecraft) then RCS controls will line up with the camera orientation.

RCS thrusters have low impact tolerance.


Between their introduction in 0.11 and 0.17.1 (including the old 0.13.3 demo), all RCS thrusters were massless and dragless, despite the CFG file values. Between 0.18 and 0.23 (including the 0.18.3 demo), RCS thrusters have the expected mass values. They were explicitly made massless parts again in 0.23.5.


RCS Fuel Density is 4 kg/unit Mass
(Units of fuel)
Image Part Radial size Cost
Full Empty Max. Temp.
FL-R20 RCS Fuel Tank Tiny 200
0.10 0.02 2 000 12 50 20
FL-R25 FT.png
FL-R120 RCS Fuel Tank Small 330
0.56 0.08 2 000 12 50 120
FL-R1 Yellow.png
FL-R750 RCS Fuel Tank Large 1 800
3.4 0.4 2 000 12 50 750
Mk2 Monopropellant Tank.png
Mk2 Monopropellant Tank Mk2 750
1.89 0.29 2 500 50 50 400
Mk3 Monopropellant Tank.png
Mk3 Monopropellant Tank Mk3 5 040
(2 520)
9.8 1.4 2 700 50 50 2 100
Stratus-v roundified monopropellant tank.png
Stratus-V Roundified Monopropellant Tank X 200
0.10 0.02 2 000 12 50 20
Stratus-V Cylindrified.png
Stratus-V Cylindrified Monopropellant Tank X 250
0.23 0.03 2 000 12 50 50


Image Part Radial size Cost
Max. Temp.
(Units of fuel/s)
Isp (s) (atm) Isp (s) (vac)
RV-1X Variable Thruster Block Radial mounted 30 0.005 1 500 12 50 0.1 0.01 100 240
Place Anywhere 1 Linear RCS Port Radial mounted 15 0.001 1 500 12 50 0.2 0.02 100 240
RV-105 RCS Thruster Block Radial mounted 45 0.04 1 500 15 50 1.0 0.11 100 240
Linear RCS.png
Place-Anywhere 7 Linear RCS Port Radial mounted 25 0.02 2 600 15 50 2.0 0.21 100 240
Vernor Engine[Note 1] Radial mounted 150 0.08 2 000 15 50 12.0 0.94 140 260
  1. The Vernor Engine uses a liquid fuel/oxidizer mixture.

Place Anywhere 1 Linear RCS Port

See also