Reaction Control System

The Reaction Control System, mostly just called RCS, is a set of monopropellant-fuelled (with the exception of the Vernor Engine) small thrusters primarily intended for orienting vessels in vacuum. It is toggled on and off by pressing the R key (default key binding). In order to use RCS, a dedicated RCS fuel tank along with some RCS thrusters are needed. The fuel tanks for RCS are different from solid or liquid fuels tank, and currently come in five options, one for each radius, and two side-mounted types.

It should also be noted that the SAS system will make use of the thrusters. 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.

In real life it's the system used to control the attitude of a spacecraft rather than its orbital velocity.

Placement

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. It is advisable to place the thrusters as far away from your centre of mass as possible, thus maximising the torque (turning force) they generate.

It is typically most practical to place them with four way symmetry as this can provide the best control. In theory, you can provide suitable roll, pitch and yaw control with just one set of four way thruster blocks. Translation manoeuvres however are much easier when the thrusters for one 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 one at the bottom of your rocket. The SAS may dampen unintentional changes of heading during translation movement.

Beware, they will be fatally damaged if they brush against the ground.

Trivia

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.

Fuel

RCS Fuel Density is 4 kg/unit				Mass (t)					Monopropellant ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Monopropellant ()
	FL-R20 RCS Fuel Tank	Tiny	200 (176)	0.10	0.02	2 000	12	50	20
	FL-R120 RCS Fuel Tank	Small	330 (186)	0.56	0.08	2 000	12	50	120
	FL-R750 RCS Fuel Tank	Large	1 800 (900)	3.4	0.4	2 000	12	50	750
	Mk2 Monopropellant Tank	Mk2	750 (270)	1.89	0.29	2 500	50	50	400
	Mk3 Monopropellant Tank	Mk3	5 040 (2 520)	9.8	1.4	2 700	50	50	2 100
	Stratus-V Roundified Monopropellant Tank	X	200 (176)	0.10	0.02	2 000	12	50	20
	Stratus-V Cylindrified Monopropellant Tank	X	250 (190)	0.23	0.03	2 000	12	50	50

Thrusters

Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Thrust (kN)	Fuel (/s)	I_sp (s) (atm)	I_sp (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
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