High Quality Content by WIKIPEDIA articles! A reaction control system (RCS) is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. 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 (roll, pitch, and yaw). This is in contrast to a spacecraft`s main engine, which is only capable of providing thrust in one direction, but is much more powerful.