Electromagnetic thruster. Computer illustration of a radio frequency (RF) resonant cavity thruster a form of electromagnetic (EM) thruster. EM thrusters are being investigated as a means of electrically powered spacecraft propulsion that uses electrical energy to change the velocity of a spacecraft. This proposed type of EM thruster would use no reaction mass and would emit no directional radiation. However, they are controversial as their design apparently violates the law of conservation of momentum, dictated by Newton's laws of motion.