Electrothermal propulsion is a form of electric propulsion in which electrical energy is used to heat a suitable propellant causing it to expand through a supersonic nozzle and generate thrust. Two basic types of electrothermal thruster are in use today: the resistojet and the arcjet. In both, material characteristics limit the effective exhaust velocity to values similar to those of chemical rockets. A third, experimental type is the microwave plasma thruster which potentially could achieve somewhat higher exhaust velocities.