Abstract: The present invention relates to a Microwave Electrothermal Thruster(MET) adapted for in-space electrothermal propulsion comprising a tunable frequency Microwave Electrothermal Thruster propulsion module enabling primary propulsion and altitude control of a satellite/spacecraft wherein RF Semiconductors is introduced for the first time as microwave generator inside cavity body to increase its efficiency and respond time and to make the thruster capable of operating in two frequencies by mechanically tuning its cavity making such thruster compact and light weight. The free-floating plasma within the resonant cavity couples the incident electrical power directly to the tangentially injected propellant gas. The plasma forms by focusing the microwave energy into the first transverse magnetic mode and operates independent of the type of propellant gas used.