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The research to develop the microwave electro-thermal (MET) thruster at Research Support Instruments, Inc. (RSI) using a variety of gases as fuel is described. The MET has undergone dramatic evolution since its first inception, and it is now moving toward flight development. The MET uses an electrodeless, vortex-stabilized microwave discharge to superheat gas for propulsion. In its simplest design, the MET uses a directly driven resonant cavity empty of anything except gaseous propellant and the microwave fields that heat it. It is a robust, simple, inexpensive thruster with high efficiency, and has been scaled successfully to operate at 100 W, 1 kW, and 50 kW using 7.5-, 2.45-, and 0.915-GHz microwaves respectively. The 50-KW, 0.915-GHz test was perhaps the highest power demonstration of any steady-state Electric thruster. The MET can use a variety of gases for fuel but the use of water vapor has been shown to give superior performance, with a measured specific impulse (Isp) of greater than 800 s. When this added to the safety, ease of storage and transfer, and wide availability of water in space, the potential exists for using a water-fueled MET as the core propulsion system for refuelable space platforms.