Tunable Microwave Electrothermal Thruster Performance on Water

Abstract

Thrust, specific impulse, thrust efficiency, and microwave coupling efficiency were measured for a microwave electrothermal thruster (MET) operating on water in a non-tunable configuration, and on helium and water in a tunable configuration. In the nontunable configuration, attempts to improve water performance by repositioning the nozzle to counteract buoyancy and by changes in nozzle geometry were unsuccessful. In the tunable configuration, microwave coupling efficiencies near 100% were achieved for both helium and water at all operating conditions. With helium, improved coupling at constant specific energy resulted in increased specific impulse and thrust efficiency, however water displayed no performance improvement. Measurements with a residual gas analyzer indicate that the plume of the water MET may be 50 to 70% dissociated, and that the observed lack of dependence of specific impulse on specific energy may be the result of a trade-off between power loss to dissociation at low specific energy and to heating of the thruster body at high specific energy.