Direct Thrust Measurements of an 8-GHz Microwave Electrothermal Thruster

Direct thrust measurements of an 8-GHz microwave electrothermal thruster were performed. Experimental work involved the redesign of the thruster cavity to minimize its weight and the construction of a vertical thrust stand. Thrust measurements were made using ammonia and simulated hydrazine (50% and 100% decomposed) as propellants. Testing was performed over a range of 100-250 W of microwave power input into the thruster. For all three propellants, the thrust increased with the increasing mass flow at higher microwave power levels able to sustain plasma at higher flow rates. Ammonia specific impulse increased with increasing cavity input power, reaching 264 s at 250 W. The 50% and 100% simulated decomposed hydrazine exhibited an initial increase in specific impulse with increasing power; however, further increases in power did not result in increased specific impulse. Higher levels of thrust and specific impulse at the same microwave input power were measured for the 100% decomposed simulated hydrazine compared to the 50% decomposed due to the absence of ammonia in the 100% decomposed hydrazine.