Microwave Electrothermal Thruster Performance

Thrust, specific impulse, thrust efficiency, and coupling efficiency were measured for a nominally 1 kW microwave electrothermal thruster operating on He, N 2 , and N 2 O, and for a 5 kW model operating on water. For He, N 2 , and N 2 O, thrust varied from approximately 100 to 700 mN at discharge pressures from 6.7 x 10 4 to 2.7 x 105 Pa and magnetron input powers from 900 to 1500 W. Thrust measurements agree well with calculations assuming one-dimensional, isentropic flow of a perfect gas with constant specific heats. Peak specific impulses for He, N 2 , and N 2 O were 418, 243, and 209 s, respectively. Water was run at magnetron input powers from 2.1 to 4.1 kW and discharge pressures from 4.0 x 10 4 to 1.5 x 10 5 Pa. Thrust varied from approximately 100 to 250 mN with a peak specific impulse of 428 s. Incorporating an impedance matching capability improved coupling efficiency with water to over 96%, but did not increase specific impulse. Measurements with a residual gas analyzer indicate that the water plume may be 50 to 70% dissociated, and that the lack of dependence of specific impulse on specific energy results from a tradeoff between dissociation losses at low specific energy and heating of the thruster body at high specific energy.