Microwave electrothermal thruster performance in helium gas

Abstract

The description and experimental performance of a microwave electrothermal thruster are presented. This thruster makes use of an internally tuned, single-mode cylindrical cavity applicator to focus and match micro-wave energy into an electrodeless, high-pressure, flowing gas discharge located inside a quart/ discharge chamber. The cavity TM011, or TM012 modes produced a discharge adjacent to the quart/ nozzle and the combination of single-mode focus control and variable, internal cavity matching allowed the continuous experimental operation and measurement over discharge pressures of 40-1100 Torr and flow rates of 8×106 to 150×10−6 kg/s. Experimental measurements of microwave coupling efficiency, thruster energy efficiency, and specific impulse are presented for nitrogen and helium discharges. Measured microwave coupling efficiencies to the discharge are in excess of 95%; i.e., the transfer of microwave energy to the discharge is a very efficient process. Experiments which measured hot and cold thruster pressures in helium gas with 200-1200 W of 2.45-GHz input power yielded calculated energy efficiencies of 10-50% and a specific impulse of 200-600s. Nozzle melting and erosion limited the input power and specific impulse. The measured performance compares favorably with other electrothermal thruster measurements and suggests that design improvements that employ higher-temperature nozzle materials and more efficient discharge chambers could yield a much improved performance. © 1987 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.