Review of research and development on the microwave electrothermal thruster

Abstract

The microwave electrothermal thruster shows promise for spacecraft propulsion and maneuvering. It promises advantages over other electrothermal thrusters in the areas of operating life, efficiency, and propellant selection. In the microwave thermal thruster, the electric power is first converted to microwave-fre quency radiation. In a specially designed microwave cavity system, the electromagnetic energy of the radiation is transferred to the electrons in a plasma sustained in the working fluid. The resulting high-energy electrons transfer their energy to the atoms and molecules of the working fluid by collisions. The heated working fluid expands through a nozzle to generate thrust. In the microwave electrothermal thruster, no electrodes are in contact with the working fluid, since nonthermal, radiative mechanisms transfer the energy into the working fluid. The main requirements for the materials of construction are that the walls of the discharge chamber be insulating and, at least in part, transparent to microwave radiation at operating conditions. Several experimental configurations of microwave electrothermal thrusters are described and compared. Diagnostic methods used to study microwave plasmas under conditions used in the thruster are described and selected results presented for titration, spectroscopy, calorimetry, electric field measurements, and gas-dynamic methods. Estimated performance efficiencies are reported and compared with other electrothermal systems. Results of computer modeling of the plasma and of the gas flowing from the plasma are summarized.