Microwave electrothermal thruster chamber temperature measurements and performance calculations

The microwave electrothermal thruster (MET) uses microwave frequency energy to create and sustain a resonant cavity plasma to heat a propellant. A 2.45-GHz aluminum cylindrical thruster with converging copper-alloy nozzles was used for this study. A spectroscopic system was used to collect light emitted through a window in the plasma chamber. A Schumann-Runge oxygen-emission model was developed assuming an anharmonically vibrating, nonrigid rotating oxygen molecule. The commercially available LIFBASE software was used to model the ionized molecular nitrogen first negative system emission from the nitrogen plasmas. Experimental data were compared to the temperature-dependent models using least-squared difference summation schemes. Oxygen rotational temperatures of 2000 K and ionized nitrogen rotational temperatures of 5500 K were measured. These measurements were nearly constant for all chamber pressures and investigated absorbed specific powers. CEA2 code equilibrium thermochemical calculations show the relationship among enthalpy addition, temperature, and specific impulse for realistic operating conditions. Nitrogen was found to be an excellent choice as a propellant or propellant component, whereas oxygen was found to be a poor choice because of the temperatures achieved for the respective gases in the MET chamber.