Gasdynamic Expansion and Preliminary Heat Transfer Analysis in the Nozzle of a Microwave Electrothermal Thruster Using Green Propellants

The microwave electrothermal thruster (MET) is an emerging technology for space propulsion that uses microwave power to heat a gas inside a cylindrical resonant cavity, before accelerating it through a converging-diverging nozzle to produce thrust. The gasdynamic expansion determines the nozzle exit conditions and the propulsive performance of the thruster. Chemical reactions in MET nozzles may be favored by the high temperatures generated in the resonant cavity, but the high gas velocities and the small size of the nozzles themselves reduce the time available for chemical reactions. Moreover, heat transfer to the nozzle walls and temperature limits of its materials can impose severe constraints on the working conditions and the maximum achievable thruster performance. A model employing the Bray sudden freezing criterion is developed and applied to estimate the role of non-equilibrium chemistry in the gasdynamic expansion of N2, N2O and H2O, for various nozzle inlet conditions. The results locate the freezing point close to the nozzle throat, shifted towards the converging section for all propellants and most chamber conditions. This behavior indicates that most of the expansion is expected to be performed under frozen chemistry conditions. A preliminary nozzle heat transfer analysis is performed for the expansion of H2O from a stagnation temperature and pressure of 6000 K and 1 atm, respectively. The results indicate large power losses to the nozzle walls, suggesting the need to include them in the gasdynamic expansion model for more accurate performance predictions.