Performance evaluation of the microwave electrothermal thruster using nitrogen, simulated hydrazine, and ammonia

The Microwave Electrothermal Thruster (MET) is an electric propulsion (EP) device that uses an electromagnetic resonant cavity within which a free-floating plasma is ignited and sustained in a propellant gas. Microwave energy is coupled to the propellant gas through collisions between free electrons and heavy particles in the plasma. The heated propellant is accelerated though a gasdynamic nozzle and exhausted to generate thrust. This heating mechanism is similar to that of an arcjet, which utilizes an arc discharge formed between two electrodes to heat a propellant gas. The main difference is that the MET plasma is free-floating and thus the system does not suffer from the lifetime-limiting electrode erosion problems that are characteristic of the arcjet. The MET potentially offers thrust and specific impulse comparable to arcjets with higher efficiency at low power levels and longer lifetimes.;Research was initiated to examine the feasibility of operating the MET using the products of hydrazine decomposition as the propellant gas. The goal of this research was to improve the performance of a hydrazine chemical system by combining it with an EP system that can outperform the arcjet and does not suffer from erosion problems. Operation with hydrazine propellant allows for integration with a conventional chemical propulsion system onboard a spacecraft. In addition, such a system could possibly be used for multimode operation, thereby enhancing the operational capabilities of the spacecraft. For example, it could be operated in a high specific impulse mode, suitable for stationkeeping, with microwave energy sustaining a high temperature plasma at moderately low pressures, or operated in a high thrust mode, suitable for rapid spacecraft repositioning, at high pressures without microwave energy input. Operation of the MET using pure ammonia, another lightweight liquid-storable propellant, was also examined to determine how well the MET performs compared to the arcjet using ammonia. The feasibility of operating the MET at various frequencies and power levels using simulated hydrazine and ammonia has been demonstrated.;In the MET plasma, microwave energy is coupled to the propellant gas through free electrons in the plasma. The electric field accelerates free electrons, which then transfer their kinetic energy to heavy particles through collision. Thus, electric field strength and chamber pressure play important roles in the power deposition and energy exchange mechanisms. These roles were examined theoretically through numerical modeling of the cavity electric field and experimentally through the variation of several MET components and parameters. In this program, testing was conducted on the 7.5-GHz MET at a power level of 70--100 W using pure nitrogen and various mixtures of N2, H2, and NH3 to simulate decomposed hydrazine. Parametric studies of the effects of nozzle throat diameter, microwave frequency, and microwave power were performed. Testing was also conducted on the 2.45-GHz MET at a power level of 1--2 kW using pure nitrogen, simulated hydrazine, and pure ammonia. Parametric studies of the effects of nozzle throat diameter, antenna probe depth, propellant injector diameter, and the inclusion of an impedance matching unit were performed. Thrust and specific impulse measurements for the 2.45- and 7.5-GHz METs were obtained using thrust stands. At the present time, the 2.45- and 7.5-GHz METs are not optimized for operation with simulated hydrazine or ammonia, but key areas of study that have potential for significant performance enhancement have been identified. For the 2.45-GHz thruster, calculated specific impulses with ammonia and simulated hydrazine approach 400 s and 425 s, respectively, whereas, for the 7.5-GHz thruster, calculated specific impulse with simulated hydrazine approaches 220 s. However, experimental performance measurements were up to 40% lower than theoretical calculations. Numerical electromagnetic modeling of the existing 7.5- and 2.45-GHz thrusters was performed using the commercially available finite element analysis software COMSOL Multiphysics. This modeling yielded insight on the effects of variation of antenna depth, microwave frequency, and microwave power. The results agreed well with experimental measurements. Numerical electromagnetic modeling was also performed to design a new 8-GHz MET. This thruster was built and a preliminary performance evaluation was conducted. Calculated specific impulse with simulated hydrazine reached approximately 300 s with 95% coupling efficiency at a forward power of 250 W.