Investigation of propellants for electrothermal propulsion systems: Application to radio-frequency inductively coupled plasma thrusters

Radio-frequency (RF) inductively coupled plasma (ICP) thrusters are an emerging electric propulsion technology that electrodelessly heat propellant gas to high temperatures via plasma-gas collisional processes. Thrust is then produced by accelerating this hot gas through a converging–diverging nozzle. Propellant selection is an important factor that affects not only the thruster operation and performance but also the design and complexity of the overall propulsion system. In this work, we study several propellant options for RF ICP thrusters, considering propulsive performance, mission suitability, and storage requirements. The analysis accounts for important high-temperature considerations such as propellant dissociation at the elevated temperatures encountered inside the thruster and frozen flow losses in the nozzle. While low atomic/molecular mass propellants such as hydrogen exhibit the highest conventional specific impulse, their low storage density results in a poor total impulse mass density (impulse per propulsion system wet mass). By contrast, liquid propellants such as water and ammonia are found to be attractive alternatives because of their higher storage density and relatively low molecular mass, which is further reduced due to thermal dissociation.