Performance of Alternative Propellants in an Inductive Electric Propulsion System

Abstract

Spacecraft fitted with electric propulsion systems can achieve a greater final velocity than chemical rockets, thus facilitating a range of new trajectories and exploratory mission capabilities. However, conventional propellants for electric propulsion are largely rare noble gases such as xenon, exhibiting a low natural abundance and requiring expensive manufacturing methods to obtain the necessary quantities. In contrast, alternative propellants are those which exist in abundance both on Earth and at locations of interest within the solar system such as moons, planets, and asteroids. The ability to utilise such resources for propulsion purposes thus provides the opportunity to decrease reliance on Earth as the sole source of propellant in the solar system and execute more flexible, higher reliability operation. As a result of these benefits, alternative propellants have been investigated in a number of conventional and novel electric propulsion systems. However, the design of these systems, as well as the dissimilarity in chemical properties of conventional and alternative propellants, have often prohibited their implementation following significant demands on increased supply power or the degradation of critical thruster components such as the electrodes. One class of electric propulsion which does not exhibit these restrictions is inductive propulsion. These systems possess the greatest flexibility in propellants, able to operate with chemically reactive species such as O₂ and CO₂ without damage to thruster components. This thesis reports on the implementation of various alternative propellants (O₂, CO₂, and N₂) in an inductive propulsion system, measuring their thrust and exhaust velocity outputs as well as developing novel techniques to characterise their behaviour using gas-independent properties. Thrust, specific impulse, and the thrust-to-power ratio were measured for each of the propellants, as well as propellant mixtures combining the molecular species with a monatomic gas, argon. The purpose of this mixing was to exploit the complimentary chemical properties of the respective species to increase the performance of the system through both the electromagnetic and gasdynamic driving mechanisms. The benefit of altering propellant properties is clear, with these propellant configurations increasing almost all performance metrics of the thruster. Thrust was shown to almost triple for a mixed gas propellant in comparison to single gas propellants at the same input power level. The final thrust values increased by factors of 1.35, 2.70, and 3.86 for O₂, CO₂, and N₂, respectively. This sharp increase in performance is attributed to a transition to discharge regimes of greater coupling efficiency, occurring at lower input powers for mixed propellant conditions. In order to better observe and characterise these transitions, a number of non-intrusive measurement techniques were developed during this project. The first of these techniques was an improved method to assess the inductive coil current, obtaining temporally-resolved information on the discharge including shifts in the coil driving frequency (in response to an increased flow ionisation linked to regime transitions) and the proportion of time spent in both the capacitive and inductive regimes during a single discharge cycle. This latter result revealed information on the stability of the discharge, with mixed propellant configurations staying in the inductive regime for up to 94 % of the cycle, compared to 71 % for the single propellant condition. Additionally, a method was developed and tested to measure the discharge skin depth experimentally, which is a fundamental characteristic of inductive discharges. While measurements of the visible skin depth component were successful, observations of the true skin depth using the magnetic field strength distribution were not. This result yielded information on the axial location of the ionisation region within the discharge tube, taking place at a significant distance from the beginning of the inductive coil. This theory is supported by spatially-resolved measurements of the discharge tube wall temperature. Measurements of the visible skin depth were compared to numerical results, indicating that the two may be reconciled by considering the definition of the visible skin depth and adjusting it to account for electrons not participating in stimulating visible radiation. These results, combined with an investigation of the impact of discharge chamber wall thickness on chemically dissimilar propellants, provide a comprehensive understanding of the performance of alternative propellants in an inductive electric propulsion system, as well as means through which future thruster designs may be improved and new techniques which can be used to characterise their performance.