Alternative Solid Propellants for Electrothermal Plasma Thrusters

Abstract

Nanosatellites, particularly CubeSats, have transformed the space industry with their low-cost and compact form factor. Their operational lifetime in Low Earth Orbit (LEO) is, nevertheless, quite limited by nature because of orbital decay through atmospheric drag, and onboard propulsion is therefore obligatory for advanced missions. For power and mass-constrained CubeSats, electrothermal radio-frequency (RF) plasma thrusters offer a promising solution as they use low power to rapidly heat a propellant and enhance thrust. While the development of such thrusters has evolved through strategies like optimising discharge geometries and coupling modes, experimental work has remained focused on traditional gaseous propellants like argon or xenon that require heavy and high-pressure storage tanks. The objective of this thesis is therefore to explore solid-state propellants as a viable alternative for the small-scale electrothermal thruster.

While some solid options like iodine are corrosive and buckminsterfullerene are expensive, this work focuses on five storable, non-corrosive, and abundant solid hydrocarbons: naphthalene, adamantane, camphor, borneol, and hexamine. The initial phase of this research involved a detailed cold-gas characterisation of these candidates. The results confirmed that their mass flow rates are exponentially dependent on temperature and can be controlled with only a few watts of heating power (with the exception of hexamine). Although the cold-gas thrust from the solid propellants was lower than that of argon, the analysis revealed their better effective specific impulse and delta-v performance, which accounts for the significant mass savings from eliminating high-pressure tanks. Based on these cold-gas comparisons, camphor and naphthalene were identified as promising candidates in terms of their power efficiency and thrust generation.

The thesis then proceeds to the measurement of plasma performance, where additional RF power is used to heat the propellant flow. The results revealed a substantial additional thrust gain for the solid propellants, significantly greater than that observed for a monoatomic gas such as argon. This enhancement resulted in a comparable total thrust of up to 1.1 mN at a 3 mg/s flow rate with 45 W of RF power, compared to a cold-gas thrust of only 0.6 mN under the same flow conditions. While the plasma thrust in argon is well-understood to be enhanced by ion-neutral charge exchange (I-N CEX) collisions, the large thrust enhancement in the hydrocarbons case is attributed to exothermic reactions resulting from their molecular dissociation, which leads to a greater kinetic energy release into the bulk gas. These experimental findings were supported by analytical estimations and relevant external literature focused on the dissociation of these specific hydrocarbons.

Finally, this work continues to develop a non-invasive diagnostic technique using Optical Emission Spectroscopy (OES) to further investigate plasma physics, specifically the neutral gas temperature, using adamantane as a case study. By analysing the rovibrational spectra of the N2 Second Positive System (as a tracer gas) and the intrinsic C2 Swan band, the neutral gas temperature was determined. The results demonstrate that by adding a trace amount of N2, the gas temperature of an adamantane plasma can be reliably estimated and is observed to increase quasi-linearly with RF power. As the C2 band was not readily measurable in a pure adamantane plasma, helium was introduced as a buffer gas to mimic plasma processing conditions. The resulting gas temperature was found to be in agreement with the temperature obtained from the N2 tracer method. These temperature results were then used to predict the thrust performance of the electrothermal plasma thruster, and the predictions were found to be in good agreement with the direct thrust measurements. Show more