Alternative Solid Hydrocarbons as Propellants for Electrothermal Plasma Radio-frequency Thrusters

Onboard propulsion is vital for extending the operational lifespan of CubeSat missions. Although electrothermal plasma thrusters are a promising low-power solution, their development has historically focused on gaseous propellants, leaving alternative options largely unexplored. This work addresses this gap by investigating the thrust performance of four solid hydrocarbons: naphthalene, adamantane, camphor, and borneol, in a prototype radio-frequency (RF) electrothermal thruster. Direct thrust measurements were performed using a calibrated thrust balance in two operational modes: unpowered (cold-gas) and powered (plasma). The performance of the solid propellants was then benchmarked against previous data obtained with argon, a commonly used inert gas. Due to their higher molecular mass, solid propellants exhibited lower exhaust velocities, resulting in lower cold-gas thrust performance at equivalent mass flow rates. Plasma thrust, however, was significantly greater because of additional heating from molecular dissociation. Around 45 W RF power nearly doubled the thrust for the solid propellants, resulting in a total thrust of up to 1.1 mN and a specific impulse of 37 s, respectively. These values are comparable to those attained using argon but without the need for heavy, high-pressure storage tanks. The findings demonstrate that sublimating solid propellants are a viable alternative for CubeSat propulsion systems that require a constrained mass and volume.