Experimental Validation of a Naphthalene-Fueled Electrothermal Plasma Thruster

A direct thrust measurement of an electrothermal radio-frequency (RF) plasma thruster, also known as a Pocket-Rocket, using naphthalene as a propellant, is experimentally performed. Naphthalene plasma is successfully ignited with only 5 W of RF power, resulting in a 32% increase in total thrust compared to cold-gas operation, which increased further to 71% as the power increased to 30 W. The total thrust is shown to increase proportionally with mass flow rate, achieving 0.84 mN at 4 mg/s. Several explanations for the thrust gain from RF power are discussed, with the most plausible mechanisms identified as neutral gas heating via ion-neutral charge exchange (I-N CEX) collisions and electron-impact dissociation. Although the total thrust measured using naphthalene remains lower than that achieved with argon, the calculated effective specific impulse, which accounts for propellant storage density, indicates that naphthalene is 1.3 to 1.7 times more efficient than argon. Moreover, at RF powers exceeding 26 W, 2 mg/s naphthalene demonstrates a better thrust-to-power ratio. These results demonstrate that sublimating solid propellants like naphthalene are a promising alternative propellant for CubeSat propulsion.