Miniature Electrothermal Thruster

A report proposes a miniature electrothermal spacecraft engine. The engine would include a chamber with an inside length of ≈1.5 cm and inside diameter of ≈0.25 cm, with a dielectric (ideally diamond) sidewall lining, a metal-coated expansion nozzle at one end, and a metal electrode at the other end. A propellant liquid (ammonia) would be vaporized into the cavity. To heat the NH3 vapor and dissociate it to a nitrogen/hydrogen plasma, the cavity would be excited with an electromagnetic field at a cavity resonance frequency of about 25 GHz (also the frequency of a dielectric resonance of NH3). The expansion of the plasma through the nozzle would generate thrust. The electric field would be of such a strength and configuration as to prevent contact between the plasma and the inner surface of the chamber. The plasma skin depth would be great enough that the plasma could absorb a large proportion of the electromagnetic energy. By use of refractory electrode and dielectric materials, pulsed operation, and, preferably, evaporative cooling of the chamber wall by the propellant liquid, it should be possible to achieve high plasma temperature and pressure and, thus, high thrust.