Multi-stage Ionization for Air-Breathing Electric-Propulsion*

The very low Earth orbit (VLEO) environment requires propulsion systems to generate significant thrust levels to maintain altitude, limiting the orbital lifetime of satellites and necessitating innovative solutions to store and utilize propellant for sustained operations. A promising approach involves multi-stage air-breathing thruster designs that can operate with low flow rates of air and with minimal compression of VLEO pressures. This research investigates multi-stage air-breathing propulsion systems, consisting of an inlet, preionization, and acceleration stage. Different types of preionization are used as sources of seed electrons to identify a pathway to breakdown of air at mTorr pressures, including RF discharges, field emitters, and optical pulses. A model is also developed to validate these breakdown experiments and extrapolate them to design thrusters to operate in VLEO with compression ratios < 100. To evaluate the effectiveness of this approach, we conduct firing tests on a pulsed electromagnetic thruster with and without preionization, measuring thrust magnitudes using a pendulum thrust stand paired with a Michelson interferometer. Mass bit and inlet pressures are varied to simulate orbital conditions ranging from 100 km to 400 km. With the preionization stage, thrust-to-power (T/P) ratios are measured at 4 mN/kW for mass bits as low as 2 µg for air, argon, and nitrogen, with specific impulses up to 10,000 s. Without preionization, T/P is 50% lower, and the minimum mass bit increases to 40 µg with specific impulses < 1,000 s. These results provide crucial insights into the optimization of propulsion technologies for VLEO missions.