The Air-breathing Electric Rocket

Air-breathing electric rockets have the potential to enable space missions at very low altitudes. This innovative propulsion method integrates a rarefied flow air intake, engineered to efficiently collect the thin atmosphere in front of a spacecraft, with an electric thruster that ionizes and accelerates the collected gas molecules. This thesis provides a review of recent studies in air-breathing electric propulsion systems and plasma modelling techniques, and introduces to a novel 0D-hybrid formulation for describing the coupled intake and thruster physics of air-breathing electric rockets. Model derivation is then used to derive formally the main system’s key performance indicators and to estimate the figure of merit for the design of rarefied flow air intakes and air-breathing plasma thrusters. The performance achievable by conical intake shapes is evaluated by Monte Carlo simulations, and the influence of inlet flow properties is assessed by dedicated sensitivity analyses. These well highlight the impact of operating in air-breathing mode as compared to on-board propellant storage and flow management via propellant feeding systems. As an experimental benchmark to verify the devised model formulation, the HT5k Hall thruster was characterized with air propellant in six operating conditions, ranging from 5 mg/s to 7 mg/s of 0.56N2+0.44O2 mass flow rate and 225 V to 300 V of discharge voltage. The cathode was operated with N2 at mass flow rates between 0.5 mg/s and 0.7 mg/s. Verified performance ranged between 30 mN to 120 mN in thrust, 1.2 kW to 5.2 kW in discharge power, and 8% to 18% in anodic efficiency. The calibrated model comparison against experimental data resulted in a mean absolute error of 3.7% in thrust and 7.6% in discharge power. A simplified plume expansion model, making use of the 0D-hybrid discharge model output as initial condition, is compared with the data acquired from a diagnostic system composed of Faraday probes, a triple Langmuir probe, and a Retarding Potential Analyzer. Building on the methodology and results presented in this study, future directions for further research are outlined and discussed.