Performance and flow characteristics of an active ABEP intake

Abstract

An Atmosphere-Breathing Electric Propulsion (ABEP) system is a novel concept in Satellite propulsion that involves capturing and compressing the rarefied atmosphere at Very Low Earth Orbit and using the captured gas to generate thrust using conventional electric propulsion methods. For such a system, the gas intake system needs to provide sufficient compressibility and mass flow to sustain the operation of the thruster. A numerical investigation into the flow characteristics and performance parameters of an active ABEP device has been conducted using the Direct Simulation Monte Carlo (DSMC) method. The Intake presented here comprises of a passive converging part and an active Knudsen Pump. The presented work initially recreates two studies one each of a passive intake and a Knudsen Pump from literature to validate the DSMC simulation methodology. Subsequently additional simulations are done based on the results of these benchmark cases.

Knudsen Pumps are usually operated in the transition flow regimes for its superior performance compared to the free-molecular regime. This study initially investigates the operation of Knudsen Pumps in free-molecular flow regime. This is done because in ABEP applications, the gases are usually in the free-molecular flow regimes. The channel lengths and the ratio of the size of the narrow channel to that of the wide channel is varied in the presented work. The variation of macroscopic properties along the axis of the Knudsen Pump is obtained from the results of the DSMC simulations. The results are compared to 1D analytical solution for some limit cases. It was concluded that increasing the channel lengths and maintaining a transition flow regime inside the wide channel of a Knudsen Pump brings up its compression characteristics while operating in a free-molecular flow regime.

In the second stage of this work, a hybrid intake consisting of a passive converging diffuse wall connected to an active Knudsen pump is investigated. A numerical DSMC model for such a setup is realised and verified using convergence studies. Two different configurations of single stage and five stage Knudsen Pump is studied. In the former case, the mass flow rate through the intake was varied using a given transmissivity for the intake exit and the temperature difference within each channels is varied. It was found that higher compressibility was observed when the total mass flow rate through the thruster was lower than the maximum thermal creep mass flow rate. Whereas the compressibility increased when the temperature difference was increased from Δ𝑇 = 500 to Δ𝑇 = 1000. However, further increase in temperature difference did not result in increased compressibility. In the five stage Knudsen Pump case, two different outlet transmissivities were studied. It was observed that the pressure and mass density drops down after consecutive stages of the Knudsen pump when the mass flow rate through the Knudsen Pump was higher than the maximum thermal creep mass flow rate. This resulted in no significant pressure or density compressibility. However, when the mass flow rate through the thruster was less than the maximum thermal creep mass flow rate, there was progressive increase in the mass density while the pressure continued to drop after consecutive stages. It was concluded that in-order to obtain significant compressibility the mass flow rate through the hybrid intake needs to be very low which in-turn might not yield sufficient thrust. A closed configuration where there is only mass flow during intermittent thruster firing could be ideal for obtaining higher compressibility using the Knudsen Pump