Transmission Probabilities of Rarefied Flows in the Application of Atmosphere-Breathing Electric Propulsion

Atmosphere-Breathing Electric Propulsion systems (ABEP) are currently investigated to utilize the residual atmosphere as propellant for drag-compensating thrusters on spacecraft in (very) low orbits. The key concept for an efficient intake of such a system is to feed a large fraction of the incoming flow to the thruster by a high transmission probability Theta for the inflow while Theta for the backflow should be as low as possible. This is the case for rarefied flows through tube-like structures of arbitrary cross section when assuming diffuse wall reflections inside and after these ducts, and entrance velocities u larger than thermal velocities v(th) proportional to root k(B)T/m. The theory of transmission for free molecular flow through cylinders is well known for u = 0, but less research results are available for u > 0. In this paper, the desired theoretical characteristics of intakes for ABEP are pointed out, a short review of transmission probabilities is given, and results of Monte Carlo simulations concerning Theta are presented. Based on simple algebraic relations, an intake can be optimized in terms of collection efficiency by choosing optimal ducts. It is shown that Theta depends only on non-dimensional values of the duct geometry combined with v(th) and u. The simulation results of a complete exemplary ABEP configuration illustrate the influence of modeling quality in terms of inflow conditions and inter-particle collisions.