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 Θ for the inflow while Θ 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 . 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 Θ 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 Θ depends only on non-dimensional values of the duct geometry combined with vth 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.
