Analysis and Simulation of an Intake for Air-Breathing Electric Propulsion Systems

The Air-Breathing Electric Propulsion (ABEP) concept employs an air-intake to collect the residual atmospheric gas and feed it to an electric thruster. In this way, these systems could compensate for the drag force affecting spacecraft in Very Low Earth Orbit and enable low altitude missions with acceptable lifetimes. The objective of this thesis is to investigate the physical phenomena involved in the operation of the intake and evaluate their impact on its performance through models of increasing complexity. First, the orbital environment experienced by a spacecraft in VLEO is simulated through the use of an orbital propagator coupled with the available atmospheric models. The results highlight the large variation of the flow conditions due to latitudinal, seasonal, and solar cycle related effects. Then, the intake is analyzed using a simple lumped parameter model, which enables to estimate the performance of the intake and derive scaling laws for the device. The model shows that, to provide acceptable flow rate and compression, the intake must be matched to the transmissivity characteristics of the thruster. The condition of drag compensation is very stringent and requires a high collection efficiency to be satisfied. Next, the Direct Simulation Monte Carlo (DSMC) method is used to investigate complex flow effects. The degree of rarefaction of the gas has an effect on the performance. For realistically low densities, increasing the amount of collisions obstructs the particles incoming from the freestream, causing a decrease of the performance. Moreover, the misalignment of the incoming flow causes a performance degradation. It can reach 20% for a 5° misalignment angle and exceed 40% for a 10° misalignment. Increasing the amount of specular reflections increases the collection efficiency and decreases the density ratio, by as much as 5% for an accommodation coefficient α of 0.9. Finally, a method based on view-factors for the simulation of free molecular flows is developed and verified. Tests on the intake geometry show good agreement with DSMC. The capability of changing boundary conditions at very low computational cost makes it a promising tool for the multipoint optimization of the geometry