Numerical study and semi-analytical model of rocket motor exhaust backflow in rarefied atmosphere

The prediction of backflow from multi-species high density rocket engine plume at high altitude, i.e., plume gases going upstream of the vehicle in rarefied atmospheric conditions remains a challenging numerical problem. Direct Simulation Monte Carlo computations are used to assess the sensitivity of backflow to plume and atmosphere inflow properties, and ultimately to derive a semi-analytical backflow model. It is found that backflow is independent of plume density in the thermal equilibrium limit at the nozzle exit plane, which allows for huge computational cost reductions in simulations to determine the backflow model parameters. The backflow behavior also appears to be dependent on two Knudsen numbers, representing the density effect in the far field atmosphere and in the compressed region in front of the vehicle. The ability of the model to estimate the backflow of specific species is finally demonstrated on a meter sized solid rocket case.