Numerical Investigation of Two Interacting Parallel Thruster-Plumes and Comparison to Experiment

Abstract

Clusters of orbital thrusters are an attractive option to achieve graduated thrust levels and increased redundancy with available hardware, but the heavily under-expanded plumes of chemical attitude control thrusters placed in close proximity will interact, leading to a local amplification of downstream fluxes and of back-flow onto the spacecraft. As ground-based experimental investigation of most technically relevant hot plume interaction flow-fields is not feasible, a joint approach is necessary to understand and predict the non-linear effects of plume-plume interaction, involving model experiments to gain confidence in the results of numerical methods. The interaction of two similar, parallel, axisymmetric cold-gas model thrusters has recently been studied in the DLR High-Vacuum Plume Test Facility STG under space-like vacuum conditions, employing a Patterson-type impact pressure probe with slot orifice. In this paper, we reproduce a selection of these experiments numerically, using a 3D parallelised implementation of the DSMC method to compute the plume expansion and interaction, while the flow-field inside the nozzles is obtained by the DLR Navier-Stokes solver TAU. We emphasise that a comparison of numerical results to the measured data is not straight-forward. The signal of the probe used in the experiments must be interpreted according to the degree of rarefaction and local flow Mach number, and both vary dramatically thoughout the flow-field. We present a procedure to reconstruct the probe signal by post-processing the numerically obtained flow-field data and show that good agreement to the experimental results is then achieved. On the basis of the numerical results, the features of the investigated cold-gas thruster plume interaction and are discussed.