Europa Lander Plume-Induced Contamination: DSMC Modeling of Monopropellant Plume Testing

Abstract

Powered spacecraft landings onto airless bodies – like Jupiter’s icy moon Europa, or Saturn’s icy moon Enceladus – can expose scientific sampling sites to contamination via the byproducts of chemical propulsion systems. It is critical to advance and validate predictive analysis capabilities for thruster plume-induced contamination in such landings to mature the technologies required for the success of the Europa Lander mission concept and other future exploration missions with landing and surface sampling elements. Therefore, the Jet Propulsion Laboratory (JPL) and the German Aerospace Center (DLR) are working together to develop physics-based plume contamination models for icy moon landings with an experimental testing campaign of monopropellant hydrazine thrusters underway in DLR’s STG-CT vacuum chamber facility. Recent numerical efforts have focused on the development by both JPL and DLR of steady-state plume models within STG-CT, intended to predict and interpret the results of the testing campaign. JPL employed a hybrid, one-way-coupled computational fluid dynamics (CFD) / direct simulation Monte Carlo (DSMC) scheme, in which rarefied and free-molecular flow in the far-field was calculated with the DSMC solver SPARTA and initialized at a coupling surface generated with STAR CCM+. DLR used a Fokker-Planck approach to model the same STG-CT plume flows, and preliminary comparisons between the JPL CFD / DSMC and DLR FP schemes generally show good agreement in both nozzle exit plane and far-field properties. In the DSMC approach, JPL compared the serial particle-selection implementation of Bird and the particle-selection prohibiting double relaxation method of Haas. Unless double relaxation of both particles is prohibited, the Bird method is not well-suited for gas mixtures with differing relaxation rates and internal DOF per polyatomic species. It works best when gases are vibrationally frozen, the rotational energies of colliding species have the same number of DOF and relaxation rates.