Multiple Approach for Ejecta Mapping in PSI: Millimeter-Wave Tomography Coupled with PTV and Predictive Modeling

This study investigates plume-surface interactions (PSI) during spacecraft landings using a reduced-scale experiment. The setup consist of a cold nitrogen jet impinging on a granular surface of glass microspheres under sub-atmospheric conditions. Two complementary measurement techniques were employed: millimeter-wave radar interferometry for quantitative measurement of particle concentrations, and particle tracking velocimetry (PTV) using multiple high-speed cameras. A Monte Carlo model of ejecta trajectory was developed to predict ejecta velocities and trajectories from radar ejecta concentration measurements. The experiments explore the effects of varying thruster height between 3, 7 and 10 nozzle diameters under ambient pressure representative of the lunar environment (6.7 Pa). The results reveal two distinct cratering mechanisms dependent on nozzle height. At a height of 3 nozzle diameters, particles are ejected at high angles (39 degrees) with decreasing velocities (2.3 to 1.3 m/s), which is consistent with the formation of deep craters. In contrast, at greater altitudes (7 and 10 nozzle diameters), particles follow near-horizontal trajectories at consistently higher velocities (4.4 m/s), which suggests shallow cratering. Discrepancies between PTV measurements and radar-based model predictions motivate future work to integrate sparse data from both techniques into a common optimization problem to accurately reconstruct the complete particle cloud evolution.