Mitigation of Plume-Surface Interaction Effects via Landing Mats.

The potential of regolith-based landing mats to mitigate the adverse effects of operating rocket-based propulsive systems near unconsolidated granular surfaces is studied in a sub-scale cold flow plume-surface interactions (PSI) facility. The ejecta cloud and surface crater produced by an underexpanded sonic jet directly impinging on a granular media bed are compared to those in the presence of a landing mat above the granular surface. Mats with varying diameters are investigated under the same plume characteristics using a converging nozzle at a pressure ratio of 2.96 discharging at atmospheric conditions, and at a nozzle exit plane to surface distance of 20 nozzle diameters, De. Glass particles of about 100 micrometers in diameter are used as granular simulant. The landing mats, when not completely perforated by the impinging jet, reduce the pressure at the surface, which is quantified in separate experiments using an instrumented impingement plate. Post-test X-ray micro-CT imaging is used to characterize the mats before and after jet impingement. The evolution of the ejecta cloud in granular tests is recorded by two cameras with orthogonal fields-of-view and path-integrated particle concentrations are measured using a mm-wave radar interferometric technique. The presence of the landing mats is highly effective at preventing ejecta lift-off, with particles leaving the surface at radial positions beyond the mat diameter, at lower ejection angles, and resulting in craters with less eroded material. Results show a reduction of test averaged ejecta concentration along a surface-parallel path crossing the jet axis at an elevation of 10De, of 97% and over 99% for mats with diameter of 40De and 70De, respectively. Post-firing craters, reconstructed from scanner measurements, are wider and shallower in the absence of landing mats, with diameters reduced by 11% and 21% by the introduction of the 40De and 70De mats, respectively.