Ejecta Tracking During Plume-Surface Interactions at Sub-Atmospheric Conditions

The thruster plume of a lunar or Martian lander necessarily impinges on and interacts with the regolith surface during landing. Regolith is lofted and ejected from the surface, which results in a crater at the landing site. The dynamics of this process are not well understood. The work reported here explores the velocity (both magnitude and direction) of ejecta particles in a sub-scale experiment with a Mach 5 cold-gas nitrogen jet impinging on a bed of glass microspheres. The ejected particle velocity is measured using a planar laser PTV system with four high-speed cameras and studied as a function of the jet nozzle altitude above the surface. Results show that, as nozzle altitude decreases, the initial burst of ejected particles has larger speeds and a steeper (more vertical) ejection angle for both lunar and Martian ambient conditions. At low altitudes, particles ejecting at steeper angles between 40 and 50 degrees move at speeds up to 15 m/s faster than those traveling close to the surface between 0 and 10 degrees. Results also show that during tests with Martian ambient conditions, ejection speeds are slower by up to 10 m/s than tests done with lunar ambient pressures.