(270c) Assessment of Collision-Less Vs. Collisional Simulation of Lunar Regolith Ejection During Spacecraft Landing

When a spacecraft lands on the Moon (or other extraterrestrial body; i.e. Mars, asteroids, etc.), the rocket exhaust causes regolith (soil-like material) to be ejected. Due to the lack of atmosphere and corresponding drag on regolith motion, the ejection can be extremely hazardous to objects both close and far from the landing point. For example, the Apollo 12 landed ~160 meters from the deactivated Surveyor III lander. An examination of the Surveyor revealed that it had been sandblasted by the ejection of fine regolith (order of microns at speeds up to 2 km/s) during the Apollo landing. Current models used to describe such ejection involve single-particle trajectories and thus do not consider particle collisions. Here, we seek to critically assess the impact of a collision-less vs. collisional description on regolith erosion. To accomplish this goal, the discrete element method (DEM), which incorporates collisions directly, is used to simulate the many-particle system. The system examined here is located 6m from the impingement point. Lift and drag forces from the plume exhaust are included in the simulation, as well as the Moon’s gravity. A one-way coupling is used such that the particles are affected by the plume but the plume is unhindered by particles. Two versions of the DEM are utilized, namely one which resolves collisions using a soft-sphere model and the other which ignores collisions and allows for the (unrealistic) crossing of particle trajectories.  Moreover, two cases of the collisional description are also considered – one with non-dissipative collisions and the other with dissipative (inelastic and frictional) collisions.  Somewhat surprisingly, the erosion rate of the non-collisional case lies between that of the dissipative and non-dissipative collisional cases.  Specifically, the non-dissipative collisional case exhibits the highest erosion rate since it allows for highly-effective collisional momentum transfer from the interior of the regolith layer to the regolith surface.  In addition, a sensitivity analysis to collisional input parameters is also performed.  Finally, a polydisperse system is examined and compared to the results of the monodisperse system.