  Previously we presented a numerical model that predicts trajectories of lunar
dust, soil, and gravel blown by the engine exhaust of a lunar lander. The model
uses the gas density, velocity vector field, and temperature predicted by
computational fluid dynamics (CFD) or Direct Simulation Monte Carlo (DSMC)
simulations to compute the forces and accelerations acting on the regolith
particles, one particle at a time (ignoring particle collisions until more
advanced models are developed). Here we present significant improvements to the
model, including the implementation of particle drag and lift formulas to
account for the rarefaction and compressibility of the flow. It turns out that
the drag force is reduced due to the rarefaction, but the lift is increased due
to several effects such as particle rotation. A data matrix of particle sizes,
engine thrusts (descent and ascent values for Altair), horizontal and vertical
starting distances, and lander height above ground, have been tested using the
latest version of the software. These results suggest that the previously
reported 3 degree trajectory angle limit can be exceeded in several cases by as
much as a factor of five. Particles that originate at a height of 1 cm above
the surface from an outer crater rim can be propelled to angles of 5 degrees or
more. Particles that start at 10 cm above the surface can be ejected with
angles of up to 15 degrees. Mechanisms responsible for placing particles at
starting heights above the surface may include the kinetics of horizontal
collisions, as suggested by Discrete Element Method (DEM) simulations. We also
present results showing the distance particles travel and their impact
velocities. We then use the model to evaluate the effectiveness of berms or
other methods to block the spray of soil at a lunar landing site.
