  The interactions between rocket exhaust plumes and the surface of
extraterrestrial bodies during spacecraft landings involve complex multiphase
flow dynamics that pose significant risk to space exploration missions. The
two-phase flow is characterized by high Reynolds and Mach number conditions
with particle concentrations ranging from dilute to close-packing. Low
atmospheric pressure and gravity typically encountered in landing environments
combined with reduced optical access by the granular material pose significant
challenges for experimental investigations. Consequently, numerical modeling is
expected to play an increasingly important role for future missions. This
article presents a review and perspectives on modeling high-speed disperse
two-phase flows relevant to plume-surface interactions (PSI). We present an
overview of existing drag laws, with origins from 18th-century cannon fire
experiments and new insights from particle-resolved numerical simulations.
While the focus here is on multiphase flows relevant to PSI, much of the same
physics are shared by other compressible gas-particle flows, such as coal-dust
explosions, volcanic eruptions, and detonation of solid material.
