Nonintrusive Diagnosis of Ejecta Cloud from Plume Surface Interactions using High-Speed Digital Holography

The physics of plume surface interactions (PSI) when a rocket-powered spacecraft performs an extraterrestrial surface landing involves producing soil erosion/cratering and high-speed ejecta which are fundamentally coupled. The crater shape and growth effects the speed and direction of the ejecta particles that may in turn be entrained by the flow field to alter the plume aerodynamic structure, the cratering process and even a space lander’s orientation. A recent touch-down event by the Odysseus lunar lander where the lander tripped and fell onto its side on the cratered lunar surface may have resulted from a PSI phenomenon, highlighting the extent of the problem. Relevant mitigation techniques require understanding the physics of PSI events. More specifically, accurate 3D characterization of the high-speed ejecta particles in terms of particle size distributions, trajectories and velocities is needed by NASA to assess mitigation measures and for benchmark validation of modeling and simulation (M&S) tools. The current state of experimental findings on PSI is scarce and the in-situ diagnostics used to obtain data only provide two-dimensional information that are highly dependent on ambient light and visibility conditions making measurements difficult close to touchdown. Given these shortcomings, this work proposes to demonstrate the feasibility of high-speed digital holography (DH) in providing high resolution time-resolved 3D trajectories, velocities and particle size distributions of ejecta particles generated by PSI events of rocket powered spacecraft landings. Since digital holography makes it possible to numerically focus on discrete depth planes, a single digital holographic video is equivalent to 1000 conventional videos, each recorded at a different plane of focus. The effort will provide a prototype instrument for physics focused ground testing and a preliminary design for a compact DH flight instrument (HoloCam) for NASA’s future mission needs.