Static Wall Pressure Measurements of Supersonic Under expanded Jet Impingement

This paper summarizes an experimental study of an underexpanded, axisymmetric jet impinging on a solid flat plate at Lunar and Martian atmospheric conditions for a varying nozzle-to-impingement surface distance. The jet angle of incidence is maintained at 90°, the ideal landing condition for a payload with a central retrograde rocket. Tests have shown that large transient pressure spikes with amplitudes ranging from between 6.1 and 8.8 kPa for Lunar impingement and between 17.2 and 39.7 kPa for Martian impingement form on the surface of the flat plate. Pressure spike duration is directly proportional to the startup and shutdown rates of the pulsed supersonic jet and corresponds to the formation or collapse of a strong normal plate shock above the surface. Schlieren imaging corroborates this result by correlating flow structure dynamics at the plate with surface pressure measurements. Experimental measurements show that peak steady state surface pressure has a nonlinear response to the impingement distance for jets that are moderately underexpanded (the nozzle exit pressure to ambient pressure ratio is on the order of 2-3). Large transient pressure spikes can create explosive erosion events that can rapidly deplete the soil beneath a lander, introducing the risk of unstable landing and the scouring of nearby objects. Our goal is to better understand the complex coupled multiphysics that occurs within the interactions of a supersonic jet, the cratering surface, and the eroding topsoil. Characterizing the flow dynamics and impingement pressure distribution at incremental points during the terminal descent stage of EDL forms the starting basis for this goal.