Characterization of Onset and Transition to Instability in Supersonic Retropropulsion

Abstract

There is a need in supersonic retropropulsion (SRP) to understand the characteristics of shock unsteadiness and the parameters that determine transition to unsteady regimes. Data collected in the NASA Unitary Wind Tunnel were studied to determine instability characteristics in shocks as angle of attack, Mach number, and nozzle configuration was varied. Data were also collected at the University of Washington in order to better understand the point at which a shock will transition from steadiness to unsteadiness as forebody size, gas combination, and jet Mach number vary. The transition data taken at UW consisted of nozzle forebody diameters at 1/8 inch, 1/4 inch, and 3/8 inch nozzles at both Nitrogen-Nitrogen and Carbon Dioxide-Helium gas combinations, with several Carbon Dioxide-Helium 1/8 inch nozzle cases taken at a freestream Mach of 3. Shock standoff distance and oscillation amplitude was increased with increasing forebody diameter and increasing freestream Mach number. Carbon Dioxide-Helium cases also exhibited both higher oscillation amplitude and shock standoff distance than corresponding Nitrogen-Nitrogen cases. Transition to unsteadiness occurred at lower pressures than previously thought: between Pe/P02 of 0.2 and 0.8, depending on forebody diameter, gas combination, and freestream Mach number. Transition to unsteadiness does not always follow the previous predictions of a total pressure ratio unity or Pe/P02 of 1. A range of unsteadiness modalities was observed, including bimodal unsteady cases and cases exhibiting intermittent unstable behavior.