Scaling of multi-gas supersonic retropropulsion dynamics

Experimental studies of supersonic retropropulsion (SRP) dynamics have almost exclusively employed matching gasses for the freestream and retropropulsive jets, hampering our ability to extrapolate these results to conditions more similar to those of engines operating with Earth or Mars’ atmosphere. In this study we explore the influences of gas molecular weight, temperature and ratio of specific heats on SRP standoff distances. Combinations of Helium, Argon, Carbon Dioxide and Nitrogen gasses are explored for single nozzle retrojets with minimal forebody at zero angle of attack at Mach 2. Control volume analysis suggests a dependence of shock standoff distance on the parameter Ψ = MW_∞T_0,j/(MW_j*T_0,∞), and the ratio of specific heats for each gas. The sensitivity of SRP interactions to these parameters is explored for ranges of jet thrust and pressure. The flow is revealed to be a strong function of Ψ, but not the ratio of specific heats. Our results suggest SRP shock standoff distances can be successfully scaled to account for gas composition and flow properties.