Thermodynamic Scaling of Supersonic Retropropulsion Flowfields

The influences of gas composition and temperature on supersonic retropropulsion (SRP) flowfields are experimentally explored. It is revealed that the standoff distance of the bow shock produced by SRP can be scaled to account for changes in thrust, mass flow rate, forebody size, gas composition and temperature within the high-thrust, steady flow regime. These parameters were systematically varied for Mach 2 and 3 heated jets at zero angle of attack, employing nitrogen, helium and argon within a Mach 2 nitrogen or carbon dioxide freestream. Comparisons are also made with higher freestream Mach number data from the literature for similar geometries The momentum ratio is shown to successfully account for changes in gas composition and temperature. Similarly, the mass flow rate ratio can account for these variables, down to a function of Mach numbers when multiplied by functions of gas molecular weight, temperature, and ratio of specific heats derived from mass conservation control volume analysis. Despite the remarkable collapse of shock standoff data, some scatter might be expected at low thrust conditions as the bow shock transitions between one fully dominated by the freestream and forebody geometry to one dominated by the jet exhaust. Shock radius was seen to be more variable at lower thrust levels, potentially indicative of this effect. This work will greatly aid the extrapolation of SRP between experiments, simulations and flight conditions.