Radiative Heat Flux Measurements for Superorbital Earth Re-entries Using an Expansion Tube

Experiments were performed using the University of Queensland's X2 expansion tube with the aim of measuring stagnation point radiative heat flux to a blunt reentry body in superorbital Earth reentry air test flow. The selected flow condition produced a flight equivalent velocity of 14.9 km/s, representative of a Mars return trajectory. A radiation gauge was developed which consisted of a thin-film heat flux gauge placed behind an optical window, enabling isolation of the radiative heat flux component. Magnesium fluoride and B270 windows were used which made it possible to isolate the vacuum ultraviolet and infrared contributions, which are the main radiating wavelength regions at Mars return conditions. Absorbance calibration of the gauges had to account for the spectral dependence of the Nickel sensing element, and relied on radiation simulations using NEQAIR to predict the spectral distribution of the radiative heat flux. Experimental measurements yielded an average calibrated radiative heat flux of 10.7 MW/m2 over the 120 - 7000 nm wavelength range, of which 60% was predicted to be in the vacuum ultraviolet. Experimental radiative heat flux was around four times higher than the NEQAIR prediction, which, based on CFD did not, however, account for precursor radiation and excitation of the high temperature freestream.