Shock Radiation Tests for Saturn and Uranus Entry Probes

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Shock Radiation Tests for Saturn and Uranus Entry Probes

Cruden, Brett A. / Bogdanoff, David W.

This paper describes a test series in the Electric Arc Shock Tube at NASA Ames Research Center with the objective of quantifying shock-layer radiative heating magnitudes for future probe entries into Saturn and Uranus atmospheres. Normal shock waves are measured in hydrogen–helium mixtures ( $89 ∶ 11 %$ by volume) at freestream pressures between 13 and 66 Pa (0.1 and 0.5 torr) and velocities from $20 - 30 km / s$ . No shock-layer radiation is detected within measurement limits below $25 km / s$ , a finding consistent with predictions for Uranus entries. Between 25 and $30 km / s$ , radiance is quantified from the vacuum ultraviolet through near infrared, with focus on the Lyman- $α$ and Balmer series lines of hydrogen. Shock profiles are analyzed for electron number density and electronic state distribution. The shocks do not equilibrate over several centimeters and in many cases the state distributions are non-Boltzmann. Radiation data are compared to simulations of Decadal Survey entries for Saturn and shown to be as much as eight times lower than predicted with the Boltzmann radiation model. Radiance is observed in front of the shock layer, the characteristics of which match the expected diffusion length.