Self-absorption and electron number density measurements on Saturn entry radiation

The aerodynamic heating experienced by capsules entering into the atmospheres of Saturn, Uranus, and Neptune is greatly affected by chemically non-equilibrium processes occurring in the shock layers. There are two orders of magnitude discrepancies among reaction schemes used in numerical predictions for hydrogen dissociation and ionization, and it is not yet known which set of reaction schemes would predict these entry conditions more accurately. This paper reports the results of electron number density measurements conducted in the X2 expansion tube at the University of Queensland using a condition representative of a proposed Saturn entry where a significant non-equilibrium shock layer is expected. Electron number density along the stagnation streamline was obtained from Stark broadening by deconvolving the measured H-beta spectral line width obtained from the contributions of other dominant broadening mechanisms. The data presented here provides an independent measurements for evaluating the reaction schemes for the conditions created. It was found that the experimental data were well modeled by contemporary kinetic models. Quantitative agreement between experimental and numerical data was found by adjusting the ionization rate coefficients from an existing reaction scheme by a factor of 25.