Electron Number Density Measurements in a Saturn Entry Condition

The aerodynamic heating experienced by capsules entering into the atmospheres of Saturn, Uranus, and Neptune is greatly affected by chemically nonequilibrium processes occurring in the shock layers. There are several reaction schemes available in numerical predictions for hydrogen dissociation and ionization, and more experimental data would assist verifying these existing models. 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 significant nonequilibrium effects in the shock layer are expected. Electron number density along the stagnation streamline was obtained from Stark broadening. The data presented here provide independent measurements for evaluating the reaction schemes for the conditions created. It was found that the experimental data were qualitatively modeled by a contemporary kinetic model. 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. This updated reaction rate set was also cross-validated with electron number density measurements in NASA's shock tube tests. The adjusted rates had better agreement than using the original rates, and quantitative agreement can be found in the high-density cases.