Experimental validation of a test gas substitution for simulating non-equilibrium giant planet entry conditions in impulse facilities

Scaled giant planet entry testing is beyond the simulation capability of most impulse facilities, which limits the ground testing data available for computational model validation. Substituting the He in their predominately H-2/He atmospheres with Ne has been used in the previous experiments. This enables higher-temperature shock layers to be recreated using the same facility performance. For binary hydrogen dissociation and ionization reactions, the substitution gives similarity with non-equilibrium processes, but has never been experimentally validated by direct comparison with H-2/He relaxation data in a region where the facility simulation capabilities in using H-2/He and H-2/Ne overlap. This work demonstrates and validates a scaling method based on the substitution in terms of flow relaxation. A H-2/Ne condition was designed to recreate the post-shock relaxation of a H-2/He condition in our X2 expansion tube. The two conditions were experimentally tested with cylindrical test models, and shock layer radiation from the Balmer series was measured. The spectroscopic data using the H-2/Ne condition show a successful recreation of the non-equilibrium processes of the target H-2/He condition, experimentally validating the scaling method. High-speed images demonstrate that the associated radiation field is also correctly reproduced. With the use of this substitution, the facility can now be used to simulate high-speed giant planet entries with confidence, and the available ground testing envelope is extended to much higher speeds.