Impact of Trace CH4 on Shock Layer Radiation in Outer Planet Entry

This paper reports experimental characterization of shock layer radiation from H2/He mixtures with a small amount of methane (CH4) addition in the Electric Arc Shock Tube at NASA Ames Research Center. The work follows on a previous test series for entry probes to Saturn and Uranus, which quantified the expected radiance in the absence of methane. However, the Saturn stratosphere is estimated to have approximately 0.4% of CH4. The current test series utilized mixtures of 0.886/0.110/0.004 H2/He/CH4 (by mole.) Freestream pressures included 0.1 and 0.5 Torr and velocities from 15-26 km/s. Data compares favorably to similar conditions collected in the Oxford T6 facility. Additional radiation features attributable to CH and C2 Swan bands are observed, albeit at low intensities. Comparisons to data collected in previous EAST tests suggest that the CH4 does not significantly alter reaction kinetics below 26 km/s.