Radiative and Total Heat Transfer Measurements to a Titan Explorer Model

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed on a 70 degrees sphere-cone in the X3 expansion tube at average freestream velocity, pitot pressure, static pressure, and density of 5.83 km/s, 76 kPa, 1.15 kPa, and 2.43 x 10(-3) kg/m(3), respectively. The radiation gauges used thin-film-sensing elements mounted behind a borosilicate window on the windward surface and were therefore shielded from the convective heat transfer. The assembled gauges are uniformly sensitive for radiation in the wavelength range from 0.347 to 2 mu m and give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer and the ratio of radiant to the total heat flux found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that, without the methane content, negligible radiation was produced, which is consistent with cyanogen being the primary radiator. Results from this study demonstrate that the gauges and the superorbital expansion tube facility are useful resources for the study of nonequilibrium radiating flows.