Radiation is a significant factor for heat loads during atmospheric entry. Especially slender configurations are affected by an extremely close shock layer. The influence of carbon-dioxide and argon in the free stream of an entry vehicle on radiative properties is investigated. The Fire II flight experiment is recalculated and compared to reference data. Additionally, a triple cone slender configuration is investigated. This includes line-by-line Monte Carlo ray tracing transport calculations. Since carbon-dioxide and argon influence on radiative properties strongly depends on spectral wavelength, special emphasis is given to the MWIR region. Equilibrium and non-equilibrium computation on radiative heat flux is demonstrated and a good match between reference data and the recalculation is obtained. However, radiation from a non-equilibrium computation is significant lager. A variation of the Monte Carlo method is given to overcome its weakness in optically thin media. Hence, evaluational size could be reduced. The modified forward computation is identified to be superior in optically thin media.
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