Investigation of in-depth penetration of radiative heating in thermal protection system (TPS)

Radiative heat flux on space vehicles is increasingly relevant for several entry missions in the future. Capturing the in-depth penetration of radiative heat flux into thermal protection systems (TPS) requires an accurate description of the radiative properties of the material at all relevant wavelengths. To this end, a radiative Monte Carlo code is being developed to compute the extinction, scattering, and absorption coefficients of various TPS materials. It uses the microstructure of TPS materials as the input geometry and computes the photon mean free path inside the material, which is converted into radiative coefficients. The code is validated against analytical solutions of spherical microstructures and then extended to compute radiative properties of porous fibrous insulators. Properties are generated for wavelengths ranging from 1-100 microns, where it is observed that the absorption coefficient significantly increases in this range. The refractive index of carbon plays an important role in the profiles of the radiative properties for spherical microstructures, and a wavelength-dependent refractive index was required to compute the radiative properties. However, for fibrous microstructures, modeled as cylindrical fibers, properties computed are not strongly dependent on the refractive index. A constant refractive index with a complex part of 0.75 resulted in the same values as using a wavelength-dependent refractive index. A key advantage of developing a radiative Monte Carlo approach over past theoretical approaches is that any arbitrarily complex microstructure can be imported into the solver, including partially ablated microstructures to provide a comprehensive database of radiative properties that can then be used to solve radiative transport equation in computational fluid dynamics or material response codes.