Influence of Ablation Products on the Radiation at the Surface of a Blunt Hypersonic Vehicle at 10 km/s

Upcoming missions to the Moon have prompted renewed attention to the radiative heat fluxes on the surface of hypersonic vehicles during the reentry phase in the Earth atmosphere. The influence of species produced by heat shield ablation is a special concern because these species can be responsible for an inc rease of emission and absorption within the boundary layer. In this paper, a preliminary a ssessment is made for the case of a blunt, Apollo-shaped vehicle entering at 10 km/s with a fl ight path angle of -17 degrees. We assume an ablative carbon/carbon heat shield with a thickness of 15 mm. We consider the instant of peak radiative heating and determine the total radiative flux produced by the shock layer between 80 and 1600 nm at the stagnation point. First, the rate of production of ablating species is computed with the Astrium-BA188 code. Then, the flow field is determined with the FGE-TINA code. Finally, line-by-line computations are made with the SPECAIR radiation code of the total radiative flux with and without the presence of ablative species. Spectral regions where the influence of a blating products can be observed are noted. I. Introduction uring reentry, a bow shock forms in front of the re entry vehicle. The main part of the radiation is g enerated in the post shock region. Depending on the size and t he equivalent nose radius of the reentry vehicle th e stand off distance of the shock will vary. Directly behind t he shock, a nonequilibrium region is generated that evolves into a stationary flow region that may be close to equilib rium. In the boundary layer close to the wall, the plasma temperatures are lower than in the post-shock regio n. Thus, only weak radiation is generated here. H owever, the surface boundary layer is of particular interest si nce a significant portion of the incident radiation can be absorbed there, yielding an increase of the plasma temperatu re and hence an increase of the convective heat flu x to the wall. The presence of ablation products may strongly incr ease absorption in the boundary layer. The paper w ill address four questions. First, what is the contribution of the VUV region (80-200 nm) to the total heat flux? Second, what is the fraction of radiation absorbed in the bounda ry layer? Third, what is the effect of ablated spe cies on the radiative heat flux and spectra? Fourth, what is th e contribution of the nonequilibrium shock region t o the flux at the wall? These questions are examined for the two cas es with and without ablation products. The final o bjective is to determine how the presence of ablative species can be detected in ground testing facilities or in flig ht experiments.