Flexible thermal protection systems are of interest due to their necessity for the success of future atmospheric entry vehicles. Current non-ablative flexible designs incorporate a two-dimensional woven fabric on the leading surface of the vehicle. The focus of this research investigation was to characterize the aerothermal performance of silicon carbide fabric using the 30 kW Inductively Coupled Plasma Torch located at the University of Vermont. Experimental results have shown that SiC fabric test coupons achieving surface temperatures between 1000°C and 1500°C formed an amorphous silicon dioxide layer within seconds after insertion into air plasmas. The transient morphological changes that occurred during oxidation caused a time dependence in the gas / surface interactions which may detrimentally affect the in-flight performance. Room temperature tensile tests of the SiC coupons have shown a rapid strength loss for durations less than 240 seconds due to oxidation. Catastrophic failure and temperature spikes were observed on almost all SiC coupons when exposed to air plasmas at heat fluxes above 80 W/cm2. Interestingly, simulation of entry into the Mars atmosphere using a carbon dioxide plasma caused a material response that was vastly different than the predictable silica layer observed during air plasma exposure.

Lors d’une rentrée atmosphérique, les boucliers thermiques des véhicules spatiaux subissent un échauffement considérable dont une fraction significative (~30%) est attribuée aux réactions chimiques à leur surface. Cette thèse contribue à la compréhension de cette réactivité hétérogène, la catalycité, au moyen des outils de la chimie théorique. Une méthode de construction de surface d’énergie potentielle globale est développée et appliquée à l’étude de la dynamique de processus élémentaires (adsorption moléculaire dissociative, absorption atomique, recombinaison Eley-Rideal …) pour les systèmes chimiques N,N2/W(100,110) et O,O2/Cu(100). Ces approches sont ensuite couplées à un modèle cinétique permettant de quantifier la catalycité. / During an atmospheric re-entry, the thermal shields of spacecrafts undergo an important heating, a significant fraction (~30%) of which is due to the chemical reactions at their surface. This thesis is a contribution to the understanding of this heterogeneous reactivity, catalycity, with the tools of theoretical chemistry. A method to build a global potential energy surface is developed and applied to the study of elementary processes dynamics (dissociative molecular adsorption, atomic absorption, Eley-Rideal recombination …) for the N,N2/W(100,110) and O,O2/Cu(100) chemical systems. These approaches are then coupled with a kinetic model quantifying catalycity.

In the design of a thermal protection system for atmospheric entry, aerothermal heating presents a major impediment to efficient heat shield design. Recombination of atomic species in the boundary layer results in highly exothermic surface-catalyzed recombination reactions and an increase in the heat flux experienced at the surface. The degree to which these reactions increase the surface heat flux is partly a function of the heat shield material. Characterization of the catalytic behavior of these materials takes place in experimental facilities, however there is a dearth of detailed computational models for the fluid dynamic and chemical behavior of such facilities. A numerical model coupling finite rate chemical kinetics and high temperature thermodynamic and transport properties with a computational fluid dynamics flow solver has been developed to model the chemically reacting flow in the inductively coupled plasma torch facility at the University of Vermont. Simulations were performed modeling the plasma jet for hybrid oxygen-argon and nitrogen plasmas in order to validate the models developed in this work by comparison to experimentally-obtained data for temperature and relative species concentrations in the boundary layer above test articles. Surface boundary conditions for wall temperature and catalytic efficiency were utilized to represent the different test article materials used in the experimental facility. Good agreement between measured and computed data is observed. In addition, a code-to-code validation exercise was performed benchmarking the performance of the models developed in this dissertation by comparison to previously published results. Results obtained show good agreement for boundary layer temperature and species concentrations despite significant differences in the codes. Lastly, a series of simulations were performed investigating the effects of recombination reaction rates and pressure on the composition of a nitrogen plasma jet in chemical nonequilibrium in order to better understand the composition at the boundary layer edge above a test article. Results from this study suggest that, for typical test conditions, the boundary layer edge will be in a state of chemical nonequilibrium, leading to a nonequilibrium condition across the entire boundary layer for test article materials with high catalytic efficiencies.