Development of Finite-Rate Ablation Toolset for Hypersonic Vehicles

Hypersonic boost-glide vehicles experience extreme thermal conditions, which require a thermal protection system (TPS) that can withstand extreme heat fluxes and temperatures. This is typically accomplished through the use of ablative carbon based materials which rely on the oxidation of carbon.  Predictive modeling of the oxidation process is challenging because the gas-phase chemistry and diffusion within the boundary layer are strongly coupled to the gas–surface chemistry, and all processes occur under nonequilibrium conditions. Legacy modeling approaches for carbon oxidation under hypersonic conditions have large uncertainties which often lead to overdesigning the TPS. Recent molecular beam experiments on vitreous carbon which included both oxidation and nitridation have provided the data needed to develop improved finite-rate air carbon ablation models relevant to hypersonic flight. The technical approach for the Phase I effort involves making enhancements to a well-established CFD code to allow for efficient implementation of current and future finite-rate ablation models, validating the models through comparison of results with existing data for a simple material (vitreous carbon), using temperature distributions and ablation rates from the CFD analysis to drive the thermal stress analysis for a representative hypersonic vehicle shape to quantify the effects of the improved ablation models, and evaluating microstructure effects and candidate test methods to guide the Phase II planning effort. The proposed innovation is a tool that allows for ablation of C/C materials under hypersonic flow conditions to accurately be accounted for within a CFD analysis to allow for the shape change based on the ablation rates, the changes in surface temperatures and the ablated species to appropriately impact the flow to determine the effect that each has on the resulting heat flux and pressure loads. In addition, MR&D will begin to lay the ground work to allow for the tool to also be coupled with a thermo-structural analysis to evaluate the anticipated stresses and their effect on vehicle survivability.