Arc-jet facilities play a primary role in recreating aerothermal conditions experienced by atmospheric entry vehicles and are widely used to test the performance of thermal protection materials. In this work, we utilize a developed coupled framework between an overset flow solver CHAMPS NBS-Cart, and a material solver KATS-MR to study the ablation of graphite under arc-jet conditions. We implement a 12-species gas phase model to accurately represent the air-carbon mixture, including argon species present in the flow. The gas phase is modeled with a two-temperature thermo-chemical non-equilibrium model without considering electronic and ionization effects. The gas-surface interactions are modeled with a newly developed air-carbon ablation model accounting for oxidation, nitridation, and recombination reactions. In addition, the model is augmented with carbon sublimation reactions experienced at high heating conditions. The chemical state at the surface is tightly coupled with the flow solver, resulting in the improved accuracy and effectiveness of the simulation. The coupled approach is applied to study two experimental test cases conducted at the IHF arc-jet facility at NASA Ames. The predicted results are validated against measured recession, surface, and in-depth temperatures and compared to the prediction of the uncoupled, equilibrium-based approach. Finally, the accuracy of the prediction is explored with respect to the environmental properties, such as the diffusion coefficient, and material thermal conductivity.

View Video Presentation: https://doi.org/10.2514/6.2022-3730.vid Experiments were conducted using both the Interaction Heating Facility (IHF) and Aerodynamic Heating Facility (AHF), two hypersonic arc-jet facilities at NASA Ames Research Center. These tests were performed to understand the material response of ablative thermal protection system materials by exposing them to high-enthalpy flows. Using in-depth thermocouple measurements from a graphite test article, test conditions were determined using NASA's one-dimensional inverse heat conduction model FIAT-Opt. A least squares fit function was found to recreate these boundary conditions as a reproducible case. Comparisons were also made with results from the one-dimensional model FIAT using CFD-derived facility target conditions. The two cases were additionally compared and validated using thermocouple measurements from another material, as well as calorimetry measurements during the same test. Additional validation against data other test series confirmed the applicability of this approach to modeling cases in other facilities.

NASA has successfully developed a new and innovative Heatshield for Extreme Entry Environments Technology, or HEEET, which, at a Technology Readiness Level (TRL) of 6, is ready for use in Ice Giant missions. HEEET is not just a replacement for the legacy full-density carbon-phenolic (FDCP) material, which was used in NASA's Pioneer-Venus and Galileo missions; it is also a more mass efficient and robust alternative, and a technology that has a sustainable manufacturing base. HEEET is a dual-layer, 3-dimensionally woven material. It has a dense outer layer, made of pure carbon fibers, that comes into contact with and protects against extreme entry environments. Below this layer is an integrally woven, lower density insulating layer, made of a blend of carbon and phenolic yarn, that reduces heat-conduction to the carrier structure. The present paper describes development of this material, its thermal, structural, and aerothermal testing, production of an engineering test unit at flight scale, and maturation for infusion into missions to various planetary destinations, with a focus on Ice Giant in situ missions. Finally, for representative entry velocities at Uranus and Neptune, and a range of entry masses and flight path angles, margined thicknesses of HEEET are computed. When the limits of heat fluxes and pressures that can be achieved in ground-test facilities, and loom limits, are imposed on these thickness estimates, it is shown that several atmospheric entry missions are possible at the two destinations.