S.R. Swan
Sandia National Laboratories,
United States
Keywords: ablation, UHTC, hypersonics, torch testing
Summary:
Hypersonics has emerged to create some of the biggest technical materials challenges in decades due to the high aerothermal heating and extended glide phase that is distinct from the ballistic re-entry conditions experienced by space craft and conventional re-entry vehicles. Composite materials play an essential role in the thermal protection system (TPS) of hypersonic vehicles due to a relatively low density, high thermal stability, and superior mechanical properties compared to monolithic ceramics and refractory metals. However, many challenges remain in our ability to meaningfully ground test these materials and understand how data from ground tests will inform performance in real environments. This talk will begin with a brief but thorough overview on the mechanisms of aerothermal heating that delineate the line between fast supersonic and hypersonic regimes and how that affects the environment these materials must be designed for. A review of the ablative and non-ablative types of TPS and the current state and limitations with these materials will be discussed. In the literature, advanced carbon/carbon (C/C) composites modified with ultra-high temperature carbides (UHTCs) such as silicon carbide (SiC) and zirconium carbide (ZrC) have shown excellent performance over densified C/C. An experimental review of the performance of these materials by torch testing at Sandia’s Oxy-Acetylene Materials Evaluation and Ablation Lab (OATMEAL) will be presented. Many of the conclusions drawn from the literature discuss a passive oxidation zone where stable oxides at the surface protect the underlying carbon fibers. However, an inherent limitation of torch testing is the diffuse flame impinging on the sample leading to limited mass conductance of hot reactive gases to the surface and a highly localized recession zone. To overcome that drawback and enhance our ability to draw meaningful conclusions from torch testing, ways to increase mass conductance and evenly distribute heat are required. Ongoing work to develop such improvements will be presented. Finally, a brief overview of laser absorption spectroscopy experiments in Sandia’s hypersonic shock tube will be presented to highlight higher fidelity ground testing methods that help to create more representative environments for composite TPS testing.