Combined Computational and Experimental Study Of UHTCs for Thermal Protection of Hypersonic Vehicles
Abstract
The proposed research is motivated by the recent resurgence of interest of the U.S. Navy in the development of hypersonic vehicles. Hypersonic systems fly faster than Mach 5 and a key aspect that distinguishes them from other flying vehicles is the need for management of the high levels of heat transfer that they experience on their external surfaces. Hypersonic vehicles mustemploy a Thermal Protection System (TPS) designed specifically to ensure survival of the vehicle throughout its operational envelope.A variety of approaches are employed for thermal protection of hypersonic vehicles including passive and active techniques. For example, the Space Shuttle used silica tiles with low thermal conductivity and high emissivity to radiate much of the heat away during its relatively long re-entry trajectory. Re-entry vehicles such as missiles and NASA capsules use thick heatshields made of ablative materials that absorb some of the energy and carry it away from the vehicle. For hypersonic systems of interest to the Navy, however, that are designed to operate over large distances in the atmosphere, such as mid-range and tactical-range boost-glide missiles, neither of these thermal protection approaches is applicable. Tiles will not withstand the highaerodynamic loads of missiles, and thick heat shields are too heavy. An alternative approach involves the use of Ultra High Temperature Ceramics (UHTCs) that are made of refractory compounds, such as ZrB2 and HfB2, combined with SiC into a ceramic. Such refractory compounds have very high melting temperatures, e.g., around 3300 oC for ZrB2 and HfB2.One of the key challenges in use of UHTCs is their performance under oxidation. Such phenomena are highly relevant because the elevated temperatures generated in a hypersonic flow dissociate the oxygen molecules in air into atoms that are extremely reactive with many materials. Tests of ZrB2-SiC and HfB2-SiC have indicated two oxidation regimes: passive oxidation of the SiC at relatively low surface temperatures transitioning to development of a thick surface oxide layer and active oxidation of SiC at higher temperatures. For both UHTCs and carbon-composites, the active agent governing the transition is thought to be SiC based on the observation that a stable layer of SiO2 forms on the material surface at less aggressive heating conditions. While these oxidation regimes have been observed recently in UHTC experiments, afull understanding of the behavior has not yet been developed.To better understand and evaluate the performance of UHTCs for thermal protection of hypersonic vehicles, it is proposed to conduct a coordinated computational and experimental study. The experimental studies will expose UHTC samples to the temperature gas flow conditions representative of hypersonic vehicles. The ability to study different materials and varythe freestream composition will generate a number of important data sets. The application of spatially resolved, non-intrusive, optical diagnostics that yield species-resolved information will provide unique insights into the most important physico-chemical processes occurring in the gasflow, on the material surface, and in-depth within the material. The computational models involve a comprehensive framework that includes the high enthalpy gas flow, the surface chemistry, and the in-depth material response. The detailed experimental data sets provide a trove of information that should make possible significant advances in the modeling of the gasflow, surface chemistry, and in-depth material response of UHTCs under hypersonic flow conditions.Through successful execution of the proposed research we will answer the key,fundamental questions raised about the suitability of UHTCs for thermal protection of hypersonic vehicles. Namely, how does the transition from passive to active oxidation occur and what leads to increases in surface temperature with continued oxidation exposure.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 17, 2020
- Source ID
- N000142012173
Entities
People
- Iain D. Boyd
Organizations
- Office of Naval Research
- Regents of the University of Colorado
- United States Navy