Material Response & Ablation
One of the most important aspects of space and planetary exploration is the ability to safely enter a planet's atmosphere, decelerate properly and land smoothly. Because entry vehicles travel long distances, they reach their destination with tremendous speed, which directly translates into a vast amount of kinetic energy. Although most of this energy dissipates into the surrounding atmosphere, a fraction still reaches the vehicle through convective heating and radiation. In order to protect the payload and ensure safe landing, it is critical to equip the vehicle with an appropriate Thermal Protection System (TPS).
The TPS of a entry vehicle is therefore one of the key components of its design. The material used for the TPS can be classified into two main categories: ablative materials, as in the one used on Apollo missions, and non-ablative materials, such as the ceramic tiles used on the space shuttle. The former can also be divided into two sub-categories: charring (also know as pyrolyzing) and non-charring ablators. The theory behind the use of ablators is quite simple; the energy absorbed by the removal of material from the surface is not used to heat the TPS, thus keeping the vehicle at a reasonably "cold" temperature. In the case of charring ablators, the ablative material is a resin that fills the pores of a carbon matrix. Although the matrix might ablate, it usually does not, thus preserving the original geometry of the aerodynamic surface during re-entry.
Modeling TPS is not a trivial task as it involves several physical phenomena in various scientific fields. First, the flow field needs to be modeled, which means solving an hypersonic flow with strong shocks, viscous effects, boundary layer effects, turbulence, radiation, thermo-chemical non-equilibrium, and much more. The surface must also be properly taken into account, as it involves surface chemical reactions, catalysis, ablation, recessing surfaces, surface roughness, etc. Finally, the TPS material must be modeled, which entails multi-layered material, multiphase flow, thermal stress, pressure stress, spallation, porous flow, chemical non-equilibrium, as well as other processes.
Introduction
The design of hypersonic flight vehicles is characterized by stringent requirements like achieving sharp leading edges to maximize lift-to-drag ratios while experiencing extreme external gas temperatures, which can reach upwards of 10,000K. Designers must contend with the reality that convective heat transfer rates between this hot gas and vehicle surface increase with decreased size of these leading edges, making most common engineering materials such as aluminum or titanium, warp, melt or even vaporize. This reality has motivated fundamental research into a new class of materials known as Ultra-High Temperature Ceramics (UHTC). These materials exhibit a unique combination of refractory and oxidation-resistant properties which allow them to survive the extreme heating environment encountered in hypersonic flight.
Building on the latest theories and modeling techniques available in the NGPDL, new material-environment interaction models are being explored which will aid the hypersonic vehicle designer and improve understanding of high temperature material capabilities. Of particular interest are various carbides, such as silicon-carbide (SiC) and di-borides, such as zirconium-diboride (ZrB2). A number of UHTC candidate materials are shown in the figure below, plotting each in terms of their melting point and the melting point of their corresponding oxidation states.
Figure 1. Melting temperatures of refractory compounds versus that of their oxides limited to those materials wherein both exceed 2000â—¦C [2]
There are two parts to modeling material-environment interactions - the material model itself, and the Computational Fluid Dynamics (CFD) or material response framework. Material models describe the surface chemistry, morphology of the microstructure, change of phases, and any in-depth chemical processes that ultimately affect the surface chemistry, such as in the figure below. Parameters such as the oxidation rate and composition of the oxidation products are important for predicting the resulting surface heating.
Figure 2. Schematic for 1-D steady-state analysis of ZrB2/SiC [3]
The underlying material-environment interaction is treated as a coupled process, involving both the material itself, and the hypersonic flowfield environment, typically modeled using high-fidelity CFD techniques. Detailed gas-surface and gas-phase chemistry are modeled in the shock layer, including any thermal and chemical nonequilibrium between the chemically reacting surface and the hypersonic environment. The figure below illustrates the predicted flowfield adjacent to the reacting surface of a SiC-coated leading edge in a hypersonic environment, showing the density of SiO, which is an oxidation product.
Figure 3. Density prediction for SiO [2]
As new combinations of UHTC materials are developed, in-depth material response modeling is needed to reproduce and understand high temperature test results. The figure below shows a snapshot of the simulated thermal response of a HfB2 sample, after experiencing hypersonic flow conditions for 10 seconds.
Figure 4. Thermal response of a HfB2-SiC material sample subjected to the surface loads after 10s [3]
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Investigators
Dr. Samuel Y. Chen, Dr. Daniil Andrienko, & Dr. Nicholas S. Campbell
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Acknowledgments
The NGPDL gratefully acknowledges the Office of Naval Research for funding this work.
References
[1] Chen, S. Y.,ÌýModeling of Material-Environment Interactions for Hypersonic Thermal Protection Systems, Dissertation,Ìý University ofÌý Michigan, 2020
[2] Poerschke, D. L., Novak, M. D., Abdul-Jabbar, N., Kramer, S., and Levi, C. G.,ÌýSelective active oxidation in hafnium boride-silicon carbide composites above 2000C, Journal of the European Ceramic Society, Vol. 36, No. 15, 2016, pp. 3697-3707.
[3] Campbell, N. S., Chen, S. Y., Boyd, I. D., Gasch, M. J., and Ellerby, D. T., Unified Material-Environment Interaction Model for Binary UHTC Composites, AIAA paper 2021-3168. 2021.