Thermal Shielding of a Reentry Vehicle by Ultra-High-Tempreature Ceramic Materials
ABSTRACT Reentry vehicles With enhanced aerodynamic performances and high maneuverability require sharp leading edges for the wings and control surfaces and a sharp tip of the fuselage nose where high localized heat fluxes occur. Ultra-high-temperature ceramics, for example, zirconium, hafnium, or titanium diborides, are candidate materials for the sharp edges of reentry Vehicles that make use of new thermal protection systems, positioning massive thermal protection systems only at the leading edge of the wings (or at the fuselage tip). The boundary-layer thermal protection concept is illustrated, and the requirements for the geometry and materials of the fuselage nose are identified. It is shown how a sharp nose will protect the fuselage, acting as a lightning rod for the rest of the structure when the vehicle flies at relatively low angles of attack. Systematic numerical analyses are shown for the sphere-cone nose vehicle to compute temperature distributions along the surface and inside the nose structure at different angles of attack. The effects of the chemistry and of the surface catalysis are discussed.
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ABSTRACT: Microstructure modifications of an ultra-high temperature ZrB2–SiC ceramic exposed to ground simulated atmospheric re-entry conditions were investigated and discussed. Fluid dynamic numerical calculations were carried out to correlate and explain the experimental results. The cross-sectioning of the ceramic models after exposure (examined by SEM) showed a compact scale of zirconia (20 μm thick) underlying an external silica thin coating. A partially SiC-depleted region, a few microns thick, underneath the zirconia sub-scale was also seen. The post-test analyses confirmed the potential of the ZrB2–SiC composite to endure re-entry conditions with temperature approaching 2000 °C, thanks to the formation of a steady-state external multiphase oxide scale. Numerical calculations, which simulated the chemical non-equilibrium flow around the ceramic model, matched well the experimental results only assuming a very low catalytic surface behavior.Journal of the European Ceramic Society 01/2007; 27(16-27):4797-4805. DOI:10.1016/j.jeurceramsoc.2007.02.201 · 2.95 Impact Factor
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ABSTRACT: The convective heat flux distributions over the surface are reproduced with stagnation point to simulate free-flight aerothermodynamic conditions in a wind tunnel such as a plasma wind tunnel (PWT) to get a complete duplication of all relevant flight conditions. The most appropriate wind tunnel settings simulating free-flight conditions are identified by approximating heat flux as a function of curvature radius and hypersonic speed as a function of density and velocity in freestream conditions. Heat flux distributions for sharp and blunt bodies at local radiative equilibrium shows that bow shock detaches from the nose and dissociation reactions take place in the shock layer for a blunt body. The sharp body numerical model suggests that the chemical reactions take place in a thin shock layer that surrounds leading edge, but near the stagnation point there is a nonegligible difference between catalytic and noncatalytic wall.Journal of Thermophysics and Heat Transfer 07/2007; 21(3):660-664. DOI:10.2514/1.26465 · 0.87 Impact Factor
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ABSTRACT: The behaviour of pressureless sintered HfC and HfB2 ceramics, when exposed to high enthalpy plasma flows typical of atmospheric re-entry environment, was investigated with an arc-jet facility at temperatures exceeding 2000 °C. The surface temperature and emissivity of the materials were evaluated during the test. The microstructure modifications were analysed after exposure. Fluid dynamic numerical simulations were carried out to evaluate the catalytic atom recombination efficiencies of the materials at the experimental conditions. Surface and cross sections of the samples showed the formation of scales mainly consisting of HfO2 and SiO2. For the HfB2-based composite numerical results correlated quite well with experimental ones assuming a low catalytic surface behaviour. For the HfC-based material the surface behaviour changed from low catalytic to partially catalytic as the temperature increased. The post-test analyses confirm the potential of these composites to endure re-entry conditions with temperature approaching 2000 °C or even higher.Journal of the European Ceramic Society 01/2008; 28(9):1899–1907. DOI:10.1016/j.jeurceramsoc.2007.11.021 · 2.95 Impact Factor