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Catalytic recombination of dissociated atmospheric oxygen and nitrogen is a major heat source for re-entry vehicles beyond Mach 10. Present thermal protection systems are designed to the extreme assumption of full catalycity. However mass, performance, and cost considerations of future reusable launch vehicles require that their thermal protection system is designed to the more realistic assumption of partial catalycity. With this in mind, the authors of this paper have studied the catalytic behavior of a coated C/C material as it was used for the heat shield of the OREX re-entry capsule. In this presentation. we will report on the laboratory measurements of the atomic oxygen recombination coefficient and its associated coefficient for thermal accommodation. The recombination coefficient was determined in plasma test chambers by actinometry and in plasmatron at 200 and 2000 Pa, respectively. The results show an effective recombination coefficient of the order of 1% (600 K - 1900 K). Unlike Stewart and the Bruno models, we did not observe a strong temperature dependency of the effective recombination coefficient, neither in the laboratory data nor in the flight data. The observed behaviour can be explained by the Cacciatore model on silica. [ABSTRACT FROM AUTHOR]

During the atmospheric re-entry phase of aerospace vehicles, the physico-chemical phenomena taking place on the hot parts (nose cap and wing leading edges) can lead to an important excess of heating and a damage of the protective materials. One aim of the researches undertaken at the FROMES-CNRS laboratory is the study of the catalytic recombination of atomic oxygen under conditions nearby Earth or Mars atmospheric re-entry. Four parameters allow to fully describe the catalycity phenomena: the recombination coefficient γ, the accommodation coefficient β the catalycily rate kw linked to the apparent recombination coefficient γ′ (equal to γ.β) and the recombination thermal flux transferred to the material. Experimental determination of these four parameters, simulation and modeling are presented according to a multi-scale approach: for the experimental part, microscopic scale for the recombination coefficient, mesoscopic scale for both the thermal flux of recombination and accommodation coefficient, and for the theoretical part, atomic scale for molecular dynamic simulation and macroscopic scale for kinetic modeling. Examples of results are given for silica and alumina ceramics in the temperature range 800-2300 K. [ABSTRACT FROM AUTHOR]

When a space vehicle enters in planetary atmospheres like Earth (air) and/or Mars (CO2), oxidation and catalycity coupled phenoraena occur on the thermal protective materials of the nose cap and wing leading edges conducting to an important excess of heating and possible damage of the materials. At the PROMES-CNRS laboratory (forraerly IMP-GNRS), in the research field "Interaction high solar fluxes-matter", oxidation under plasma conditions, and particularly the active-passive transition in the oxidation of silicon-based ceramics and the catalytic recombination of atomic oxygen under the same conditions are studied. The most important conditions for the ground simulation (high temperature, non-equilibriura plasma flow) have been realized in the MESOX set-up which associates a solar radiation concentrator and a microwave generator. In this paper, are presented some results about the transition between the active and passive oxidation regimes and the recombination of atomic oxygen and/or carbon monoxide at the surface of sintered silicon carbide at high temperatures and in a total pressure range of 102-104 Pa. The comparison is made between the two atmospheres: air (Earth entry) and CO2 (Mars entry) for the oxidation transition domain and for the atoms recombination. [ABSTRACT FROM AUTHOR]