Influence of heterogeneous catalytic processes on heat fluxes to heat shield coatings of space vehicles entering into atmospheres of the Earth and Mars

Kovalev V.L.

The Moscow state university named after M.V. Lomonosov, Moscow, Russia

In the hypersonic flight of bodies in the atmospheres of the Earth and Mars, nonequilibrium processes in the gas phase and on the body surface exert a considerable influence on the heat fluxes to the surface. Thus,  whereas the difference in heat flux values for different models of the  homogeneous processes can be as high as 25 %, the values obtained under different assumptions concerning the catalytic properties of the surface can differ by several times. Therefore, a fairly accurate description of the surface processes is important for developing new spacecraft.

The efforts made to understand and simulate the catalytic processes on reusable vehicle surfaces are reflected the many heterogeneous recombination models developed in recent years [1].  There have been numerous studies, both experimental and theoretical, of  high-temperature catalysis in dissociated air in the context of the development of thermal insulation systems for Space Shuttle and Buran aeroassisted orbital transfer vehicles. Originally, in the theoretical models heterogeneous catalysis was described by first-order reactions with experimentally obtained rate constants. Recently, more accurate models  based on the ideally adsorbed layer theory of  Langmuir  have been developed. These models have made it possible adequately to describe  for a suitable choice of the relevant parameters  the aerodynamic heating on the windward side of reusable vehicles along the entire re-entry path in the Earth's atmosphere. Much less research has been done on the high-temperature heterogeneous catalysis in dissociated carbon dioxide on the basis of a consideration of the detailed mechanism of heterogeneous catalytic processes.  Usually, for evaluating the heat fluxes during entry into the Martian atmosphere, the limiting cases of an ideal catalytic surface (with a maximum rate of heterogeneous recombination of the dissociated carbon dioxide components) and a noncatalytic surface were considered. New  models for heat-shield coatings on spacecraft entering the Martian atmosphere, based on a detailed analysis of the heterogeneous catalytic reaction mechanism, were suggested in [2- 4]. The first results of measurements of the dissociated carbon dioxide flow parameters in a high-frequency plasma generator, as well as the heat fluxes to a catalytic surface and the temperature of the latter, were published in [5] for three types of coating materials and quartz.

But still, the processes of heterogeneous recombination are ill studied on both levels - theoretical and experimental ones. CFD models available from the literature do not provide strong enough capability to predict heat transfer rates with sufficient accuracy and some time led to the inconsistent results due to ignoring the formation of the exited molecules on the surface in the heterogeneous recombination. Data of the energy accommodation have quite a large scattering. Moreover, these data are practically absent for the reusable thermal protection materials the Earth and Martian atmospheres entry conditions. The development of the advanced TPM requires studying the processes of the heterogeneous relaxation and energy accommodation on high-temperature surface.

In this study, we evaluate the effect of different heterogeneous catalysis mechanisms in air and dissociated carbon dioxide on the surface of glassy coating materials, such as chemical and physical adsorption, Eley-Rideal  and  Langmuir-Hinshelwood reactions involving physically and chemically adsorbed particles, and heterogeneous recombination of carbon atoms. Estimates are obtained on the basis of certain models of these processes suggested in the literature and a  comparison with the experimental data. We examine a simple physicochemical model leading to closed-form predictions of the quenching and energy transfer effects and relevant parameters. This model, together with experimental data on the production and quenching of electronically excited molecules during heterogeneous O-atom recombination, are used to estimate the importance of gas phase and wall quenching in experimental determinations of atom recombination coefficients or the apparent heat of atom recombination at catalyst surfaces, and the likelihood of appreciable reduction in the aerodynamic heating of hypersonic glide vehicles.