Application of reusable heat-shielding coatings at the entrance

in the Mars atmosphere

Valery L.Kovalev, Nataly E.Afonina, Valery G.Gromov

Moscow State University named after M.V.Lomonosov

Vorobyevy Gory, Moscow, GSP-2, 119992, Russia

Extensive experimental and theoretical studies on high-temperature catalysis in dissociated air and in dissociated carbon dioxide have been conducted in connection with the development of heat shields for the reusable spacecrafts Space Shuttle and Buran as well as for space vehicles entering the Martian atmosphere. A large number of heterogeneous recombination models were proposed in recent years to gain a better understanding of surface catalytic processes for reusable spacecrafts. A thorough survey of works in this direction of research is given in [1]. The models that take into account the detailed mechanism of heterogeneous catalytic processes allow one (if the model parameters are chosen appropriately) to adequately describe the aerodynamic heating of windward surfaces of reusable spacecrafts along their entry trajectories in the Earth's atmosphere [2] and to predict the level of heat fluxes for the case of Martian atmospheric entry [3, 4].

In this paper we study flow near a space vehicle entering the Martian atmosphere. The vehicle consists of a heat shield and a cylinder located at the rear of the shield (Figure 1). The shield has the shape of a spherically blunted cone whose half-angle is equal to 60њ. In our computations we used the following parameter values: R_n = 1 m, R_s = 0.15 m, R_c = 0.8 m,љљ L_c = 1 m, and D_b = 2R_b = 3.4 m.

When the space vehicle enters the atmosphere, the level of heat fluxes on the cylindrical surface is much lower than near the shield. Nevertheless, this level should be known accurately enough in order to optimize the total mass of heat-shielding coatings. Our computations were performed in the framework of Navier-Stokes equations on the basis of

Fig. 1

the thermally equilibrium gas-phase model with the 11 components: п, у, N, Ar, O₂ љN₂ , CO, C₂, CN, NO, and CO₂. It was assumed that all the components were in the electronic state in the form of thermally perfect gases with equal translational temperatures. The rotary and oscillatory motions of molecules were described with the aid of the model "rigid rotator-harmonic oscillator". The kinetic model includes 22 chemical reactions of dissociation-recombination and exchange. When the transfer properties of gaseous mixtures in diffusion-type elastic collision cross sections are analyzed, two-parameter interpolation formulas based on the cross-section values at ф₁ = 300 л and ф₂ = 20000 л are used. These values are determined with the aid of the Lennard-Johns potential for low temperatures and the Born-Meyer potential for high temperatures. The viscosity and thermal conductivity coefficients are specified by the modified Wilky-Vasiljev formulas, whereas the diffusion fluxes are specified by the Stefan-Maxwell relations.

The effect of catalytic surface properties on heat exchange is analyzed with the aid of the numerical method developed in [5].

The effect of catalytic surface properties on heat exchange near the bottom surface of a space vehicle entering the Martian atmosphere was studied. It was shown that the catalytic properties of the frontal and bottom surfaces might significantly influence (up to several times) the levels of heat fluxes. It was found that the physical adsorption of oxygen atoms leads to an increase of heat fluxes on the surface regions with a temperature lower than 600 K; their levels, however, remain low.

1.      V.L. Kovalev, Heterogeneous Catalytic Processes in Aerothermodynamics (in Russian), Moscow, 2002.

2.      V.L. Kovalev, O.N. Suslov, and G.A. Tirskiy, "Phenomenological theory for heterogeneous recombination of partially dissociated air on high-temperature surfaces", in: Molecular Physics and Hypersonic Flows, pp. 193-201, Dordrecht, 1996.

3.љ N.E. Afonina, V.G. Gromov, and V.L. Kovalev, "Modeling of catalytic properties of coatings for high-temperature thermal protective materials in dissociated mixtures of carbon dioxide and nitrogen", Izv. Ross. Akad. Nauk. Mekhan.љ Zhidkosti З Gaza, I: 106-116, 2000.

4.љ N.E. Afonina, V.G. Gromov, and V.L. Kovalev, "Study on the effect of different mechanisms of heterogeneous recombination on heat fluxes directed toward a catalytic surface in dissociated carbon dioxide", Izv. Ross. Akad. Nauk. Mekhan. Zhidkosti З Gaza, I: 132-140, 2002.

5.љљљљљљљљ N.E. Afonina and V.G. Gromov, "Thermochemical nonequilibrium computations for a MARS EXPRESS PROBE", in: 3d European Symposium on Aerothermodynamics for Space Vehicles, pp. 179-186, Noordwijk, 1998.