TIN-ALLOY COATINGS FOR TEMPERATURE CONTROL OF SPACE VESSELS

The problem of combining the temperature control of a space vehicle with the mechanical and chemical stability of the surface is addressed. With the absorption of solar radiation and the emission of thermal radiation considered the static and dominant factors that determine the equilibrium temperature of a spherical object, a simple model is formulated. Realistic variations of the two material-dependent parameters, solar absorptance α and hemispherical emittance e, permit a large change in the equilibrium temperature, from less than -50 to more than +150 °C. It is pointed out that for a generalized gray surface, i.e., one made from a material whose reflectance–emittance has the same value within the visible and the thermal wavelength regions, the equilibrium temperature is ≈5 °C, independent of the numerical value of the reflectance. With the requirement for electrical conductivity also taken into account, TiN alloys are identified as candidate materials. Measurements and calculations of some of them indicate that this group may contain a material that fulfills all the requirements. The experimental reflectance spectrum is used in estimating the equilibrium temperature for different TiN alloys; one Ti–Al–N-alloy with a gradient content of Al is found to have the lowest equilibrium temperature, 66 °C.