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* Use 0.27 if the Sun is In the orbit plane. This is were albedo heat loads are the most significant

* Use 0.27 if the Sun is In the orbit plane. This is were albedo heat loads are the most significant

11.5.2 Thermal Control Components Surface Finishes

In spacecraft thermal designs, wavelength-dependent thermal control coatings are used for various purposes. Solar reflectors such as second-surface mirrors and white paints or silver- or aluminum-backed teflon are used to minimize absorbed solar energy, yet emit energy almost like an ideal blackbody. To minimize both the absorbed solar energy and infrared emission, polished metal such as aluminum foil or gold plating is used. On the interior of the vehicle, if it is desired to exchange energy with the compartment or other equipment, black paint is commonly used. Thus, the existing state of die art uses a rather wide variety of coatings which have wavelength dependent thermal properties. The problems of in-space stability, outgassing, and mechanical adhesion to the substrate have been resolved for most coatings. There are many fully qualified coatings, so development and qualification of a new coating for a new design is normally unnecessary.

The external surfaces of a spacecraft radiatively couple the spacecraft to space. Because these surfaces are also exposed to external sources of energy such as sunlight and Earth-emitted IR, their radiative properties must be selected to achieve an energy balance at the desired temperature between spacecraft internal dissipation, external sources of heat, and reradiation to space, as illustrated in Fig. 11-16.

(Steady state)

Fig. 11-16. Radiator Energy Balance (no «tonal blockage). Note that we must , select radiative properties of the spacecraft surface to achieve an energy balance among spacecraft internal dissipation, external heat sources and reradiation to space to obtain the desired temperature.

(Steady state)

Fig. 11-16. Radiator Energy Balance (no «tonal blockage). Note that we must , select radiative properties of the spacecraft surface to achieve an energy balance among spacecraft internal dissipation, external heat sources and reradiation to space to obtain the desired temperature.

The two primary surface properties of importance are the IR emissivity, e, and the solar absorptivity, a. Table 11-46 shows the range of properties available for some common surface finishes. Two or more coatings are sometimes combined in a checkerboard or striped pattern to obtain the desired combination of average absorptivity and emissivity if it can not be obtained in a single material.

As an example, the average temperature of a sphere at 1 AU from the Sun can be calculated as follows:

where a is the Stefan Boltzmann's constant, 5.670 51 x 10"8 W/m2 K4, A- is the projected area, A is the total area, and S = 1367 W/m2 (the solar constant). For a sphere, Ap/A = ni2/ 4m2=0.25. If the sphere is painted with Z93 white, ale=0.17 / 0.92 and T= -90 °C. For a sphere painted with Z306 black, ale = 0.92 / 0.89 and 7=8 °C.

Thermal control finishes are affected in orbit by charged particles, ultraviolet radiation, high vacuum, and the contaminant films that deposit out on almost all spacecraft surfaces. The general result of these processes is an increase in solar absorptivity with little or no effect on infrared emissivity. This is normally undesirable from a

TABLE 11-46. Properties of Common Finishes. The absorptivity and emlssivity of typical spacecraft finishes are shown here. Note that a combination of finishes can be made to create the desired absorptivity to emlssivity ratios.

Surface Finish

a (Beginning of Life)

e

Optical Solar Reflectors

Solar Panel Basics

Solar Panel Basics

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