planets, and the geometry of the orbital system), meteoroid protection requirements, surface coating properties, material properties of the fins and tubes, properties of the heat transport fluid, transport rates, etc. Fortunately, radiator-design techniques are well-developed, and extensive computer codes exist for the design of any type system required. An interesting consideration, however, is to simplify the problem to get an idea of the area required, for example, for the Brayton cycle system, by deriving for geosynchronous orbit the "equivalent sink temperature," which is the apparent environment temperature for radiation "seen" by an isothermal plane of given surface coating properties in a given orbital attitude. This equivalent sink temperature is derived for two panel attitude configurations as shown in Figure IV-B-l-c-10. The first is for a panel whose normal vector is parallel to the sun vector. The normal for the second configuration is perpendicular to the sun vector. In both cases the radiator panel will always be facing away from direct solar incidence, and the panel will always be irrotational with respect to its own axis since the SPS reflectors are precisely oriented to the sun at all times. The equivalent sink temperature variations for these two configurations in geosynchronous orbit are shown in Figure IV-B-l-c-11, for (“Vs) (ratio of solar absorptivity to thermal emissivity for the surface coating) values from 0 to 1. A good radiator surface coating will have a low solar absorptivity with a high thermal emissivity. The Apollo radiator coating had an (*0 ) ratio of (0.18/0.92), or about 0.2. As the coating degrades in orbit, its solar absorptivity rises, making it a less effective barrier to sunlight reflected off the earth. The influence of solar absorptivity becomes less important as radiator operating temperature increases. Thus solar absorptivity for the Apollo radiator panels (operating temperature on the order of 100F) is much more critical than it would be for the Brayton cycle radiator, which would operate at 300 to 500°F. One early radiator concept for the Brayton system assumed an integrated reflector-radiator wherein the radiator panels were mounted on the back side of the reflector to reduce structural requirements. In this case the curves of configuration (a) are applicable in Figure IV-B-l-c-11. In this configuration, however, the radiator is very far from the conversion system, which causes increased pressure drop and pumping power problems. While these problems would be serious for a gas radiator system, they may be offset for a liquid system by the weight saving effected through integration of these two large-area system components. The maximum equivalent sink temperature for this configuration is approximately 220°R for (*76) = 1.0. This maximum sink temperature occurs when the SPS is in line with the earth-sun vector (0=0°). The curves of configuration (b) correspond to the present Boeing radiator system concept, and in this case the maximum equivalent sink temperature is approximately 190°R. This maximum occurs when the SPS is 60° removed from the earth-sun vector. The required radiator area for a radiator with a thermal emissivity of 0.85 and a varying solar absorptivity (from 0.25 for a new surface to 0.85 for a degraded surface) for radiator effectiveness values of 0.7
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