Differential expansion refers to different changes in size of connected structures. This can result from using materials with different coefficients of thermal expansion or from connecting structures that have different operating temperatures. In the SPS most of the structure is shielded from the sun by the solar cell panels. The face structure, however, which supports the solar cell panels receives direct sunlight and has an operating temperature which is estimated to be 100 degrees higher than the rest of the structure. As a result of this, when the SPS is eclipsed, the face contracts more than the back, causing the SPS to warp. Even though the differential contraction is small it is significant because it causes changes that are not axial with respect to the structural members. As the SPS is eclipsed this effect could cause permanent warpage or even failure in some structural components. There are two potentially useful passive devices for dealing with this problem, high emissivity coatings and sun shields. The back surface of the exposed beams could be coated with aluminum oxide to achieve a high emissivity. As the emissivity of aluminum oxide ranges from about 0.1 to 0.8 depending on the thickness and anodization process, an emissivity may be chosen such that the shielded and unshielded structure will have approximately the same operating temperature. This effectively minimizes the differential contraction (assuming a uniform cooling rate) during an eclipse. The maximum linear contraction, however, would not be significantly reduced. If it proves too high then the additional step of adding sun shields to the exposed beams may be warranted. This would reduce the maximum temperature of these beams considerably. As the beams shielded by the solar cells would now be the 'hot* portion of the structure, it would be necessary to anodize them to return the system to a near uniform (but now lower) operating temperature. It is believed that by adding heat shields to all of the major structure and further adjusting the high emissivity coatings, an operating temperature near the minimum (or eclipsed) temperature may be achieved, at which point all size changes would be very small. Nearly uniform temperatures will only exist during static operating conditions. Immediately after entering and exiting the eclipse this will not be the case and significant distortions will still occur. This is added incentive to decrease the maximum operating temperature as the coefficient of thermal expansion for aluminum decreases significantly as the absolute temperature decreases. All of these estimates were supported by computer models developed by SRA to simulate the thermal behavior of objects in space. These calculations were based on formulas for radiative heat transfer between bodies of various geometries.(11,p242-254) Though complex, these models provided only a low level approximation of an SPS. As such, they served to support the qualitative conclusions presented above but could not provide precise numerical results. Any attempt to adequately determine the behavior of an SPS during eclipse (and the thermal control required thereby) would necessitate the development of a much more sophisticated model. Such an effort was beyond the scope of this project.
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