As the limit of curvature must be held to 1 arc-min or 0.0166°, a further analysis is required to determine probability of this worst case occurring. 3.3.3.4 Materials Three materials have been considered for the antenna structure: aluminum, graphite/epoxy and polyimide composites. Each material has its advantages and disadvantages. Aluminum offers low material cost and established processing, manufacturing and assembly techniques but suffers from relatively high coefficient of thermal expansion. Graphite/epoxy, an organic composite that Grumman is developing extensive experience with, exhibits the attractive properties of a low coefficient of thermal expansion and a high strength-to-weight ratio. On the debit side, the material cost of graphite/epoxy runs 25 to 200 times that of aluminum. Also on the debit side is the relatively low maximum recommended service temperature for graphite/epoxy (conservative designers limit graphite/epoxy to 450°K, i.e., 350°F, the same as aluminum). Based on the temperature predictions of Subsection 3.3.2, the use of either aluminum or graphite/epoxy for the columns near the center of the antenna is precluded (see Fig. 3.3-26); the use of these materials for the beam cap elements near the center is marginally satisfactory. Polyimide composites such as graphite/polyimide or Kevlar/polyimide offer relief to the temperature problem as they have maximum recommended service temperatures in the range 530 to 645°K (500 to 700°F). In addition to the higher allowable temperature, graphite/polyimide offers the same main advantages of a graphite/epoxy matrix, namely high strength-to-weight ratio and low coefficient of thermal expansion. Figure 3.3-41 shows these properties for various graphite composite systems. Little data exists for the strength characteristics of epoxy and polyimide composites at elevated temperatures. However, Fig. 3.3-42 sheds light on the performance of these materials when used as adhesives. The superiority of polyimide over epoxy at 533°K (500°F) is obvious. But 5 concern exists as to what the performance of polyimide will be after 30 years (2.6 x 10 hrs) of operation at 533 K. Figure 3.3-42 shows that after (4.6 years) the lap shear strength is only 65% of its value after 10 hours. Suitable tests and extrapolation procedures are required to resolve this concern. On the debit side, the material and processing costs for polyimides are considerable in comparison with epoxy. Futhermore, as Fig. 3.3-43 shows, polyimides have a high volatile content. More than likely the polyimides will be processed in space and, therefore, a suitable bleeder system must be provided to prevent contaminating items such as microwave converters and parts that have been thermally coated.
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