With any of the composites, the question of outgassing naturally arises. Tests run at Grumman on graphite/epoxy laminates showed essentially no outgassing when the specimens were exposed to high vacuum at room temperature for 72 hours. Tests at elevated temperatures need to be run. Polyimides, not being free of volatiles as are epoxys, may present an outgassing problem. The vapors given off by a polyimide composite structure may inhibit proper operation of the microwave converters. Tests are required to establish the vapor pressure characteristics of candidate composites at elevated temperatures and then, if necessary, suitable coatings to minimize the outgassing must be found. Up to this point, any reference to temperature has always been along the lines of high temperature. The MPTS, however, will experience cold temperatures around the time of the two equinoxes each year. The structure will cool down to temperatures in the neighborhood of 75°K (-325°F). This cold temperature will not present any problem to aluminum which has been used to virtually 0°K. How the composites hold up is an unknown. Tests to establish their cold temperature performance and their response to temperature cycling (from 75 to say 600°K) are required. Structural members manufactured from composite materials will have a white thermal control paint applied to the side of the member that faces space while the opposite side which faces the hot antenna surface will have an aluminum foil bonded into it to provide a good heat reflector. Thus, the composite materials will not be directly subjected to ultraviolet radiation. However, tests to establish the ultraviolet degradation that the white paint and aluminum foil will undergo during the 30-year MPTS life are required. Figure 3.3-44 summarizes pertinent properties of the three materials: aluminum, graphite/epoxy and graphite/polyimide. At the risk of over simplification, the material to use for the antenna support structure should have the best available strength-to-weight ratio and be capable of operating at 600“K for 30 years. This statement can be made because material and processing costs should play a secondary role in material selection since transportation costs dominate the overall cost picture. Furtheremore, it can be assumed that any tendancy towards outgassing or ultraviolet degradation will be aptly prevented by application of suitable coatings. A low thermal expansion coefficient is desirable but should never play a dominant role in the material selection process since a mechanical adjustment device will most likely be utilized to remove manufacturing tolerances. This same device, properly controlled can remove deflections caused by differential thermal expansions. In conclusion, the polyimide matrices have much to offer but appropriate test data for a 30-year life are required on their low and high temperature performance as well as their vapor pressure characteristics.
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