Figure 3. Thermal Load Implications Figure 4. Frame Thermal Deflections would be .8 m with an aluminum design of the same depth and with a coating of comparable a/e . This data, in conjunction with an optimized frame compression load stability analysis, resulted in the data presented in Figure 5. To maintain a surface flatness to within 18 cm without active figure control, an antenna of 850-m aperture radius will utilize a graphite composite tri-beam 70 m deep and have a total structure mass of 95,000 kg. The issue of selection of the primary structures material has received considerable attention since 1976. Graphite composite issues (compared to aluminum) pertinent to the hexagonal frame for the MPTS are: 1. Advantages—flatness maintained to required value of 18 cm without active control; eclipse thermal loads negligible (3 to 5%) without constant load devices; minimum weight; minimal thermal load variations during construction (simplification). 2. Disadvantages—higher reduced material unit cost; material suitability risk. The last advantage pertains to the markedly reduced construction built-in loads. It is recognized that these loads can be essentially eliminated in an aluminum structure that is statically determinate and therefore presents a viable design for a non-critical figure control structure such as a solar blanket support frame. The implications of static determinancy are: (1) no thermally-induced axial loads; (2) no preload (potential elimination of member length adjustments); (3) increased design confidence with reduced analysis effort; (4) reduced number of members to fabricate, handle, place, and attach; (5) weight, depending on loading regime; (6) simplification of qualification testing; (7) loss of redundancy (not expected to be significant); (8) construction fixtures required to accommodate temporarily unrigidized structural assemblies; and (9) not necessarily compatible with automated construction. The latter two items constitute prime disadvantages that have apparently deterred its use. Also, while a 100% statically determinate structure is not achievable with stable union designs, the achievable degree of determinancy is expected to be sufficient to realize the listed advantages. The results of modal analysis performed upon the MPTS hexagonal frame and overall solid state configuration are shown in Figure 6. The overall configuration minimum modal frequency of 0.00196 Hz permits the use of classical control techniques. Also shown is the significant separation between the frequency of the configuration and that of the geosynchronous orbit solar-imposed gradient so that essentially no magnification of solar-induced deflections occur in the absence of any damping. The affect of frequency separation with gravity gradient disturbances is comparable.
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