• Weight • Electrical conductivity • Space longevity - Outgassing — Fatigue allowables — Radiation resistance • Producibility • MW transparency In the discussion of the baseline configuration, it was shown that in certain material selections (such as 6061 aluminum for the bus/structure and mica-glass ceramic for the carry-through structure surrounding the MW antenna), because of very particular requirements in these areas, very little variation can be permitted in the materials selected. Other areas in the configuration, such as the non-conductive stmts, did permit variation in material selection. In analyzing a structure for its elastic body characteristics, the only criterion from the above list to be considered is the material's modulus of elasticity. For the non-conductive struts, variations in their modulus of elasticity were inputted into the symmetrical dynamic model and the effects on the total structure's natural frequencies were plotted in Figures 14 and 15. As can be seen in the curves, when considering symmetrical modes, variations in the non-conductive struts' modulus of elasticity affect only torsional modes; even with these modes, increasing the value of the modulus over that used in the baseline configuration has practically no effect on the total structure frequencies. In further analyses, variations of the modulus should be inputted into the anti-symmetrical dynamic model. It can be estimated, however, that variations would probably again not have much effect on the bending modes. However, since torsion is more predominant with anti-symmetrical modes than with symmetrical, a greater change in frequencies can be anticipated than is shown in Figure 14. Identification of Areas Requiring Further Structure and Dynamic Analysis.— The following items are areas in which future analytical analysis is needed: a. Improved Method for Reduction of Number of Variables Used in Equations of Motion It is likely that the attitude-control jets will excite a number of high-frequency vibration modes as well as the lower-frequency and rigid-body modes. These motions, in combination, may result in waves which emanate from each jet and damp out as they proceed through the structure. To predict the dynamic behavior of the structure accurately, it appears that an unusually large number of modes will be required; thus, the computer time and storage requirements would be excessive. For these reasons a study is recommended to determine more effective dynamic-analysis methods for large flexible space structures. The improved techniques developed in this study provide increased confidence in the capability of the control system to achieve the required control.
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