For different values of solar concentration ratio, the structural cross section geometry must change, since the primary structural members align with the reflectors and solar cell blanket. The geometrical variations indicated here are based upon maintaining a constant spacing between the upper longitudinal members. With increasing concentration ratio, the height of the structure, H, in Figure 7-9 increases, thus increasing structural moment of inertia and improving structural efficiency. However, with increasing concentration ratio, the total planform area must increase for a given power output to account for the decreasing solar cell efficiency that occurs as a result of increased temperature. It should not be concluded that options do not exist or cannot be developed that allow increased concentration ratio while retaining relatively high efficiency (Fig. 7-4, e). For the structural cross sectional geometries indicated previously, the change in first mode bending period was determined as a function of the structural member size, indicated by longeron cross sectional area. As can be seen in Figure 7-10, the structural member size increases with decreasing concentration ratio for given stiffness criteria. A longeron area of 0. 605 cm2 with an overall structural height of 214 m results in a first mode bending frequency of less than 25 min, which satisfies the 30 min upper limit. The basic structure, which was shown earlier for the photovoltaic power system, is a beam truss. This beam truss is triangular in cross section and consists of nine V-hat cross section longerons. These longerons are packaged into three small beams, nominally 1 m in height, located at the vertices of the truss beam. The truss beam longitudinal loads are carried through the V-hat section longerons which are sized at 0.038 cm thickness and are shaped as shown in Figure 7-6. These longerons are fabricated from 2219-T62 aluminum and fail by local crippling at a compression load of approximately 4000 N. This failure mode is assumed independent of length, and the value is plotted as a horizontal dashed line in Figure 7-11. The Euler buckling failure mode is length dependent as shown on the same plot. The two curves intersect at a point corresponding to a longeron unsupported length of approximately 1. 8 m. This point can be described as a design point where longeron failure could be expected to occur in crippling and/or buckling under a 4000 N compressive load. To increase the longeron unsupported length beyond 1.8 m would lower the load bearing capability of the longeron, according to the Euler buckling curve. A family of curves representing Euler buckling loads for the small triangular beams located at the vertices of the basic truss beam and consisting of three longerons is presented in Figure 7-12 for beam heights ranging from
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