figures IV-B-3-4 and 5. Detailed dynamic and thermal analyses have not been performed; however, the columns could withstand 15 times the anticipated static loads. All structural weights for the cable/column configuration were multiplied by two to allow for joints, cross bracing, fittings and our lack of experience with such a structure. To re-emphasize the importance of cost, it should be noted here that a less precise, heavier, but less sensitive column (lower L/p) may be desirable. This can only be ascertained in the detailed design stage. Truss Configuration: The "truss" configuration shown in figure IV-B-3-3 is sized for the same electrical power and microwave transmission parameters as the cable/column configuration. This planar truss is a three-dimensional structure composed of axial load carrying members. These members are arranged in repeating pyramidal modules as illustrated in figure IV-B-3-3. This structure does not "shadow" any of the solar cells or reflectors. Thus, the structure is continuous at full depth as required for dynamic stability in the "long" dimension (27.5 km) and at half depth beneath the solar cells. Although optical shadowing is not significant for the dimensions involved, a full depth structure is not required for dynamic stability in the transverse direction (5.2 km). The planar truss possesses hard points at the intersection of each member. Thus, a distributed mass (such as the solar cell blanket) is afforded a relatively short and direct load path. The redundant nature of the planar truss offers a failsafe structure in the event of individual member failure. To accommodate an occultation, however, thermal strain relief connectors would be required between the solar blanket or reflectors and the hard points. Progression in the technology of lightweight structures generally assumed that elastic buckling did not occur during the life cycle of a structure. The ultra light structure required for the truss type configurations of the SPS cannot be realized with conventional aerospace structural technology. This is understood in engineering terms by reference to the elastic buckling curves for columns, figure IV-B-3-4. The SPS truss structure may operate at compressive stresses near the buckling stress. Although elastic buckling might occur from unique loadings (e.g., construction, maintenance) the structure would return to an operational shape on removal of the unique loading. The deformation of a Venetian blind is an example of this type of structure. The truss structure in the Glaser concept is built up from Venetian blind type elements as suggested by the Grumman Aerospace Corporation (reference 4). It should be noted that the concept of allowing local elastic buckling is not possible for the cable/column configuration where buckling would be a failure. The planar truss module shown in figure IV-B-3-3 is built up from compression elements as given in figure IV-B-3-6. A Venetian blind type element was designed for an L/p of 200. This 25 cm long element has a cross-section formed from a parabola segment with a height of 0.3 cm and a 5 cm width. A graphite composite material of .127 mm (5 mil) thickness is suggested. Despite the seeming frailty of this Venetian blind type element, incorporation in a three-dimensional form will result in a more than adequate structure for the low anticipated loadings. It should be noted that based on the two examples calculated (figures IV-B-3-5 and 6) a rather long space truss or column can be obtained at roughly 1 kg/m mass per unit length. This truss member may be used with minor modifications in a number of configurations. The 16 m wide truss can be 1 km long with an L/p of 200. This provides a 1 km size pyramidal module for
RkJQdWJsaXNoZXIy MTU5NjU0Mg==