D. Structures and Materials 1. Satel 1i te General Characteristics - A major advantage of the SPS concept for energy is the minimal structural requirements for a very large satellite. Although this has been recognized from the onset (ref. 1), it is not always obvious in conceptual portrayals of the system where the structure may appear as a prominent feature. Studies to date indicate that the entire structural mass is generally less than ten percent and very likely on the order of five percent of the total system mass. The reason is the extremely low load environment of orbiting systems, particularly in geosynchronous orbit. This characteristic is somewhat alien to terrestrial engineering experience where structures can dominate mass and energy investment requirements. In light of the rather benign load environment, the prime structural function is one of providing adequate stiffness for attitude control and pointing. Feasibility assessments and system studies have focused on passive structures which meet overall system requirements as a result of an underlying philosophy that adequate, simple approaches will be cost effective. This approach is seemingly achievable, even for the stringent dimensional control tolerances of the MPTS (ref. 28). A possible exception is the structural joint between the MPTS and array which might best achieve dynamic isolation through a "smart" structure. The basic features of a representative SPS structure is one of minimum gauge material operating under low stress, tiered into a truss element of rather large dimensions and sized on the basis of an adequate margin for elastic buckling. The structural design and configuration should reflect the requirements of construction, system operation and the environment. Structural design approaches are evolving with ideas generated as a result of an improving understanding of the relatively novel requirements of the SPS. Loads - Earth orbit is fundamentally a balance of the body forces associated with gravitational attraction (which is inversely proportional to the square of the distance from the center of the earth) and centrifugal acceleration
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