SPS STRUCTURES AND CONTROL: A PERSPECTIVE Robert C. Ried NASA/JSC, Houston, Texas 77058 Current concepts for a Solar Power Satellite (SPS) are inherently large systems. In the relatively benign external load environment of space; however, the characteristics and design requirements for the structure and control systems are quite different from a terrestrial system. To provide a perspective on these systems, and to provide some background for the more comprehensive papers which follow, a rather simplistic but indicative analysis on a representative configuration has been developed. It should be emphasized that this approach addresses a particular concept only as a mechanism for providing insight. The first figure illustrates the representative configuration masses and dimensions in convenient approximate magnitudes. The largest magnitude external influences are illustrated in the second figure. There are, of course, many smaller external disturbances which must be considered in the design of a real system as well as control forces, forces between conductors, inertial and gravity loading and the most significant internal loading, isometric stress for stiffness. The lightest weight structure can be achieved through the use of the most efficient structural elements (axially loaded members). A cable or membrane represents the most efficient structural elements, however these elements are limited to tension. To take compression, a column or truss must be designed to be stable from buckling. For lightly loaded and long columns, the most efficient approach is to build a tier of smaller efficient elements as illustrated in Figure 3. Note that as the structure is tiered the mass characteristics approach a proportionality to material density and load and an inverse proportionality to Young's Modulus (E). In general, the structural mass can be reduced by configurations which use fewer but larger compression members. A significant point is that in spite of the large scale of SPS structures, the lightly loaded columns can be designed by minimum gauge material considerations. The prime design consideration for the SPS structure and control systems is dynamic stability. The classical approach for achieving a dynamically stable system is to employ a frequency separation as illustrated by the frequency hierarchy in Figure 4. The greatest magnitude disturbance is the gravity gradient torque which cycles only twice a day. A simple attitude control law approach, as illustrated in Figure 5, gives a control correction frequency which is proportional to the square root of the disturbance torque derived by the allowable angular momentum impulse deadband. The system dynamic frequencies, as governed by the structural stiffness and overal system mass, are dependent primarily on geometry as illustrated in Figure 6. The largest component of this SPS mass is the solar cell blankets which, to first order, behave as membranes with a frequency dependence as illustrated in Figure 6. Classical frequency separation is possible for the isolated major system components of the example configuration as illustrated in Figure 7. If the array and the antenna were isolated for this example configuration, the structure control interaction would be minimal. Since these components are connected by the rotary joint, the dynamics and control characterization requires a more in-depth analysis. The significance of structure control interaction and the significance of stiffness to the minimization of dynamic energy is illustrated in Figure 8 by the dimensionless plot of energy against time for a step function input to a simple beam. There do not appear to be any insoluble problems associated with the dynamics and control of an SPS but it is an area requiring more in-depth
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