When the SSPS is eclipsed by the Earth, the temperature of the blankets and mirrors will drop more rapidly than the supporting structure. The result is that a cold blanket or mirror will coexist with a warm structure. As the SSPS comes out of eclipse, the opposite condition exists. These differences in temperature cause differences in thermal distortions between the blankets and mirrors and their supporting structure. To compensate for the relative differences in distortions and also to keep the blankets and mirrors from wrinkling, the use of pretensioning devices between the blankets and mirrors and their supporting structure is anticipated. The magnitude of load that these devices supply and their locations are given in Reference 33. Input of Flight and Control System Loadings into Structural Model to Determine Internal Loads and Structural Deflections.— Unit forces were applied to the structural math model at the grid points that correspond to the various attitude control actuator locations. These control forces were balanced by inertia forces acting at the various mass points. Internal member loads and structural deflections were derived for these unit forces using the NASTRAN Static Analysis Solution given in Reference 28. Solutions for 100-kg control forces applied at actuators located at the outboard end of the mast are given in Figures 12 and 13. Assessment of Baseline Configuration for Internal Loads and Deflections. - The primary purpose of this portion of the study was to investigate whether a large-area, light-weight space structure (represented by the baseline SSPS configuration) could be controlled in space, and then to check the structural integrity of the spacecraft for the resulting actuator control forces. The results of the performance evaluation of the baseline system performed in Reference 30 has shown that the SSPS can be controlled to well within a ±1 deg pointing accuracy with end thrusters of only 4.5 kg (10 lbs). These thruster loads counterbalance the gravity-gradient disturbance torques discussed earlier. Internal member loads and structural deflections resulting from these forces are negligible. However, in addition to attitude control forces, additional external control forces are required for corrections due to orbital drift resulting from such disturbances as solar pressure. The magnitude of these control forces is dependent upon the system's duty cycle and corresponding propellant limitations. No attempt was made to define the recommended duty cycle for this mission. Instead, a structural analysis was performed [Reference 34] which indicated that, to prevent localized bending of the solar array structure from exceeding ±1 deg, the end thruster force must not exceed 303 kg (667 lbs). Since the orbital correction force, dependent upon its duty cycle, could exceed this value, the results of the structural analysis show that the system's duty cycle must be selected so that the control force does not exceed 303 kg. The end thruster force of 303 kg in the Z-direction is therefore the critical loading condition acting on the SSPS structure. The maximum internal load resulting from these control forces was calculated in Reference 34 to be 290 kg (640 lbs) limit. This load occurs in the carry-through structure surrounding the MW antenna. Using a structural model area of 1.74.cm2 (0.27 in.2 ) gives a maximum compressive stress of 167 kg/cm2 (2375 psi). Ignoring the local buckling allowables, to obtain a column allowable of 167 kg/cm2 for a member having a modulus of elasticity (E) of 0.64 x 106 kg/cm2 (9 x 106 psi), would require that the L/p of this column not exceed 194. This L/p value implies that a column to support this load could probably be constructed.
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