The amplitude of 18.8° may be excessive for sensitive axes. The effectiveness of applying some control to reduce the amplitude of oscillations without disturbing the basic motion was tested. The control chosen is proportional to the gravity gradient torque but in the opposite direction. Figure 2.4-16 illustrates the effectiveness of this control. Consider the case where a = 1. Reducing the amplitude to 1/2 its natural value requires 80 percent of the propellant required to reduce the amplitude to zero. On the other hand, permitting small amplitude oscillations, say 1° saves only about 2 percent of the propellant. Hence, controlling the amplitude of the oscillation is not very effective. A vehicle configuration that can accept the natural amplitude of the oscillation is required to take advantage of this concept. 2.4.5 Alternative Vehicle Configurations Two alternative vehicle configurations are proposed. The first one, called Configuration A, is identical to the reference concept except for two features. It is canted to eliminate the gravity gradient torques about the POP axis. The thruster attitude control system is replaced with a three-axis momentum control system. The advantages of this configuration relative to the reference are summarized in Table 2.4-3. The total mass required to be placed in orbit over the lifetime of the vehicle can be reduced by an estimated 920,000 kg or 3.8 percent of the vehicle mass. Even more importantly, the complexity of the on-orbit system and its operational support are greatly reduced. The second one, called Configuration B, is a long narrow vehicle like the reference concept. However, the long axis is placed in the orbit plane. Gravity gradient torques are eliminated about the long axis by canting so that the apparent north-south motion of the sun can be tracked without large torques. Free oscillations are used to eliminate concern over the in-plane gravity gradient torques. The active control system consists of a 3-axis momentum control system. The microwave antenna is mounted on the side of the vehicle. Figure 2.4-17 illustrates the configuration. Configuration B has several advantages over the other configurations. Large torque and angular momentum requirements for the attitude control system are eliminated. The worst case misalignment with respect to the sun line of the solar array is reduced from 23 1/2° to 18.8° resulting in an illumination efficiency improvement of from 92 to 95 percent. There are no seasonal variations in power output (except for eclipsing). However, there is a daily variation of 5 percent. Since the microwave antenna pivot is moved away from the primary structure, it can be designed to the optimal size and configuration. Also, there is no microwave look-through of the structure if the antenna is located on the north side of the vehicle. Table 2.4-4 summarizes the advantages of Configuration B relative to the reference. The advantages of this concept are similar to those for Configuration A except that the improved efficiency permits a significant reduction in the size of the vehicle. Further, the required control system can be smaller since smaller torques and angular momentum must be generated.
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