about 4.1 percent. Since the sun's average position is in the orbit plane, the array should be oriented perpendicular to the orbit plane (POP). Lateral misalignment relative to the concentrator troughs results in losses much higher than the simple cosine relation of a flat array (figure IV-B-4-6). POP orientation of the array, therefore, requires that the concentrators also be POP rather than the more convenient diagonal mounting that was possible with solar orientation (figure IV-B-4-7). To maintain the same average power output, the solar array must be enlarged to compensate for the loss due to POP orientation. This penalty is 1.48 x 10® kg for solar cells, concentrators and supporting structure. An additional penalty arises from the considerably greater length of power bus required, as illustrated in figure IV-B-4-7; this is estimated at 2.07 x 10® kg. On the other hand, solar orientation with b/a = 2 would require an additional 300,000 kg of propellant per year (ISp = 98,000 m/s) compared toPOP orientation. Over a 30-year life, therefore, the POP orientation offers a substantial weight savings. The preceding discussion implicitly assumed that no other disturbances acted on the SPS. In reality, this will not be the case. Antenna tilt for non-zero rectenna latitudes will produce a gravity gradient torque on the antenna, non-un1form mass distribution can produce a radiation pressure torque, and random and second-order disturbances will have a virtually unpredictable effect. Accordingly, provision must be made to maintain the desired attitude within some deadband. Since the Y axis (minimum moment of inertia) is POP, the attitude is unstable. For the column/cable configuration, the average torque is 1.04 x 10®o N-m, where © is the departure of the Y axis from POP in degrees (© small). The thruster moment arm for this configuration is 7212 m about the Z axis and 14424 m about the X axis. The thrust required is then 144© N or 72© N for the Z and X axes, respectively. Assuming the same error about both axes and a specific impulse of 98,000 m/s, total propellant required is 70,000© kg per year, where © is the average error angle. Clearly, it is desirable to keep © as small as possible; a target of 0.2 degree appears reasonable and has been used for propellant budgeting purposes. In summary, the preferred attitude for the column/cable configuration is POP to eliminate long-term torque combined with a relatively slender configuration (long axis POP) to minimize short-term torque. Short-term torque will be counteracted by propulsion or counterweights, depending on the specific impulse that can be achieved.
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