Because of the large saving in attitude control propellant, all studies have adopted this attitude. The antenna joint can also be simplified in the POP attitude, requiring two axes of rotation (one continuous) instead of three in the solar orientation. The second gravity gradient effect arises from the continuous attitude change of the SECS relative to the earth (figure A-3). This torque is cyclic with a 12-hour period. Because continous rotation is required, a fixed attitude cannot be used as in the previous case. Consequently, the torque has been minimized in most studies to date by reducing the difference in moments of inertia about the two axes in the orbit plane. This can be done by increasing the length of the SECS (perpendicular to the orbit plane) and reducing the width. The moment of inertia difference can also be reduced or eliminated by departing from a flat solar energy collector. Several approaches have been proposed for both photovoltaic and thermal conversion, although the geometrical constraints of thermal systems make them less amenable to such treatment. Power distribution paths are generally longer in inertially "balanced" configurations, tending to reduce the total-mass-to-orbit advantage. Most, but not all, iner- tially balanced configurations appear to be more difficult to construct. Since speed and ease of construction are significant factors in the practicality of the SPS concept, most inertially balanced configurations have not survived evaluation. Since the gravity gradient torque is cyclic, momentum storage devices such as momentum wheels appear attractive. However, preliminary studies have indicated a cost disadvantage compared to a reaction control system unless the high specific impulse projected cannot be achieved. Satellite Sizing - For a minimum cost of electricity per kilowatt-hour, the output per antenna should be as large as possible, although studies indicate that the cost per kilowatt-hour increases only slightly for outputs substantially below maximum. Maximum output is constrained by two factors, the power density at the transmitting antenna (heat rejection) and the power density at the ionosphere (ionosphere disruption). The best current definitions of these limits
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