Figure 10.4.5 Outline of Launch and Assembly Sequence. Basic Topology Trades There were a variety of basic platform topologies that were considered. A large planar array with gravity gradient stabilization; a conventional planar array in which the long dimension of the array flies parallel to the flight vector; and finally, die current choice is a large prismatic concept. Recent studies (SPS-2000) have explored the prismatic concept and it appears attractive because of the greater stiffness that results. However, the planar array that is currently explored in this section utilizes space station derived trusses combined with a tension wire concept which may result in sufficient stiffness. The choice for the current prismatic demonstrator concept is not necessarily due to the stiffness but to the flight stability which results The planar structure in LEO would be sensitive to gravity gradient perturbations and would therefore require an active control system or an extremely large boom to ensure passive stability control. Furthermore, the low time of visibility from the receiving rectenna to the transmitting antenna (less than 10 min for 1000 km) precludes, or at least significantly penalizes, the use of the delivered power at ground level or requires the use of huge movable solar arrays or antenna.. Power Collection: Photovoltaic vs. Solar Dynamic The solar dynamic concepts have certain advantages, in particular relating to increased efficiency which reduces the size of the overall collecting surface (that is, for a similar power output). In addition, the cost per unit area of the reflecting material is lightweight and much of the array does not have electrical subsystem connections. However, past studies have indicated that the radiator subsystem required to dissipate the thermal energy of any particular thermodynamic cycle is relatively massive, and perhaps requires an actively pumped coolant in order to adequately transport the thermal energy. In contrast, the steady development of lightweight and easily deployable photovoltaic arrays over the last decade has resulted in systems which are ready to be used for large scale space applications. For these reasons, the selection of a photovoltaic array for this class of demonstration platform seemed the most conservative and appropriate choice given the budget constraints associated with this project. This choice allows the sizing of the solar arrays and then the sizing of the spacecraft: • The total efficiency of the chain is the product of each element efficiency: solar array, power converter, antenna, and atmospheric path. The rectenna efficiency is taken into account at the ground level.
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