expense of overall complexity in system design and operations. As this concept evolves to its full growth potential, a number of equally spaced Sun Towers operating in LEO sun-synchronous orbit would continuously collect solar energy and intermittently deliver power to ground receiver stations or to equally spaced relay satellites emplaced in MEO (medium altitude equatorial orbit). The relay satellites would, in turn, process the received energy for subsequent power transmission to ground stations. The delivered power scale is in the range 10 -100 MW. Space-to-ground power transmission is at microwave frequencies, while the space-to-space link can be at mm wave frequencies if the relay satellite employs frequency down-conversion. Alternatively, the relay satellites may be configured as ReflectArrays to avoid the efficiency loss in frequency conversion and retransmission (but paying the mass penalty and expense of potentially much larger satellite transmitter and/or reflecting arrays), in which case the space-to-space and space-to-ground frequencies are the same. A schematic drawing of a LEO sun-synchronous Sun Tower is provided in Figure 3-6. This representation is similar to the Sun Tower that might be used in a relay context, but the transmitting array would not be oriented as shown, ie. for transmission to ground, but configured vertically so that its transmitting axis is pointing out of the paper in the same direction as the solar collectors. The other major space element of the LEO-MEO Relay architecture is the relay satellite, which in the Phase II study has been baselined as the ReflectArray. Figure 3-7 illustrates the basic technique behind the ReflectArray concept and provides several usefill technical references as well A full description of the ReflectArray is given below in the discussion of the LEO-MEO Relay Space Segment. Finally, a diagram portraying the architectural context of the LEO-MEO Relay concept is given in Figure 3-8. LEO-MEO Relay: Development & Manufacturing Both the Sun Tower and relay satellites are designed with extensive modularity so that relatively small individual components of the system can be developed and tested at a moderate cost. Efficient techniques for automated assembly in space must also be worked out during the development phase, and built in to the hardware design, packaging schemes, and operational procedures. System elements would be mass produced at low unit cost in customized manufacturing facilities. Ground Launch Infrastructure No concept-unique infrastructure is required beyond those facilities necessary to support extremely low launch costs (on the order of $200 per kg). However, because both near-polar and equatorial orbits are involved, separate WTR and ETR launch sites (or new site options) need to be considered. Earth-to-Orbit Transportation No concept-unique ETO transportation system is required beyond that necessary to achieve the low launch cost objective. Nominal payload delivery capability in the range 10 - 25 metric tons per launch is consistent with the Highly Reusable Space Transportation (HRST) system concepts currently being studied. Increased capability in the range 25 - 50 metric tons might be beneficial if low launch costs could be preserved.
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