system. The secondary reflector is fixed to the sandwich array, while the primary reflectors point at the sun continuously. Although the reflector area is large, the reflector mass is low since it is fabricated from 1/2 mil aluminized kapton. The concept as illustrated in Figure 2 has two microwave antennas, each of which transmits 1.2 GW of power to the utility interface on the ground. This concept is not practical with silicon solar cells because it is necessary to have concentrated sunlight (5.2 suns in this concept) on the solar cells to provide adequate power density to the transmitters. Either GaAs or multibandgap solar arrays are feasible under these conditions. The sandwich panel concept designed by Rockwell is shown in Figure 3. The solar array is bonded to a fiber honeycomb core. The back of the solar array serves as a ground plane for the power amplifier drive distribution system located midway through the honeycomb core. Another ground plane is located on the other side of the sandwich. This ground plane serves both the amplifier drive system and the transmitter. The honeycomb core assembly is attached to a truss structure that supports the antenna and power amplifiers. The power amplifiers are shown mounted at the front of the truss structure to beryllium oxide wafers that dissipate waste heat from the amplifiers. A single amplifier drives each dipole. The total mass per unit area of the sandwich is only 1.68 kg/m^ Magnetrons appear to have significant advantages compared to klystrons, including increased lifetime (up to 30 years compared to ten years for klystrons between replacements), a single rather than multiple operating voltages (thus eliminating dc/dc conversion), a simpler waste heat rejection system (use conduction/radiation of heat rather than heat pipes and radiators), and possible a higher dc/RF conversion efficiency (e.g., 90 percent compared to 85 percent). The resulting design concept is shown in Figure 4. Although the satellite appears very similar to the klystron reference concept, there are some significant differences. Power at the utility interface is 5.6 GW compared to 5.0 GW for the reference concept. The mass is only 26.7 million kg compared to 31.6 million kg for the reference concept. Table 1 compares some of the most significant characteristics of the above described concepts. As indicated, the magnetron concept has much lower specific mass compared to the reference concept, the solid-state end-mounted antenna concept has higher specific mass, and the solid-state sandwich concept, using multibandgap solar cells, has a specific mass that is similar to the reference concept using multibandgap solar cells. The multibandgap solar cells significantly reduce specific mass. They •are particularly effective on the sandwich concept in this regard. Laser power transmission concepts have not yet been defined to the level of detail of microwav< concepts. The Boeing Company is currently conducting initial studies of this concept. Three approaches are being considered: electric discharge, solar pumped, and free-electron lasers. Figure 2. Solid-State Sandwich Satellite Concept Figure 4. Magnetron-Antenna Satellite Concept
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