The cost breakdown for Plan 1 in Figure 39 shows that only about 10% is applicable to the orbital assembly task, so that major cost reduction is possible if a more economical basic transportation mode to low earth orbit were developed. If, as forecasted, an unmanned fully reusable, fly back heavy lift launch vehicle (HLLV) with a 180,000 kg payload can be launched at a unit cost comparable to the Shuttle Orbiter, which has a 30,000 kg payload, then the transportation and total costs would approach 125 $/kg and 150 $/kg respectively. Assembly operations of course would still require a manned Shuttle. 6. RBCSIVING ANTENNA A review of options for the antenna design at the ground receiving site quickly confirmed that an array of solid state diode rectifier elements each combined with individual dipole antenna and suitable filter was the only choice combining both high efficiency and low cost. These and other significant factors are noted in a comparison of approaches in Figure 40. This integrated reception-collection-rectification antenna concept is termed a rectenna, and its technology is the farthest advanced of those related to efficient transmission of microwave power. The most recent evidence of this was the achievement of an 82% efficiency at an output power level of 32 kW in a demonstration at the Goldstone, California facility of the Jet Propulsion Laboratory [Raytheon Co., 1975]. This subsystem efficiency is very close to the individual element efficiency plotted as a function of frequency in Figure 41. Overall construction of the rectenna which covers an area of about 100 km was shown in Figure 1. The panels aro trited to normality with the incoming phase front, but the accuracy need not be great since the individual antenna elements have broad dipole gain patterns; and for the same reason the phase front can be distorted by the atmosphere or ionosphere without appreciably affecting efficiency. Figure 42 shows the detailed organization of the rectenna where the DC power is collected at each element in parallel. At the next level it is summed in a series connection to reach voltage levels at which efficient conversion or distribution can be made. The ground plane is open metal construction for low cost and low wind resistance. Sealing of the rectenna elements within a plastic tube is suggested as a means to achieve economical environmental protection. Principal concern as regards weather phenomena would be damage due to large hailstones and this must be considered in site selection. The rectenna’s large scale demands a very low cost, mass production approach to manufacture of the several billion elements and the supporting structure. Cost estimates for a 2.45 GHz operating frequency are:
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