n. Laser Transmitter Orbital Mechanics In order to provide continuous power to the lunar traverse mission described above, it is necessary to have three laser power stations equally spaced in an orbit inclined 20° to the lunar equator. Figure 2 shows the power station orbit. The orbit altitude is 1,815 km, the minimum laser transmission distance, while the maximum transmission distance to the lunar horizon is 3,100 km. Such an orbit will provide ± 12° of coverage on each side of the orbit plane which allows beaming to the rover at any point on the specific traverse described above. The station orbital period is 5.3 hours. Each station will provide power to the rover for approximately 1.76 hours. The laser would require propulsion power to minimize orbit precession. With this orbital configuration, only one laser system is used at a time to beam power to the rover. The reserve power capacity would be used to power other missions at other lunar locations. The orbital power infrastructure is not tied to any specific mission but provides a flexible method of powering diverse missions. An additional advantage of this laser-transmitter-design concept is that the laser system could be assembled and completely checked out in low-Earth orbit before being transported to lunar orbit. HI. Laser Transmitter System The prime power source for this specific study was chosen to be the SP-100 nuclear reactor. This system is in the development stage and is an ideal source of electrical power to the laser transmitter chosen. The reactor provides 100 kW of electrical power at ± 200 VDC via a cable across a 20-m boom to the laser device. The reactor mass is approximately 4,000 kg, including the reactor shield. Since the reactor would remain in lunar orbit, there would be very little possibility of contamination of the lunar surface, and no vehicle for transportation to the surface is needed, thus saving mass and cost. The laser transmitter system would be located on the end of a 20-m boom opposite the reactor where the heat loading from the reactor is 0.14 W/cm2, the gamma dose is 5 x 105 rads, and the (7 full power years) fast neutron fluence is 4 x 1012 neutrons/cm2 [6]. There are a number of laser transmitter systems that could be used with the SP-100 reactor prime power source. For this system study, we have chosen the laser diode array concept because of its rapidly maturing technology base which has produced single diode lasers with approximately 50 percent electric-to-laser conversion efficiency [7]. They are also low mass devicesthat can be mass-produced. The laser wavelength is typically 0.8 /zm, which is a reasonably good wavelength to minimize the size of the laser transmission optics, and solid-state photovoltaic converters on the rover can readily convert the beam into electricity. Figure 3 shows a physical diagram of the laser transmitter. The 20-m truss connects the laser to the SP-100 reactor power source. A heat shield reflects heat from the SP-100 away from the laser diode array. The array, surrounded by a 126
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