Figure 9 shows a cutaway view of the laser-to- electric converter as it would appear on the top of the lunar rover. Cooling fluid flows up the center tube, then is diverted across the converter, and finally out the holes of the lower channel and onto the thermal radiator. The active area diameter is approximately 0.3 m, and a 2-m diameter reflective dish surrounds the active area to refocus stray laser light onto the active area. Figure 10 shows some of the design curves used with this system. The 50-kW incident laser beam produces a power density of 42 W/cm2 (at a maximum range of 3,100 km). This power density, and other factors, fix the efficiency at approximately 45 percent and requires the device to be maintained at 320 K. Figure 11 shows the converter efficiency across the incident laser-beam profile. The efficiency is fairly constant across the beam profile produced at the maximum transmission range of 3,100 km. A heat-rejection system is needed to extract heat (27.5 kW) from the converter. Figure 12 is a block diagram of the thermal control system using Freon-12 coolant. Table IV shows the best- and worst-case scenarios. During lunar-day at high noon, the surface temperature is maximum at 370 K (radiator sink temperature), thus more power must be sent to the compressor (7 kW). At lunar night, the surface temperature is 120 K (radiator sink temperature), thus only 400 W of compressor power is required, the balance going to the rover load (22.1 kW). The mass of the laser-to-electric power system components is shown in table V. The mass of the photovoltaic device is very small. The major mass component is the thermal radiator with specific mass of 2.7 kg/m2. The total power system mass is 520 kg, which translates into a specific mass of 21 kg/kW, significantly less than onboard rover power systems studied by others. The safety issues associated with a 0.8-pm laser at 42 W/cm2 are eye damage, damage to rover vehicle, and spacesuit damage. The laser beam would be optically locked onto the photovoltaic array, and any movement off of the array would immediately shut down the beam. Thus, exposure times would be less than several seconds. Spacesuit heating would also be insignificant. To prevent eye damage, all visual windows on the rover or spacesuits would need to be reflectively coated for the 0.8-pm laser wavelength. V. Conclusion A conceptual design of a long-range lunar rover powered by a laser beam has been proposed. The laser system is an array of 5,926 individual laser diode amplifiers producing a total of 50 kW of laser power at 0.8 pm. The electrical input power is derived from a 100-kW electric SP-100 reactor prime power source. Three such laser transmitters would give full-time coverage to a 4000-km lunar rover mission, where laser transmission distances would be between 1,800 and 3,100 km. The mass of one laser transmitter system is approximately 5000 kg. The lunar rover would receive a maximum of 50 kW, converting 45 percent (22.5 kW) to electrical power for science, locomotion, heat rejection, and crew needs. The
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