alloy at the cathodes. The two products, separated by the weak lunar gravity, would be removed continuously from the top and bottom of the cells. Periodically, when the oxides are exhausted, calcium and magnesium compounds accumulating in the electrolyte would be removed by additional electrolysis at higher voltages. The remaining cryolite would be recirculated as the electrolyte for continuing operations. The near vacuum on the Lunar surface, absence of oxidizing gases and widely differing vapor pressures of the four elements in the alloy product make it feasible to separate the elements by fractional distillation. Aluminum and silicon in purified form, along with oxygen and other products, are shipped from the moon to lunar orbit by means of a mass driver. Because the Moon's gravity is weak (one sixth that of the Earth), it is practical to literally fire containers and blocks of material as projectiles into lunar orbit. The mass driver operates on the principle of the linear induction motor, providing constant acceleration to a projectile by electromagnetic induction. Superconducting magnets in a "truck" that travels on the mass driver track suspend the truck and its payload above the track without friction. Magnets in the track, turned on in sequence as the truck passes over them, impart the acceleration needed to bring the truck and its payload to escape velocity for the moon - 2400 meters per second. When escape velocity has been reached, the truck slows, continuing to follow the moon's surface curvature on the track, while the payload continues outward to lunar orbit. Freed of its payload, the truck circles back on the track for another load, returning energy to the system as it is braked electromagnetically. Lunar Orbit Factory The factory for producing components for the Solar Power Satellites is located in lunar orbit where it can receive raw materials from Moon Base and take advantage of microgravity conditions to process them. By carefully maintainingorbit in relation to the mass driver's orientation, it is possible to position the Lunar Orbit Factory so that the trajectory of projectiles is nearby and the relative speed of approaching projectiles is low. A mass "catcher" snags the projectiles of silicon, aluminum and other materials as they approach the Factory. Stretched within an opening approximately one square kilometer in area, is a target held by cables on reels on an oval track. The cables can be reeled in and out to enable the target to move into the path of a projectile and to move with it as it makes impact. Incoming projectiles from the mass driver are located by radar far enough from the mass catcher that the target can be positioned to center the catch. When a projectile is caught, projectile, target and reels roll around the mass catcher's track, slowing the projectile to a speed only slightly faster than that of the orbiting factory. At the end of the track, the target and reel system speed up and circle around to return to the front for another catch. The projectile, free of the mass catching system, continues into the factory's storage area.
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