will be needed or relay mirrors, perhaps as far as synchronous orbit, will have to be used. Initial operations may require the scheduling of launches only when the power system is available. Figure 5 shows a constellation of two laser-power satellites combined with four relay mirrors which would support continuous launch operations. The transmitting and receiving mirrors are separated at most by only 18,000 km, and modest mirror sizes not exceeding 10 m will ensure little loss of transmitted energy. The mirror design problem is more likely to be in the handling of high flux levels than in size per se. It is not clear, once we have a laser power station system, what the exhaust velocity of the rocket should be under the nontrivial assumption that the beam, after arriving at the rocket, can be concentrated to any desirable degree and yield any exhaust velocity. Figure 3 shows that an exhaust velocity roughly equal to the mission velocity will result in the least expenditure of energy. This is not necessarily the pertinent criteria, however, for orbital power stations will be heavy, and it is likely to be more important to minimize orbital weight, at least in the early years. Figure 6 shows the power required per unit payload as a function of exhaust velocity for the orbital transport. The power in the exhaust was calculated simply from 'h the product of thrust and velocity, and the various weight ratios come from Fig. 1. Internal efficiency was assumed as 50% and the thrust/weight ratio was taken as 1.25 times launch weight. The minimum power occurs at a specific impulse of about 1200 s which is a ratio of exhaust to mission velocity of 1.2. The power is high. A megawatt must be delivered to the transport for every kilogram taken to orbit.
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