narrow beam which can be intercepted with a modest sized mirror on the rocket. Since the laser is virtually a point source, and certainly does not have the angular diameter of the sun, the beam can theoretically be concentrated inside the rocket chamber so intensely as to generate temperatures far beyond those achievable with chemical combustion. Such a rocket would not need to carry the heavy reactor or shielding of a gaseous-core nuclear rocket. Even though it must carry the weight penalty of the energy collecting system, it could easily turn out to be the best combination of high-exhaust velocity and high thrust/weight ratio that we have yet been able to envision. A large station could conceivably be used to drive orbital transport rockets (Fig. 4). Of course, it is first necessary to have the laser power stations already in position, wherever that may be. It is likely that some, if not all, power stations will be in space. This paper makes that assumption although a similar line of reasoning can plausibly be developed assuming ground-based lasers with additional relay mirrors. The power stations will be powered either by fission or fusion reactors. It is difficult politically, as previously stated, to envision them as fission power systems. If we ever solve the problems of controlled fusion, then perhaps we could have fusion reactors at the power stations. Until that happens, we will simply have to make due with the fusion reactors in the sun and use solar-pumped lasers. A logical location for such a solar-driven laser power station is in sun-synchronous orbit so that the station is continuously in sunlight. Such orbits are near polar retrograde orbits so that a single station is not available to support launches from the United States continuously. Either more than one station at suitably high altitudes
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