This leads directly to the thought that if lasers can be used to achieve a greater energy concentration in rocket exhaust streams, perhaps they can likewise be used to remove some of the restrictions on high velocity air breathing. If a laser-driven air breather could breathe air and get useful thrust the whole way to orbital velocity, then we would have an Earth launch vehicle which required no working fluid whatsoever. It would, furthermore, only use the energy supply from the orbiting laser station. A space transportation system which would use no Earth energy except that used to build the vehicles is a fascinating thought. It is not clear, regrettably, that this can be done. At high velocities, the vehicle must fly at extremely high altitudes in order to constrain the aerodynamic heating of the airframe surfaces to a reasonable value. This leads to an incoming airstream of extremely rarified gas. It may not be possible to get enough into the vehicle to create the necessary thrust. Previous work (3) indicated promise for an airbreather using hydrogen preheated by a nuclear reactor. Thus, adding laser preheated hydrogen at high velocities could be effective. Another hybrid vehicle can be envisioned which could be very attractive. It would be a laser-powered airbreather up to about half of orbital velocity after which it would switch to a laser-heated rocket, possibly using water as propellant. For such a vehicle, the working fluid supply would be only a small fraction of the initial weight and volume of the vehicle. The vehicle would be acceptably economical, and an environmental dream. In studies done a few years ago (4,5), laser power stations were sized both for propulsion stations and for transmitting solar power energy to Earth (as opposed to microwaves). These analyses covered a variety of different possible lasers including
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