Thus, if a pure rocket of 30 metric tons payload (roughly equivalent to the shuttle) were driven to orbit by a laser power station, 30 GW would have to be delivered to the collecting mirrors on the transport. Structural improvements can lower this, at the most, by a factor of 3. One much investigated way of getting around the presumed defficiencies of rockets is to utilize air breathers. We are very comfortable with air breathing, which we have done for some time. Air breathing, unfortunately, suffers from the same kind of limitations as combustion rockets but, again, for different reasons. The thrust of an air breather is created by the difference between the momentum of the outgoing exhaust and that of the incoming airstream. In the engine, energy is added to this airstream. The adding of energy to an airstream works very well at low velocity. As the flight velocity gets higher, however, the incoming stream is already so energetic that not a great deal can be done to increase its temperature. The net result is that all such systems suffer a reduction of propulsion efficiency with increasing flight velocity. Usually, by the time one has reached 20% of orbital velocity, the alleged advantages of air breathers have pretty well disappeared. Even at those speeds, supersonic combustion systems are required, for if one slows the incoming air stream to subsonic speed, then all extra energy is being added to a very high temperature, stagnated gas and little can be achieved. To extend air breathing efficiently to higher speeds, then, requires the same higher concentration of energy per unit mass in the stream as is required for improved rockets.
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