required for orbit-to-orbit laser-driven rockets of different mission velocities is shown in Fig. 8. Minimum power occurs at a ratio of exhaust to mission velocity of 1.25. For many missions of interest, chemical rockets are very efficient. The lightsail requires approximately 26,000 times the power of the laser-heated rocket for the same thrust. The power required for lightsails with very small payloads can easily exceed the power requirements for heavy transport rockets. The utility of laser- driven lightsails versus laser-heated rockets remains to be seen. Another form of direct momentum transfer which has been discussed recently is the skyhook (9,10). One form of this is a beam built the entire distance from the surface of the Earth to geostationary orbit plus an even greater distance on the other side of geostationary orbit in order to balance the forces in the beam. Then one can presumably climb up the beam, admittedly using substantial energy, but at least not having to carry rocket working fluid. One gets off at geostationary orbit if that is the destination, or climbs even higher, eventually reaching a point where stepping off the beam results in an escape trajectory from Earth. One must watch one's step there. Such a structure is fabulously large and heavy. There is some doubt, furthermore, as to whether materials can be found strong enough for such construction. The skyhook is gravity stabilized since the portions of it below geostationary orbit are in suborbital velocities and pulling down while those beyond the geostationary point are in supercircular velocities and pulling up. Thus, tension forces in the beam must support the weight of the beam. Some estimates show that the beam could only be constructed of pure crystals, if even then. Beam construction must start at geostationary orbit, working both up and down at the same time. A large amount of heavy rocket transportation is required. A recent
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