variation of the skyhook, known as the star bridge (11), uses a totally different principle. It can be built from the ground up. It is an actively supported structure in which a magnetic accelerator at the Earth's surface shoots rings vertically within the structure. The deceleration of the rings plus their subsequent acceleration back down to the Earth creates reaction forces which support the structure. The rings are turned around and shot up again when they return to the Earth station. Skyhook systems work because they project a rigid spoke from the rotating Earth. As a vehicle climbs up the spoke, its lateral velocity increases with increasing distance from the Earth. This is precisely the opposite of what happens with a body in free flight. There, angular momentum is conserved and lateral velocity decreases inversely proportional to the distance from the Earth. The angular momentum relationship in the two systems is precisely opposite. One can also consider gravity gradient stabilized beams — not nearly as long or massive as the skyhook — which are installed in Earth orbit rather than being limited by the angular rotation of the Earth’s surface. By climbing up and down such beams, one could achieve subcircular or supercircular velocity. Such systems need not be rigid beams, but can be tethers (12). The tether, since it cannot support bending moments, must be extended and retracted in properly programed ways in order to achieve the desired orbital configurations. This amounts to redistributing the angular momentum between the two bodies such that the velocities are proportional to their radius from the Earth, as in the skyhook. If one then releases the upper body from the tether it will go into a higher orbit while the lower body will go into a lower orbit. This is a way of exchanging the linear momentum of two bodies by first redistributing their angular momentum and their distance from the gravitational center. It’s a rocket system in which the payload and the launch vehicle supply the reaction mass for each other's maneuver. It could have considerable utility in the future. Another version of the general class of device which stores and transfers momentum would be to use orbital carousels made of rotating beams or large disks. If a rotating beam is placed in circular orbit and its dimensions are long enough, then it is possible for space vehicles to rendezvous with its lower tip by arriving at a suborbital velocity which just matches that of the beam. There would not be much time to hook on, but the relative velocities would be zero. By releasing the space vehicle after the carousel has rotated through 180°, the vehicle will now have a super orbital velocity essentially equivalent to its previous suborbital velocity, and will go into a higher orbit. If at the apogee of that orbit another carousel were waiting, one could go to an even higher orbit. It has also been suggested (13) that a large rotating beam could be made to walk around the Earth's equator dipping down to the surface periodically. The relative sizes of tethers and carousels are shown in Fig. 9 as a function of the velocity increment each can impart to a payload. Carousels have much smaller dimensions than tethers, but are likely to be very much more massive. Both mass and dimensions of either are insignificant compared to skyhooks or star bridges. In any of these schemes, it makes a great deal of difference whether one is going only in one direction (either to or from the Earth) or whether there is an equal payload flux in both directions. One does not circumvent the laws of conservation of momentum. The skyhook only remains erect if the lateral forces for outgoing payloads are supplied by an equal flux of incoming payloads. The velocity increment given to a payload by a rotating beam will have to be canceled by a retro momentum input to the beam itself. If this is continued in the same direction for a large enough number of payloads, the beam will eventually deorbit or fly out into space depending upon whether you were coming or going from the Earth. Actually, the same thing is
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