true in planetary swingbys. When a planet changes the direction of the space vehicle, the planet itself actually slows down or speeds up by what is truly an infinitesimal amount. Thus, any rotating structures which are used in the manner described have to be massive enough so that they are not terribly disturbed every time they boost or deboost a payload. If there is a net flux either out or in, then the momentum loss of the stations would have to be made up by rocket impulse. It is quite possible that by the time we understand the implications of all such systems, and at the same time have learned to build proper rockets, it may be that the momentum transfer and storage systems will only be of use if there is almost equal traffic to and from the body about which the stations orbit. Rockets, too, can benefit if there is both an outflow and inflow of mass. In the rocket case, there is no need to carry propellant all the way, but it can be stored partway and picked up on return. This is precisely what the Apollo Program did by using lunar orbit rendezvous to place the first men on the moon. Figure 10 shows various options for lunar and return rockets (14). Propellant caches are very useful and, of course, propellant supplies on other celestial bodies would make many things easy. Rockets are also free to make use of the natural consequences of the gravity field. Particularly if plane changes are required, the rocket frequently benefits by overshooting its final altitude so that conservation of angular momentum slows the vehicle and orbit changes require less velocity. Figure 11 (15) shows optimum maneuvers as a function at final altitude and plane change. Regions where transfers with two- impulse, three-impulse, or at infinity are optimum are shown. THE LARGE THROUGHPUT QUESTION Many people extolling skyhooks do so because of the belief that only such sys-
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