Страница 66 из 67
We must tether the lower end of the beanstalk cable at the equator. As a fringe benefit of the system, if we send mass all the way to the end of the beanstalk, far beyond geostationary orbit, then we will also have a free launch system. A mass released from 100,000 kilometers out can be thrown to any part of the solar system. The energy for this is, incidentally, free. It is provided by the rotational energy of the Earth itself.
Using the beanstalk
A load-bearing cable is not a transportation system, any more than an isolated elevator cable is an elevator. To make the transportation system, several additional steps are needed. First, we strengthen the tether, down on Earth’s equator, so that it can support a pull of many thousands of tons without coming loose. Next we go out to the far end of the cable and hang a big ballast weight there. The ballast pulls outward, so the whole cable is under an added tension, balancing the pull of the ballast against the tether.
We are going to attach a superconducting drive train to the cable. This will employ linear synchronous motors to move payloads up and down the length of the beanstalk. These motors are well-established in both principles and practice, so we can use off-the-shelf fixtures — except that we will want about 100,000 kilometers of drive ladder, and will need appropriate construction facilities and abundant materials. Here we will find a use for an asteroid of different composition, one high in metallic ores.
The motors will drive cargo cars up and down the beanstalk. Passengers, too, if the traveler is willing to put up with a rather long journey. At a uniform travel speed of 300 kms an hour, a journey to synchronous orbit will take five days. Much slower than a rocket but a lot more restful, and with spectacular scenery, this trip may resemble a leisurely transatlantic crossing on one of the great ocean liners.
The added tension provided by the ballast is very important. Each time a payload is attached to the drive train, the upward force on the tether is reduced by the weight of the payload. However, provided that the payload weighs less than the outward pull of the ballast weight, the whole system is stable. If the payload weighed more than the ballast’s pull, we would be in trouble. The whole beanstalk would be dragged down towards the Earth.
There is another advantage to using a really massive ballast. It allows use of a shorter cable. If we hang a big ballast weight out at, say 80,000 kilometers, there is no need to extend cable beyond that point. Another modest-sized asteroid, say a kilometer across, will do nicely for ballast. It will mass up to a billion tons.
We have not mentioned the source of energy to power the whole system. That could be provided by a solar power satellite, but will more likely be a fusion plant, sitting on the beanstalk at a geostationary orbit location. Superconducting cables run the length of the beanstalk, and can if appropriate provide power to the ground as well as ru
Any engineering structure has vulnerabilities, and the beanstalk is no exception. It easily withstands the buffeting of winds, since its cross-sectional area is minute compared with its strength; and the perturbing forces introduced by the attraction of the Sun and Moon are not enough to cause trouble, provided that resonance effects on the structure are avoided in its design. Accidental severing of the cable by impact with an incoming meteorite would certainly be catastrophic, but again the small cross-section of the cable makes that a most unlikely event.
In fact, by far the most likely cause of danger is a man-made problem: sabotage. A bomb exploding halfway up the beanstalk would create unimaginable havoc in both the upper and lower sections of the structure. All security measures will be designed to prevent this.
Alternative forms of beanstalk
There are two pacing items that decide when we can we build a beanstalk: the availability of strong enough materials, and a substantial off-Earth manufacturing capability. However, the first of these applies only to the “basic beanstalk” used in this novel. We now consider some interesting alternatives which remove the need for super-strong materials. We will term these alternatives the rotating beanstalk and the dynamic beanstalk.
The rotating, or non-synchronous, beanstalk was suggested in 1977, by Hans Moravec. It is a shorter stalk, non-tethered, that moves around the Earth in low orbit and dips its ends into the Earth’s atmosphere and back a few times a revolution. The easiest way to visualize this rotating structure is to imagine that it rolls around the Earth’s equator, touching down like the spoke of a wheel, vertically, with no movement relative to the surface.
Payloads are attached to the ends of the stalk at the moment of closest approach to the ground. But you have to be quick. The end of the stalk comes in at about 1.4 gees, then whips up and away again at the same acceleration.
The great virtue of the rotating beanstalk is that it can be made with less strong materials, and it would be possible to construct one today using graphite whiskers in the main cable. The taper factor is about ten. There is, of course, no need to have such a rotating stalk in orbit around the Earth. It could be sitting in free space, and as such it would serve as a “momentum bank.” It can provide momentum to spacecraft and thus forms a handy method for transferring materials around the solar system.
The dynamic beanstalk, which I think of an “Indian Rope Trick” for reasons I will give later, is an even nicer concept than the rotating beanstalk. It is not clear who first had the idea. Marvin Minsky, Robert Forward, and John McCarthy all seem to have had a hand in it, and I think I did the first analysis of its stability.
The dynamic beanstalk works as follows.
Consider a continuous stream of objects, such as steel bullets, launched up the center of a long, evacuated vertical tube. Suppose that the initial speed of these bullets is very high, faster than Earth’s escape velocity. This could be arranged using an electromagnetic accelerator at and below ground level. Suppose also that the tube is surrounded by the coils of a linear induction motor, so that there is electromagnetic coupling between the motor’s coils and moving objects within the tube.
Now, as the bullets ascend they are slowed by gravity; however, they can be given additional slowing by electromagnetic coupling. When this is done, the rising bullets transfer upward momentum to the surrounding coils.
At the top of the long tube (it can be any length, but let us say that it runs to geosynchronous altitude) the bullets are slowed and brought to a halt. Then they are moved over to another evacuated tube, parallel to the first one, and allowed to drop down toward the surface. As they fall they are accelerated downward by another set of coils surrounding the tube. Again, the result is an upward transfer of momentum to the coils. At the bottom, the bullets are slowed, caught, given a large upward velocity, and moved back into the original tube to be fired upward again. We thus have a continuous stream of bullets, ascending and descending in a closed loop.
If we arrange the initial velocity and the rate of slowing of the bullets correctly, the upward force contributed by the bullets at any height can be made to match the total downward gravitational force at that height. The whole structure stands in dynamic equilibrium, and it has no need for any super-strong materials.