Vectors (25 page)

Building the Stalk well away from Earth helps the problem of material supply. We certainly don't want to use Earth materials for construction, since getting them up there would be an enormous task. Fortunately, two of the promising substances that we found in the table of strong materials are graphite and silicon carbide. Coincidentally, two of the main categories of asteroid are termed the carbonaceous and the silicaceous types. They can be the source of our raw materials.

The way to build the Beanstalk is now apparent. We fly a smallish (a couple of kilometers in diameter) asteroid in from the Asteroid Belt and settle it at L-4. We build a solar power satellite or a fusion plant out there, too, to provide the energy that we need. Then we fabricate the Beanstalk, the whole thing: load-bearing cable, superconducting power cables, and drive train (more on these in a moment). And we fly it on down to Earth.

The final descent speed need not be high. We can use the inertia of the whole length of the Stalk to slow the arrival of its lower end.

The demand on the raw material resources of Earth in this whole operation will be minimal.

USING THE BEANSTALK.

A couple of paragraphs back, I threw in reference to superconducting power cables and drive train. These are the key to making the Beanstalk useful. Let us look in more detail at the whole structure of the Stalk.

We will have a load-bearing cable, perhaps a couple of meters across at the lower end, stretching up from the equator to out past geosynchronous altitude. It will be tethered at its lower end to prevent it from moving about around the Earth. It will be strong enough to support a load of millions of tons. What else do we need to do to make it useful?

First, we will strengthen the tether, to make sure that it can stand a pull of many millions of tons without coming loose from Earth. Next, we will go out to the far end of the cable, and hang a really big ballast weight there. The ballast weight pulls outwards, so that the whole cable is now under an added tension, balancing the pull of the ballast against the tether down on Earth.

We really need that tension.

Why? Well, suppose that we want to send a million tons of cargo up the Beanstalk. The first thing we will do is hang it on the cable near the ground tether. If the tension down near the lower end is a couple of million tons, when we hang the cargo on the cable we simply reduce the upward force on the tether from two million tons to one million tons. The cargo itself is providing some of the downward pull needed to balance the upward tug of the ballast at the far end. The whole system is still stable.

But if we had used a smaller ballast weight, enough to give us a pull at the tether of only half a million tons, we would be in trouble. If we hang a million tons of cargo on the cable, it will pull the ballast weight downwards. There is just not enough ballast to provide the required upward pull. We must provide an initial ballast weight that is sufficient to give a tension more than any weight that we will ever try and send up the cable.

There is another advantage to a massive ballast weight. We can use a shorter cable. We can hang a really big ballast at, say, a hundred thousand kilometers out, and it will not be necessary to have more cable beyond that point. The ballast weight provides the upward pull that balances the downward pull of the cable below geostationary height. We have to be a little careful here. A ballast that has a
mass
of ten million tons will not be enough to allow you to raise a
weight
of ten million tons up from Earth. The ballast will not pull outwards as hard as the weight pulls downwards, unless it is out at a distance where the net
outward
acceleration due to combined centrifugal and gravitational forces is one gee. This requires that the ballast be more than 1.8 million kilometers out from Earth—far past the Moon's distance of 400,000 kilometers.

We conclude from this that the ballast will be a massive one. This is no real problem. After all, even a modest sized asteroid, a kilometer across, will mass anything up to a billion tons.

Once we have a taut cable, suitably anchored, we need a power source for the activities on the Beanstalk. We put a solar power satellite or a fusion plant out at the far end and run cables all the way down, attaching them to the main loadbearing cable. Superconducting cables make sense, but we will have to be sure that they are suitably insulated—near-Earth space isn't
that
cold. But perhaps by the time we build the Stalk we will have superconductors that operate up to higher critical temperatures. The ones available now remain superconductors only up to about 23 degrees Kelvin.

There is a fringe benefit to running cables down the Beanstalk. We can carry down power from space without worrying about the effects of microwave radiation on Earth—which is a serious worry with present solar power satellite designs.

Once we have the power cables installed, we can build the drive train, again attaching it to the load cable for its support. The easiest system for a drive train is probably a linear synchronous motor. The principles and the practice for that are well-established, which means it will all be off-the-shelf fixtures—except that we will want fifty to a hundred thousand kilometers of drive ladder. But remember, all this construction work will be done before we fly the Beanstalk in for a landing, and the abundant raw materials of the asteroid at L-4 will still be available to us.

Assuming that we drive cars up and down the Stalk at the uniform speed of 300 kilometers an hour, the journey up to synchronous altitude will take five days. That's a lot slower than a rocket, but it will be a lot more restful—and look at some of the other advantages.

First, we will have a completely non-polluting system, one that uses no reaction mass at all. This may appear a detail, until you look at the effects of frequent rocket launches on the delicate balance of the upper atmosphere and ionosphere of Earth.

Second, we will have a potentially
energy-free
system. Any energy that you use in the drive train in taking a mass up to synchronous height can in principle be recovered by making returning masses provide energy to the drive train as they descend to Earth. Even allowing for inevitable friction and energy conversion losses, a remarkably efficient system will be possible.

In some ways, the Stalk offers something even better than an energy-free system. When a mass begins its ascent from the surface of the Earth, it is moving with the speed of a point on the Earth's equator—a thousand miles an hour. When it reaches synchronous height, it will be travelling at 6,600 miles an hour. And if, from that point on, you let it "fall outwards" to the end of the Stalk, it will be launched on its way with a speed of more than 33,000 miles an hour, relative to the Earth. That's enough to throw it clear out of the Solar System.

Where did all the energy come from to speed up the mass?

The natural first answer might be, from the drive train. That is not the case. The energy comes from the rotational energy of the Earth itself. When you send a mass up the Beanstalk, you slow the Earth in its rotation by an infinitesimal amount, and when you send something back down, you speed it up a little. We don't need to worry about the effects on the planet, though. You'd have to take an awful lot of mass up there before you could make an appreciable effect on the rotation rate of Earth. The total rotational energy of Earth amounts to only about one thousandth of the planet's gravitational self-energy, but that is still an incredibly big number. We can use the Beanstalk without worrying about the effects that it will have on the Earth.

The converse of this is much less obvious. What about the effects of the Earth on the Beanstalk? Will we have to be worried about weather, earthquakes, and other natural events?

Earthquakes sound nasty. We certainly want the tether to be secure. If it came loose the whole Beanstalk would shoot off out into space, following the ballast. However, it is quite easy to protect ourselves. We simply arrange that the tether be held down by a mass that is itself a part of the lower end of the Stalk. Then the tether is provided by the simple weight of the bottom of the Beanstalk, and that will be a stable situation as long as the force at that point remains "down"—which will certainly be true unless something were to blow the whole Earth apart; in which case, we might expect to have other things to worry about.

Weather should be no problem. The Stalk presents so small a cross-sectional area compared with its strength that no storm we can imagine would trouble it. The same is true for perturbations from the gravity of the Sun and the Moon. Proper design of the Stalk will avoid any resonance effects, in which the period of the forces on the structure might coincide with any of its natural vibration frequencies.

In fact, by far the biggest danger we can conceive of is a man-made one—sabotage. A bomb, exploding halfway up the Beanstalk, would create unimaginable havoc in both the upper and lower sections of the structure. That would be the thing against which all security measures would be designed.

WHEN CAN WE BUILD A BEANSTALK?

We need two things before we can go ahead with a Beanstalk construction project: a strong enough material, and an off-Earth source of supplies. Both of these ought to be available in the next fifty to one hundred years. The general superiority of Beanstalks to rockets is so great that I expect to see the prototype built by the year 2050.

I do not regard this estimate as very adventuresome. It is certainly less so than Orville Wright's statement, when in 1911 he startled the world by predicting that we would eventually have passenger air service between cities as much as a hundred miles apart.

Unless we blow ourselves up, bog down in the Prox-mire, or find some other way to begin the slide back to the technological Dark Ages, normal engineering progress will give us the tools that we need to build a Beanstalk, by the middle of the next century. The economic impetus to deploy those tools will be provided by a recognition of the value of the off-Earth energy and raw materials, and it will be with us long before then.

This discussion seems to me to be so much a part of an inevitable future that I feel obliged to speculate a little further, just to make the subject matter less pedestrian. Let us look further out.

Non-synchronous Beanstalks have already been proposed for the Earth. These are shorter Stalks, non-tethered, that move around the Earth in low orbits and dip their ends into Earth's atmosphere and back out again a few times a revolution. They are a delightful and new idea that was developed in detail in a 1977 paper by Hans Moravec. The logical next step is free-space Beanstalks. These are revolving about their own center of mass, and they can be used to provide momentum transfer to spacecraft. They thus form a handy way to move materials about the Solar System.

Look ahead now a few thousand years. Civilization has largely moved off Earth, into free-space colonies. There are many thousands of these, each self-sustaining and self-contained, constructed from materials available in the Asteroid Belt. Although they are self-supporting, travel among them will be common, for commerce and recreation. Naturally enough, this travel will be accomplished without the use of reaction mass, via an extensive system of free-space Beanstalks which provide the velocity increases and decreases needed to move travellers around from colony to colony. There will be hundreds of thousands of these in a spherical region centered on the Sun, and they will all be freely orbiting.

The whole civilization will be stable and organized, but there will be one continuing source of perturbation and danger. Certain singularities of the gravitational field exist, disturbing the movements of the colonists and their free transfer through the Solar System.

The singularities sweep their disorderly way around the Sun, upsetting the orbits of the colonies and the Beanstalks with their powerful gravity fields and presenting a real threat of capture to any who get too close to them.

It seems inevitable that, in some future Forum on one of the colonies, a speaker will one day arise to voice the will of the people. He will talk about the problem presented by the singularities, about the need to remove them. About the danger they offer, and about the inconvenience they cause. And finally, as a newly-arisen Cato he may mimic the words of his predecessor to pronounce judgment on one or more of those gravity singularities of the Solar System, the planets.

"Terra delenda est"—Earth must be destroyed!

BEANSTALK TIME—A FINAL NOTE.

Beanstalks, originally called skyhooks, are an idea of the 1960's whose time may at last have come. They are used as important elements of at least two novels published in 1979, Arthur Clarke's "The Fountains Of Paradise" and my own "The Web Between The Worlds." I suspect that they will become a standard element of most projected futures, as a rational alternative to the rocketry that has served sf writers so long and so well.

 
Afterword.

This was the first semi-technical account of Beanstalk construction (principles and design factors) to appear in print. I know it will not be the last one.

I wrote the article for two reasons. The first was to get the idea of the Beanstalk (or Skyhook, Orbital Tower, Space Elevator, Cosmic Funicular—all the same thing) into wider general circulation. As I mentioned in the Afterword to "Skystalk," most of that story had to be taken up with the description of the Stalk itself. Little space was left over to go far beyond the basic idea. Once the concept is as familiar to the reader as the rocket, though, we'll be able to go a lot further, to build on the idea and write stories that are much more speculative and intellectually demanding. Once you accept Beanstalks, a dozen things are possible on the interplanetary and even interstellar scale, and most of them have a good story in them. In another couple of years we'll be able to write those stories.

That was the other reason that I wrote this article. When black holes became an idea familiar to the general reader, is seemed as though every sf writer rushed off to his or her typewriter. The result was a great flood of black hole stories, many of them scientific gibberish. I would like to do my bit to stop the same thing happening to Beanstalks. No Beanstalks tethered at the North Pole, please, and no Beanstalks on Venus or Mercury unless you are willing to make them a million kilometers long.