Origins of the Siphon: The Coal Planet

Both the Siphon and Idle Theory originated in one thought experiment in 1991. I had imagined that there existed, somewhere in the solar system, a small black planet, so small that its mass produced next to no perturbations in the motion of the major planets, and so dark that it could never be seen by any telescope. This was the coal planet. One day, by accident, the coal planet was discovered, and it was estimated that its hydrocarbon reserves were of the order of a thousand times those of the planet Earth. It was, potentially, a huge source of fuel, plastics, and a whole range of materials which were getting in very short back on Earth. The coal planet itself was uninhabitable. As well as coal, substantial reserves of water and metals, and a thin atmosphere partly made up of oxygen. Occasionally, when oxygen levels rose too high, the coal planet would catch fire, and burn furiously until planetary oxygen levels fell to extinguish it. It was during one of these brief incandescent periods that the coal planet was discovered. It was noticed that the coal planet rotated very rapidly, spinning on its axis once every ten minutes.

The problem was: how to ferry coal from the Coal planet to earth? Should there be a fleet of space cargo ships plying between the two, much as on terrestrial oceans? But cargo ships imply two-way trade, and the movement of material from the coal planet was in one direction.

An ingenious solution was proposed. A single rocket would be launched from Earth to the coal planet. The rocket would carry a tracked mechanical digger and transporter, which was able, using the metals available on the coal planet, to make copies of itself. When the rocket landed on the equator of the coal planet, the coal-fired steam digger lumbered off across the planet's surface, digging coal, and carrying it back to the point on the equator where it had landed. Gradually, as the diggers reproduced, there were first two, then four, then eight, sixteen, thirty-two of them. And eventually thousands of them, all chugging across the coal planet and bringing back a load of coal to deposit at the equator. This made for a growing mountain of coal. And the coal mountain eventually grew so high that, at the top, the centrifugal forces acting on the coal exceeded the gravitational pull of the planet, and it flew off into space.

The Siphon was a simplication of the coal mountain. A rigid pipe was extended vertically up from the equator, and filled with a slurry of coal. When the tube had extended far enough out from the equator, the net centrifugal forces acting outwards on the coal slurry exceeded the net gravitational forces pulling inwards, and the entire column of coal slurry rose of its own accord, as in a siphon, and was thrown out into space. Since the coal planet rotated on an axis perpendicular to the plane of the solar system, if the coal slurry was collected during each revolution at the top of the tube as it came oozing out, and was released at one particular moment, a 'day's output of coal would be sent, in one spinning mass, into an orbit that would intersect with the orbit of the Earth. The result would be that every ten minutes a considerable mass of coal slurry would be fired off in the direction of the Earth, and all that was needed was some means, on Earth, to capture this supply of packets of coal. The multiplying diggers on the coal planet just had to continuously feed in a continuous flow of coal slurry into the base of the siphon.

The idea for this siphon was sketched out in a letter to Howard Marks. Marks, who had studied Physics and the philosophy of science at Oxford, saw no difficulty with it. The discussion turned to what was happening with the reproducing (and mutating) diggers. For I had argued that, after a while, some of the diggers would forget their task of carrying coal to the siphon, and simply roam around the planet doing nothing. Since the hardworking diggers which kept supplying the siphon tended to wear out and 'die' more rapidly than the idle layabout diggers, the population of diggers ultimately came to consist entirely of the latter. Once it had ceased to be fed with coal, the siphon stopped working. (In some variants of this story, the siphon had never worked in the first place, because it was a physical impossibility, and the diggers had all perforce become idle layabouts.) The diggers evolved into parasites and predators. The coal planet diggers provided the basis of a variant theory of evolution - the Idle Theory of Evolution.

The idea for the siphon lay dormant for several years, until Andrew Gay, FRAS, * sketched out, over a couple of beers, how to write an orbital simulation model. Together, over a few weeks, we co-wrote the computer model. Andy tested it by finding the Lagrangian points of the Earth-Moon system. But Andy was sceptical about the siphon - he reckoned that the slurry would just trickle back down.

With a working model that could calculate the orbital paths of satellites around the Earth, I wondered if the model could be extended to deal with equatorial radial towers. The simplest route, it seemed to me, was to model satellites that were tied to each other by elastic cables, and connect one end to the surface of the earth. All that was needed, in addition to Newton's law of gravity and the laws of motion, was Hooke's law. Using a system of connected satellites, I was able to show that the results matched fairly well with the existing literature of orbital towers - which included Arthur C. Clarke's Fountains of Paradise.

But the proposed siphon was larger than both Clarke's and Pearson's orbital tower, and a different kind of beast. Experiments with releasing a high tower at the base, and feeding in new bodies into a chain of satellites, resulted in the collapse of the tower. It was only when the bodies in the chain of connected satellites were constrained laterally, as if held within a rigid tube, that the siphon began to work. In fact, it worked too well: it drew up new bodies at an ever-accelerating rate, and broke apart. It was only by applying brakes to the material entering the siphon that a steady flow of material could be generated.

I'm not an astrophysicist. I'm a layman who is interested in science, and understands some science. For me, building a computer simulation model of the siphon was more about trying to use science to explore something I didn't understand, rather than simply use it to prove again what was already known. The siphon, was new, and huge - 300,000 km high -. If it worked it offered a more-or-less free mass launch system. And it was fun - particularly when nobody else believed it would work - to try to find out whether it could work, whether my intuition was right or wrong. It was even fun to use differential calculus, rusted since school days, to figure out when two bodies would collide.

Scientists complain of lack of public interest in science. One reason for that may be that for too long science has been a one-way street, with scientists doing the talking and laymen doing the listening. But it should be a dialogue, not a monologue. Sometimes the layman should do the talking, and the scientists do the listening.

The orbital siphon is about a layman venturing into science, working out something for himself, rather than waiting for scientists to tell him. As a presentation, it probably wouldn't make it into a respectable journal. If the siphon hasn't been dreamt up already (and I've yet to find it), it'll most likely be a professional scientist who gets credited with the idea. At the time of writing, the siphon web page has taken about 200 hits from all round the world, often from academic institutions, computer departments, and the like. The idea is out there now.

Chris Davis. 27 July 1998.


The professional scientist showed up early in 2005. But he didn't take the credit. Instead Professor Colin McInnes of Strathclyde University made sure that I got it, by offering me co-authorship of a paper he was presenting at IAC2005 in Fukuoka, Japan, in October 2005.

I only met him the night before the presentation. And the presentation went well. The Space Elevator people will now begin to consider the siphon, I hope, entirely thanks to Colin McInnes drawing it to their attention.

I should be able to publish the paper here soon, given IAF permission.

Chris Davis. 24 October 2005