Читать книгу The Starship and the Canoe - Kenneth Brower - Страница 22

The idea of a bomb-propelled spaceship began with Stanislaw Ulam, the inventor of the hydrogen bomb. He and Cornelius Everett worked the notion out in a rough way at Los Alamos in 1955. It was taken over by Theodore Taylor, a former colleague of Ulam’s at Los Alamos. Taylor was a physicist who had spent much of his career designing bombs, and who wished that he hadn’t. His gift was for the concrete. He did with Ulam’s spaceship what he had done previously with a succession of bombs; he rendered an abstract notion practical.

“Ulam is very much a man of my own type,” says Freeman Dyson. “Basically a mathematician. He’s more like me than like Ted Taylor. Ulam and I never stick with anything very long. Ted does. Ted got hold of the idea and made it a lot better. Ted designed it, understood how to do it in detail. He could organize. He got the project going, as Ulam could never have done.”

It was Taylor who gave the project its name. “I just picked it out of the sky,” he says of Orion.

Taylor, like Dyson a former student of Hans Bethe’s, met with their old teacher in 1956 in San Diego at a conference on atomic energy. Also on hand was Edward Teller, Alvin Weinberg, Marshall Rosenbluth, and other nuclear heavyweights. The conference was organized by Frederic de Hoffmann, a former Los Alamos physicist who now headed General Atomic, a new division of General Dynamics Corporation. Secrecy had just been lifted from nuclear reactors, and de Hoffmann wanted a free discussion on what might be done with them. Edward Teller had one idea. He believed that what the world needed was a reactor so safe it was “not just foolproof, but Ph.D.-proof.” Taylor and Dyson liked the idea, and they joined the safe-reactor team. They found they worked well together. They collaborated in the design of a small nuclear reactor called TRIGA, its purpose the production of medical isotopes. That was how they spent their summer vacations. Then they went their separate ways, Freeman back to Princeton, Taylor to work at General Atomic.

In winter of 1957, Taylor called Dyson in Princeton and explained Ulam’s new idea. Taylor wanted to build a ship that would blast them into space with atomic bombs. This did not for a moment sound crazy to Freeman.

“It sounded good. It didn’t frighten me. The immediate reaction of everybody is that it will blow the ship to pieces. I wasn’t bothered by that. The thing made sense on a technical level. It sounded like what we’d all been waiting for. This was an alternative to chemical rocketry that could work.”

Freeman has little enthusiasm for chemical rockets. In a rocket, velocity is severely limited by the heat-tolerance of the engine alloys. In a chemical rocket, temperature limitations hold the velocity of the ejected gas to about four kilometers per second. In a nuclear rocket the limit is about eight kilometers per second. A nuclear-powered rocket remains an inviting idea, however, in that nuclear fuel is the most compact energy source known, with a million times the energy of any chemical fuel. A lot of high-velocity human thought has gone into nuclear engines and possible ways to circumvent their temperature limitations. “Gas-core” systems would cheat by insulating the engine with a gas that, upon being heated, becomes the propellant. “Nuclear-electric” systems would use a nuclear reactor to generate electromagnetic energy and produce a jet of ions—the plasma-drive that powers much of science fiction. Plasma-drive has yet to power a real engine, but it’s a promising idea, and someday Freeman would like to give some time to it. “There’s no problem in plasma drive, except in the energy source,” he says. “It’s one of the things I’d like to build—a nuclear-electric engine for a spaceship.” A plasma-drive engine would be sharply limited in its thrust. The ship would accelerate slowly, and thus would be most valuable in long-range, unmanned voyages.

Unmanned voyages do not particularly excite Freeman Dyson. The voyages that Ted Taylor was planning did excite him. Taylor’s manned spaceship would move its crew and equipment rapidly around the solar system. It presented fewer problems to be worked out than other systems presented, Freeman thought, and he believed these could be resolved in his lifetime. In spring of 1958 he took a leave of absence from the Institute for Advanced Studies, moved his family to California, and began work on Orion. In July 1958 he wrote this manifesto:

From my childhood it has been my conviction that men would reach the planets in my lifetime, and that I should help in the enterprise. If I try to rationalize this conviction, I suppose it rests on two beliefs, one scientific and one political.

1.There are more things in heaven and earth than are dreamed of in our present-day science. And we shall only find out what they are if we go out and look for them.

2.It is in the long run essential to the growth of any new and high civilization that small groups of men can escape from their neighbors and from their governments, to go and live as they please in the wilderness. A truly isolated, small, and creative society will never again be possible on this planet.

To these two articles of faith I have now to add a third.

3.We have for the first time imagined a way to use the huge stockpiles of our bombs for better purpose than for murdering people. My purpose, and my belief, is that the bombs which killed and maimed at Hiroshima and Nagasaki shall one day open the skies to man.

The Orion spaceship would escape temperature limitations by fleeing the heat. The time during which each bomb blast interacted with Orion’s pusher plate would be reduced to a millisecond or less. The explosions would transfer their momentum to the spacecraft—blow it away—before the heat could penetrate. Common sense has the spaceship blowing away, all right, but common sense is wrong. In explaining their idea to doubters, the Orion men used the coal-on-the-rug analogy. A hot coal pops from the fire onto the rug. If you convey it carefully between thumb and forefinger back to the fireplace, you scream. If you flick it into the fireplace, you get away free. The bombs would flick Orion through space. Aluminum and steel can withstand surface temperatures of more than 80,000° K for short periods, losing only a thin epithelium of metal to ablation. An external-combustion engine like Orion’s can operate at those temperatures, whereas the internal-combustion engines of rockets are limited to propellant temperatures of around 4,000° K.

Ablation in Orion could be stopped entirely, the Orion men discovered, if the pusher plate was greased between detonations.

The pusher plate would move in jerks every half second. The plate would be made of aluminum. It would be lens-shaped and very heavy, about a third of the weight of the vehicle. It would be connected to the ship by pneumatic shock-absorbers, which would even out the ride, leaving it lurchy but not unpleasant. Greased like a channel swimmer, Orion would frog-kick through the void.

The shock absorbers were crucial, clearly.

“Above the pusher plate,” explains Ted Taylor, “there was a set of flexible gas-filled doughnuts about three feet high, sort of like a stack of tires. Then came a set of aluminum cylinders about twenty feet high, filled with compressed nitrogen, and they worked like pistons. Those really smoothed out the shock. The peak acceleration of the ship proper was about three or four Gs, which is lower than what the people in Apollo got.

“We had two ways of running the shock absorbers. One was in what we called the ‘dissipative mode.’ There, the shock absorbers compress and then expand, reverberating dissipatively until they stop. That meant a bouncy ride—you get kicked up to about three or four Gs every second, then down again. We were willing to bet that everyone would get violently seasick.

“But there was another way of doing it, and this was what we finally settled on. It was to drive the shock absorbers in synchronism, the result of which was that the acceleration of the ship proper was steady, at about a G and a half or two Gs. That would have been quite comfortable. It took some careful timing and got a little bit tricky, but it seemed to be worth it.”

Orion would move so fast that few of the detonations would occur in the atmosphere. Somewhere out past the ionosphere, Orion would hang a right and head for Saturn. The atmospheric detonations would add an increment to the fallout from the current bomb testing, but not a big one. Orion’s saving grace was that the spaceship, unlike the testing, was at least going somewhere. The Orion men guessed that pure fusion bombs would be invented by the time they were ready to depart, so they didn’t worry much about sprinkling plutonium over the planet they were leaving behind.

They didn’t worry, either, about space travel’s small niceties. No one bothered to calculate how much shielding they would need from cosmic rays. Orion would have to be such a thick hunk of metal, what with atomic bombs going off regularly a hundred feet away, and gamma rays pounding its abdomen, that a few wandering cosmic rays would not be a problem. The Orion men did not waste time designing interior accommodations. Orion in its enormous power could haul such excesses of freight that no cleverness was necessary in planning staterooms and storage. The crew would not need to recycle their urine, for they could afford to carry hundreds of tons of water. They would simply vent their wastes into space. They would not have to squeeze bland meals from tubes, for they could carry whole sides of beef in Orion’s freezers.

Freeman became Orion’s chief theoretician, and he shared with Ted Taylor the responsibility for the overview. His special area, insofar as any of the Orion men had special areas, was the physics of the explosions. He and Taylor spent much of their time thinking about that. The shock wave alone was not enough to drive the ship, they knew. The bombs had to be packaged with some sort of propellant-material that would vaporize and strike the pusher plate.

“If the bomb explodes in all directions equally, you’ve wasted most of the propellant,” says Freeman. “To make it efficient it was important that all the debris go forward and backward. Half of it was supposed to hit the ship, and half was supposed to fly out backward. That’s the most efficient arrangement. To achieve it you have to design the bomb-propellant arrangement very carefully.

“For bigger ships, using existing stockpiles of weapons would have worked. Just put enough propellant around, and it didn’t matter that the charge was not shaped. That was characteristic of everything we did. It was always easier if you made the thing big enough.

“You can use anything you like as propellant. Water was clearly very good. That was another reason it was very important to go to a place with water. From Earth, the propellant most likely was paraffin wax. One thing that would not work was rock. That’s why the moon looked bad. In a way, it was easier to go on long trips to Mars and Saturn. Rocks would have increased the ablation problem. It wasn’t clear that rocks would vaporize. You didn’t want bits of rock punching holes in your pusher plate.

“I think we all had conventional ideas about where to go. The moon certainly was first. We wanted to know if there was water there. That’s important if you’re serious. I’ve been discouraged by the lack of water found. But there’s still a chance to find it. On the north or south pole, you might find ice in some dark cave.

“Second was Mars. We would look for the same thing—water. We wanted to go to the north pole of Mars. There it’s really certain that there’s ice. We would have built a permanent base on the north pole of Mars.

“Third was the rings of Saturn. Both for practical and for aesthetic reasons. We all wanted to have a look at those. We thought, then, that they were a fog of ice crystals. Radar now suggests big chunks of ice at least a few feet in size. We would have stopped and collected some. That’s one of the beauties of Orion—it can refuel. For each hundred pounds of bomb, you need nine hundred pounds of propellant, and ice will do fine.”

Dyson and Taylor planned to be on Mars by 1964, on Saturn by 1970.

Taylor wanted a few rocks from Mars on his mantelpiece. He hoped Orion would make Martian rocks so common on Earth that you could just leave them lying around. Freeman wanted to know why Saturn’s moon Iapetus was black on one side and white on the other.

Taylor and Dyson did most of the mental space traveling for the Orion team. Their cerebral voyages were peculiar: boyish in enthusiasm, but not in detail. They spent little time imagining themselves clunking around in weighted boots. They imagined instead the worlds they would see, and the phenomena. They went as disembodied intellects, or as wandering eyes.

Freeman was especially eager to visit the satellites of the outer planets. A lot of interesting real estate orbits out there. Jupiter’s moon Ganymede is larger than the planet Mercury, and three of Ganymede’s sisters are larger than our moon. The moons of Jupiter and Saturn, Freeman thought, would be good spots from which to observe those enormous worlds.

Powerful gravitational forces made landing on the big planets themselves difficult, but the same forces would be a help in landing on their satellites. Freeman explained this in a paper he wrote for General Atomic, GAMD-1012, “The Accessibility of the Outer Planets to a High-Thrust Nuclear Spaceship.” In it he calculated that an Orion ship, operating with an exhaust velocity of thirty kilometers per second, could make a round trip to the satellites of Jupiter in two years, and to the satellites of Saturn in three, with takeoff and landing at both ends. It would accomplish this by making use of gravity and of Orion’s remarkable capacity to decelerate quickly.

On a trip to Jupiter’s moon Callisto, for example, Orion would rumble off Earth on a course parallel to Earth’s orbital velocity, on a day when such a course put it into a hyperbolic orbit that would intercept Jupiter. The spaceship would expend a great load of bombs in the vicinity of Earth, then sail in silence on the long voyage to Jupiter. As Orion grazed Jupiter at sixty-seven kilometers per second, it would retrofire a salvo of bombs, decelerating at a rate of seven kilometers per second and allowing itself to be captured by the planet’s gravitational field. Because Jupiter’s radius is seventy-one thousand kilometers, only about one thousand seconds would be available for the maneuver. For a low-thrust spaceship like a nuclearelectric rocket, this is not nearly time enough. For Orion, it’s a piece of cake. Once captured by Jupiter, Orion has a free ride. Its elliptical orbit is chosen to bring the ship tangentially to Callisto’s orbit. A final velocity change would be necessary at Callisto, but a small one, for the gravity there is slight. With a last, polite clearing of its mighty throat, Orion would settle on Callisto’s surface.

Freeman was most interested in the inner satellites of the outer planets. The inner moons of Jupiter and Saturn would give the best views of those beautiful, frigid, poisonous spheres. Mimas, Saturn’s innermost moon, is only 115,000 miles from the planet’s surface. Seen from Mimas, Saturn would fill much of the sky. Orion’s crew could watch Saturn’s atmospheric bands rotate at their different speeds. They could study the thin tropic of shadow cast by Saturn’s rings. From one of Jupiter’s inner moons, they could watch that planet’s rapid spin, the centrifugal force bulging the equator and flattening the poles. They could watch the Jovian Red Spot change color from its salmon red to its pale green.

Freeman figured the velocity increments necessary for trips to various moons and he set down the results in tables. “The inner satellites,” he summarized, “which are much the most interesting for observing the planets, were substantially harder to reach than the outer satellites. Fortunately, the help available from refueling is most certain where it is most needed, namely at the inner satellites.”

Happy coincidence. If Orion could refuel, everything became easier. For propellant, Orion could use ice, ammonia, or hydrocarbons, and “these substances are certainly to be found on Mimas, which has density 0.52 and is probably composed mainly of ice or snow.” (The density of water is 1.) “The mass of Jupiter 5 is unknown, but it is likely to have a density and composition similar to those of Mimas. The big intermediate satellites, Callisto and Titan, have densities 1.7 and 2.1. For comparison, the earth’s moon, which is of comparable size and is made of rock, has density 3.3. It is, therefore, almost certain that Callisto and Titan have thick outer layers of ice which would be available as propellant. In addition, we could, if necessary, convert the methane in Titan’s atmosphere into propellant.”