The Starship and the Canoe. Kenneth Brower
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,