The Last Theorem - Arthur Charles Clarke [34]
Statistics wasn’t quite as boring as Ranjit had feared it might be. It wasn’t much fun, either. Long before Ranjit entered the class, he had a pretty good understanding of the differences between mean, median, and mode and knew what a standard deviation was, and it didn’t take him long to learn how to draw any kind of histogram the teacher wanted. But the teacher surprisingly turned out to have a sense of humor, and when she wasn’t teaching the class what stem-and-leaf and other statistical plots were, she was almost—well, sometimes she was almost—as entertaining to listen to as Joris Vorhulst himself.
But on second thought, Ranjit told himself, no. That was going too far. She was a nice enough person, but she just didn’t have the material of Astronomy 101 to work with. Such as the space elevator and its wondrous ways.
Even Artsutanov’s lift wasn’t the only game in town. What (asked one of the students one day in astronomy class) about something like the Lofstrom loop? For that you didn’t have to start by putting some humongous satellite into orbit because the thing just sat on Earth’s surface, from which it flung your space capsules into orbit.
But there Dr. Vorhulst began to rein in the class’s speculation. “Friction,” he said succinctly. “Don’t forget friction. Remember what reentry did to a lot of the early spacecraft. If you used a Lofstrom loop, you’d need to accelerate your capsule to that seven miles a second of escape velocity that I was talking about the other day before you let go of it, and then the air friction would burn it right up.”
He paused, eyes roving over his class, expression as good-natured as always but with a faint twinkle that made Ranjit expect some sort of surprise was coming. “So,” the teacher said sociably, “have any of you junior-grade astronauts figured out what kind of rocket drive your ship is going to have yet?”
Ranjit hadn’t thought of anything beyond the usual fuel-plus-oxidizer. He kept his mouth shut, though, because he knew, from the fact that the professor had raised the question in the first place, that Dr. Vorhulst had something else in mind.
So did his seatmate, but he responded in a different way. His hand went right up. “You’re not talking about a chemical rocket, are you, Dr. Vorhulst? So, then, what do you think? Maybe a nuclear-powered one?”
“Good guess,” the teacher said, “but, no, I don’t think a nuclear-powered rocket would be your best bet, at least not the kind of nuclear power you mean. Oh, there are designs around for rockets that were driven by exploding atom bombs, one after another. We can talk about them if you like, but for getting from LEO to Mars I think there are two much better possibilities. Both of them are tailor-made to be used with some kind of space elevator to boost them into low earth orbit because they’re both way too feeble to lift anything off Earth’s surface and into space. One is the solar sail. The other is the electric rocket.”
Ten minutes later Dr. Vorhulst had given short and convincing reasons for avoiding nuclear explosions as rocket propellants—the need for heavy shielding to protect astronauts from the deadly radiation, and anyway who wants to shoot a few hundred atomic bombs off into space? Solar sails had a lot to recommend them, he conceded, but they were dreadfully slow and not very maneuverable. However, the electric rocket, while also pretty slow to accelerate, required no fuel storage and produced no undesired byproducts. Where did the electricity come from? Perhaps from an onboard nuclear power plant, Vorhulst conceded, but just as easily from solar power—that is, from solar power in space, where there were no nights or cloudy weather, so that the sun always shone. “And what do you do with that electricity? You use it to ionize some working fluid—a gas like xenon, for instance—and the gas fires itself out of your rocket nozzles at very high velocity, and off you go.”
He paused for a breath. “All right,” he admitted, “an electric rocket would not accelerate very fast.” But