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thermal, you launch the reactor fuel piecemeal in Soyuz or Dragon capsules with a man-rated launch escape system, then fuel the reactor once it’s in orbit. No, nuclear-thermal is fine. Not very efficient, but it’s not going to kill anyone. If you want efficient propulsion technology, you’ve got to look elsewhere. But unfortunately the best rocket tech we know of is far too dangerous to use.”
    â€œHow dangerous?”
    Jim winked at me. “Let me introduce you to Leonard,” he said as we drifted toward the poolside cocktail bar. “He’ll fill you in on it…”

    Leonard Hansen—not his real name—is a tanned seventy-something rocket scientist who spent the 1950s in California and New Mexico, as a graduate student researching rocket fuels under John D. Clark and then as a fuels scientist working on various missile programs. Today he lives in semiretirement in Florida, but he retains a keen interest in the field of rocket fuel design.
    â€œWhat you need to understand is that in order to go faster, you need to increase the exhaust velocity,” he explained. “You can do this by making it much hotter, or by using lighter exhaust particles. If you want to make it hotter, however, you need to pump more energy into it. So if you’re using chemical rockets, you need to use very energetic reagents—fuel and oxidizer.”
    He paused for a mouthful of lime margarita. “Take the space shuttle,” he said wistfully. “With just two tweaks, we could have put a hundred tons into its payload bay!”
    â€œTwo tweaks?” I asked doubtfully. A hundred-ton payload (in a vehicle already massing close to a hundred tons) would have put the shuttle in the same bracket as the Saturn V.
    â€œYes.” He smiled sourly. “They could have stretched it, given it a bigger thermal protection system as well—the Columbia disaster wouldn’t have happened. But they rejected my proposal. The first part, to upgrade the SRBs, would have been trivially easy! Although the alternate oxidizer for the space shuttle main engines would have presented certain handling difficulties, that much is true…”
    â€œTell him about the SRBs first,” Jim suggested. He leaned forward expectantly; at a guess, he’d heard this before.
    â€œAll right. First, the solid rocket boosters. Regular SRBs run on a mixture of ammonium perchlorate—the oxidizer—and finely powdered aluminum, suspended in a rubbery polymer that holds everything together and provides additional reaction mass. When they ignite you get aluminum oxide and ammonium chloride and lots of energy. But it’s not really enough! We could make them about twenty percent more efficient if we just replaced the aluminum with powdered beryllium. It’s a lighter atom and the redox reaction is more energetic—”
    â€œHang on!” I stared at him. “Beryllium is really poisonous. Wouldn’t that—”
    Leonard shook his head. “Nonsense.” A small smile. “You see, then there was my second proposal. If you replace the oxidizer in the space shuttle main engines with liquid fluorine, you could also get an extra twenty percent out of them. And I know what you’re going to say next: wouldn’t that give rise to an exhaust plume of extremely hot hydrofluoric acid? You’re absolutely right: it would! But hydrofluoric acid reacts with beryllium oxide to give you beryllium fluoride—which is almost inert in comparison—and hydrochloric acid, which is neither here nor there.” A shadow crossed his face. “It’s totally safe, compared to some of the other projects I’ve worked on. But NASA took one look at the environmental impact statement and, and…” His shoulders began to shake; whether with laughter or tears, I couldn’t tell.
    Now, I have a background in chemistry. And I think I should explain at this point that