A Tall Tail

 

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This is true, I swear:

I was in Orlando in October, staying in one of those big, bland conference hotels. DARPA, the Pentagon department tasked with nurturing Mad Science in all its most speculative forms, had decided to throw a brainstorming conference on the 100 Year Starship—a mind-meld to try and figure out what research they'd have to conduct in order to have a hope of beginning to build a starship some time in the twenty-second century. And for no reason I clearly understood, they decided to fly in a bunch of SF authors from all over the world. I'm not sure
why
the Pentagon might want a starship, but I was glad someone was paying for me to go to Orlando and kibitz on their conference, and I was happy to bloviate about such things from a hard SF point of view.

The 100YSS conference exceeded all my expectations—and everyone else's. But the sheer amount of information on tap made the experience feel a bit like trying to drink from a fire hose. It turns out there's a lot we don't know about how to build a starship, but also a lot that we
do
know, and this was the mother of all networking opportunities for folks with an interest in the field.

Like all networking sessions, a lot of the interesting stuff happens among small groups by the poolside bar, or over a dinner table in a nearby restaurant. You get talking to some interesting-sounding folks who ply you with beer, and the next thing you know you discover you've been drafted into some kind of DARPA-funded think tank, or wake up with a hangover in a North Korean labour camp, doomed to spend the next two years coaching the Great Leader's son through writing the Nobel Prize–winning SF novel that daddy expects him to produce.

Luckily that's never happened to me, but I
have
had an eye-opening experience or two. Like the chat I had on Sunday evening by the swimming pool.

The Orlando conference center where they held the 100 Year Starship event had a resort-grade outdoor water attraction. Not only were there hot tubs and a regular swimming pool and a water slide, this hotel had an artificial river about a quarter of a mile long, down which you could drift along on a truck tire, propelled by aquajets and drenched by fountains. On a muggy October evening, after a long day of listening to talks about liquid indium ion drives and aneutronic boron fusion reactors, this was exactly the right place to hang out if you wanted to bump into inebriated floating rocket scientists. Like Greg Benford's twin brother, Jim.

You probably know Greg Benford best as a physicist and hard SF writer, one of the “Killer B's” who dominated the field in the 1980s and who is still actively writing novels and research papers. But you might not be aware that Greg has an identical twin, or that his sibling Jim is a card-carrying rocket scientist. Back in 2011, Jim was doing impressive things with microwave sails, and Greg was talking up the applications of a lot of recently declassified Russian research into nuclear thermal rockets: “They built this underground test complex near Semipalatinsk, so that they could capture the exhaust. And then they ran their motor for nearly five hundred hours. That's about a thousand times longer than Project NERVA managed, in total—in one burn! It's safe, and it's reliable, and it's the best way of getting to the outer planets.” His tire spun slowly round as he drifted under one of the fountains, and because we were orbiting near opposite sides of the artificial river, our paths diverged. Intrigued, I paddled to catch up.

“Don't you think launching a nuclear reactor might be a bit problematic?” I asked, as I closed to within hailing distance. “I mean, the antinuclear protests when Cassini launched…”

Jim waved dismissively. “It's safe as houses,” he assured me. “You're looking for safety, right? Nuclear 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.”

“How 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.

“What 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!”

“Two 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.

“Yes.” 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…”

“Tell him about the SRBs first,” Jim suggested. He leaned forward expectantly; at a guess, he'd heard this before.

“All 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—”

“Hang 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 liquid fluorine is
not
your friend. Fluorine is the most active elemental oxidizing agent in the periodic table, and liquid fluorine makes liquid oxygen look inert and unreactive. It likes to oxidize things we don't usually think of as oxidizable, like water—which it reacts with to product hydrofluoric acid, which is in turn nasty enough that sane people avoid working with it because if you forget for one second what you're dealing with it's liable to dissolve your bones.

Back in the nineteenth century, chemists used to joke that you could tell who had just discovered elemental fluorine by reading the obituary columns. But liquid fluorine and hydrofluoric acid are themselves not the worst oxidizing agents out there. Elemental fluorine may be the thuggish hit-man of the halide world, but if you torment it with chloride ions you can turn it into the chemical equivalent of Hannibal Lecter: chlorine trifluoride, an oxidizing agent so malignant that it will set fire to water and burn explosively on contact with sand, asbestos, or rocket scientists.

Jim Benford smiled. “Now why don't you tell him about the proposal for NAIL SPIKE?”

“What?” Leonard looked confused for a moment. “I thought he'd be more interested in D-SLAM—”

“Wasn't that Project PLUTO?” I asked. I'm enough of a crazy cold war projects geek to have heard of the atomic powered cruise missile—a device that only Dr. Strangelove could have loved—from the 1960s.

“Yes, D-SLAM was PLUTO,” said Jim. “But it's not that interesting, unless you want a power source for a drone that can explore the atmosphere of Jupiter for years at a time. NAIL SPIKE, on the other hand—”

“NAIL SPIKE was a CIA project.” Leonard polished off his cocktail and put the glass down on the bar, just loudly enough to get my attention. He caught my eye. I sighed, and waved at the bartender.

“Are you supposed to be talking to me about it, then?” I asked. “Me being a foreigner, remember?” Not that I wasn't curious, but I wasn't particularly enthusiastic about unintentionally having my stay in the US extended by a few years due to someone else's loose lips.

“Oh, it's old hat.” Jim waved my question away. “We're talking about 1970s projects here. It was declassified in the 90s, after the end of the cold war.”

“The
CIA
were into rocketry?” I couldn't help myself.

“After a fashion.” Our new round of drinks arrived. Leonard took a cautious sip of his. “Who was that writer friend of yours, Jim, from California…?”

“Larry, Larry Niven. ‘Any reaction motor is a weapon of efficiency proportional to its efficiency as a rocket.' That's what you were thinking of?”

“Yes, that's the one. He was spot-on, you know. NAIL SPIKE was about building a
really efficient
rocket motor and then marketing it to the opposition.”

“Wait, what?” (I can't easily describe the experience of involuntary nasal irrigation with a lime margarita, so I won't bother. Just try not to do it.)

When I stopped convulsing Leonard continued: "The idea is quite simple. During the 1940s and 1950s we experimented with a number of really quite unpleasant substances before settling in the 1960s on a handful of slightly less unpleasant stuff as our propellants of choice—liquid oxygen, liquid hydrogen, kerosene, and solids based on powdered aluminum. Of course, there were exceptions; Titan ran on dinitrogen tetroxide and a hydrazine/UDMH mixture, for example. Hydrazine isn't just explosive, it's corrosive and poisonous too. There's a reason the EPA-bullied wimps at NASA won't use it for launch vehicles these days. Dinitrogen tetroxide is explosive and corrosive. But we more or less stopped using really nasty stuff like red fuming nitric acid—we leave that to the Russians these days.

"But during the 1960s some bright spark at ARPA got a bright idea and handed it to the CIA: why not pretend we were using some extremely high reactivity oxidizers and fuels in our latest missiles, and leak plans and blueprints to the bad guys' spies? Obviously this wouldn't play with the Soviets, but small fry like East Germany or North Korea or Iraq might fall for it. Worst case, it would send them on a wild goose chase; best case, they might
really
damage themselves trying to build and fly this stuff.

“So we brainstormed the most suicidal rocket motor we could come up with. And you wouldn't believe just how mad it was.”

I grabbed another mouthful of iced alcohol; being slightly numb seemed like a good idea under the circumstances. “So what
did
you come up with?”

“Well, there's the stuff we ruled out first. Leaking Project Orion—” the nuclear bomb–powered pulse-detonation space drive “—was a nonstarter; the test-ban treaty put the kibosh on that. The PLUTO nuclear ramjet likewise wasn't an option. We had to stick to chemical rockets. But it turns out there are chemical rockets as nasty as anything nuclear. Nastier, even.

“First, there's an oxidizing agent that's even nastier than chlorine trifluoride.” Dr Hansen grinned. “It's called FOOF, dioxygen difluoride. You make it by reacting liquid fluorine and liquid oxygen in a cryogenic steel reaction vessel under X-ray bombardment. I say ‘you' make it because I'm not stupid enough to go anywhere near the stuff myself. I hear they cancel your life insurance if they catch wind that you're working with it. FOOF is unstable and tends to explode if you let it get much warmer than the boiling point of liquid nitrogen, or if you look at it funny. It's mostly used in producing uranium hexafluoride from—” he coughed. “Well, anyway, it's a
great
oxidizing agent!”