¶Chemistry and rockets, rehashed. A recent video by Scott Manley about hypergolic fuels (which are both useful and generally terrifying) reminded us of an entry we wrote all the way back in Issue #3, reprised here, about “Ignition!: An Informal History of Liquid Rocket Propellants” by John D. Clark. It’s more than one would expect of a book from 1972 about rocket propellants—dense, but surprisingly hilarious. It is now back in print (probably due to Musk calling it “fun”), but you can still find a free PDF of the original. Writing about chlorine trifluoride—one of many terrifying substances people have proposed putting into rockets—Clark writes: “It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals–steel, copper, aluminium, etc.–because of the formation of a thin film of insoluble metal fluoride[...]. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.” Sort of related: using discrete metal powders as fuel.