Fourmilog: None Dare Call It Reason

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Monday, April 9, 2012

Reading List: Ignition!

Clark, John D. Ignition! New Brunswick, NJ: Rutgers University Press, 1972. ISBN 978-0-8135-0725-5.

This may be the funniest book about chemistry ever written. In the golden age of science fiction, one recurring theme was the search for a super “rocket fuel” (with “fuel” used to mean “propellant”) which would enable the exploits depicted in the stories. In the years between the end of World War II and the winding down of the great space enterprise with the conclusion of the Apollo project, a small band of researchers (no more than 200 in the U.S., of whom around fifty were lead scientists), many of whom had grown up reading golden age science fiction, found themselves tasked to make their boyhood dreams real—to discover exotic propellants which would allow rockets to accomplish missions envisioned not just by visionaries but also the hard headed military men who, for the most part, paid the bills.

Propulsion chemists are a rare and special breed. As Isaac Asimov (who worked with the author during World War II) writes in a short memoir at the start of the book:

Now, it is clear that anyone working with rocket fuels is outstandingly mad. I don't mean garden-variety crazy or merely raving lunatic. I mean a record-shattering exponent of far-out insanity.

There are, after all, some chemicals that explode shatteringly, some that flame ravenously, some that corrode hellishly, some that poison sneakily, and some that stink stenchily. As far as I know, though, only liquid rocket fuels have all these delightful properties combined into one delectable whole.

And yet amazingly, as head of propulsion research at the Naval Air Rocket Test Station and its successor organisation for seventeen years, the author not only managed to emerge with all of his limbs and digits intact, his laboratory never suffered a single time-lost mishap. This, despite routinely working with substances such as:

Chlorine trifluoride, ClF₃, or “CTF” as the engineers insist on calling it, is a colorless gas, a greenish liquid, or a white solid. … It is also quite probably the most vigorous fluorinating agent in existence—much more vigorous than fluorine itself. … 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, aluminum, etc.—because the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, 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. (p. 73)

And ClF₃ is pretty benign compared to some of the other dark corners of chemistry into which their research led them. There is extensive coverage of the quest for a high energy monopropellant, the discovery of which would greatly simplify the design of turbomachinery, injectors, and eliminate problems with differential thermal behaviour and mixture ratio over the operating range of an engine which used it. However, the author reminds us:

A monopropellant is a liquid which contains in itself both the fuel and the oxidizer…. But! Any intimate mixture of a fuel and an oxidizer is a potential explosive, and a molecule with one reducing (fuel) end and one oxidizing end, separated by a pair of firmly crossed fingers, is an invitation to disaster. (p. 10)

One gets an excellent sense of just how empirical all of this was. For example, in the quest for “exotic fuel” (which the author defines as “It's expensive, it's got boron in it, and it probably doesn't work.”), straightforward inorganic chemistry suggested that burning a borane with hydrazine, for example:

2B₅H₉ + 5N₂H₄ ⟶ 10BN + 19H₂

would be a storable propellant with a specific impulse (I_sp) of 326 seconds with a combustion chamber temperature of just 2000°K. But this reaction and the calculation of its performance assumes equilibrium conditions and, apart from a detonation (something else with which propulsion chemists are well acquainted), there are few environments as far from equilibrium as a rocket combustion chamber. In fact, when you try to fire these propellants in an engine, you discover the reaction products actually include elemental boron and ammonia, which result in disappointing performance. Check another one off the list.

Other promising propellants ran afoul of economic considerations and engineering constraints. The lithium, fluorine, and hydrogen tripropellant system has been measured (not theoretically calculated) to have a vacuum I_sp of an astonishing 542 seconds at a chamber pressure of only 500 psi and temperature of 2200°K. (By comparison, the space shuttle main engine has a vacuum I_sp of 452.3 sec. with a chamber pressure of 2994 psi and temperature of 3588°K; a nuclear thermal rocket would have an I_sp in the 850–1000 sec. range. Recall that the relationship between I_sp and mass ratio is exponential.) This level of engine performance makes a single stage to orbit vehicle not only feasible but relatively straightforward to engineer. Unfortunately, there is a catch or, to be precise, a list of catches. Lithium and fluorine are both relatively scarce and very expensive in the quantities which would be required to launch from the Earth's surface. They are also famously corrosive and toxic, and then you have to cope with designing an engine in which two of the propellants are cryogenic fluids and the third is a metal which is solid below 180°C. In the end, the performance (which is breathtaking for a chemical rocket) just isn't worth the aggravation.

In the final chapter, the author looks toward the future of liquid rocket propulsion and predicts, entirely correctly from a perspective four decades removed, that chemical propulsion was likely to continue to use the technologies upon which almost all rockets had settled by 1970: LOX/hydrocarbon for large first stages, LOX/LH₂ for upper stages, and N₂O₄/hydrazine for storable missiles and in-space propulsion. In the end economics won out over the potential performance gains to be had from the exotic (and often far too exciting) propellants the author and his colleagues devoted their careers to exploring. He concludes as follows.

There appears to be little left to do in liquid propellant chemistry, and very few important developments to be anticipated. In short, we propellant chemists have worked ourselves out of a job. The heroic age is over.

But it was great fun while it lasted. (p. 192)

Now if you've decided that you just have to read this book and innocently click on the title above to buy a copy, you may be at as much risk of a heart attack as those toiling in the author's laboratory. This book has been out of print for decades and is considered such a classic, both for its unique coverage of the golden age of liquid propellant research, comprehensive description of the many avenues explored and eventually abandoned, hands-on chemist-to-chemist presentation of the motivation for projects and the adventures in synthesising and working with these frisky molecules, not to mention the often laugh out loud writing, that used copies, when they are available, sell for hundreds of dollars. As I am writing these remarks, seven copies are offered at Amazon at prices ranging from US$300–595. Now, this is a superb book, but it isn't that good!

If, however, you type the author's name and the title of the book into an Internet search engine, you will probably quickly come across a PDF edition consisting of scanned pages of the original book. I'm not going to link to it here, both because I don't link to works which violate copyright as a matter of principle and since my linking to a copy of the PDF edition might increase its visibility and risk of being taken down. I am not one of those people who believes “information wants to be free”, but I also doubt John Clark would have wanted his unique memoir and invaluable reference to be priced entirely beyond the means of the vast majority of those who would enjoy and be enlightened by reading it. In the case of “orphaned works”, I believe the moral situation is ambiguous (consider: if you do spend a fortune for a used copy of an out of print book, none of the proceeds benefit the author or publisher in any way). You make the call.

Posted at 22:06