Monday, May 20, 2019

It's all in the timing: a tale of two detonations

Athens Double-Barrelled Cannon

You might think that this double-barrelled cannon sitting next to City Hall in Athens, Georgia could be prop in some sort of misguided Civil War comedy. And, if you did, you wouldn’t be far from right. According to a plaque nearby, the cannon, the only one of its kind, was the brainchild of a Mr. John Gilleland, a private in the “Mitchell Thunderbolts,” an elite “home guard” unit of business and professional men ineligible because of age or disability for service in the Confederate army. Here I use the the words “brainchild” and “elite” quite loosely.

The double-barrel design was intended to fire simultaneously two balls connected by a chain which would “mow down the enemy somewhat as a scythe cuts wheat.” It failed for lack of a means of firing both barrels at the exact same instant. Apparently, “the lack of precise simultaneity caused uneven explosion of the propelling charges, which snapped the chain and gave each ball an erratic and unpredictable trajectory.”

My first reaction to reading this story of this unusual weapon was, “well, duh!” I ran the numbers in my head, first estimating that the velocity of a ball leaving the muzzle of a cannon was on the order of a a thousand feet per second. This meant that if the two detonations occurred even a millisecond apart, the balls would be separated by a foot when exiting the mouth of the cannon, and that separation would increase by an additional foot for each additional millisecond difference in the timing of the detonations. It’s no wonder the chain broke

It’s hard to imagine how 1860s technology, utilizing sputtering fuses and hand-packed gun power, could achieve such precise simultaneous detonations. It’s also hard for me to imagine how the “elite” men of the Mitchell Thunderbolts failed to do the simple arithmetic required to determine the feasibility of their ambitious Yankee-killer.

Oddly enough, this misadventure in weapons development brought to mind a more recent development in military technology that faced a timing challenge far more daunting than the one that faced the Mitchell Thunderbolts. Fast forward only eighty years, and a genuinely elite international team of scientists and engineers found themselves engaged in the development of the first atomic bombs as part of the American Manhattan Project.

Most popular discussion of the technological hurdles faced by J. Robert Oppenheimer and his crew based at Los Alamos centers on the difficulties in creating the necessary amounts of enriched uranium and plutonium to serve as the cores of the fission bombs they wanted to build. But there were other formidable problems, including some having to do with then as yet uninvented electronics.

In particular, design for one of the first two bombs, dubbed Fatman, employed a spherical plutonium core and required that it be compressed by an implosion that would squeeze that core to the critical density needed to initiate the sustained chain reaction that would result in the desired nuclear detonation. To accomplish this, the small plutonium core was nestled in a spherical structure whose outer layer was formed of facets of very powerful chemical explosives. For this arrangement to work, these facets — or lenses as they were called — had themselves to be detonated within a microsecond of each other.

Failure to achieve this kind of “precise simultaneity,” to borrow words from the Athens double-barreled cannon plaque, would lead to an “uneven explosion of the propelling charges.” In the case of Fatman, this would mean that the plutonium core would not be compressed uniformly to the required density and, as a result, the necessary chain reaction would be muted or, perhaps, not occur at all.

As we know from history, whether for good or ill is a matter still hotly disputed in some quarters, the Manhattan Project team succeeded in achieving the exquisite timing required for an implosion bomb.The 40,000–80,000 deaths in 1945 resulting from the detonation of such a device above the city of Nagasaki, Japan on August 9 of that year serve as lasting reminder of their tragic success.

In closing, it is interesting to note that the Athens-area Confederates failed attempt at constructing a weapon of enhanced destruction resulted in a public “object of curiosity.” More telling, as it says on the plaque, the double-barrelled cannon “performed sturdy service for many years in celebrating political victories.” Somehow the cruel madness of the Jim Crow South saw this ill-conceived cannon as something to be proud of. Go figure.

I can only hope that one day our own more successful weapons of mass destruction will be stripped on their nuclear cores and their husks distributed to city halls far and wide as objects of curiosity where they can perform sturdy service celebrating the victory of those fighting for complete nuclear disarmament.

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