Saturday, April 15, 2017

A Brief Moabite Digression

     It’s somewhat unfortunate that the U.S. had occasion to use its GBU-43 “MOAB” bomb just now, for a reason that few other than observant Jews will be familiar with: Moses, the greatest of the Old Testament prophets, died on a mountaintop in the land of Moab, overlooking the promised land – Israel, for those of you who don’t have a program – which God had forbidden him to enter. Yet we just dropped a MOAB on a mountain in Afghanistan, and in Passover season, at that. Well, military timing isn’t always convenient for the rest of us.

     Anyway, the Left’s carping about the bomb being somehow “inhumane” (alternately, “obscene”) left me briefly chuckling. How could a bomb be “humane?” Its purpose is to kill and destroy. This particular bomb was ideally suited to its target: a complex of caves and tunnels embedded in the Afghan mountains. The overpressure its nine tons of RDX explosive generated in that confined space would kill everyone in those caves more or less instantaneously. If you want an essentially painless way of dispatching a group of enemies who’ve concentrated underground to their eternal reward, I can’t think of a better one. A better non-nuclear one, at least.

     To me, the GBU-43 is an impressive device. Very few persons appreciate how difficult it is to build a chemical explosive that large and have it go off as a single event. There have been larger chemical bombs – the Allies mined a German emplacement with a 500 ton (1,000,000 pound) bomb during World War I – but the records don’t indicate whether it exploded singly or as a succession of smaller blasts. At any rate, it appears not to have mattered to the Germans.

     The problem lies in combustion front speed versus shockwave speed. A chemical explosive is really a material that burns very, very fast. The expansion of the combustion products – hot gases, mostly – creates a wave of overpressure called a shockwave. The overpressure created by the shockwave is isotropic. As it burns, the combustion front is partly retarded by the shockwave, which is simultaneously pushing the material around it outward. Thus, if the bomb is large enough, the combustion front that expands from the point of ignition will eventually reach a void: a gap between the combustion front and the unexploded part of the bomb. If the unexploded part has been pushed too far from the combustion front to ignite, combustion ceases at that point.

     Let it suffice to say that a really big chemical-explosive bomb must be very carefully designed and constructed. The explosive must be special; the ignition mechanism must be special; the container must be special; and the men who decide whether, when, and where to use it must be very special indeed.

     Yes, there are some analogous problems with fission and fusion bombs, but in those cases there are some offsetting advantages. The “combustion front” in a fission weapon is a wave of fast neutrons. In a fusion weapon, which requires a fission explosion to set off the fusion component, reflectors are used to redirect those neutrons into a compressive force that squeezes the fusion core to the pressure and temperature required to initiate a self-sustaining fusion reaction. Solving the problems involved took a considerable amount of brainpower from some really heavy dudes.

     Now, to those on the Left who don’t reflexively condemn anything the American military does and have retained some vestige of rationality: Analyze the “humanity” of being killed instantaneously by a GBU-43 MOAB — or a nuclear weapon, for that matter – in comparison with death on a cross. Show all the relevant steps in your equations and sign your work at the bottom. While you’re busy with that, I’ll be praying. It’s Holy Saturday, don’t y’know.

1 comment:

Alien said...

I've occasionally wondered about the maximum practical size of conventional explosives. IIRC, the scientists at Los Alamos detonated a 1,000 ton pile of TNT as a test to determine the measurement tools necessary for determining the output of the Trinity test. I haven't seen anything on how that detonation was managed, but I'm confident it wasn't some guy with a match lighting a fuse.

As for the MOAB, I've seen grain silos after a dust explosion, so ignition of highly flammable particles in a dense cloud is a technique that's been around a while. I read somewhere that fission detonations stop after something like a dozen or so generations of the chain reaction complete because the energy release disperses the fissionable material over a volume sufficiently large to terminate the chain reaction. Seems like a dozen or so generations is enough to produce the desired effect, though.