Monday, January 25, 2016

Exploding Headwire

     I was watching Manh(A)ttan last night, the not at all real story of U.S. development of atomic weapons during WW II, when a major character just tossed out the line, "We'll use exploding-bridgewire detonators...," as if it was the most obvious thing in the world, been around forever, yadda-yadda--

     Except it's not.  It was brand-spanking new and seriously classified.  These characters have been wrestling with the problems of building an implosion-type device for a year or more, including running explosive experiments akin to RaLa (only without the radiation hazard) and one of the major problems is setting off all the implosion charges at the same time.  It's a huge big deal -- conventional detonators, then and now, are sloppy things, three or four orders of magnitude* slower than EBW, and you can expect that order-of-magnitude variation within a single batch. Use them to fire off your implosion-type bomb and there's a very loud "bang!" with farty overtones as it barfs out one or more plumes of vaporized radioactive material at the low-pressure points -- a fairly slow plume, as such things go: it's not a bomb, just a very nasty mess.

     The need for rapid and synchronous detonation became obvious during the development of the implosion-type atom bomb; Dr. Luis W. Alvarez and his student Lawrence H. Johnston invented (or possibly re-invented) them at Los Alamos, probably in 1944: it was a very big deal.  It would not have been taken for granted.

     Then again, that same series of episodes has the Nazis send the severed head of an Allied spy in the German nuclear program to the President Roosevelt, and I'm irked about exploding-bridgewire detonators?  Yes; after spending so much time on the very real technical challenges of  building a implosion-type bomb, the show throws away a major problem to concentrate on the fictional issue of not having a supply of primers for experimental work.  It completely misses the point.

     There has been, by the way, all manner of soap-opera interaction between the characters; diagramming who's-doing-who would be crowded on a whole sheet of paper.  This builds dramatic tension and engages the viewer, just as it should -- is it too much to ask that they not bugger the technological history while they're at it?

     (A much smaller quibble: most of the pencils seen onscreen have been Mirado "Black Warriors," a plausible choice since they were in wide use at the time [as Eagle "Black Warrior"] and are still made today; the black body and brass [probably anodized aluminum these days] ferrule with a maroon strip are very recognizable.  Now I'm seeing some aluminum-ferrule versions, and I think that's an anachronism. [ETA: is it ever! Plastic ferrules replaced metal during WW II. We can safely assume Uncle Sam might be using up existing stock but silvery-metal ferrules were still rare.]  Given the show's tendency to use "scientist-calculating-away-with-pencil" as a recurring image, it can be jarring. In an historical film, there's someone on the Internet who knows the history of every single class of object seen on the screen.  With a lot of time and a huge budget, it can all look right.  In reality, close often has to be good enough.)  
* These would be engineering orders of magnitude, each 10n per step, and not astronomical ones, which I'm not going to be able to explain in a footnote.


Karl said...

Comrade Roberta! Have you seen the Nuclear Secrecy blog? He helped on the MP tv show and has some posts on it. Great blog!

I stopped watching after 6-8 episodes - too much drama for me. And as you say, not completely accurate.

wheelgun said...

"is it too much to ask that they not bugger the technological history while they're at it?" Of course it is...

I haven't seen the program in question, but I find it is pretty much impossible for Hollywood to do anything but hand-wave when stuff gets complicated.

And besides, math is hard. (And physics too!)

Anonymous said...

From a cousin who worked at Hanford for a number of years after he retired from the Navy as a submarine reactor tech, the need for a very high quality capacitor for the detonator led to the development of the ultra-high quality audiophile crossover capacitors in use today. The paper-electrolyte soaked caps of the day weren't nearly consistent enough.

He also noted that it was of high importance that the propagation delay of the wires to the individual detonators was accounted for to a tiny fraction of a millimeter...

Roberta X said...

Very interesting comments! Exploding bridgewire detonators are picky things and the HV generators (where the capacitors are used) that drive them are not the least of it.

Karl, I have not seen that blog. I'll have a look.

Wheelgun, they actually aren't horrible at the broad sweep of things, which may be why I come back to the show. But it's far off on the history!