[AR] Re: Damascus AR Incident
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 22 Jan 2017 01:38:08 -0500 (EST)
On Tue, 17 Jan 2017, David Weinshenker wrote:
Early on there was an effort to design a plutonium "gun" explosive
(I've seen various sources refer to this as the "Thin Man" or "Big
Boy", presumably depending on whether it was being compared to the
"Fat Man" radial implosion system, or to the "Little Boy" linear
uranium gun configuration)...
There were originally two design sketches made, well before detailed bomb
design was done, as information for other work that needed to get started
in parallel -- aerodynamic design, fuzing systems, aircraft modifications,
etc. Thin Man was a nominal gun bomb, basically a long thin cylinder to
house the gun barrel, with a bulge at one end; Fat Man was a nominal
implosion bomb, basically a sphere. The original plan was Thin Man for
both uranium and plutonium, because gun bombs were better understood and
much easier to develop -- Fat Man was included as a precaution and a
possible future direction.
Then there was good news about U-235 and bad news about Pu-239. Some of
what had been thought to be spontaneous fission in U-235 turned out to be
due to cosmic-ray neutrons, which could be shielded against, reducing the
required gun velocity and making a shorter barrel possible; that shortened
the uranium Thin Man into Little Boy. And first samples of reactor-made
plutonium turned out to have a rather higher spontaneous-fission rate than
the original cyclotron-made samples, because Pu-240's s-f rate was much
higher than expected; that made a plutonium Thin Man essentially
impossible, and required a massive last-minute effort to turn vague
notions about implosion bombs into a practical Fat Man design.
Gun bombs are inherently inefficient, burning only a tiny percentage of
their fission fuel, because they don't *compress* it before ignition. So
afterward, Fat-Man-derived implosion bombs were preferred even for U-235,
with gun bombs seen only in special applications that needed a very thin
shape or a very robust structure. So Thin Man did not reappear.
Henry
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