I’ve just watched Fat Man And Little Boy. I can’t believe it came out in 1989. I remember it coming out, but I don’t remember it coming out so long ago! :eek: In any case…
I’d known for at least a decade before the movie was released (and three decades from the time I actually watched it) that Little Boy was a ‘gun-type’ atomic device. But this only occurred to me just now: In a gun-type atomic weapon, what happens to the air between the ‘target’ half-core and the half-core that is fired down the barrel? Is it compressed so much that it doesn’t matter? Are the half-cores not airtight in the barrel? Or something else?
I do not know the answer to the question but I do know one goal of the designers was to keep the two pieces of uranium together for as long as possible (which was not long at all but still a goal). If the air was compressed and resisting the two pieces coming together (pushing back as it were) that would be undesirable.
So I would guess either there was a way to exhaust the air as it traveled down the tube or that part of the weapon was a vacuum.
But maybe that would be negligible given the forces involved and not worth extra complexity in the design. IAMANuclearWeaponDesigner.
The “barrel” was indeed not airtight. However, my intuition is that air resistance would have been pretty much negligible in the situation - a gunpowder explosion forcing a lump of heavy metal over a fairly short distance.
A simple Little Boy gun-type design involves bringing together a cylindrical piece and a hollow annular piece of fissionable material. So, no, the pieces are not airtight in the barrel, and there are vents to regulate the pressure. This design is not very efficient, but it was foolproof enough that they didn’t even bother to test it.
I thought about mentioning this in my previous post. For when that New Yorker article appeared in 2008, I was the Londoner who thus legged it down the Northern Line to have a look at the IWM casing in question on the specific point before the museum authorities could react to the press coverage. So I got to see the hole.
They then did fairly promptly remove the casing from public display. When it did go back on show, as part of their general overhaul, it was with a label explaining that it’d been maintained with the help of Aldermaston (the UK’s nuclear weapons lab) and a little circle of metal soldiered over the hole. Which is how it’s currently displayed.
For the record, John Coster-Mullen has been known to very rarely post on the Dope.
The cylindrical projectile assembly was definitely air tight. I think a series of brass rings around the projectile’s steel back ensured this, otherwise there would be “blow by” from the cordite detonation.
However the OP question was good – the interior barrel cannot be air tight or that would likely have interfered with the merge of the core assembly and the dwell time before the nuclear chain reaction took over. Adiabatic compression would have caused an extreme temperature rise that could have compromised the process or core materials. It would be like an automobile engine’s compression cycle. I don’t know what the device’s theoretical compression ratio was but it was probably far beyond a car engine.
The only solution was to either have a high vacuum in the barrel or have vents in the bottom. A vacuum would probably have been finicky and difficult to maintain, so the process of elimination indicated it must have been vented someplace in the region of the target. But I don’t recall the vents ever being mentioned anywhere before the 2008 New Yorker article.
The hole of concern in 2008 is actually utterly irrelevant to the question under discussion, beyond the general point that the design wasn’t airtight, which doesn’t turn on that particular hole.
For the gun-type Uranium weapon, the most expensive part was the Uranium. It took an extremely long time to be able to refine Uranium to the high enough degree needed to be used in the bomb, time that people that worked on the machines that did it wanted to be paid for. A lot of labor and fuel went into enriching that Uranium that they were not going to waste it testing it when the design seemed relatively foolproof. It wasn’t not “bothering” to test it - it was a matter of economics in a war that was decided by the economic might of the US.
The most expensive part of the implosion-type Plutonium weapon was the time and knowledge that went into the engineering getting the implosion exactly right. They definitely needed to test it to make sure they had it engineered it correctly, and the Plutonium required was relatively inexpensive to create.
I visited Oak Ridge National Labs a few years ago. Very little of the original construction remains-but lots of stories.
One story I remember is the mystery, to the workers during the war, of what happened to the output of the Lab. Thousands of people working 24/7, the world’s largest building, a significant fraction of the entire nation’s electrical output consumed by the plant, intensive work all around-and nothing came out. None of the workers ever saw anything leave the plant. Lots of stuff came in, lots of activity, but no output. 99%+ of the workers had no idea that every couple of months a lone courier would go down to the Knoxville train station with a briefcase, take the train to Chicago and another train to New Mexico-and on to Los Alamos.
Why was the plutonium bomb an implosion type rather than a gun type? I assume it’s because of some difference in the physics of the two different elements.
PU-239 has a much higher spontaneous fission rate than U-235. Because of this, gun-type assembly is much too slow. Before the mass is anywhere near full assembly, a spontaneous neutron will have started the chain reaction, resulting in a “fizzle.” U-235 is “calm” enough that a gun-type assembly will work. The neutrons are generated by a (still classified) device that creates them when it gets crushed during the last moments of assembly. A similar device is used in PU-235 implosion bombs, too, to insure that the reaction starts at the right time.
Everything about nuclear weapons is still classified. But good guesses at most of the information have nonetheless been made by people not cleared to know, for pretty much every aspect.
You can’t classify the laws of physics. The secrets are in the engineering, and that can be worked out by anyone with sufficient knowledge and resources.
Fortunately, obtaining sufficient concentrations of the necessary isotopes requires a difficult to hide large scale industrial operation.
