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Old 06-29-2018, 11:22 PM
Johnny L.A. Johnny L.A. is offline
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Gun-type atomic weapon

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! 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?
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Old 06-29-2018, 11:25 PM
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IIRC, not air-tight.
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Old 06-30-2018, 12:29 AM
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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.

Last edited by Whack-a-Mole; 06-30-2018 at 12:31 AM.
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Old 06-30-2018, 06:26 AM
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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.
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Old 06-30-2018, 06:55 AM
Lucas Jackson Lucas Jackson is offline
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IAMANuclearWeaponDesigner.
This is a term that thankfully you don’t see a lot on the internet.
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Old 06-30-2018, 07:43 AM
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Yeah, fortunately most folks on the Internet are nuclear weapon designers.
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Old 06-30-2018, 08:30 AM
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Yeah, fortunately most folks on the Internet are nuclear weapon designers.
Thanks to the answers to my question in this thread, I am now!
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Old 06-30-2018, 08:40 AM
Llama Llogophile Llama Llogophile is offline
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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.
Here's a great article about an amateur atomic bomb historian, and he talks about finding vent openings on one of the casings.
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Old 06-30-2018, 01:36 PM
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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.

References: http://nuclearweaponarchive.org/Nwfaq/Nfaq2.html
https://en.wikipedia.org/wiki/File:G...thin_lines.svg
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Old 06-30-2018, 01:51 PM
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Here's a great article about an amateur atomic bomb historian, and he talks about finding vent openings on one of the casings.
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.

Last edited by bonzer; 06-30-2018 at 01:53 PM. Reason: Grammer
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Old 06-30-2018, 02:38 PM
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...the pieces are not airtight in the barrel, and there are vents to regulate the pressure...
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.
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Old 06-30-2018, 02:49 PM
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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.
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Old 06-30-2018, 04:54 PM
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This design is not very efficient, but it was foolproof enough that they didn't even bother to test it.
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.
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Old 06-30-2018, 05:14 PM
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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.
Plus, they didn't have enough enriched U-235 for another bomb. Testing it would have delayed deployment of the weapon for months.
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Old 07-01-2018, 03:50 AM
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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.
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Old 07-01-2018, 07:19 AM
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BTW--in the 1960s, Dick Tracey was on tghe trail of a mobster, who had stolen a different kind of gun type atomic weapon.

A pistol.

Careless handling destroyed weapon & mobster both.
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Old 07-01-2018, 07:50 AM
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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.
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Old 07-01-2018, 08:08 AM
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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.
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Old 07-01-2018, 01:46 PM
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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.
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Old 07-01-2018, 02:03 PM
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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.
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Old 07-01-2018, 02:43 PM
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You can classify the laws of physics. It's just not likely to be very effective when you do.
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Old 07-01-2018, 02:47 PM
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You can't classify the laws of physics.
You need to say it like this: Ye canna change the laws of physics!
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Old 07-01-2018, 03:12 PM
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Pretty much a lot of the 1945 designs of nuclear are declassified.

The big snag - particularly compared to anything else from the period - is that 1945-style weapons are exactly what hostile powers want to currently build. So there is the level of 1945-detail that remains supposedly deeply classified. Yet, while obviously personally biased, I suspect these are areas where it's pretty much been figured out online.
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Old 07-01-2018, 05:02 PM
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By "1945-style", do you mean a basic concept like an implosion design to achieve supercriticality? Because a nuclear weapon designed today would not be a straightforward copy of anything from 1945. You would be engineering it from scratch (or a more modern design) anyway, with the benefit of modern simulation software.
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Old 07-01-2018, 05:15 PM
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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.
[nitpickle]The problem was actually with impurities in the supply of PU-239, namely minute amounts of PU-240 that couldn't be easily extracted. But the effect was the same. There was work on a gun-type Plutonium bomb called Thin Man that was abandoned when this was figured out. [/nitpickle]

Reading around about the U.S. atomic weapons program, it's amazing at how ad hoc everything was. I mean, I guess they HAD to be that way, because they were doing things never done before, but geez. They dropped only the SECOND bomb ever made on Japan, without even testing the design?! And attention to security was so mediocre that the U.S.S.R. stole all kinds of info to help them build their own??!

Last edited by Lizard; 07-01-2018 at 05:16 PM.
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Old 07-01-2018, 05:28 PM
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[nitpickle]The problem was actually with impurities in the supply of PU-239, namely minute amounts of PU-240 that couldn't be easily extracted. But the effect was the same. There was work on a gun-type Plutonium bomb called Thin Man that was abandoned when this was figured out. [/nitpickle]

Reading around about the U.S. atomic weapons program, it's amazing at how ad hoc everything was. I mean, I guess they HAD to be that way, because they were doing things never done before, but geez. They dropped only the SECOND bomb ever made on Japan, without even testing the design?! And attention to security was so mediocre that the U.S.S.R. stole all kinds of info to help them build their own??!
Well, PU-240 has a horrible spontaneous fission rate, but PU-239 is pretty bad. I don’t feel like doing the calculations to determine if pure PU-239 is low enough to use in a gun-type bomb. I suspect it isn’t.

As for security, it was extremely tight (all mail was censored, for example). But, spies have their ways.

As an aside - tight security is a security risk in itself. Just like the “Spectre” flaw in Intel chips, the fact that mail from Los Alamos was being censored leaks information that something important is happening there.


ETA: if the PU-240 contamination is bad enough, then even an implosion-type bomb won’t work. This is a major component of nuclear non-proliferation - long fuel-cycle Uranium can’t be diverted to make bombs, because it’s too contaminated by PU-40.

Last edited by beowulff; 07-01-2018 at 05:31 PM.
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Old 07-01-2018, 05:30 PM
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I forgot to add, a simple nuclear gravity bomb is a lot less useful today than it was in 1945.
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Old 07-01-2018, 09:00 PM
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The illusion of security was very tight. The reality was different. Like, everyone had a combination-locked filing cabinet, with customizable combinations... which means that half of the combos were just left at the default, and the rest were all things you could guess if you knew the owner of the safe. Or not even need to guess, if you were ever in that office when it was open.
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Old 07-01-2018, 09:09 PM
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The illusion of security was very tight. The reality was different. Like, everyone had a combination-locked filing cabinet, with customizable combinations... which means that half of the combos were just left at the default, and the rest were all things you could guess if you knew the owner of the safe. Or not even need to guess, if you were ever in that office when it was open.
Well, yes, Dr. Feynman.
But, the secrets that were stolen weren’t pilfered from a safe - they were deliberately given to the soviets by one of the senior scientists, working under Hans Bethe.
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Old 07-01-2018, 09:22 PM
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And Richard Feynman famously picked those locks.

For those interested in the history and the atmosphere of Los Alamos during the Manhattan Project, I cannot recommend this book highly enough:

109 East Palace: Robert Oppenheimer and the Secret City of Los Alamos
by Jennet Connant.

Not only is Ms. Connant a skilled and entertaining writer, but also her grandfather, James B. Connant, was an administrator at Los Alamos during the Manhattan Project.
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Old 07-01-2018, 09:56 PM
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I forgot to add, a simple nuclear gravity bomb is a lot less useful today than it was in 1945.
Why is that so?
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Old 07-02-2018, 11:17 AM
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I thought this thread was going to be about Gerald Bull
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Old 07-02-2018, 11:52 AM
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In response to the claim that a gun-type gravity bomb is less useful than it was in 1945:

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Why is that so?
Atomic/nuclear gravity bombs of any kind are less useful because it's comparatively easy to to defend against the aircraft required to drop a gravity bomb. The preferred delivery mechanism nowadays is an ICBM with multiple-re-entry warheads.

What's more, hydrogen bombs (involving fission in one way or another) are all the rage these days, and while you might be able to use a gun-type fission weapon as a primary (to set off the fusion secondary), I'm not aware of any design that has done this. Then again, I'm not aware of any particular design beyond what's publicly available on the interwebs.
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Old 07-02-2018, 12:38 PM
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One of the reasons that gun-type assembly is no longer used is it’s extreme inefficiency. Fat Man was about 10x as efficient as Little Boy. Also, since Plutonium is easier to make than HEU, it’s pretty clear that nobody is going to be making gun-type weapons anymore.
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Old 07-02-2018, 01:35 PM
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One of the reasons that gun-type assembly is no longer used is it’s extreme inefficiency. Fat Man was about 10x as efficient as Little Boy. Also, since Plutonium is easier to make than HEU, it’s pretty clear that nobody is going to be making gun-type weapons anymore.
Unless of course, they intend to fire them out of a gun, or use them in some other kind of rugged application- the last uses of the gun-type weapons was in nuclear artillery shells for that reason.

The thing with the gun-type bombs is that they're not quite as finicky about the exact timing as implosion-type weapons are. As a result, they're more rugged and more reliable, and the Manhattan Project scientists were confident enough of the U235 gun type weapon ("Little Boy") that they didn't think they needed to test it prior to use.

Of course the tradeoff is that gun-type weapons are dreadfully inefficient relative to implosion type weapons. Little Boy used 64 kilograms of U235 to get a yield of 15 kilotons, while Fat Man (implosion type) used 6.4 kilograms of Pu-239 to generate a 21 kiloton yield. So ten times as much fissile material in a gun-type bomb generated 3/4 the yield of the first implosion design.

As far as the venting of the gun barrel goes, the air itself isn't important, but rather that an unvented barrel and consequent pressure rise could decelerate the projectile significantly and result in a lower yield, or even a fizzle due to predetonation*, if the two parts weren't "assembled" fast enough.

* - Predetonation is the phenomenon where neutrons start fissioning material before the bomb is in the right configuration, and these early fissions destroy enough of the fissile material to render the bomb useless, or significantly reduce the yield. In the case of gun weapons, if you put the two parts together too slowly, they start to predetonate before they come together, and you end up with drastically reduced yield. Also, with plutonium, the Pu-240 impurities have such high spontaneous fission rates that gun type weapons cannot put the two parts together fast enough to prevent predetonation because of all the neutrons whizzing around.

Last edited by bump; 07-02-2018 at 01:40 PM.
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