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Extravoice
2006-Mar-06, 06:55 PM
Hi All:

I just finished reading the "How Stuff Works (http://people.howstuffworks.com/nuclear-bomb.htm)" article on nuclear bombs and it got me wondering.

Now, I know very little about atomic weapons design but the article indicates that the "little boy" gun-style approach to a fission bomb is far simpler, albiet less efficient, than the "fat man" implosion design.

It makes sense that the Manhattan Project worked to develop both styles of bomb, but as far as I know, only an implosion bomb was fully tested prior to use in war. Were the scientists so sure that a gun-style bomb would work that they didn't bother to run a full-up test? If so, why didn't they divert resources from the fat-man design in an effort to get little boy into the arsenal sooner?

I'm sure I'm missing a *lot* of backstory regarding designs, availability of materials, etc. Would anyone on this list care to fill-in some of the gaps in my understanding?

Thanks.

Doodler
2006-Mar-06, 07:09 PM
Hi All:

I just finished reading the "How Stuff Works (http://people.howstuffworks.com/nuclear-bomb.htm)" article on nuclear bombs and it got me wondering.

Now, I know very little about atomic weapons design but the article indicates that the "little boy" gun-style approach to a fission bomb is far simpler, albiet less efficient, than the "fat man" implosion design.

It makes sense that the Manhattan Project worked to develop both styles of bomb, but as far as I know, only an implosion bomb was fully tested prior to use in war. Were the scientists so sure that a gun-style bomb would work that they didn't bother to run a full-up test? If so, why didn't they divert resources from the fat-man design in an effort to get little boy into the arsenal sooner?

I'm sure I'm missing a *lot* of backstory regarding designs, availability of materials, etc. Would anyone on this list care to fill-in some of the gaps in my understanding?

Thanks.

Just venturing a guess, but the Trinity test was a proof of concept for nuclear detonation (could we actually do it) and the implosion method. I would suppose the Hiroshima bomb would have been a proof of concept in situ for the gun style. Nagasaki was simply a follow up to ensure we had the attention of the Japanese leadership.

Eta C
2006-Mar-06, 07:31 PM
Fat Man and Little Boy were very different bomb designs. Fat Man was an implosion device using plutonium. Little Boy was a simple gun design using enriched uranium (that is, the fraction of highly fissile U-235 compared to less fissile U-238 was increased compared to naturally occuring uranium.) Basically, a slug of uranium was fired into a target at the other end of the bomb to assemble the critical mass.

The designers were confident that the gun design for the uranium bomb would work. Also, there wasn't enough enriched U for a second bomb so a test of that one would be a waste. Tests with Pu determined that it was too reactive for the gun mechanism to work (the device would go off prematurely before developing its full yield, a so-called fizzle). So the implosion mechanism was the way to go. This, however, was not a sure thing. Since Pu was more plentiful, it was clear that the implsion mechaism would have to work. Before dropping an implosion-based weapon, they wanted to make sure it would work. Thus the Trinity test.

The shapes of the bombs give away the mechanism. Little Boy was long and narrow along shape of the gun. Fat Man was... rotund.

For other good references Richard Rhodes' book "The Making of the Atomic Bomb" is a good history of the Manhattan Project. If you want a more technical description, check out Serber's "Los Alamos Primer." These are the actual lectures given at the start of the Project, with annotations.

Sammy
2006-Mar-06, 11:19 PM
Right on, EtaC.

The uranium isotope seperation process was a huge bottleneck , and consumed vast resources. Two approachs were used--electromagnetic speration and gaseous diffusion. The latter involved an enormous facility processing highly corresive unranium hexaflouride gas through micropore filters to separate the fissile isotope. The former used hundreds of "Calutrons," which were essentially large mass spectrometers that accelerated beams of ionized uranium around a circular track; the minute mass difference of the two isotopes caused their paths to differ, and the fissile material ended up in a collector placed in the critical path.

The plutonium weapon was tested because of the uncertainty of the implosion method, and the fear that a "dud" weapon weapon would give the Japanese a gift of plutonium worth hundreds of millions of dollars, and which they could possible use against us. There was so much concern about the Trinity test being a fizzle and scattering the plutonium that a giant steel vessel (Jumbo) with massive walls was built; the bomb was to be detonated inside Jumbo so that if only a low-order explosion occured, the vital plutonium could be salvalged. By the time of the actual test, confidence in the implosion technique had risen, and Jumbo was not used.

cjl
2006-Mar-07, 01:11 AM
The shapes of the bombs give away the mechanism. Little Boy was long and narrow along shape of the gun. Fat Man was... rotund.


Indeed...

Fat Man: http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB162/fat-man-1.jpg
Little Boy: http://www.pirateplanet.com/nm/Los_Alamos_Museum_Little_Boy.jpg

Blob
2006-Mar-07, 01:30 AM
Ahhhrrr,
you may be interested in the strange tale of the last voyage of the submarine U 234... that set course for Japan from Norway near the end of the war in a resupply role with 10 German and two Japanese passengers...

It carried plans and prototypes of the latest nazi weapons.
Also on board were sealed boxes with 560 kg of uranium oxide. The boat avoided the Canadian navy and surrendered to the US Navy and taken to Portsmouth New Hampshire. The extra uranium and information onboard no doubt hastened the Manhattan project .
I would even go as far as to say, that without the extra uranium the project would have been extended by at least a year.

http://www.orau.org/shiver_me_timbers.htm (http://www.orau.org/PTP/articlesstories/u234.htm)

Captain Kidd
2006-Mar-07, 01:31 AM
Please - do not climb on Little Boy.
That sign is cracking me up!

According to Building the Bombs: A History of the Nuclear Weapons Complex by Charles Loeber (heh, I'm holding a copy without a UPC or ISBN, holy cow they want a lot of money for it; I wonder how much this pre-release copy would bring... but I digress.):
In 1942, the proposed bomb had a variety of names such as "The Gadget," "The Device," and "The Thing." Later, when the probable dimensions of the wespon began to evolve, the scientists looked to President Roosevelt and Prime Minister Churchill for the source of their private language. The uranium bomb, since it was designed on the gun-barrel principle, was named the "Thin Man" after Roosevelt. The plutonium bomb would have to be the shape of a sphere and was called the "Fat MAn" in line with the proportions of Churchill. The scientists reasoned that anyone over hearing conversations about the Thin Man and the Fat Man would conclude that it involved another Roosevelt-Churchill converence. In 1943, Emilio Segre determined that the subcritical uranium masses would not have to be brought together as quickly as previously thought, so the gun barrel could be shorter and lighter. When the barrel of the "Thin Man" was shortened, the name was changed to "Little Boy." (page 32 with a couple references to other works, all typos mine)

Sammy
2006-Mar-07, 04:12 AM
Ahhhrrr,
you may be interested in the strange tale of the last voyage of the submarine U 234... that set course for Japan from Norway near the end of the war in a resupply role with 10 German and two Japanese passengers...

It carried plans and prototypes of the latest nazi weapons.
Also on board were sealed boxes with 560 kg of uranium oxide. The boat avoided the Canadian navy and surrendered to the US Navy and taken to Portsmouth New Hampshire. The extra uranium and information onboard no doubt hastened the Manhattan project .
I would even go as far as to say, that without the extra uranium the project would have been extended by at least a year.

http://www.orau.org/shiver_me_timbers.htm

Sorry, but the last two statements are dead wrong. There is no evidence that the captured ore was put to any use whatsoever. There was no shortage of uranium ore, and it's supply was never an issue for the Manhattan Project. We managed to acquire large supplies of ore owned by Belgium (from their mines in the (then) Blegian Congo, and from mines in Canada.

Even if we had needed the ore, there is no way it could have been processed through the isotope seperation systems and fabricated in time for use in the August attacks on Japan.

As for information, the Germans never got a viable bomb program going, and were pursuing a totally wrong approach. They had no useful information for us to acquire.

Eta C
2006-Mar-07, 05:00 PM
Any use of the uranium ore on U-234 would have been too late, as Sammy indicates. The German surrender was on May 7. Little Boy (the uranium bomb) was used on August 6 and was shipped out on USS Indianapolis around 12 July. Given the time required to enrich the ore, there's no way any of the U-234 stock made it's way into the Hiroshima bomb.

It was also too little. Clay Blair, in Hitler's U-Boat War: The Hunted quotes an estimate of what that ore could have contributed (pg 694). When processed, it would have yielded about 3.5 kilograms of U-235, about 1/5 of what was required to make a nuclear bomb.

Sammy is also right in pointing out that the German bomb program was nowhere near being able to make a weapon. Whether this was due to deliberate stalling by Heisenberg (as some speculate) or simply due to an early wrong turn is still debated. In any case, the US was far, far ahead of them in this respect.

This brings up one of the myths of WWII. That is, that the Germans were far ahead of the Allies technologically, and that we only won due to numbers. Balderdash. While true in some areas (jet and rocket propulsion in particular) it was not true in others. Besides the Manhattan project, we had better radars, better submarines (Blair is rather damming in his assessment of the Type XXI "electro-boat"), and developed proximity fuzed shells.

The surrender of U-234 had no impact at all on the progress of the Manhattan Project, both in terms of information and material.

Extravoice
2006-Mar-07, 05:56 PM
The surrender of U-234 had no impact at all on the progress of the Manhattan Project, both in terms of information and material.

I imagine it had an *emotional* impact. Here we have enemy #1 shipping the raw material for an atomic bomb to enemy #2. That ought to get your attention!

Edited to add: Thanks to everyone for your thoughtful and informative responses to my original questions.

Sammy
2006-Mar-07, 06:48 PM
I imagine it had an *emotional* impact. Here we have enemy #1 shipping the raw material for an atomic bomb to enemy #2. That ought to get your attention!

Edited to add: Thanks to everyone for your thoughtful and informative responses to my original questions.

I must again disagree. Anyone who knew about the cargo of the sub would also know how far it was from raw ore to fissile material. They would also know that Japan, by that time, had been largely reduced to an industrial wasteland. The B-29 firebomb raids, which killed more people than both A-bomb attacks, had devasted all major cities except for those "saved" as possible nuclear targets. Those aware of the VAST industrial infrastructure required to produce atomic bombs were pretty confident (by mid-1945) that Japan had no hope of producing such a weapon.

The U 234 incident is an interesting historical sideshow, nothing more.

RE EtaC's comment on Heisenberg and the Nazi nuclear effort, there has been much speculation that Heisenberg purposely failed to pursue the correct approach in order to assure that Hitler would not gain a nuclear weapon. Based on recent scholorship, it appears that as so often is the case, incompentence trumps conspiracy. A good summary is provided in a new book, Before The Fallout (From Marie Curie to Hiroshima) by Diana Preston. It provides a fascinating look at the history of nuclear physics and the personalities of the leading scientists, and does not require a PhD to understand the science. The book is a great read. Preston makes a very strong case that Heisenberg's post-war statements were self-serving attempts to avoid blame and to justify the failure of the Nazi effort. Comments by Neils Bohr, who was in touch with Heisenberg before he (Bohr) was extracted and brought to England and then the U.S. are particularly dammning.