Some considerations from my behalf:
Hartmann10 wrote: the USA would not accept this, but in fact it had not enough U235 to finish the "Little Boy" in time, because they used a part of the U235 in the Hanford piles when they changed to water cooling.
WW2 production piles in Hanford used water cooling, and were designed for that from outset. The prototype X-10 used air-cooling, but that was in Oak Ridge and was an miniscule scale compared to production piles.
Also Hanford piles did run on natural U metal during WW2 (not Oxide BTW) - at their original 250 MWt power level. They started to use enriched Uranium to boost up power levels (and Pu production) only after WW2, when U enrichment capacity had gained available surplus. During wartime production that was not the case, like you well know.
http://www.childrenofthemanhattanprojec ... H-06b4.htm
http://www.atomicarchive.com/History/si ... ctor.shtml
http://www.hanford.gov/doe/history/file ... fBArea.pdf
Hartmann10 wrote:If you cool a pile with water, it is because you use slow or medium enriched uranium oxide.
To some degree you are right. Natural water cooled reactors are usually run on low-enriched uranium (pressurized water reactors, boiling water reactors, and Russian RBMK type).
BUT that does not mean that graphite moderated, natural water cooled reactors can not run with natural U.
Hartmann10 wrote:In other state, the "little boy" is a curious "White elephant" in the nuclear bombs history by some reasons:
1) It was supposedly designed before the year 1945 (when the Manhattan project had not been still determined the U235 critical mass) but only there were another 5 unassembled bomb hulls, all finished post war.
Right. It was an white elephant - an inefficient design, wasting fissionable material. But it did work. Untested. And could be almost quarantee to work even with the information that was available in turn of 1944/45. In that respect it had its role in Manhattan Project. The way it looked from outside does not have any significance.
The low number of bomb casings built was natural. Americans knew in early-1945 well that if implosion does not work, they have lots and lots of time to built more bomb casings before enough U would be available for second Little Boy (since Pu would be worthless and gun-type weapon needs lots of U). It never materialized. Implosion worked, making Pu an usable bomb material, and making sure that Little Boy design would never be needed again, not even for U weapon. The fact that Little Boy was used, was because they were in hurry and U (or composite) pits for implosion weapons were not ready. It maximized the available weapons for August-1945.
About critical mass: Gun-type weapon will "forgive" some uncertainty during design phase. Nothing could be easier than making corrections to mass of "target" if necessary. Just make target a bit larger in diameter and mill some material off from the tamper.
In implosion design such change would need whole bomb assembly to be redesigned/rebuilt from scratch.
Hartmann10 wrote:The gun was supposedly made reboring a 3 inches AA gun barrel to fit the projectile (very miser or "hurried adaptation" solution giving the importance of the matter, my god an atomic bomb¡¡¡).
The 3" AA-gun barrel is made from very high quality weapon steel, designed to withstand high pressures.
What could be better than that, if it fit the bill for needed measurements of "bullet". Should they had made it from titanium or something else to make it "worth the weapon".....
I don't think so.
Belive me, they surely grinded some material of from that barrel, to make proper fittings, and then rebored it to smooth-bore, and cut it to lenght they needed.
Should they have ordered an exotic new design and possibly postpone the use of weapon in case the specially ordered one-off barrel would not be available because possible failure in production ?? Well, THAT would be crazy. Thankfully Americans were far too practical and sober not to fall for such pits in critically important weapon. And what advantage specially designed "firing tube" would had served ?? Weight of barrel ?? - seriously - the bomb was laden with U and Tungsten anyway....
There is nothing
wrong with starting material they used for barrel material. It was plentifull and no hick-ups for delivery was possible.
Hartmann10 wrote:2) The U235 rings of the core where only enriched to the 50% while the "projectile" or "bullet" rings gunned to the core, where enriched to the 89%, something very curious giving the fact that it had to be a fool proof design.
Considering the complexity of U enrichment cycle in early-mid 1945 i don't find that surprising. Multiple stages and plants for successive production of feedstock for next plant, and the very nature of last enrichment method, calutrons. Batch-like system requiring runs again and again. Not really "production-line". And same time time
being an essance they did not had. In that regard having batches of U with lower enrichment than Oralloy by dead-line set for finalizing the design and starting assembling the bomb was more than natural, it was unavoidable.
Giving this, and without previous testing, how in the hell did the Scientifics of the Manhattan project predicted the right yield and altitude detonation burst of the bomb?
They did not.
Little Boy was detonated too low for maximum 5psi overpressure in ground. Somewhere between 1850-1900 feet altitude.
5psi is the optimum for attacking city target and detonation altitude is normally set to maximize the area of 5psi overpressure. It almost can pierce ear-drum of human, and destroy most unhardened city structures.
Little Boy actually maximized the 12psi overpressure area. The detonation altitude was set conservatively because the uncertainty of yield. That altitude is optimum for 5kt weapon and way too low for 11kt weapon.
American were conservative, wisely, as setting the altitude too high rapidly diminishes results. Little too low detonation altitude is not so critical error.
For example, if Little Boy would had delivered only 5kt and it would had been detonated say, 3000feet (far better for 11kt than the altitude they used), the effect in ground would had been greatly diminished.
Not quite "dud" and "American flash-in-the-sky weapon" in Japanese point-of-view, but not far from that.
Yours, Mark V