[lelio]

How much was in the Hiroshima bomb?
About 140 pounds, but that was back when the equipment and methods were pretty crude.

According to this page it was 140# of HEU but only 1.38% of the uranium fissioned.

The 140# is split up into 85 "target" and 55 "projectile" pounds. Anyone know what that means?

[Krafty123]

Wasn't it an implosion device? IE the center is fissionable U-235 (85 pounds) and around it is U-238 (85 pounds). Explosives go off around the outside, pushing the U-238 into the U-235 & making it dense enough to start a chain reaction.

[Vinnie_Vidi_Vici]

The "target was doughnut shaped and the Projectile was a perfect fit for the "hole". The projectile was fired from a short cannon barrel to ensure almost instantaneous contact of the 2 subcritical fissioable elements.

[Myrddin]

The 140# is split up into 85 "target" and 55 "projectile" pounds. Anyone know what that means?

Since the Hiroshima bomb was the "gun assembled" format, one could infer that the "assembly" took place by firing an enriched uranium projectile at an enriched uranium target to create the necessary "critical mass" to initiate the fission reaction.

[okie01]

"The 140# is split up into 85 "target" and 55 "projectile" pounds. Anyone know what that means?"

The Hiroshima bomb was a uranium bomb; Nagasaki was a plutonium bomb.

The Hiroshima bomb was built around a 5" artillery barrel (this bomb was known as "Tallboy", remember, for it's long, lean shape). There was a "donut" of enriched uranium at one end of the barrel -- which weighed 85 lbs. The "donut hole" was the 55 lb "projectile", in the barrel of the gun. At ignition, the projectile was fired down the barrel of the gun, achieving critical mass when it "filled" the donut hole in the projectile.

Whereupon, it went "kaboom".

A simple and elegant piece of engineering...

The "Trinity" test at Alamogordo was of the plutonium bomb -- to make sure that the concept and design worked. There were no tests of the Hiroshima uranium bomb -- there was absolutely no doubt that it would work. The only unknown was the yield...

[Orangedog]

The Hiroshima bomb was a one-off deal, a product of early research of the Manhattan Project. The materials developement was a little behind the curve, so the went with the Plutonium based weapon for the second bomb dropped by Bockscar. That became the prefered method because it more efficient. Since then enrichment methods have improved, as well as engineering and casing materials.

[hford02]

The simplest form of atomic weapon places a target of fisionable material at the end of a canon barrel the projectile is also made of fissionable material which is shot into the the center mass of the target creating a fissionable critical mass that produces the fission explosion instantly.