Today is the anniversary of the Little boy being dropped on Hiroshima now I’m pretty sure they were both dropped today, I could be wrong on Hiroshima and Nagasaki. But why do I always hear more about the Little Boy and not much about Fat Man, and what is the difference between the two, how does an atomic bomb work
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Little Boy was what is known as a “gun-type” bomb, and used about 64 kg / 141 lbs of highly-enriched uranium. It was a very simple bomb design, with one piece of the uranium being shot into another through an artillery gun barrel, which created the “critical mass” of material necessary for an explosion. It was very crude and not very efficient; only about 1.5% of the fuel in the bomb actually reacted.
Fat Man was what is known as an “implosion” bomb. It used a different fuel, plutonium, which is made in nuclear reactors. Originally the scientists thought they would use the plutonium in a gun-type bomb as well, but after they finally made some, they discovered that it had properties that meant it couldn’t be used that way. So they had to invent another way to set it off.
Implosion works by taking a “subcritical” ball of plutonium and compressing it perfectly on all sides with high explosives. They basically squeezed the metal ball so that it was, for an instant, half the size of its original dimensions. By doing so they increased its density, and that put it into a “supercritical” state so it would explode. Creating this design took an immense amount of engineering and physics and chemistry and metallurgy, and was the major work of the secret Los Alamos laboratory for about a year. They still weren’t sure it would actually work, or how well it would work, so they tested it first with a device called The Gadget at the Trinity test in New Mexico on July 16, 1945.
It turned out to work better than they expected, and about 20% of the material reacted. It used about 10X less fuel than the Little Boy bomb: about 6 kg / 13 lbs of plutonium. Both the Fat Man and Little Boy bombs were similarly destructive (they released 15-20 kilotons of TNT equivalent), though the Fat Man bomb was much more technically impressive (it wasted a lot less fuel, among other things).
Almost all atomic bombs after World War II were more like Fat Man bombs than Little Boy bombs because they are so much more efficient. As to why you learn more about Little Boy, it’s both because people focus on Hiroshima more than Nagasaki (they see Hiroshima as “first” and Nagasaki as “second,” though I would argue that we should see Nagasaki as “last,” which perhaps makes it as important). But it’s also because it’s easier to explain the science of the Little Boy bomb than the Fat Man bomb, and so for a simple explanation it is an easier one to go with.
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