You probably know Little Boy and Fat Man as the atomic bombs detonated over Hiroshima and Nagasaki, effectively ending WWII. The U.S. rushed headlong into the development of the bombs, trying to actualize the atomic bomb ahead of the Nazis. And we did it.

But during the Manhattan Project scientists developed at least four bombs—Thin Man, The Gadget, Little Boy and Fat Man—three of which made it to fruition (The Gadget, Little Boy, and Fat Man) with two actually being used in war (Little Boy and Fat Man). Why haven’t you heard of Thin Man? Well, it never saw the light of day.

How to Build An Atomic Bomb

There are two different ways to build an atomic bomb. One is gun-type assembly, in which a hollow, sub-critical (less mass than needed to sustain a chain reaction) fissile “bullet” is shot at a solid supercritical (able to sustain a chain reaction) core. The bullet strikes and compresses the core, fission happens, and the bomb explodes. This type of assembly is relatively simple in design, involving one thing being shot at another, and we’d been building non-nuclear things of this sort for years before the Manhattan Project.

The second type of assembly is a bit more complex. Implosion-type assembly involves a hollow sphere containing a complex arrangement of high explosives and detonators, surrounding a solid fissile, but not yet supercritical core. The high explosives are detonated in such a way that the blast wave compresses the core to a supercritical density. This causes fission, which makes the bomb explode.

Thin Man was a gun-type weapon designed to use plutonium as fuel. But scientists found that the reactor-produced plutonium available at the time contained too many impurities, causing a greatly increased spontaneous fission rate, which basically means that the fuel would pre-detonate and blow itself apart while trying to attain criticality, rather than after, which was kind of putting the nuclear cart before the horse. So the bomb was scrapped, and focus turned to Little Boy.

Cue Uranium

Little Boy was the same type of weapon as Thin Man—except for the fact that it used uranium rather than plutonium. Since uranium isn’t as prone to pre-detonation, a much simpler weapon was designed, without many of the safeguards of the Thin Man design.

Even though plutonium wasn’t a good choice for gun-type weapons, it worked well in the implosion design, which included more safety features that prevented pre-detonation of the plutonium. While Thin Man and Little Boy were being developed, the scientists were also working on The Gadget and Fat Man, which were both implosion-type devices, using plutonium (and uranium) as fuel.

Since the implosion design was more complex, it was tested before being put into action. The Gadget was detonated in the New Mexico desert during the Trinity Test on July 16, 1945. The test was successful, and became the first man-made nuclear explosion in history, and we were now relatively certain that Fat Man would work as expected since it used the same design.

No Tests Necessary

We were so confident of our scientific prowess that we sent the pieces for Little Boy to the Mariana Islands for final assembly two hours before the Trinity Test. Now remember, this weapon design was completely different, and we had never done any of this before. So why didn’t we test the design for Little Boy?

Well, the gun-type assembly was thought to be simple enough that it would detonate without problem. And enriched uranium was much harder to come by than plutonium, so we didn’t want to waste the rare stuff on a test. Time was also a factor: It had taken years to enrich enough uranium to use in this one bomb, and the war was raging on. We simply didn’t want to wait the months or years necessary to accumulate enough uranium for another bomb.

Less than three weeks after the Trinity Test, Little Boy was successfully detonated over Hiroshima on August 6, 1945. Three days later, Fat Man was detonated over Nagasaki, adding atomic weapons to mankind’s arsenal for the first time.