Why were "Fat Man" and "Little Boy" so comically oversized?

Question

the bombs dropped on nagasaki and hiroshima in 1945 almost looked like bombs from warner brothers cartoons, was there any reason for them being the size of your average SUV?

WOW! i'm amazed by all these technical responses.
i'm assuming there are far more amateur atomic experts than i initially suspected.

Answer 1

I believe that there had to be a minimum separation between the two separate masses of radioactive material inside to prevent neutron radiation from the two masses reaching each other and causing premature detonation. The two masses were driven together by high explosive in order to close the distance between them fast enough to prevent a premature partial explosion that would cause the bomb to fizzle out before a critical mass had been achieved.

I believe that modern nuclear weapons use different tricks to produce the required number of neutrons needed to kick start a chain reaction, so they can now be made small enough to fit inside a missile.

The B-52 bomber was designed to be as big as it is because the original H-bombs where huge.

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The minimum mass of fissile material that can sustain a nuclear chain reaction is called a critical mass and depends on the density, shape, and type of fissile material, as well as the effectiveness of any surrounding material (called a reflector or tamper) at reflecting neutrons back into the fissioning mass. Critical masses in spherical geometry for weapon-grade materials are as follows:

Uranium-235 Plutonium-239

Bare sphere: 56 kg 11 kg
Thick Tamper: 15 kg 5 kg


The critical mass of compressed fissile material decreases as the inverse square of the density achieved. Since critical mass decreases rapidly as density increases, the implosion technique can make do with substantially less nuclear material than the gun-assembly method. The "Fat Man" atomic bomb that destroyed Nagasaki in 1945 used 6.2 kilograms of plutonium and produced an explosive yield of 21-23 kilotons [a 1987 reassessment of the Japanese bombings placed the yield at 21 Kt]. Until January 1994, the Department of Energy (DOE) estimated that 8 kilograms would typically be needed to make a small nuclear weapon. Subsequently, however, DOE reduced the estimate of the amount of plutonium needed to 4 kilograms. Some US scientists believe that 1 kilogram of plutonium will suffice.

If any more material is added to a critical mass a condition of supercriticality results. The chain reaction in a supercritical mass increases rapidly in intensity until the heat generated by the nuclear reactions causes the mass to expand so greatly that the assembly is no longer critical.

Answer 2

I'm not a nuclear bomb designer but the engineers and scientists of the time used the technology and nuclear fuel they had. The fat form followed the function. The two bombs used different fuel -- one used plutonium and the other uranium 235 -- and this made the designs different. Basically, all the stuff under the skin has to work together to create a critical mass using all the nuclear fuel simultaneously. The size of the critical mass and the difficulty in working with smaller units of fuel caused the bombs to be shaped the way they were.

Answer 3

They were entirely different types of devices is why. But you also have to consider this was devised in the '40s. Miniaturization wasn't even desired at that point.

Little Boy was a uranium gun-type device with a yield close to 13 kilotons of TNT.
Fat Man was a more complicated and powerful plutonium implosion weapon that exploded with a force equal to 20 kilotons of TNT.

Answer 4

Duh!
1940's technology. No IC's, no transistors, no tantalum capacitors, no printed circuit boards. No CNC machining centers, no Titanium, no Carbon Fiber, no advanced ceramic composites.
These were handbuilt prototypes, and had to be built so that everything could be reached for maintenance and checking.
Also, they didn't know the best way to achieve initiation of the nuclear reaction, and so 'overbuilt' the mechanisms and support structure to be as stable as possible (don't want it to go off by accident when the plane hits turbulence, eh?)

Answer 5

two words


tech nology


HAHAHAHAHA egads i crack myself up...