3 ms·

> I was under the impression that to actually get the powerful chain reaction you have to spend a lot of energy bringing pieces of fuel together (to the point t

> I was under the impression that to actually get the powerful chain reaction you have to spend a lot of energy bringing pieces of fuel together (to the point that explosives are used for that) otherwise they'd just melt and not explode. When it comes to the original fission weapons, you’d use a spherical arrangement of high explosives to force a carefully crafted, solid core to implode and achieve the critical mass (fat man) or you’d fire a gun with fissionable material as the bullet into a fissionable target that would achieve critical mass once the bullet whacked into it (little boy). Your description is super interesting because it sounds like maybe you were conflating the two. I can’t imagine the gun method really takes a lot of energy (more than a shotgun shell, but not much more?) but the high explosive arrangement for the spherical method was probably a scary, powerful work of art, in its way. I expect with the more developed weapons, not to mention the hydrogen bombs, the warheads are quite sturdy and it’s the arming systems and fuses that prevent disaster. The bombs might sometimes be expected to hit the earth from a great height and then detonate, after all.

puffoflogic 4y ago
> The bombs might sometimes be expected to hit the earth from a great height and then detonate, after all. I'm under the impression that this is not how aircraft-carried nuclear bombs would be detonated. The Little Boy detonated at Hiroshima did so 580 meters from the ground.
- detaro 4y ago
  Depends on the goal. Air burst optimizes for area of blast effect on the surface, ground burst is more effective against underground targets and causes way more fallout and contamination.
sbierwagen 4y ago
>into a fissionable target that would achieve critical mass once the bullet whacked into it [...] I can’t imagine the gun method really takes a lot of energy (more than a shotgun shell, but not much more?) The gun type is mechanically simple, but impossible to scale up. If you're smacking two subcritical spheres together, each sphere can't be more than 15kg.¹ But wait, Little Boy used 64kg of uranium. How did it do that? "Critical mass" is a function of mass and surface area. A sphere packs the most atoms together, a long skinny rod spreads them out. To increase the amount of uranium used and thus the explosive yield, Little Boy actually fired a 38.5kg cylinder onto a stationary 25.6kg plug. Each half of the pit would have been supercritical, if they were spherical. Gun types also need to assemble quickly, since as soon as any part of the assembly reaches critical mass it will start blowing itself apart. It had 64kg of uranium but 2,300kg of neutron reflecting material and tamper, and the launching charge used 3.6 kilograms of cordite! That would be a hell of a shotgun shell. --- ¹: For pure U-233, which Little Boy didn't actually have, since it was only enriched to 80%. Wartime contingencies and all.
- justin66 4y ago
  > Little Boy actually fired a 38.5kg cylinder onto a stationary 25.6kg plug. Each half of the pit would have been supercritical, if they were spherical. I’d forgotten that, if I ever knew it. Super interesting.

puffoflogic 4y ago

> The bombs might sometimes be expected to hit the earth from a great height and then detonate, after all. I'm under the impression that this is not how aircraft-carried nuclear bombs would be detonated. The Little Boy detonated at Hiroshima did so 580 meters from the ground.

detaro 4y ago
Depends on the goal. Air burst optimizes for area of blast effect on the surface, ground burst is more effective against underground targets and causes way more fallout and contamination.

sbierwagen 4y ago

>into a fissionable target that would achieve critical mass once the bullet whacked into it [...] I can’t imagine the gun method really takes a lot of energy (more than a shotgun shell, but not much more?) The gun type is mechanically simple, but impossible to scale up. If you're smacking two subcritical spheres together, each sphere can't be more than 15kg.¹ But wait, Little Boy used 64kg of uranium. How did it do that? "Critical mass" is a function of mass and surface area. A sphere packs the most atoms together, a long skinny rod spreads them out. To increase the amount of uranium used and thus the explosive yield, Little Boy actually fired a 38.5kg cylinder onto a stationary 25.6kg plug. Each half of the pit would have been supercritical, if they were spherical. Gun types also need to assemble quickly, since as soon as any part of the assembly reaches critical mass it will start blowing itself apart. It had 64kg of uranium but 2,300kg of neutron reflecting material and tamper, and the launching charge used 3.6 kilograms of cordite! That would be a hell of a shotgun shell. --- ¹: For pure U-233, which Little Boy didn't actually have, since it was only enriched to 80%. Wartime contingencies and all.

justin66 4y ago
> Little Boy actually fired a 38.5kg cylinder onto a stationary 25.6kg plug. Each half of the pit would have been supercritical, if they were spherical. I’d forgotten that, if I ever knew it. Super interesting.