Can anyone more knowledgeable chime in here?
Without the force you get various intensities of fizzle explosions and radiation bursts.
Hitting the ground with a "doink" after falling out of a plane is more likely to break something than trigger a full-scale explosion.
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.
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.
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¹: For pure U-233, which Little Boy didn't actually have, since it was only enriched to 80%. Wartime contingencies and all.
I’d forgotten that, if I ever knew it. Super interesting.
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.
This isn't really how it works because you are not "putting energy" into the system via explosives.. You are compressing a plutonium core to cause it to become "supercritical".
To your point, this is very difficult because if the explosive lenses are not perfectly timed, the core will not compress properly. This could cause anything from a total "fizzle" where the plutonium is spread over a large area due to "conventional explosives" to a yield far below expectations.
To get the "powerful chain reaction" you need to hold this compressed "supercritical" core together as long as possible, to consume the plutonium fuel.
This is challenging because the fission reactions increase at an exponential rate, so obviously it cant be held supercritical for long.