No, I do not “need an answer, fast.” But it IS something I’ve puzzled over, now and again, mostly when tinkering with steampunk stories and settings.
The thing is, I can’t recall a single nuclear weapon design, built or proposed, that didn’t incorporate high explosives. Even the Cordite used in Little Boy, while apparently classified as a “low” explosive, contains enough nitroglycerin that I’d have to consider that “cheating” for these purposes.
The closest I can think of would be the British “Green Grass” and US Mk. 18 bomb designs, which both contained more than one critical mass worth of HEU, and could have been detonated by crushing or a partial explosive firing. Perhaps an (inefficient) warhead along those lines could be set off by simply dropping it from height, or be delivered by artillery, for a ground burst?
I dunno…anyone better in the know want to weigh in? I’ll just busy myself, looking over used Zeppelins…
A fission bomb relies on a chain reaction to release all that energy. Yet, the very moment you start a chain reaction, the incredible energies released will violently throw the parts of the device away from each other.
For instance, in nuclear accidents, you end up with a big pile of fissionable material in close proximity. The heat generated, if the reaction is vigorous enough, will boil or vaporize the material and the reaction stops.
This is where the explosives come in. In a fission bomb, you need a way to violently force all the components together to create a HUGELY powerful chain reaction. If you did this slowly, the components of the weapon would turn to gas long before they got to this state. This, in the nuclear weapons trade, is called a fizzle.
Explosives aren’t the only way to do this, obviously. You could make a simple fission bomb work by placing one part of the bomb at a great height on a sled, and the other part on the ground. The pieces would slam into each other, forming a critical mass, and you’d get an explosion.
You could create a huge force the way General Fusion is trying to do it, with electric fields and a collapsing vortex of molten lead.
And 50 other ways to do it.
However, all these methods aren’t nearly as compact or convenient as an explosive charge in the bomb itself.
The OP seems to be mostly concerned about high versus low explosive. High just means that the reaction propagates faster than sound in the explosive material. If you are building a nuclear weapon you are looking for very precisely controlled pressure fronts, and for that you are going to want an explosive that is very precise in its characteristics. When you want it all to live in a very small sized device, you also want fast propagation. An explosive that just deflagrates and creates a great deal of gas over a long period of time (relatively speaking) isn’t going to be what you want.
A gun assembly weapon (Little Boy) is much easier on the explosive needs, but you still need to assemble the weapon with extraordinary precision in timing, and a lot of energy. For a Fat Man, you have a sphere of explosive, and collapse one layer of fissile material onto a core. This requires stupid levels of timing precision, and precision in the explosives, to get the pressure wave to collapse in a sphere. Polanski’s movie Frantic had as its special device plot element a cryotron - a very fast switching device used to trigger such weapons.
The physics of a detonating nuclear weapon are astounding. Indeed it is a bit of a miracle that it was possible to step between the various constraints and create useful weapon at all. In order to get a useful yield, you don’t just slam the fissile material together, you actually compress it to about half its original volume. This gets your neutron flux up to a density that you are going to get enough fission steps occurring before the weapon disassembles. Compressing solid metal like this isn’t exactly trivial. You also have to do it very quickly, as otherwise stray neutrons will likely start the fission reaction before you are ready, and the weapon will disassemble before it reaches maximum density, and you will get a damp squib of a bang, instead of what you wanted. There is much more going on too, but in general, slow explosives are not what you want. You are looking for very fast propagation, and that by definition means high explosives.
I don’t know if just gravity could actually get the first piece moving fast enough to make a real explosion, but if so, then you could make a bomb by attaching the second piece below the first piece with carefully designed collapsable struts that would break when the bomb hit the ground. Bottom piece of fissile stuff stops, top piece keeps moving, they hit, bang.
I’m not saying it’s trivial engineering to build a bottom piece that will stop nearly dead without deforming when it hits the ground (and you’ll need to know exactly what the landing spot is going to be like), but – again if speeds are high enough – the basic physics is there.
In implosion-type bombs, you have to basically raise the density of the pit- it’s usually done using explosive lenses to create a spherical shockwave inward.
Gun-type weapons don’t rely on the compression of their pits, but rather on the really fast assembly into a super-critical mass. This is why a gun is used.
I imagine that it would be entirely possible to rig up some kind of gun-type weapon using a linear accelerator and evacuated tube to get the bullet up to a suitable speed for detonation. Similarly, I imagine if you threw enough electricity and engineering at it, there’s probably a way to spherically compress an implosion pit with electromagnetic energy.
Both seem like really arcane ways to do it versus the tried and true chemical explosives.
Krytron rather than cryotron, the latter being a liquid-helium-temperature switch involving superconductivity (and wholly unsuited to triggering nuclear bombs).
The trick to any of them is getting the fissile material, and a large part of that is because Pu-239 and U-235 are tiny fractions of most plutonium and uranium, but are the best candidates for fission.
Plus, the presence of things like Pu-240 or U-236 that gum up the works makes very exacting isotopic separation very necessary for a successful bomb.
It took the US government something like a year to purify the necessary U-235 in 1944 and 1945, and somewhat less to produce the plutonium for Trinity and Fat Man.
Suppose I had the material and wanted to blow up Houston. I could rent an abandoned warehouse and create a gun-type weapon with no gun. The receiving material is mounted into a frame, and the “slug” material is placed at the head of a piston. The piston is then set to be propelled into the receiver at high speed using a hydraulic, electrical, or pneumatic motor. Press a button, it “shoots” into the receiving material, kaboom. It’s a gun-type nuclear weapon with no gun and no high explosive.
Not fast enough. You have to get the two parts of a gun-type bomb together fast enough so that the chain reaction happens while they’re together, and not beforehand, which would cause predetonation and a fizzle. Not coincidentally, Pu-240 contamination makes it impossible to have a successful plutonium-based gun weapon since the Pu-240 spontaneous fission rate makes predetonation inevitable at any reasonable gun-type bomb assembly speed.
In Little Boy, the “bullet” was going nearly 1000 feet per second, so that’s a pretty good ballpark number for what’s necessary in terms of assembly speed for gun-type fission weapons. I don’t think that’s possible with some sort of motor.
A motor with a wheel of 1 foot in circumference turning 1000 rpm is not much of an enginering challenge. A series of them pushing a sled up to just shy of 700mph may have its challenges but a “gun” is a far more efficient.
OK, at 1000 feet per minute that’s about 680 mph. With a terminal velocity of a falling object of 200 ft per minute I would expect the kinetic energy to transfer at a much higher rate of speed upon impact. Throw some weight behind the Little Boy “bullet” to ensure enough energy is transferred.
With gun-type bombs, it’s literally the speed that’s important, not the kinetic energy that they come together with.
Really simplified, if the two pieces come together too slowly, the chain reaction starts before they’re together enough to go off like a bomb, and you get a fizzle due to predetonation and no bang.
I imagine you’re thinking of implosion weapons, where the goal is to create as close to a perfectly spherical converging shockwave to basically change the density of the pit in order to create the same runaway chain reaction. This is accomplished through a number of methods- sheer force, phase changes (particularly in plutonium), collapsing a hollow shell, etc…
Any divergence from almost spherical, and you don’t really get the same density change or chain reaction needed for the bang you were looking for. This is why the pits are very carefully machined- they have to be nearly spherical as well.
I hadn’t thought of a wheel; seems to me that you’d have to spin it up first, and then figure some way to actually assemble the core. That could be interesting.
Would a magnetic pinch work in a reasonable configuration? It may take a huge bank of capacitors, but if you don’t have to make the bomb transportable it doesn’t seem grossly impractical.
The speed of kinetic energy is going to be faster than terminal velocity as there is no wind resistance in the process. It would mean positioning the bullet at the source of material meant to split.
