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> I want a warhead that goes off correctly over the target, or not at all. That appears to be a fairly straightforward engineering challenge

That is not an engineering challenge but rather the natural state of nuclear warheads. If your device works perfectly, you get a nuclear explosion, if it does not, you don't, at best you get a fizzle that does a tiny fraction of the intended output. Of course, you do scatter some radioactive material around.

It would be a very interesting engineering challenge to design a nuclear warhead that can do a partial detonation even if damaged by some AA, but I don't think anyone has done that.

It would be a very interesting engineering challenge to design a nuclear warhead that can do a partial detonation even if damaged by some AA, but I don't think anyone has done that.

Apparently, the British managed, albeit inadvertently. In the 1950's they did not have access to American research on the behaviour of plutonium under extreme conditions any longer, so they to increase yield they built fission bombs with a larger amount of fissile material. There was the risk that during a crash landing and fire the conventional explosives cooking off would cause a fission reaction at a substantial fraction of design yield, consequently the cavity was filled with ball bearings to be removed only immediately before actual use.

The United States also had something similar -- the Mark 18[1], designed as a very large fission bomb in case the first thermonuclear bomb failed, had four critical masses of uranium-235 in its core. Even one of the implosion lens detonating could have led to a kiloton-range explosion. To prevent such an accident, a chain of aluminum and boron (the latter being an especially good neutron absorber) was inserted in the core until it was ready to be used.

[1] https://en.wikipedia.org/wiki/Mark_18_nuclear_bomb

A Intercepted supersonic re-entry vehicle is pretty much guaranteed to fail catastrophically, due to the speeds involved.

That said, special extremely low sensitivity explosives are used in nukes now a days, so it’s not clear the explosives themselves would go off at all, correct.

Either way, it’s decently likely a cloud of plutonium or enriched weapons grade uranium will be released. Not ideal.

Not as big a deal as one would expect though.

> a cloud of plutonium or enriched weapons grade uranium

Sure, I don't want to standing under a kilo of plutonium when it gets transformed into dust.

But the thing was supposed to detonate at (say) 8,000 ft; presumably it's intercepted well above that altitude. The dust is going to be spread over a wide area, and I suspect the excess deaths are likely to be about zero.

Agreed. There is also a chance of a fizzle (tiny fission reaction - in the hundreds of tons at most - with negligible energy output), but it’s not going to have much of a different outcome frankly.

Compared to a 100 kiloton to 5 megaton explosion, it’s nothing.

'does not reach criticality' means you'll not get a thermonuclear explosion.

Or a notable fission reaction.

Or a pony.

> I don't see why damaging the delivery vehicle should affect the bomb, unless the vehicle explodes catastrophically, destroying the bomb.

You no longer have a reentry vehicle, you now have a man-made meteorite burning up in the atmosphere.

I can sleep peacefully at night knowing that North Korea doesn't have a chance in hell of nuking Seattle because I have some understanding of how stupidly hard it is to keep a reentry vehicle from turning into a meteorite.

The flip side though is how stupidly hard it is to intercept something going that fast.

>But don't they go to a lot of trouble to ensure that conventional charge dowsn't go off except under the correct control?

Yes, because even without the nuke it's a respectable sized bomb and you don't want one "just going off" when someone crashes an aircraft or has a missile silo fire or whatever, even more so when it's likely to spray plutonium all over the place which is likely to be an expensive facility or vessel you care about not causing downtime for.

The charges will not be like nitroglycerin, but they will be highly explosive and will detonate when sufficiently energized. Under the right circumstances the kinetic energy from a fragment of an interceptor would suffice.

Most modern high explosives won't detonate at all in response to a kinetic insult. They need a detonator, and usually a primer too. Even TNT; you can hit it with a hammer, shoot at it, or try to set it on fire; it won't explode. To set it off, you need a supersonic shockwave.

To my circumstantial understanding, it is plausible that fragments of an interceptor detonated in proximity to the warhead could achieve supersonic velocities capable of delivering the requisite shock, but I lack the knowledge to argue the point.

And it'd still need to detonate the difference sections in exactly the right timing to get a successful nuclear explosion.

> The charges are not like nitrogycerin, they won't go off on impact.

They will most likely go off if the warhead gets exploded tho.

I doubt they test how the nuclear bombs react to getting shredded by AA

Not bloody likely. Its a very delicate balance of geometry and timing. Hit it with a hammer (blow it up) and that's all destroyed. You'd get the same effect as blowing up a pile of parts not yet formed into the missile. That is, some radioactive material vaporized and spread around.

I meant that the charges will blow off from explosion, not that they will blow off in exact way needed for nuclear bomb to happen

Ah. More likely.

> I doubt they test how the nuclear bombs react to getting shredded by AA

They don't need to bother testing against conventional AA because of how unlikely it is to ever happen.

Something like a Russian ZHU has an effective range of 1.4 km

A reentry vehicle travels around Mach 13 or 4km/s so it will only be in effective range for 350 milliseconds. Good luck shooting it down. You won't even be able to track the thing and it would just be the luckiest blind shot in the world.