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Comment by AgentK20 | Hacker News Reader

Keep in mind that in order for a nuclear explosion to occur, a very precisely timed series of explosions must occur within the device to trigger fission (and then potentially fusion, depending on the type of device). If the inbound missile is kinetically damaged or destroyed before it gets the chance to trigger, then the conventional explosives will likely detonate within the missile in a way that does not reach criticality, resulting in a conventional explosion with a small amount of fallout from the nuclear core that wasn't triggered being dispersed.

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Shaped charges are used to penetrate Russian tank hulls in Ukraine, and work by using explosives shaped in a very specific way to invert a copper cone and turn it into a hypersonic jet of hot goo.

A nuclear device is an spherical array of them, all focused inward. If any of them goes first, it pushes things out of the way, and you get a "fizzle"

You also have to shower things with a neutron pulse at just the right time to kick off the chain reaction.... while all the inbound hypersonic jets of goo are in flight.

I'm amazed they were ever able to pull it off, let alone make them reliable.

> then the conventional explosives will likely detonate within the missile in a way that does not reach criticality

Why?

The primary in a thermonuclear bomb needs careful timing of the conventional charge. But don't they go to a lot of trouble to ensure that conventional charge dowsn't go off except under the correct control? The detonators are timed and triggered electronically. The charges are not like nitrogycerin, they won't go off on impact.

I don't see why damaging the delivery vehicle should affect the bomb, unless the vehicle explodes catastrophically, destroying the bomb. And unless, I suppose, there's a failsafe installed, so that a malfunctioning vehicle automatically disables the bomb. That would make sense to me; I don't want my ICBM exploding over some place that wasn't on the target-list.

Basically, I want a warhead that goes off correctly over the target, or not at all. That appears to be a fairly straightforward engineering challenge; as they say, it's not rocket science.

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

Or a notable fission reaction.

Or a pony.

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.

> 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

> 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.

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.

>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.

> 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.

https://www.youtube.com/watch?v=zVhQOhxb1Mc has a good explanation of how nuclear weapons work.

It may be possible that a missel is disabled and when it hits the ground, produces a much lower yield explosion than designed for, in particular a plutonium device, if I recall the video and extrapolate with sufficient accuracy

It is basically impossible for a plutonium bomb to generate anything resembling what you think of as a nuclear explosion unless it is deliberately triggered and experiences ideal ignition conditions.

If it were to crash into the ground, be blown up by an intercepting missile, or otherwise malfunction, you'll just get a normal-bomb-sized regular conventional explosion and a bunch of toxic and only mildly radioactive plutonium strewn all over the place.

Just adding about the so-called "dirty bombs" (https://en.wikipedia.org/wiki/Dirty_bomb). It's not dirty because it's lethal but rather because the construction of these bombs are rather crude, which allows nearly everyone with some knowledge of bomb building to construct them. In some sense, this is more dangerous than a proper fission-type nuclear bomb.

The explanation you gave for why a dirty bomb is called as such doesn't seem to be supported by the link you provided. Rather, it's dirty because it contaminates the area around the explosion with radioactive material.

That is not correct

The vast majority of effects of a dirty bomb are foiled by such advanced technologies as windows and showers. The initial conventional explosion will kill and injure far more people than the radioactive dust. At worst, that dust will slightly increase cancer rates for people relatively close to the location of the bomb (say, upto a few hundred meters) over their lifetime.

Dirty bombs are weapons of terror, not weapons of mass destruction.

The true damage is caused by societal and economic overreaction from the target: fear leads to expensive security theatre, a desire for vengeance leads to lashing out with expensive unnecessary wars, uncertainty and political instability destabilizes financial markets, etc.

Some diseases are similar: immune system overreaction causes more harm than the actual invader.

> It's not dirty because it's lethal but rather because the construction of these bombs are rather crude

I don’t think that’s correct, and do not see that Wikipedia page say that. It says

“The purpose of the weapon is to contaminate the area around the dispersal agent/conventional explosion with radioactive material, serving primarily as an area denial device against civilians.”

I think that “serving primarily as an area denial device against civilians.” isn’t 100% correct. For use by terrorists, I think it’s more to instill fear than to make areas inaccessible to civilians (that still would happen, but isn’t the goal)

Dirty bombs are typically not nuclear. They are just regular bombs packed with radioactive materials like medical grade cesium sources.

While plutonium is mildly radioactive than fission products like strontium-90 and cesium-137, you can't discount its chemical toxicity and will accumulate in the bone marrow if ingested. Also add that alpha decaying isotopes (like plutonium-239 used in bombs) elevates the risk for lung cancer when inhaled.[1]

Definitely a lot "better" than a nuclear explosion, but it's no picnic to have fragments of a destroyed nuclear warhead to be scattered around.

[1] https://en.wikipedia.org/wiki/Plutonium#Precautions

Yeah, it would not be anything like the explosion people think when they hear nuclear. What I recall from the video is that Plutonium is ~3x more reactive than Uranium, and if you brought two 2/3 critical mass together with your hands, they would explode the room you are in and create a lot of nasty dust.

I'm hypothesizing that a falling plutonium warhead might experience more kinetic energy on impact with the ground, such that the fissible material would still chain react, more so than the example given in the video of bringing them together with one's hands.

The amount of plutonium in a bomb is subcritical. There is no way a haphazard impact would cause it to go critical. It requires a very precisely timed explosion of the spherical lensed charges surrounding the core to generate an insane amount of implosive pressure which squeezes the core to be many factors more dense and therefore reach criticality.

...to have an ideal, optimized reaction that causes a large explosion. If you watched the video I linked in another comment, you would hear someone who knows more than both of us describe how other reactions happen and play out. There's a lot more to getting good, high yield nuclear explosions than you describe as well. I explicitly said that it would not be anything like a well done explosion, but there will still be nuclear chain reactions happening, they are always happening because of nuclear decay. It's a difference of how fast

This is, imho, the correct answer. But setting aside the nuclear device, the rocket itself wouldn't survive any damage either.

Rockets aren't built to be robust because that adds mass, which requires more fuel and reduces acceleration. Better to just go faster, avoid the danger. But that means any damage to the rocket is probably going to doom it.

There's no rocket left after the boost phase. Everything separates off but the warhead and its encapsulating reentry vehicle -- a compact, tough shell designed to protect it from the stresses of reentry. So it's probably far from fragile.

Here's what those look like:

https://en.wikipedia.org/wiki/Multiple_independently_targeta...

The nuke could be programmed/signaled to actually detonate if it encounters an interceptor AND is within X distance of target territory. In which case, it is an atmospheric nuclear explosion probably with EMP effects.

This is one reason people stress missile defense that hits the nuke immediately over their launch site (boost phase).

For NK that might mean interceptor UAVs hovering off the Korean coast or ships. But for Russia or China it means a Starlink-style system with hypersonic interceptors staged in orbit around the globe.

https://wikipedia.org/wiki/Space_Development_Agency

This obviously has tons of flaws though. It would cause a feedback increase in nuclear stockpiling to counteract the fraction intercepted to ensure MAD.

>if it encounters an interceptor

How would it know? You can't have radar on it, because that would make it easier to hone in on, aiding interception.

Also, on re-entry most objects have a highly ionized shock-wave around them, making forward facing radio communications quite difficult at best (assuming you could manage the cooling, etc)

They (launcher of nuke) can use satellites to track their own missile and the interceptor.

Also separately, communication/radar works fine through reentry plasma once you go up to 20-50 GHz frequency.

that sounds like it still adds way to spot the nuke and intercept it.

My intuition says, due to the speeds and sizes of the two vehicles and latency of communications there'd be no way to figure out if the interceptor will successfully hit the nuke until it's too late.

If the interceptor misses the nuke's location by 50 microseconds it'll explode harmlessly a foot behind it. (The ICBM is probably traveling faster than the shockwave front of the explosive, depending on where it is)

I don't think a radar from satellite could be that precise, that fast, and get a signal out.

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