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Comment by daenz | Hacker News Reader

I'm not sure I fully understood the explanation. So the copper cone is turned inside out and turns into a pointed wire that drives into the target? How is it that this wire can continue through the 1ft of steel? Is the force of the explosion flowing through this wire/tube, like liquid in a straw? And somehow it can sustain this through 1ft of steel?

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Here's a vid I found that seems to show a simulation of the effect: https://www.youtube.com/watch?v=CpVVGk2OfQQ

Great find @ 0:40. The only thing it doesn't convey very well is the older man's explanation of the copper spear curling outward on itself. From the simulation, it seems like it is just punching through and maintaining its rigidity somehow.

iirc, nothing is rigid, it's just very high energy jets, so they don't deform much. like particles in water jets cutting steel.

Fun fact: water doesn't actually do the cutting, the water is just a medium for carrying an abrasive dust (ruby/sapphire/diamond) that abrades the steel.

We use 80 mesh garnet.

It appears to rely on the copper’s ductility. Curious if a gold or silver shaped charge has been modelled.

I found https://apps.dtic.mil/sti/citations/ADA278191 which has this to say about gold:

>Hypothetical shape charge liner. High density is a benefit but equally low speed of sound indicate s that it may be no better than uranium. An advantage may be found in its ductility, if it leads to greatly improved jet formation relative to its cost.

It seems that sound speed and density are considered more valuable, so tungsten, zirconium, molybdenum.

It's been used for plutonium. The explosive lenses used in implosion-type nuclear weapons are extremely-precise shaped charges, and work by focusing all the explosion's energy into compressing the plutonium down to critical mass:

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

https://en.wikipedia.org/wiki/Nuclear_weapon_design#Implosio...

Aha!

So the shape is such that all parts of the cone converge at the center, so the full energy of the explosive is focused in that thin column.

The time scale on those is very interesting.

Sorry to answer my own post, but I watched it again and picked up on the key idea at 2:09.

The inverted apex of the cone drives into the steel, pushing the steel aside, but then the apex opens up and flows back along the outside of itself. In other words, any given part of this copper "wire" interacts with the steel only long enough to push it open, then it is replaced by new copper.

From this explanation, it sounds like the tunnel that has been "bore" through the steel would be completely coated from start to finish by the inverted copper cone.

The mental image which is working well enough for me is a series of tightly packed bullets with each one digging the hole a little deeper than the last one.

Of course the shaped charge isn't a gun though and the stream of copper doesn't travel for hundreds of meters/yards. If you need to stand off from the target you'd want to mount it as the payload of a rocket and fire towards thine enemies.

EDIT: this is of course a better description:

https://www.globalsecurity.org/military/systems/munitions/bu...

So the rod is traveling so fast that it and the target act like liquids, even though it isn't molten.

Imagine a picture of the jet coming out of a black hole.

Something very roughly similar is happening here. The shape of the cone as the detonation wave goes through the explosive forces copper into the center and the only (or lowest resistance) way out is a straight jet through the target material.

due to the copper cone shape and the explosion wave propagation from the back the copper, which at those pressures is flowing like a liquid, is formed into a jet and pushed forward like a water in a power washer hose. Water at 3000bar would cut several inch steel (https://www.youtube.com/watch?v=quL14Csmi_Y). The copper in that explosion gets accelerated to the speeds of like 10km/s and as a result cuts through everything in its way.

A bit of current context - beside deep penetration the jet isn't that destructive to surroundings. And here comes design flaw of Russian tanks - getting inside a Russian tank such jet frequently hits the tank's ammo which in those tanks is stored in a carousel around/under the turret and thus it results in the whole tank ammo explosion which even throws the multi-ton turret several stories up into the air (you can see a lot of the tanks blown up in that way - 'lollypops' - in Ukraine). To compare - US Abrams tank has ammo stored in separate compartment in the back of the turret.

FYI, waterjets cut by using water to carry an abrasive media which is injected into the stream at/near the nozzle, not by the action of the water alone.

You can use an abrasive component but you don't have to, pure waterjet cutting works. And depending on the materials and the nozzle shape might actually be the optimum.

Also crucially, the munitions compartment is weaker on the outside of the tank than on the wall between it and the crew, so if it goes, it blows out of the tank (search term “blowout panels”)

Fireworks and ammunition factories and storage facilities tend to have strong walls and relatively weak roofs for the same reason: if a building goes boom, you don’t want it to make neighboring buildings go boom, too.

I'm impressed (and a little confused) at the idea the explosive is essentially sitting untethered on the target. I would have thought the entire contraption would shoot up with the explosive force being reflected off the ground, but I suppose there's enough energy in the initial blast (before the reflection) to form the copper 'bullet'. =

The reason it works is nothing has time to get out of the way - all this is well past the speed of sounds for all materials involved. So the action/reaction is happening - the gas from the explosive is recoiling for instance - but it only has time to go a few inches and the forces and counter forces are immense.

It's not molten but it could be and it would still work as the guy in the video invented water-lined shaped charges (for disabling mines and IEDs without making them explode). What's important is the force/pressure that pushes steel to the side.

It functions more like a jet of water. The speed that the solid copper moves at pushes the steel aside as if the were both liquids

The velocity of the jet is typically around 10 km/s. There is enough pressure to get trough.

A major part of it is that the copper is going extremely fast.

But also see other explanation, about being renewed.

Update: this is incorrect.

The cone becomes very hot stream of molten copper. It melts through the steel plate, much like a jet of hot water melts through a block of ice.

This happens fast enough that much heat does not have the time to escape from the impact site, despite high thermal conductivity of metals. The high pressure created by the explosion keeps the jet compressed from sides, too, so it does not fragment easily.

Various kinds of "active armor" trigger the munition by a thinner layer of metal well ahead of the real thick armor plate, then produce counter-explosions to break the jet.

The video specifically says it’s not molten.

It's not actually liquid, it only behaves like one on impact due to the magnitude of the forces. It's more accurate to say it erodes through the armor instead of melting.

> much like a jet of hot water melts through a block of ice

cold water can cut through steel also...https://www.youtube.com/watch?v=v0NVOThRooE

Waterjet cutting uses water to propel a powder which acts as an abrasive, like an infinite stream of sandpaper. I don't think water by itself will achieve much.

you learn something new everyday

Huh, thanks for mentioning this! I always thought it was just water with enough pressure.

It looks like pressure cutting with pure water exists but it is limited to softer materials. This page [0] has a fair bit of detail on it all.

[0] https://www.machinemfg.com/waterjet-cutting-guide/#Classific...

Sounds like pressure washing vs sand blasting. Only difference between the two (aside from drastically different effect) is sand blasting has an intake tube to suck sand into the water jet.

People have made DIY waterjet cutters with pressure washers and a sand feeder: https://www.youtube.com/watch?v=Lg__B6Ca3jc

Waterjeting without abrasive is a thing, it's used for soft materials like rubber... and there are some companies marking higher pressure approaches for metal (though usually with tiny amounts of abrasive rather than none at all).

It’s also used in commercial food prep.

Water is not doing the cutting there. Water is just the carrier for garnet abrasive, which does the actual cutting. Garnet is roughly three times harder than steel, so at high pressure cuts it very effectively.

Maybe that's what happens but the video explains it as a stream of copper (not molten?) that pushes the steel out of the way.

It’s not molten, and heat plays no role in the penetration ability. This is a very common misconception.

It’s simply focused kinetic energy that does it. The cone focuses the copper into a slug like object, and the slug becomes similar to an extremely powerful bullet.

So it's like that old picture of the grass straw driven through the telephone pole by nothing but hurricane wind?

The flimsy straw could do it simply because of how fast it was moving. The strength of the straw doesn't matter, simply it's mass, moving that fast, carries itself through, ie the leading edge is not being pushed from behind like a nail, more like a bullet with a string attached?

Setting aside the simplification, that probably the mass of the rest of the straw does play some part not absolutely zero, is that a reasonable way to conceptualize it?

Yes, that's reasonable. By the time the jet hits the target the explosive provides no motive force whatsoever: it's purely a momentum game.

Are you certain that a hurricane can blow a straw through a telephone pole? I hadn't heard of this phenomenon and was interested. I was unable to find a good source.

According to this article, the National Oceanic and Atmospheric Association thinks it's not possible:

https://www.livescience.com/39270-tornado-straw-into-tree-wo...

(Alternative explanation in the article: the tree twists during the hurricane, cracks temporarily open, and debris gets stuck in the cracks.)

The picture (perhaps multiple but at least one) was a piece of hay poked right through a telephone pole, in the aftermath of some kind of storm. Not a tree. It was shown in grade school in the 80s or late 70s.

What would happen if you just started with a slug?

I don't think you could deliver as much energy to it because it has such a small surface area normal to the direction of travel.

It’s really hard to get a slug Moving like that without tearing it apart. It’s easier to use the explosive to spread the force over a wider area initially but in a way that the projectile is formed by the concentration of the explosive forces mashing it all together. Weird.

A plasma technically. And it doesn't exactly penetrate, it pushes the steel atoms aside very literally like sticking your finger in butter.

Edit: obviously it penetrates, but not in the way that you would expect.

Certainly not a plasma. It's a coherent spear of hydrodynamic copper, not a bunch of charged ions floating around.

As far as I know, it could be little qanons in there stealing the steel atoms to build jfk jr's moon rocket. However, the principle physicist for the nuclear emergency search team described it using the butter analogy.

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