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Comment by nine_k | Hacker News Reader

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.

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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.

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.

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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