In my column "IEDs: the Lazy Man's Insurgency" (eXile 226), I did my best to explain how shaped charges work. And let's face it, my best wasn't that great. I'm no physicist. Luckily, some of my readers are. In fact, a reader named Ryan Caveney, who's not only a physicist but a damn good writer, sent in a great explanation of how anti-armor warheads work. It sure helped me understand how a wad of soft metal can penetrate hi-tech tank armor. If you want to understand the ultra-cool physics involved, read this article.
You know, messages like this make me realize how weird it is for a CC dropout like me to be writing for people who are smarter and more educated than I am. All I can say is, I'm grateful you rate me on my performance instead of my credentials--because I ain't got none.
And while I'm at it, I want to say thanks to all the readers who send in tips and ideas for columns, comments, site links and other info. I'm not that social a person so I don't write back much but I read everything and check out the links and info you send me. Just wanted to say I appreciate it. OK, that's enough of that. Back to business.
Dear War Nerd,
I am a physicist, as well as another fat war nerd. The shaped charge stuff, crazy as it sounds, is real. It's been around since a bit before WWII: bazookas, Panzerfausts and the like (every "HEAT" weapon) are all based on the effect. The details of the physics still aren't well understood, but I'll give the explanation a shot.
You've already got the basic idea: "blast alone doesn't do a good job".
You need to take a blast that would otherwise blow outward in all directions and channel it into the narrowest jet you can make, concentrating as much force as possible into actually hitting one point on the armor instead of just scorching the nearby grass. The metal cone (whether it's copper is not very important; see below) acts somewhat like the sabot from an APDS round. It starts out as a cone, so there is a lot of expanding gas from the burning propellant pushing on it. However, the gas pushes so hard that the cone folds up into a long, thin rod, causing all that energy to be concentrated onto an even smaller area; doing this also causes the expanding gases to form a narrow jet.
The cones actually have to be pretty thin for this to work. As a rough approximation, pressure is what penetrates, and pressure is force divided by area. This is why a very sharp knife (sharp = small area of the cutting surface) easily cuts skin even when pushed very gently, but a baseball bat has to be going a lot faster (if it can ever be said to "cut" at all). Very narrow and very fast can produce amazing results: tornados can drive pieces of hay clean through telephone poles.
As for "solid [but] behaves like molten", consider this: is clay a solid or a liquid? How about sand? Both of these comparisons are relevant. For one thing, hot metal (though still far from liquid) is a lot softer than the cold stuff; this means the squished metal cone and the superheated gases driving it put a lot of heat into one small region of the armor in a few millionths of a second, making it a lot easier to push something through it. To some extent, the hot metal and gas melt their way through the armor. Second, there is good evidence that the metal cone actually shatters into lots of tiny little pieces all traveling in a line. When it hits the armor, it acts like superfast Chinese water torture -- a whole bunch of mini-warheads hit the same spot one after the other, each one drilling a little bit further through. If you use a lead target for testing instead of real armor, you can actually see the individual craters from the train of microscopic fragments.
The metal cone doesn't need to be copper: steel, aluminum, magnesium and tin have also been used. In the weirdo realm of shaped charge physics, hardness is a lot less important than density (hence the M1's use of super-dense depleted uranium in its armor). Hardness is related to how well large regions of the metal stick together, but the HEAT impact is so narrow and so fast that it's more like a bunch of one-on-one contests between a cloud of tiny particles, in which the heavier guy has an advantage. Another strange variable is the speed of sound in the metal cone, relative to the speed of the gases produced by the detonating explosive -- the closer the match, the better the energy transfer. Hence different propellant explosives do better with different metals for the cone.
hobham armor is designed specifically to counter shaped-charge weapons. Available info on the M1 indicates that what "Chobham" means is a multilayer sandwich of metal and ceramic. As the shaped charge penetrates each layer of metal, it pulverizes the next layer of ceramic, which sprays back out the hole, disrupting the focused gas jet driving the penetrator.
This is almost exactly how explosive reactive armor works, except without the side effect of spraying nearby friendly infantry with shrapnel. Trouble is, it tends to break down on its own over time (such as while driving), so it's really expensive to maintain. The main point to remember, however, is this: a given thickness of armor can stop projectiles of up to a given power. This is why an
RPG-7's shaped-charge round is only a moderate threat to an Abrams, but a shaped-charge IED based on a big enough explosive is essentially unstoppable by anything tank-sized that isn't completely solid.
The trick to "big enough" is that the depth of explosive behind the metal cone is not important: past a certain point, it can't effectively be focused, so it just blows out uselessly to the back and sides. The effective armor penetration depth of a shaped-charge round is determined by its diameter, which is why RPG warheads are so much wider than the firing tube. Therefore, just piling a few mines under an existing HEAT round doesn't help -- but taking them apart and making a new, much wider device with a much wider and deeper metal cone is a terrifying weapon.
Thanks for letting me ramble on,