#apfsds

let me add some context here -- this ask is in response to a post i made about tank ammunition, specifically about how modern long-rod penetrators (the kind of ammunition you fire at hard targets like tanks, bunkers, etc) don't bounce. full stop. i don't mean that their tendency to bounce is so small that it effectively vanishes to zero, i mean that there is no circumstance under which the penetrator could bounce

the fast and dirty explanation is that the human tendency to interpret physics from the perspective of the types of physics our mind works in. for example, the speed of sound through air (relatively slow) may as well be instantaneous to us. the moment someone's mouth starts moving, your ears hear what they are saying. only under extreme circumstances (yelling into the grand canyon and hearing the echo, or witnessing an explosion far away only to hear the bang several moments later) can we tell the difference. and the speed of sound through solids (relatively fast) may as well be equal to the speed of light, to our human senses.

there is this tendency i've noticed, when people talk about HEAT rounds (those things i wrote about in this post) is that they try and argue about whether the HEAT jet is a solid, a liquid, a plasma (?), or otherwise to define it in the physics we deal with as humans. HEAT jets have velocities on the order of 10,000 meters per second, which is like north of 22,000 miles per hour. the fastest F1 race cars travel on the order of like, 100 m/s or several hundred mph, and that is probably pushing the human limits of velocities we can "deal" with. the difference between speeds much faster than that, to us, may as well be the speed of light

imagine if you teleported, idk, aristotle to the modern day and presented him with a large submarine and demanded he explain what he's looking at. he might try to rationalize it as some sort of large whale-like sea creature, which is a hopelessly flawed basis to try and understand a submarine. looking at a subcaliber APFSDS penetrator and asking why it can't "bounce" is much the same.

that is the tldr, now let's look at the fun stuff. lets look at a "subcaliber APFSDS penetrator":

M829A4, armor-piercing fin-stabilized, discarding-sabot round. fired from a tank. this is the long rod penetrator i mentioned, although you can only see the very tip of it in this photo. the rest of the body is full of propellant and primers and that sort of thing. the diameter of the round above is the same as the tank's barrel! keep this in mind. it's one of three types of ammo tanks carry, the second being one you can imagine as a "scaled up rifle cartridge with the bullet hollowed out and filled with explosive". that is what you shoot anything that isn't thick concrete/steel. we're not concerned with that today, and we're certainly not concerned about the secret third type of ammo. let's see what M829A4 looks like naked:

well, not naked, more like cut in half. see that long, shiny dart-shaped thing in the center? that's the penetrator. the black bits are propellant (explosive, "gunpowder"), and the top, less-shiny part is the sabot. we'll get to that later. this penetrator is made out of "DU" which misleadingly stands for "depleted uranium", but is more accurately described as "depleted uranium-238 alloyed with some kind of crazy, classified shit that gives it insane material properties". before this, it was some kind of tungsten carbide, and before that it was probably 4140 alloy steel. you see the trend here -- denser, heavier, stronger penetrators do better. the hallmarks of penetrators are that they are 1.) long 2.) strong 3.) gotta get the friction on

notice how the penetrator is very thin and recall earlier when i wrote that the shell itself was the diameter of the tank's gun barrel. that's curious -- if the penetrator is so much more narrow than the barrel, does it just bounce around in there before popping out? it does not. that is what the sabot is for -- it fills that space, sealing the barrel, and allowing the gas pressure from the explosives to entirely put its energy into the projectile and not just escape around it. the critical thing about this discarding sabot, is, that it discards. the moment the penetrator + sabot assembly leaves the barrel, the sabots fly off, and now the penetrator has all the energy of a f#%k-off large barrel cannon concentrated in a tiny dart, with only a fraction of the aerodynamic profile. so not only is that mofo moving faster than our natural senses can comprehend, it is moving faster than our derived, logical understanding of normal gun ballistics can comprehend as well

so, after the tank goes boom and the penetrator is now moving a bit shy of 2,000m/s, it finds its target, let's say another tank, in fact, let's say it is an strv-103 (the stank!) which i wrote about here, and looks like this:

i choose this example due to how sloped the upper profile is -- the perfect recipe for what should be a "bounce" -- this is the angle of incidence mentioned initially. the penetrator (remember, dart-shaped thing) hitting something that is sloped only a few degrees would, in a lower-energy circumstance, elastically deform upon impact, e.g. would soak up some energy from the stank's armor "pushing back", that energy would be in the form of internal tension/compression, which would eventually culminate as a change in vector (direction) of the pentrator. that is a long-winded way of describing a bounce

the reason the penetrator won't bounce is because that elastic deformation/internal tension & compression i mentioned are all subject to the speed of sound. it takes some amount of time for pressure waves to propagate through it before it can react accordingly, and that amount of time is substantially less than what it would take for the penetrator to meaningfully change its vector (its "direction"). more simply put, the ass end of the penetrator does not even "know" the the front end of it has hit anything until it is too late.

you can make the angle of impact as thin as you want, it won't make a difference. things moving at these velocities do not react like things that move at human velocities do. remember that old tumblr blog that posted GIFs of like, early 2000's/late 90's physics simulations? the famous "pear wiggler"? if you look at simulations involving very high velocities, you'll notice that things just sort of "mush" into each other as if they were both play-dough

i hope that makes enough sense. let me cover one other nit though, which comes from people trying to comprehend APFSDS penetrators as if they were bullets fired from a pistol or rifle or something. those bullets fail to meet criteria #1 of the penetrator: they are not long. they are short, they have a small moment of inertia, and therefore we fire them out of rifled barrels to make them spin very quickly in addition to move forward very quickly, which stabilizes them in flight. a penetrator is long, has a high moment of inertia, and in every case but a geometrically-perfect computer simulation, any imparted spin's error will quickly compound on itself and cause the penetrator to tumble through air, no good. the thing with bullets though, is that rotational inertia will cause its trajectory to "corkscrew" upon impact which kind of over-exaggerates what we'd see as a bounce or ricochet. APFSDS rounds are shot out of smooth-bored* gun tubes and don't spin**, and use fins to achieve stabilization instead

tanks for reading. writing all of this has helped evaporate some mental stress that might have otherwise caused me to make a very poor financial decision today.

Also your art is great! and one of my gfs has used it to tease me in the past... and probably will again so, thanks for helping with that c:

You have to tell me which piece(s) now. Its the law

Newly completed on #Twitch stream: River hefts an APFSDS round as she poses.