Ricochets very slow?

[Bulletpoint]

I've noticed that when tank armour deflects an incoming AT-round, often the shell will bounce upwards in a very slow, lazy arc, and eventuallly land nearby.

 

If I remember my basic physics, real-life high velocity projectiles will continue at almost their initial velocity (but in a different direction) when deflected. If the round is not deflected, either the armour or the shell or both will be destroyed.

 

Again assuming I understand the laws of physics, the slow deflection we see in the game would only be possible if the projectile converted most of its inertia into rotation. Is this what happens? Or is it just that the game engine is simplified to consider most of the shell's energy "lost" if the projectile fails to penetrate the armour?

Edited May 10, 2015 by Bulletpoint

[poesel]

There is also energy converted into 'deformation energy'. I guess this is a non-negligible amount but I cannot say how much.

[Bulletpoint]

There is also energy converted into 'deformation energy'. I guess this is a non-negligible amount but I cannot say how much.

 

Yes, but wouldn't that mean that the projectile shattered to bits?

[umlaut]

It has always been my impression that these slow ricochets were indeed rotating.

[womble]

Yes, but wouldn't that mean that the projectile shattered to bits?

No. Things can deform without shattering. The penetrators in AP rounds are designed to be very resistant to shattering, since that failure mode would be prejudicial to good penetration performance. Imagine throwing a tennis ball at a bank of sand. The ball will not bounce off the sand as fast as it hit it, the sand will be marked and the ball will be a bit warmer, having soaked up some energy in rubber hysteresis. And as umlaut says, they could be spinning end-over-end, very fast indeed. If an AP round doesn't penetrate, it will have spent a chunk of energy deforming the armour (and making a hit decal), some energy will have gone in sound, some will have been used to deform the penetrator, some will have transferred as KE to the target, to be bled off as heat in springs and shock absorbers. The spot where the round hits will probably be a fair bit warmer than it was, too. All these things combine to reduce the speed the ricochet is travelling at.

[Bulletpoint]

No. Things can deform without shattering. The penetrators in AP rounds are designed to be very resistant to shattering, since that failure mode would be prejudicial to good penetration performance. Imagine throwing a tennis ball at a bank of sand. The ball will not bounce off the sand as fast as it hit it, the sand will be marked and the ball will be a bit warmer, having soaked up some energy in rubber hysteresis. And as umlaut says, they could be spinning end-over-end, very fast indeed. If an AP round doesn't penetrate, it will have spent a chunk of energy deforming the armour (and making a hit decal), some energy will have gone in sound, some will have been used to deform the penetrator, some will have transferred as KE to the target, to be bled off as heat in springs and shock absorbers. The spot where the round hits will probably be a fair bit warmer than it was, too. All these things combine to reduce the speed the ricochet is travelling at.

 

As I understand it, AP-shells were designed for hardness. Hardness means less likely to deform, but also more likely to shatter. Of course, you don't want your shell to do either, but I think it's one or the other. I also don't think these shells were elastic, at least not like tennis balls that deform and then spring back. The most probable thing to my mind is the rotation scenario, with a small proportion of energy lost to the factors you mention. But I would surely like to ask a tank armour designer about this

[Thewood1]

AP rounds can and do penetrate without shattering in real life.  Ricochets are no different.  Sometimes they might shatter, sometimes they just gouge out some armor and deflect off.  A lot of energy gets lost on deflection, especially if a significant mark is left on the armor.

[womble]

As I understand it, AP-shells were designed for hardness. Hardness means less likely to deform, but also more likely to shatter. Of course, you don't want your shell to do either, but I think it's one or the other. I also don't think these shells were elastic, at least not like tennis balls that deform and then spring back. The most probable thing to my mind is the rotation scenario, with a small proportion of energy lost to the factors you mention. But I would surely like to ask a tank armour designer about this

No, they're designed for strength, both hardness and toughness. It's possible, with good materials tech, to have metals that are both tough and hard. Simple processes, yes, tend to favour one over the other, but a brittle cast iron penetrator would be as useless as a soft wrought iron one (two use two diametrically different Fe alloys) against good armour plate. The former would shatter before transferring its energy to the target (i.e. expend all its energy breaking itself up), and the latter would squidge into a lump (i.e. expend all its energy smearing itself across the target). So penetrators have to have good hardness and good toughness. Sometimes it goes wrong, and there are phenomena like the "shatter gap", but that's restricted to a fairly narrow set of circumstances. The penetrator-armour battle is a contest of who can build a penetrator that is both hard enough and tough enough to beat the increasing toughness and hardness of the target plate. Sometimes the armour has what turns out to be suboptimal characteristics, as the face-hardened plate the germans used for a while - that turned out to be too brittle for the sake of the hardness.

 

Any material is elastic. It's entirely possible for an AP round to be elasticly deformed by a glancing impact. The energies involved, though, pretty much ensure that any deformation of an AP round striking square and not penetrating is going to be greater than the elastic limit of the materials involved; this is not true of thrown tennis balls. Perhaps the example of a squash ball splitting would be easier to get your head round? Or a table tennis ball denting. You'd have to fire such things at sand pretty fast to plasticly deform them though, and that'd probably bury the projectile: score one for that delivery system in the projectile v armour war. Whether the projectile returns to its original shape or not is completely irrelevant. Any deformation takes energy, and that energy isn't necessarily returned to the projectile as KE, if at all. The energy needed to scoop a dent out of 80mm steel armour plates is pretty large.

[Bulletpoint]

Good points, Womble. I find it difficult to get my head around this intuitively. Maybe because the forces involved are so far beyond anything we find in daily (civilian) life.

 

I've seen lots of pics of AP-shells, and lots of pics of holes in tanks, but I've never seen any pic of AP-shells after impact (apart from one that shows a shell imbedded in a half-penetrated armour plate).

[womble]

Just for giggles, I've seen footage (on the old BBC science programme "Tomorrow's World", I think) of an ordinary paraffin wax candle fired out of a shotgun (replacing the birdshot in a normal cartridge with the candle) through an oak door. All that was left intact of the candle was the wick, with a few bits of wax on it. But it made holes in the door. Can't find equivalent footage, but a swift google shows the loony german slingshot maker guy shooting a candle through some chipboard with a 1000N slingshot...