Chemistry of super heated AP rounds, a questions

[sgtabell]

Back in chemistry class, I had professor giving a demonstration of thermite, and more importantly, the explosion which is caused when iron/steel is superheated past a particular degree point, and it comes in contact with water.

He then moved to the point that when AP rounds went through a tank, and then came in contact with water, i.e....a human body, an explosion would result that was deadly in nature due to the concussion.

Was this a myth or real? I haven't seen this referenced anywhere, and reading the post below of "what is it like to be in a tank when it got hit?" thread reminded me of it.

Anectdote question....

Talked to someone in Desert storm who was in a tank. After combat, they were driving back to the base, and the driver was saying that something got hit, because the tank was driving funny.

They got out, and there were a number of shells embedded in the armor, which was causing a weight imbalance....is this a wive's tale also? (Would tank shells "stick" in the armor like a spoon in ice cream?

SgtAbell

[offtaskagain]

Shells certainly have been known to stick in armor. The weight imbalance thing doesn't sound very plausible though.

[MikeyD]

Modern long rod penetrators impart so much energy/pressure per sq cm of steel target that calulations that resemble fluid dynamics need to be used to model the effects.

As for ap shot versus human tissue -- I rather suspect besides the whopping big hole it would produce initially, the supersonic shockwave following in the wake of the shell would indeed cause the equivalent of an 'explosion'. A much larger scale version of what happens with a normal rifle round. Thev temperature of the income/outgoing round would have very little to do with the effect.

[ February 06, 2003, 02:38 PM: Message edited by: MikeyD ]

[flamingknives]

The explosion that you're talking about is essentially flash-vapourisation, i.e. the water is heated beyond boiling point so fast that the resultant expansion, due to Boyle's Law, is effectively an explosion.

I wouldn't have thought that it would happen when a human body was hit witha tank shell though, as the velocities are so high.

IIRC, water has a specific heat capacity of 4200 J/kg/K (joules per kilogram per kelvin), so it takes 4200 J to increase the temperature of 1 kg of water. Therefore, to convert the water in human tissue to steam(temperature rise 37 to 100 = 63kelvin) you need 264600 Joules for each kg of water, or 264.6 for each gramme. Unfortunately I don't know how much energy a tank shell carries. Help please?

Note: 1 cm cubed of sea water heated in an enclosed space will wreck a science lab

[offtaskagain]

Take a full can of soda and place on rock. Grab rifle. Point at can and shoot. Observe effects.

[demoss]

Therefore, to convert the water in human tissue to steam(temperature rise 37 to 100 = 63kelvin) you need 264600 Joules for each kg of water, or 264.6 for each gramme.

Actually, that only gets you to 100 degrees Celsius but still liquid. It takes still more heat to turn it into water vapor at 100 degrees Celsius, and I know it's a LOT compared to the heat required to increase the temperature one degree. So the amount is higher than you've estimated, but not dramatically so.

I'd also swear that the thermite reaction involved an aluminum/iron mixture, but it's been a long time.

[MikeyD]

Remember, the 'shock wave' is a rapid rise and fall of pressure as the shell parts the air, and boiling point is related to air pressure. So in the micro-environment behind the moving shell the airpressure drops to near zero for an instant. Still, any 'flash vaporization' would only be at the cellular level. They say a deer hunter will cut out and discard the meat surrounding the wound because of the destroyed tissue.

By the way, has anyone mentioned yet that this is a really grizzly subject?

[ February 06, 2003, 05:45 PM: Message edited by: MikeyD ]

[sgtabell]

Yes,,it is grisly....

I don't think I was clear. The iron,,being superheated by passing through metal, is supposedly what explodes. That somehow,,superheated iron compounds, when they hit water, have some type of explosion. (Other than superheated steam). Ok,,time to get out my transcripts and find the damn professor...

SgtAbell

[YankeeDog]

I have no idea what's going on when an AP round, superheated by friction, hits a human body inside a tank, but I can explain the "exploding soda can" phenomnenon.

One of the more odd physical properties of liquid water is that it's an incompressible medium. That's right, no matter how much pressure you apply to it, it just won't change in volume. Theoretically, there is some evidence that this may change at EXTREMELY high pressure, but the evidence is sketchy.

So anyway, when a high velocity object, like a rifle bullet, hits a mass composed mostly of liquid water (like a soda can, or a human body), interesting things happen. Since the water *can't* compress, it has to move out of the way of the bullet, VERY quickly. Hence the exploding soda can. Inside the human body, this "hydrostatic shock" is actually what causes much of the damage from a hit by a high-velocity round - the water in the body moves out of the way of the invading bullet at supersonic speed, damaging surrounding tissue severely.

I suppose that if the invading body were large enough (as in an AP shell), and the velocity were high enough, a projectile invading the human body might literally cause an 'explosion' from hydrostatic shock. Sounds messy to me.

Spontaneous creation of gases by the introduction of a superheated body to a cool liquid is another matter, and as mentioned can also cause an explosion-like effect.

Thermite is a totally different thing. Thermite is one of a class of aluminium-containing chemicals that burn at a VERY high temperatures - well above the melting point of steel. The important thing to recognize is that this is a combustion reaction, not an explosion. Burning thermite creates a blinding, flare-like flame (and LOTS of heat, of course), but no explosion. Somewhere around I have the plans for how to build a thermite bomb that will take out a tank by simply melting a hole in the armor through the massive heat generated. Kind of a weapon of desperation, but interesting nonetheless.

Cheers,

YD

[offtaskagain]

Thermite is extremely simple to make. Simply mix aluminum powder and rust powder. Being metallic it's obviously tricky to ignite. Usually a magnesium ribbon is required to ignite it. I've yet to try it but that's the common recipie for it.

[ February 07, 2003, 01:34 AM: Message edited by: panzerwerfer42 ]

[MikeyD]

I believe the HC smoke shell has some powdered aluminum content as well to initiate burning, along with the hexa... hexa... that HC stuff.

[Douglas Rudd]

If I may add, on the History Channel last year, they had a series about great weapons/equipment, one was devoted to the Sherman tank.

At one point they were talking about how weak the armor was. As they were talking about this, they showed a clip of a French Army Sherman that had just been hit by a 75mm round.

The crew had bailed out, and pulled out a badly wounded member that looked to have been hit by the round.

His right side had a chunk taken out about the size of a basket ball. <shudder>

Never did say how long he lived afterwards.

Doug

[MikeyD]

From the literature solid shot shells certainly don't 'explode' on contacting people. Too many stories about shells rattling around turret insides causing mayhem. If a round can pass through 150mm of solid steel it probably doesn't even notice passing thru that big bag-o-water in the process.