T-80 Tanks

[zraver]

Optionsss,

Ek (kinetic energy) is the total energy that is avaible for a KE round. Round design simply seeks to increase the delivery of this energy. Western rounds seem to need 13.5K or so joules to defeat each mm of RHAe based on estiamtes. Russian rounds need around 15.4K to do the same. One can infer the longer penetrators bsides adding mass also add efficency or energy transfer (13.5 vs 15.4)

 

[optionsss]

Penetrating is deforming. The thing that counts is therefore kinetic energy. (kinetic energy pr. cm^2 is more accurate)

An easy example:
3 kilo coconut moving at 1m/s: p=m*V= 3*1=3 Ke=1,5
3 gram bullet moving at 1000m/s: p=m*v= 0,003*1000=3 Ke=1500

These two have the same momentum, but I think you know which penetrates most armour. (and, yes i know the surface aerea is different, just a silly example)

I still don't follow, but the coconut and the bullet would have the same KE as well, but the bullet would go deeper because it's smaller surface area thus generate a much bigger pressure and in a shorter time frame. This could be describe by impulse.

Penetrating is deforming

I think I get this a little, the projectile will do work on the Tank to deform the thing and from that process energy is lost. Since we have a very small object hiting a very big tank, all the energy will be used up to deform the thing. This would be a very crude estimation through. A lot of energy will be used to produce heat, light and sound ect...

the US M829E3 is 830mm IIRC

Assumming they all have similiar shape and density, then their mass would be a function of the ammunition's length and radius^2. So, if the 700mm estimation is true from the previous pic then volume ratio of the chinese ammunition vs american ammunition:
700*62.5^2/830*60^2=0.915

the M1A2's 120mm L/44 M829E3 weights 10kg

10kg*0.915=9.15kg

Assuming that 17kj/mm is correct. Then:
17kj*850=0.5*1780^2*M
M=9.12Kg

So, if the pic is true, then 850mm is reasonable. Esp, the Chinese gun seems longer and have more internal volume. If all other aspects are the same, then in theory it should be able to push a projectile to a higher speed. Again these are a very very crude estimation.

 

[fishhead]

optionsss

Momentum is a very useful tool but it's mainly used to calculate the speed and direction change after hard object collision in the free space, assuming the KE converted into other forms is known or none.

In a missle punching a tank case, the tank basically doesn't move and all KE of missle is basically converted into heat or disform of metal, then the momentum calculation is useless here.

 

[Skorzeny]

I still don't follow, but the coconut and the bullet would have the same KE as well, but the bullet would go deeper because it's smaller surface area thus generate a much bigger pressure and in a shorter time frame. This could be describe by impulse.
.

No, they won`t . the coconut have Ke 1,5, while the bullet have 1500, a thousand times higher kinetic energy.

Ke=0,5*m*v^2 (V^2 Being the important factor)
so two objects having the same momentum, can have wildly different kinetic energy

 

[optionsss]

ft, I conpletely forgot about the v^2 for the coconut one...,
Yeah, that's right, the coconut would have much less kinetic energy. My bad, haven't touched physics for almost 3 years, only after my first year univeristy.

 

[fishhead]

No, they won`t . the coconut have Ke 1,5, while the bullet have 1500, a thousand times higher kinetic energy.

Ke=0,5*m*v^2 (V^2 Being the important factor)
so two objects having the same momentum, can have wildly different kinetic energy

I wouldn't endorse this kind of argument. Even the KE is same, the impact still maybe different, momentum plays some role as well.

But applying momentum in this kind of application is very hard, since momentum analysis needs to get all objects as the whole involved. A missle hit a tank, the tank impacts the earth, but the result of earth moving is usless.

The energy is more easy way to measure, but not very accurate way I say as well. Since energy can be converted into heat or other forms while momentum can't, so people choose it as an unit of conveience.

 

[Skorzeny]

I wouldn't endorse this kind of argument. Even the KE is same, the impact still maybe different, momentum plays some role as well.

But applying momentum in this kind of application is very hard, since momentum analysis needs to get all objects as the whole involved. A missle hit a tank, the tank impacts the earth, but the result of earth moving is usless.

The energy is more easy way to measure, but not very accurate way I say as well. Since energy can be converted into heat or other forms while momentum can't, so people choose it as an unit of conveience.

How in the world would you use momentum in this scenario? If you look at my silly example, you see that momentum don`t really tell you anything in this kind of impact. And momentum can`t be be lost as heat? Momentum is connected to Ke, and when Ke is lost, you loose momentum. Remember you have the same factor in both equations. reduction in Ke means reduction in velocity which means reduction in momentum.

Momentum is a useless property in this case

Energy lost to Heat, plastic deformation etc doesn`t mean its not accurate, its just impact physics

 

[fishhead]

Read my post carefully, I am saying the samething.

But I say just using energy is not enough as well, the impact depends on the time you release the energy.

 

[Skorzeny]

Read my post carefully, I am saying the samething.

But I say just using energy is not enough as well, the impact depends on the time you release the energy.

I agree.
There are countless factor being necessary to accuratly estimate of armour penetration, but to make a rough estimate, kinetic energy is the right number. Higher Ke means higher potential penetration. Penetrator weight is important because a heavier penetration wil retain its energy longer and have less loss to drag. (if similar Ke)

 

[optionsss]

How in the world would you use momentum in this scenario? If you look at my silly example, you see that momentum don`t really tell you anything in this kind of impact.

After thinking about it, if it is impact, then momentum can be used and actually in most physics book, it is the only correct one to calculate the one deminstional inelastic collision. In that scenario. we have to consider impulse, J=F*t, the bullet travelling at a much faster speed would have a much less impact time. Thus the force acting on the tank armor would be:
F=mv/t
One reasonable assumption would be since the bullet travelling 1000 times faster, the impact time would be a thousand times less than the coconut. Actually the bullet would probably have a much less impact time, because of its shape.

Then the large amount of F acting on rather small area, would have a even bigger pressure, and that is how do you have deformation.