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Why The Penetration Of Steel Rod Is Too Low?


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#1 lemd

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Posted 20 November 2017 - 1451 PM

From this image

 

v_opt.jpg

 

I tried a 10MJ with l/d = 30

 

Length: 1.42m

Diameter: 47.2mm

Volume: ~2500cc

Weight: 20kg

Velocity: 1km/s

BHN: 300

 

And it only penetrates 122mm into 300BHN target.

 

Compare to 152mm warhead

length: 1.4m vs 0.9m

area of penetration: 47mm vs 152mm, about 9 times smaller

velocity: 1km/s vs 800m/s

 

It seems all parameters of the rod are far better, but 152mm warhead can penetrate +200mm easily.

 

So how can the steel rod penetration be too low?

 

A



#2 TTK Ciar

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Posted 20 November 2017 - 2112 PM

If you look at the Lanz/Odermatt equation, you will see penetration depth in the hypervelocity domain is much much more than a matter of energy. In it, penetration is proportional to (among other factors) the square of the ratio of penetrator and target densities, and the penetrator density times the square of velocity (as opposed to energy, being mass times the square of velocity).

Steel is much less dense than DU/Ti or WHA -- 7.86 g/cc compared to 18.8 g/cc and 17.0 g/cc, respectively. This puts a stiff penalty on steel's performance as a hypervelocity penetrator.

Edited by TTK Ciar, 20 November 2017 - 2113 PM.


#3 KV7

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Posted 21 November 2017 - 0527 AM

Why does DU performance degrade at high velocity ?



#4 Josh

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Posted 21 November 2017 - 1601 PM

That is curious. I thought the entire advantage of DU was that it sharpened at high velocity where as tungsten flattened? I assume the problem is cross sectional? IE, the long rod snaps under the load?

#5 KV7

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Posted 21 November 2017 - 1641 PM

That is curious. I thought the entire advantage of DU was that it sharpened at high velocity where as tungsten flattened? I assume the problem is cross sectional? IE, the long rod snaps under the load?

The standard equations cannot handle fracturing, it is far too hard to model in such an elegant manner. Maybe this is from experimental data ?


Edited by KV7, 21 November 2017 - 1641 PM.


#6 methos

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Posted 21 November 2017 - 1728 PM

Why does DU performance degrade at high velocity ?

 

Because the graphics shows penetration at a constant 10 MJ energy and a constant L/D ratio of 30. Meaning: on the left side of the graph, the penetrator is heavier (and due to the fact that the L/D ratio is fixed, it also is longer & thicker), while on the right side of the graph the penetrator will be smaller (shorter & thinner). This image only shows the optimal velocity for L/D 30:1 penetrator with 10 MJ energy, not the relation of penetration and velocity of a constant penetrator shape.



#7 KV7

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Posted 21 November 2017 - 1838 PM

 

Why does DU performance degrade at high velocity ?

 

Because the graphics shows penetration at a constant 10 MJ energy and a constant L/D ratio of 30. Meaning: on the left side of the graph, the penetrator is heavier (and due to the fact that the L/D ratio is fixed, it also is longer & thicker), while on the right side of the graph the penetrator will be smaller (shorter & thinner). This image only shows the optimal velocity for L/D 30:1 penetrator with 10 MJ energy, not the relation of penetration and velocity of a constant penetrator shape.

 

Yes I understood that, I suppose the question really is 'why is the optimum velocity for DU lower than for Tungsten and especially steel'. I can understand the effect for steel as the lower density means the rod is physically bigger for the same velocity and energy, so there is more to be gained from higher velocity and correspondingly smaller projectile.



#8 methos

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Posted 22 November 2017 - 0653 AM

DU is denser and therefore heavier. That means at the same velocity in the graph, the DU rod will be shorter. That is one of the key factors why it seems that the optimum velocity for DU would be lower; at higher velocities (and a fixed 10 MJ), the DU rod becomes too short for optimal penetration.

 

This is also one of the reasons why the overall penetration of tungsten is displayed as being higher in the graph: the longer tungsten rod can get closer to optimum velocities.



#9 Hakka

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Posted 22 November 2017 - 0709 AM

DU is denser and therefore heavier. That means at the same velocity in the graph, the DU rod will be shorter. That is one of the key factors why it seems that the optimum velocity for DU would be lower; at higher velocities (and a fixed 10 MJ), the DU rod becomes too short for optimal penetration.

 

This is also one of the reasons why the overall penetration of tungsten is displayed as being higher in the graph: the longer tungsten rod can get closer to optimum velocities.

 

It's a very interesting question, and it appears to have little to do with energy. I played around with Willi Odermatt's penetration calculator on his website (http://www.longrods.ch/equation.php), and it doesn't seem to have anything to do with length.

 

Comparing a DU rod (18000 kg/m^3) and WHA rod (17000 kg/m^3) with identical dimensions and identical striking velocity of 1800 m/s, the WHA rod penetrates a few cm more, even though the DU rod has higher bulk density and higher energy. This is for a 300 BHN target with a density of 7850 kg/m^3 at 60 degrees obliquity. Comparing the same two rods against the same target but at a striking velocity of 1600 m/s, the DU rod penetrates a few cm more.

 

However, this changes when the obliquity changes. Comparing the DU rod (18000 kg/m^3) and WHA rod (17000 kg/m^3) with identical dimensions and identical striking velocity of 1800 m/s on the same target but at 0 degrees obliquity, the penetration is exactly the same (difference is a hundredth of a percent). For the same 0 degree target but at 1600 m/s, the DU rod penetrates a few cm more than the WHA rod.

 

So it seems that WHA rods only penetrate more than DU rods on high obliquity targets and at high velocity. Otherwise, DU is better or on par with WHA. Based on this, I guess that WHA handles lateral stress better than DU, allowing it to penetrate more steel at higher obliquity without fracturing during penetration.


Edited by Hakka, 22 November 2017 - 0710 AM.


#10 KV7

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Posted 23 November 2017 - 1730 PM

As above the equation only models plastic deformation and erosion, shattering and fracturing is a discrete process that cannot be so easily modeled.






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