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Double Layered Gun Shields.


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#1 Inhapi

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Posted 05 October 2019 - 0643 AM

At least the Pak 38 and Pak 40 had a gunshield made out of very thin plates mounted at some distance from each other.

 

Was there a special reason for this kind of construction ? 

 

 


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#2 KV7

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Posted 05 October 2019 - 0741 AM

I suspect it was so they could use harder plates whilst still being able to easily shape them into the curve.


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#3 bojan

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Posted 05 October 2019 - 1118 AM

2 x spaced angled plates were more effective than a single plate of the double thickness.


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#4 Mobius

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Posted 05 October 2019 - 2005 PM

Mostly some protection from tungsten cored anti-tank rifles.  But, it also gave protection from ball rifle bullets.

 

7.5cm PaK 40 Two 4mm Plates (Spaced Armor) @ 30° 
7.5cm PaK40 gunshield was rated for 7.92x57 s.S. Patrone heavy ball @ 100m, 7.92mm AP @ 500m.
100mm   AT gun shield    7mm @  32°.      
100mm MT-12 vs 7.62x54mm 'D' heavy ball @ 200m.

 

These ranges seem optimistic because the Mosin-Nagant booklet says that the bullet can penetrate 6mm vertical armor at 100m.  (I assume the L light bullet).  And German data gives the K98 the ability to penetrate only 5mm at 100m.  This might be at 30°.  


Edited by Mobius, 06 October 2019 - 1037 AM.

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#5 Inhapi

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Posted 06 October 2019 - 1734 PM

2 x spaced angled plates were more effective than a single plate of the double thickness.

 

Is this a rule for thin plates only or also for AT weapon proof plates ?


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#6 Mobius

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Posted 06 October 2019 - 1954 PM

 

2 x spaced angled plates were more effective than a single plate of the double thickness.

 

Is this a rule for thin plates only or also for AT weapon proof plates ?

 

Well actually two plates separated are not as good as a single plate.  A single 80mm plate is equivalent to a 40mm and a 52mm plate.   The first plate may blunt the shell.   The two plate penetration formula is 1 plate = [(1.15x1st plate)^1.4+A^1.4 X (2nd plate)^1.4]^1/1.4  according to Nathan Okun. This formula is for softer armor.  Hard armor is penetrated by plugging and will be different.

A=1.0 for AP shell.

That's why I think than something like hard core shatter may be happening.


Edited by Mobius, 06 October 2019 - 1958 PM.

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#7 KV7

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Posted 06 October 2019 - 2145 PM

 

 

2 x spaced angled plates were more effective than a single plate of the double thickness.

 

Is this a rule for thin plates only or also for AT weapon proof plates ?

 

Well actually two plates separated are not as good as a single plate.  A single 80mm plate is equivalent to a 40mm and a 52mm plate.   The first plate may blunt the shell.   The two plate penetration formula is 1 plate = [(1.15x1st plate)^1.4+A^1.4 X (2nd plate)^1.4]^1/1.4  according to Nathan Okun. This formula is for softer armor.  Hard armor is penetrated by plugging and will be different.

A=1.0 for AP shell.

That's why I think than something like hard core shatter may be happening.

 

Yes, thinner ductile plates have less efficiency as it is easier to force plate material out of the path of the projectile the thinner the plate is. But a thinner plate can be made harder before shattering is a problem, and a two plate array can use harder steel without worrying so much about spalling from non penetrating hits. Still, performance would probably rise if there was some low density filler between the plates, allowing the back plate and filler to support the front plate, and if it fails energy is lost driving the plug or fragments through the low density filler.  But in WW2 there was no good and cheap low density filler material available - only aluminum alloy really - which would be far too expensive for use on a gun shield.
 


Edited by KV7, 06 October 2019 - 2154 PM.

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#8 Ben Dejo

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Posted 09 October 2019 - 1050 AM

At least the Pak 38 and Pak 40 had a gunshield made out of very thin plates mounted at some distance from each other.

 

Was there a special reason for this kind of construction ? 

 

 

The only thing that comes to my mind is that the space between the plates gives the projectile room to rotate ( keyhole) and strike the 2nd plate at a less than optimal angle.  Almost all spitzer type projectiles will rapidly destabilize when striking a surface, much like an air-cooled VW Bug will turn around in the rain and go rear end first. ( and indeed the same mechanic for the action is in play here)


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#9 KV7

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Posted 09 October 2019 - 2224 PM

 

 

2 x spaced angled plates were more effective than a single plate of the double thickness.

 

Is this a rule for thin plates only or also for AT weapon proof plates ?

 

Well actually two plates separated are not as good as a single plate.  A single 80mm plate is equivalent to a 40mm and a 52mm plate.   The first plate may blunt the shell.   The two plate penetration formula is 1 plate = [(1.15x1st plate)^1.4+A^1.4 X (2nd plate)^1.4]^1/1.4  according to Nathan Okun. This formula is for softer armor.  Hard armor is penetrated by plugging and will be different.

A=1.0 for AP shell.

That's why I think than something like hard core shatter may be happening.

 

The formula cannot be very good, as it gives a too high value for an array which is almost all the first plate, asymptotically it gives a ME of 1.15 as all of the plate is the front plate, when it should of course be 1.



 


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#10 DougRichards

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Posted 10 October 2019 - 0441 AM

 

At least the Pak 38 and Pak 40 had a gunshield made out of very thin plates mounted at some distance from each other.

 

Was there a special reason for this kind of construction ? 

 

 

The only thing that comes to my mind is that the space between the plates gives the projectile room to rotate ( keyhole) and strike the 2nd plate at a less than optimal angle.  Almost all spitzer type projectiles will rapidly destabilize when striking a surface, much like an air-cooled VW Bug will turn around in the rain and go rear end first. ( and indeed the same mechanic for the action is in play here)

 

 

I used to drive an air cooled bug and never had even a hint of that experience


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#11 Mobius

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Posted 10 October 2019 - 0912 AM

 

 

 

2 x spaced angled plates were more effective than a single plate of the double thickness.

 

Is this a rule for thin plates only or also for AT weapon proof plates ?

 

Well actually two plates separated are not as good as a single plate.  A single 80mm plate is equivalent to a 40mm and a 52mm plate.   The first plate may blunt the shell.   The two plate penetration formula is 1 plate = [(1.15x1st plate)^1.4+A^1.4 X (2nd plate)^1.4]^1/1.4  according to Nathan Okun. This formula is for softer armor.  Hard armor is penetrated by plugging and will be different.

A=1.0 for AP shell.

That's why I think than something like hard core shatter may be happening.

 

The formula cannot be very good, as it gives a too high value for an array which is almost all the first plate, asymptotically it gives a ME of 1.15 as all of the plate is the front plate, when it should of course be 1.

You are right.   The 1.5 factor is introduced in Armor and Ballistics by Lorrin Bird.  I went to the Okun site and his formula is Tspaced = (Te11.4 + Te21.4 + ... + TeN1.4)0.71429.   So that is different.

 

I tested this out using Okun's armor penetration formula.   So the 75mm KwK 40 penetrates 5.4" of US Class B armor at point blank.  But, first penetrating 2.7" leaves enough residual velocity to penetrate another 3.2".


Edited by Mobius, 10 October 2019 - 0934 AM.

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#12 Colin

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Posted 10 October 2019 - 0927 AM

Did they have that information at the time? Weight is a major factor as well, as these guns were dragged around by their crews quite a bit.


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#13 Mobius

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Posted 10 October 2019 - 1058 AM

The PaK 36 only had a single plate of 4.4mm.   The PaK 38 had two 4mm spaced plates.   So that is the timeframe this occured.


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#14 Ssnake

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Posted 11 October 2019 - 0347 AM

I suppose the gun shields were mostly intended as protection against HE fragments, MG fire, and snipers. For the latter with AP rounds, a layered shield probably works better.


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#15 seahawk

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Posted 11 October 2019 - 1224 PM

According to the notes of my grandfather - mostly to protect against spalling from normal ball rounds.


Edited by seahawk, 11 October 2019 - 1225 PM.

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#16 KV7

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Posted 12 October 2019 - 0355 AM

According to the notes of my grandfather - mostly to protect against spalling from normal ball rounds.

Yeah, as I argued above:

'But a thinner plate can be made harder before shattering is a problem, and a two plate array can use harder steel without worrying so much about spalling from non penetrating hits.'

 


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#17 Mobius

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Posted 12 October 2019 - 1050 AM

In the same timeframe the single layer gunshields also increased in thickness. 45mm M.1937 Anti-Tank Gun 4.5mm @ 42°.

Where 45mm M.1942 Anti-Tank Gun 7mm @ 42°.


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#18 lastdingo

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Posted 16 October 2019 - 0509 AM

I remember that a book mentioned in context of mini subraines of 1944-1945 that 4 mm metal sheets were much more easily worked (bent) than thicker ones. The hulls were thus made of 4 mms teel, which limited their maximum depth.

 

I suppose the same technical limitations may have led to the twin 4 mm PaK shield.

 

Personally, I supect the outer plate was hardened and the 2nd plate was more ductile, but I've never seen this confirmed anywhere.


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#19 KV7

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Posted 16 October 2019 - 0637 AM

I remember that a book mentioned in context of mini subraines of 1944-1945 that 4 mm metal sheets were much more easily worked (bent) than thicker ones. The hulls were thus made of 4 mms teel, which limited their maximum depth.

 

I suppose the same technical limitations may have led to the twin 4 mm PaK shield.

 

Personally, I supect the outer plate was hardened and the 2nd plate was more ductile, but I've never seen this confirmed anywhere.

And 4mm sheets of quite high hardness can probably be cold worked to a large radius curve, and hence don't need additional heat treatment.

 


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#20 Colin

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Posted 25 October 2019 - 1031 AM

The Soviets issued ATR's most prolifically so I expect that suppressive fire onto any AT gun spotted would be significant?


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