Spaced armor

[Paul Lakowski]

" The Vulnerability of Armored Vehicles to Ballistic Attack" On the section on spaced armor report that against a conventional AP shot spaced armor would have little effect. However later in the book a number of case are detailed where impacting projectiles shatter as a result of the angle of impact. These include pics of the shattered projectiles and these are almost always cut in half . The point is that if this happens on the outer plate , any penetration of the inner plates is going to be reduced by half.

20mm AP M-75 impact 3/4 inch 364 BHN plate [1:1 t/d] @ the following angles and results

0° Ballistic limit 1100f/s projectile intact

10° Ballistic limit 1150f/s projectile intact

20° Ballistic limit 1400f/s front 1/4 broke off

23° Ballistic limit 2000f/s broke into two into 1/3 and 2/3

30° Ballistic limit 2100f/s broke into 3x 1/3 parts

40° Ballistic limit 2220f/s broke into 7 parts

50° Ballistic limit 2580f/s broke into many

37mm APC M-51 impact 3/4 inch 364 BHN plate [t/d 0.5:1] @ the following angles and results

20° Ballistic limit 900f/s projectile intact

30° Ballistic limit 1150f/s projectile intact

40° Ballistic limit 1400f/s projectile intact

50° Ballistic limit 1850f/s projectile intact

60° Ballistic limit 2250f/s projectile intact tip more blunted

50 Cal APM2 impact 1/2 inch 494 BHN RHA plate [1:1 t/d] @ the following angles and results

0° Ballistic limit 1240f/s projectile intact

10° Ballistic limit 1380f/s projectile intact

15° Ballistic limit 2000f/s Projectile shattered

20° Ballistic limit 2180f/s Projectile shattered

30° Ballistic limit 2480f/s Projectile shattered

50 Cal APM2 impact 1/2 inch Face Hardened plate [1:1 t/d] @ the following angles and results

0° Ballistic limit 2180f/s Projectile shattered

10° Ballistic limit 2200f/s Projectile shattered

20° Ballistic limit 2340f/s Projectile shattered

30° Ballistic limit 2560f/s Projectile shattered

37mm APC M-51 impact 1 inch[0.7:1 t/d] Face Hardened plate 648 BHN front 437 BHN back , @ the following angles and results.

0° Ballistic limit 1180f/s Projectile shattered

10° Ballistic limit 1220f/s Projectile shattered

20° Ballistic limit 1280f/s Projectile shattered

30° Ballistic limit 1410f/s Projectile shattered

40° Ballistic limit 1690f/s Projectile shattered

Last entry should read as follows

37mm APC M-51 impact 1 inch [0.7:1 t/d] Modified Face Hardened plate 481BHN front & 389bHN rear @ the following angles and results

0° Ballistic limit 800f/s Projectile Intact

10° Ballistic limit 830f/s Projectile Intact

20° Ballistic limit 960f/s Projectile shattered

30° Ballistic limit 1250f/s Projectile Intact

40° Ballistic limit 1680f/s Projectile shattered

The exception @ 30° could be that this is a statistical event , they report that in the modified plate all the results were intact until the 40° angle was reached so the 20° event could be graphical error.especially when you compare to the regular face hardened plate at that angle 1410 f/s.

The prior explaination for this behaviour was that the projectile had to turn on impact and thus the side of the projetile hit the side wall of the crater, thus shattering the projectile.

[ 12-08-2001: Message edited by: Paul Lakowski ]</p>

[Paul Lakowski]

I've been given a 1950 Research paper into spaced armor that further explores the effects of spaced armor.

Hurlich reports that in tests of 90mm APC fired through 'skirting plates '[ Heat treated T-33 & T-50 Vs 1/2 inch & 1 inch steel @ 30-60° ] , 21% [30/145 shots] of the T-50 APC shots shattered while 93%[26/28 shots] of the T-33 AP shots shattered.In the same series of tests 57mm AP& APC shots [M-70 & M86] were fired through the 1/2 inch skirting plates and the caps were removed from the APC shots but none of the 40 projectiles fired shattered . The poorer performance of the T-33 round could have been as a result of its heat treatment and shorter L/d.

The results of impacts @ 40° suggest the penetrating projectiles 'picked up' a yaw of 5-7°. Int.J.Impact Engng Vol-22 p211 shows that penetration loss per degree yaw is about 0.5-0.7 % per degree yaw for a AP type shot, thus this should result in a drop of 3-4% in penetration.

But it seems that yaw and deflection are interconnected and the penetrating projectiles struck the main armor at angles 5-7 ° , over a 16 inch travel distance. If these plates are parallel then this means less than the inital angle of impact , thus the over all penetration was increased by ~ 10%.

But this outcome seemed to be controlled by the projectile. In the case of 57mm AP/APC the change in penetration was to reduce resistance 10-12%. However in the case of the T-50 90mm APC round , the spaced plates reduced penetration by 20-23% @ 30-40° impact.

[Paul Lakowski]

MOre from the book "The vulnerablity of Armored Vehicles to Ballistic attack"

It reports that against slanted spaced armor Vs AP projectiles , the net resistance of the spaced plate arrangement was less than the resistance of the same over all thickness of solid plate [lower t/d in the spaced plate arrangement].However they conceeded that if the spaced plate is able to damage the penetrator in any serious way then spaced plate should be benifical as was the case in caped projectiles. For "D-capping" see Dr Elder's paper on Battle ship armor...another smashing good read!

They state that HVAP would definatly suffer from slanted spaced armor impacts.90mm HVAP could penetrate 11 inches of homogeneous plate @ 30° while the same projectile was defeated by a 1/2 inch and 6 inch roled plates with a 12 inch airgap set back at 30°.

HEAT M-9A1 hollow charge is reported to penetrate 4 inches of solid rolled plate but it was found that 1/2 inch RHA with a 14.5 inch air gap and a 1.5 inch RHA back plate defeated the warhead. It also reported the two 1.5 inch RHA plates with only 5 inches air gap set @ 45° could also defeat the warhead.

105mm HEAT [M67] penetrate 5 inches of RHA plate @ 0° , but this was defeated by a spaced armor arrangement of two 1.5 inch RHA plates with a 10 inch airgap set @ 0°.Also two 1.5 inch RHA plates with 5 inches of air gap set back @ 45° defeated the M67 warhead.

[Paul Lakowski]

In addition to the effects on AP & APC shots the spaced armor also reduces the effect of shaped charges.

From tests I've seen copper charges fall from 4.3 cone diameters [Cd] penetration @ 2CD standoff down to 2.8Cd penetration @ 6CD standoff..In the "Journal of Battlefield Technology" Vol 1-1 pp 1 a article was published on HEAT developement and standoff penetration chart was published for Steel,Copper and Aluminum cone shaped liners.....here are provisional figures taken from the chart.

Standoff in cone diameters

Liner----------1.0----2.0----3.0----4.0----5.0----6.0----8.0

Aluminum--1.0----1.5----2.0----2.4----2.5----2.5

Steel---------2.8----3.1----3.5----3.5----3.2----2.8

Steel---------3.0----3.5----3.5----3.1----2.6----2.1----1.9 Vs mild steel?*

Copper-------3.1----4.1----4.8----5.0----4.3----3.8----2.5 Vs mild steel?

Copper-------4.2----4.4----4.2----3.8----3.4----2.8----2.2 Vs 340BHN

result is the expected average penetration in cone diameters

* "Explosives with Lined Cavities"J of Applied Physiscs Vol-19 June 1948 pp 563.

So against a M-9 [rifle grenade] at normal impact [2 Cd standoff ] that should be 2.3 Cd penetration into RHA [3.5 into mild steel] and @ 7.5Cd standoff about 1.2Cd penetration into RHA[1.9 into mild steel].

The M-9 warhead is a 1.76 inches diameter so by the above figures it should get 4 inches into RHA ,while the same penetration into a spaced armor arrangement with 4 additional Cd standoff should do 2.1 inches penetration… "The Vulnerability of Armored Vehicles to Ballistic Attack" , reports HEAT M-9A1 hollow charge to penetrate 4 inches of solid rolled plate but it was found that 1/2 inch RHA with a 14.5 inch air gap and a 1.5 inch RHA back plate defeated the warhead.Exactly what the above figures suggest!

It also reported

105mm HEAT [M67] penetrate 5 inches of RHA plate @ 0° , but this was defeated by a spaced armor arrangement of two 1.5 inch RHA plates with a 10 inch airgap set @ 0°.Also two 1.5 inch RHA plates with 5 inches of air gap set back @ 45° defeated the M67 warhead 4.2 .Its reported in American Arsenal that the penetration is 4.5inches but this is probably the 30° penetration . This would be a 5.2inch LOS penetration but 105mm warheads have been known to penetrate Panther glacis in tests 80-85mm @ 55° =140-148mm [5.5-5.8inches].At first glance this doesn’t make sense , however the Rheinmetall Hand Book on Weaponry show the following relationship of HEAT warheads to slanted penetration.

------------10°-----30°------45°-----60°

HEAT---1.02----1.14----1.37----1.96

LOS----1.02----1.15-----1.41----2.0

1982 Rheinmetall Hand book on Weaponary....[figure 1128]relative penetration of modern 105mm projectiles @ 1000m range......

So the penetration against sloped armor is more than vertical armor of the same LOS thickness.

Now in the M-67 , 45° angle case is interesting because the solid thickness is 3 inches @ 45° or 4.3 inches LOS solid thickness suggesting that the spaced armor had little additional effect. The 1.7Cd additional standoff should have resulted in 12% drop in penetration [4.4 inches] when the actual solid LOS penetration @ 45° should have been about ~ 5.4inches. this suggests the effects of standoff on spinning warheads is more substantial than non spinning warheads or the spaced plate offers more advantage than the LOS thickness suggest...as is the case in the 1982 study [int.J.Engng. Sci Vol-20, pp 947-961[1982] "Effects of Hypervelocity jet on a layered target".

[Paul Lakowski]

Continued ...

The APC shot can be shattered all the time if the mild steel skirting plate has a t/d ratio of 0.3 compared to the attacking projectiles,thus the 5mm skirting plates around the Pz-III/IV, should be effective at decapping and shattering projectiles 12-15mm diameter. This should result in 1.2 increase in the effectiveness of the over all armor arrangement. The armor as I recall is 30mm hull plate plus the skirting plate but the travel distance over which the damaged projectiles have to travel is much more than the 16 inches in the test[4-7 projectile diameters].

Holher and Stilp showed that at sub ordance velocity a gain of 10% on the ballistic limit of a spaced plate arrangement can be gained if the airgap is ~ 20 projectile diameters.So in the above case the overall resistance may be 1.3 times the plate thicknes or

30mm hull plate plus 5mm mild steel x 1.2-1.3 = 42-46mm effective resistance.

Things get really interesting when the attacking projectile is tungsten carbide.Under the test conditions the 90mm M-304 HVAP shot shattered in almost every spaced plate impact condition resulting in enormous increases in the ballistic limits of the armor arrangement. Here are some examples...

90mm M-304 HVAP striking the following targets.

4 inch back plate -280BHN- plus 1/2 inch mild steel skirting plate set back at an angle of 30° was equivellent to 7 inches or armor at the same angle [ 1.56 times the LOS thickness].

4 inch back plate -280BHN- plus 1/2 inch mild steel skirting plate set back at an angle of 45° was equivellent to 8 inches or armor at the same angle [ 1.78 times the LOS thickness].

6 inch back plate -280BHN- plus 1/2 inch mild steel skirting plate set back at an angle of 30° was equivellent to 11.5 inches or armor at the same angle [ 1.77 times the LOS thickness]

So in the Pz-III/IV case the effectiveness of the armor against HVAP shots @ 0° impact angle should be some where in the 1.4-1.5 times the LOS thickness or 49mm-52mm equivellent armor protection @ 0°.