IS-2 - thickness of gun mantlet armor measured

[Amizaur]

 

 

And comparison of the blueprint with my drawing of mantlet profile (which was constructed only on base of measurements, I didn't use blueprint at all).

It fits suprisingly well:

 

 

My mantlet seems slightly more thick. Maybe it's an artifact of inprecise blueprint, or maybe the older 1943  (narrow) mantlet was 100mm ?

 

 

Edited December 30, 2015 by Amizaur

[Amizaur]

Looking for that info, I found few nice photos: 

 

Few new photos I found on the net - showing the turet shape under the mantlet and opening for the gun:

 

 

IS-2 turret with it's gun mantlet removed - notice the visible rubber shock absorbers on the mounting where the mantlet is fastened:

 

 

 

I wish it was a little closer than 350km from me

[Amizaur]

 

Interesting thing to notice on above photo - the armor under the mantlet is only about 40-50mm thick, because it's almost horizontal and normally also shielded by lower mantlet. But if an armor piercing projectile hit top of upper front plate or even skidded from the turret ring protector and was deflected slightly upwards, it would hit exactly there with almost full energy, probably penetrating the thin cast armor there and entering turret under the gun. Especially in late tanks with 60deg upper plate I see this as a shot trap.

After looking at some new blueprints of ture IS-2 turret (not Is-1, as before) with armor cross section visible, I have to withdraw the above statement. From those drawings I attached below,  it seems that constructors have foreseen the shot-trap problem and eliminated the weakpoint I talked about by making the IS-2 under-the-mantlet armor much thicker than corresponding upper part. I didn't measure this lower part (there was no way to measure it with tape measure, and the surface was too rough to measure it with ultrasonic meter), only the upper part. I assumed the turret armor has some kind of vertical simmetry and the lower part thickness is similar to upper part. From those drawings it seems that it isn't. The lower part under the mantlet is distinctly thicker.

so on this drawing below, the lower part of armor marked with arrow is not right (too thin):

Seems for me that there is probably no under_the_mantlet weakpoint for IS-2 (there may habe been for IS-1). The only part prone to penetration in this area is the vertical part of turret ring. But turret ring is probably about as hard to penetrate as front turret (about 100mm of same cast steel + some obstructions on the way of the projectile) so it's hard to call it a "weakpoint" really.  I'll try to verify this next time.

 

Edited January 5, 2016 by Amizaur

[Battlefront.com]

More good stuff!

The other possibility about the mantlet is there is an "early" and "late" versions, the first having the shot trap and the other not.  Or perhaps it was a factory production variation.  At this point in the war the production standards were more uniform, but earlier there were lots of variations based on factory or timeframe.  Because this tank isn't German and named after a ferocious feline there hasn't been as an exhaustive study.  Something like this could have been missed by researchers.

Steve

[Amizaur]

The shot trap I talked about would be some relatively thin front turret armor behind the lower edge of the mantlet. A shell that bounced from uper edge of hull upper front plate would hit just there, bypassing the mantlet. I say "would" because it turned out that the armor there is not thin and there is no weakpoint at all.

Such relatively thin armor is visible on IS-1 turret blueprint, but it is very small area and there is rather small chance that a bouncing shell would hit just there. On Is-2 the turret front is longer so this part of armor is longer too and more exposed. I believed the front turret design is similar like in Is-1 so this part under the lower edge of  is about 60mm thick (at best) and it would be a weakpoint then. I couldn't measure it unfortunately, it is very hard to reach from inside, and too rough to use ultrasonic meter from outside. 

But on Is-2 turret blueprint that I have found few days ago, this part is visibly thicker than in IS-1, it's about 80mm thick. I suppose they corrected it while designing new turret front for 122mm gun because this part is "thick by design", not "reinforced" -  the whole turret front armor geometry is designed in a way that makes lower part of the front turret thicker than upper part. 

Those IS-2 blueprints are probably of early IS-2 models. One shows whole old IS-2 model with two-piece nose and old narrow mantlet type and gunsight, second shows turret with new mantlet (TSh-17 gunsight visible).

On the other hand, much later piece (from very late 1944 or maybe even early 1945) of Is-2 from Lebork has only some minor modifications - on the whole it seem to perfectly agree with the early Is-2 blueprint. I doubt that during production there were any serious modifications other than known  ones (new front hull design with upper front-side belts reinforced, and wide mantlet model) as any such change would slow down the production rate for some time. Quantity was much more important than quality. Only small details (things that didn't require changes in production line tooling) were improved.

I'll try to visit some early (narrow mantlet) Is-2 model, I wonder if I find any differences in turret design. But I have to wait for springtime - we have -15deg C here now, and I can't measure anything in thick gloves ;).  

 

Edited January 6, 2016 by Amizaur

[Amizaur]

And one more picture. After comparing my photo of measurements with technical drawing of Is-2m front hull, I see that point of my most forward thickness measurement (135mm) of this part could still be behind the100-130mm transitiion area.

So it's quite possible that the 100-130mm transition is there. I'll check this next time. 

 

IMG_7442 transition shown by Marcin Ostrowski

[Amizaur]

 

100m front uppper plate - diagram by Marcin Ostrowski, on Flickr

 

 

IS-2 front hull versions with thickness by Marcin Ostrowski, on Flickr

 

(Two transition areas for upper  front-side and side armor belts are not shown on the picture above). First one is on the front where the 100mm nose plate is welded to 100mm front-side belt, then over lenght of 25cm the thickness inreases to 130mm. Second transition from 130mm to 100mm is where the front-side belt turns into side part (under side of the turret). Armor changes from 130mm to 100mm over lenght of 30cm.)

 

IS-2m cast hull z opisami by Marcin Ostrowski, on Flickr

 

IS-2 new hull UTZM, chyba RHA z opisami by Marcin Ostrowski, on Flickr

 

Edited January 9, 2016 by Amizaur

[Andrew H.]

You are a true hero!

[Amizaur]

Correction to the transition region picture: I studied the blueprint a bit more and I see it should rather look like that:

IS-2 transition region correct by Marcin Ostrowski, on Flickr

[MikeyD]

Steve wrote: 

..."early" and "late" versions, the first having the shot trap

Hmmm, I think the primary difference between 'early' and 'late' IS-2 mantlets wasn't the shot trap so much as position of the gunner's sight. Originally it was snuggled too close to the gun and was later moved some inches to the left. Interestingly, the US Stuart light tank had exactly that problem! Gunner's sight on original vehicle was tucked too close to the gun  and the mantlet needed a redesign to move the sight and give the gunner more head room. 

 

[Amizaur]

Of course - the reason for introducing the wide mantlet was moving the gunner's sight further from the gun.

But changes were done only to the left side of the turret, and to the mantlet a thinner part covering the gunsight area was added. But the central thick part of the mantlet was unchanged, as far as I know at the moment.

So if there was any shot trap, it would be unchanged by narrow -> wide modernisation. Currently I do not believe that any particualry vunerable area was under the mantlet (at least if the barrel was set in horizontal position), so there was no need to change anything there. Front turret armor under the mantlet was thicker than corresponding part of armor over the mantlet and it was done this way by design.

[Amizaur]

I found en error in my first post. The bug is in the line:

"- The most thick part of mantlet is about 130mm above the gun and gunsight axis. In other words, the gun is mounted slightly below the thickest part of the mantlet."

it should say:

"- The axis (and most thick part) of the mantlet is about 33mm above the gun / gunsight axis. In other words, the gun is mounted slightly (33mm) below thickest point of the mantlet."

Edited January 27, 2016 by Amizaur

[kch001]

Very interesting analysis. Thank you for all the work!

[Amizaur]

I got my Leeb type steel hardness tester.  First trials went well, it seems that it's suitable for measuring hardness of armored steel. Parts of armor tend to be big, thick and heavy and that is exactly kind of target for Leeb meter (it can't measure small objects).

 

The only problem is that reliable and accurate measurement requires a bit of clean, bare, smooth metal surface, even more clean and smooth than for ultrasonic thickness meter. It's very hard to find such bare and smooth metal surface on an old tank (it's either rusty, or painted, or both) and cleaning it with sandpaper usually is out of question (if it's some museum piece). 

 

I tried the meter on piece of rusty 8,8cm PzGr39 (not very reliable results from 300BHN at the base, 500 BHN on the side, near front of body, and 500-600 BHN at the tip of the cap). Second try was on the T-34/76 tank (hybrid of 1941/1942 models) that is on a monument 1,5km from my living place :). I use it to test equipment ;).

 

 

 

https://www.flickr.com/gp/136792894@N05/u3Qh0o

 

https://flic.kr/s/aHskuG6J1h

 

Finding good surface for measurements was very hard. Eventually I managed to do measurements of front upper plate, rear upper plate, gun mantlet in it's upper part, side turret armor, casting for hull MG mount and the cast bar that connects front upper and lower plates. Not all measurements were reliable, but some were. 

 

Results are as follow (in BHN = HB scale):

 

Front upper plate RHA  46mm   ~ 400 BHN (390-420 range, two higher results -  460 once, 500 once, 520 once - are probably errors, the surface was not completly smooth and clean)

 

Rear upper plate RHA  46 (?) mm   - reliable 400 BHN result, probably done from the same type of plate like the front, sides, turret, mantlet and all other 45mm thick RHA parts. I didn't measure this plate so I'm not sure it's 45mm  (and not 40), but it seems to be 45 (whole tank done from single RHA plate  thickness).

 

Side turret plate  (bent RHA plate) 46mm  - 400-420 BHN

 

Gun mantlet piece (bent RHA piece) 46mm  - 340 BHN (up to 360 BHN in some places, guess it's because of some forging action).

 

nose casting (100mm thick)   -  most readings in 460-520 range. It's a part that connects the front upper and lower plates. I got also lower and higher results (280...400....550) and I'm not sure if harndess is variable trough piece, or results are spread because of rough surface. 

 

MG mount casting, upper part 40mm thick - 300-320 BHN

 

All 45mm thick RHA parts seem to be made from the same type of armored plate (400-420 BHN) - with exeption of gun mantlet which is also 45mm thick but gave lower readings (340-360 BHN), maybe because it was formed by press bending and maybe retempered in the process, or something.  

 

Now I'm ready to go and measure high hardness castings of IS-2 mod 1944 :). It should be easier, there is lot of polished, bare metal spots on IS-2 exhibit than on T-34.

Edited May 9, 2016 by Amizaur

[John Kettler]

Amizaur,

I don't have CMRT, and I generally come over when some new post appears which is of interest to me. Consequently, it's a fluke I found this incredible thread. Not only is it first rate field and analytical work you did, but you've got the most revealing tank closeup shots I've ever seen. Consequently, if any members from IPMS who do WW II Russian armor ever find you, expect to have your shoulder dislocated, be taken out for a beer or be soundly bussed. Maybe all three! Shall have to revisit this and your amazing pic files when I'm a lot more together. Now that I think about it, I'm surprised the T-34/76 isn't the polygonal turreted M 1943. Didn't realize the earlier model was still in the fight. Live and learn! Congratulations on an astounding job, for the fresh insights you bring to us as grogs and the refinement in armor modeling now possible for CMRT and other titles to follow, such as the ever popular Combat Mission: B4 in Berlin!

.

Initially forgot to say that I cut my PC teeth on an Amiga and nearly gave a friend who'd just shelled out $4K for his gray screen Mac SE a heart attack (talk about instant buyer's remorse) when I showed him the Amiga with full color display, stereo sound and inexpensive software--for well under half of what he'd just paid. I had a 1500, and we later got a Amiga 2500. Wonderful machines, but Commodore totally botched the marketing, then did it again by failing to license the OS in time later. Sigh. Loved the Amigas and would still be using them if Commodore hadn't so stupendously screwed up! A Mac was the next best thing after that, so we started with the family's Gen One iMac, and I went from there.

Regards,

John Kettler

Edited May 24, 2016 by John Kettler

[Amizaur]

Thanks, John :).

Regards!