8,8 cm PzGr 39 Performance

[Peasant]

I've taken a close look at the chart showing the dependence of slope multipliers upon striking velocity and discovered a pattern in the data. Apparently the effective thickness changes linearly with velocity. I don't know if its actually true or if it is just a good enough approximation Krupp engineers were working with (probably the latter) but I'm sure this will be of use to us.

[critical mass]

Shouldn’t the 60deg datapoint be
0.51 for 500m/s and 0.44 for 800m/s?

The 500ms curves (which also includes the 0-30 deg realm) are very close to the natural cosine function. At higher velocities an exponent kicks in and gets stronger at 1100ms.

From that point of view the decision to proceed with larger cal guns became very feasaable. F.e. Dropping the 10.5cm l70 in lieu of the 12.8cm l55...

[Peasant]

Good catch. I unsure about the point at 800m/s, it's not drawn well on the chart but 0,44 does not fit into linear correlation. Interpolating between other two points puts it somewhere between 0,45-0,46.
I can't update the chart in my old post as I would like, so I'll leave it here:



Edit: The overall function is pretty simple. The coefficients a and b of the linear functions used to describe these lines are described by simple second degree polynomials as the functions of obliquity.
Edit2: I have whipped a quick calculator that gives you the slope multiplier from obliquity and striking velocity. Much handier to use than the tables: https://docs.google.com/spreadsheets/d/ ... sp=sharing

[critical mass]

Considering the presence of a factor of 0.44 for 60 deg in relation to 30 deg performance in another position within this document (compare tab. III in the attachment), gives some credence to the correctness of this value. There was a lot of testing done at 700-1000m/s range. However, I strongly presume, that only the 500m/s curve is shown in the 0° to 30° range of the graph. That is because they both are shown lighter (presumably originally in a lighter toned colour, such as red or green, which later became a lighter grey tone). It can be compared with Tab. III values, which fit with the 800m/s graph. This range is also, where Your datafile gives curious figures.

The cross over close to 30° obliquity is a good explenation why they preferred this obliquity generally. Its less affacted by various scale problems.
Because such a cross over does also exist within the german Navy major calibre fire effect tables for homogenious armor, it may points towards an underlying mechanism of penetration.

[Tenkist]

At 500m/s 8.8cm pzgr 39 penetrate ~66mm at 30degrees, so if i understand the graph correctly it penetrate ~33mm at 60 degrees ? T/D =0.375, at 830 m/s ? penetrate ~140mm at 30, and ~62mm at 60. T/D=0.7 I think that T/D should be same to say that the velocity of impact is important variable at high obliquity.

[critical mass]

The amount of armor which can be penetrated at high obliquity varies with the inverse of plate hardness, too. The memos above concern german test RHA, which was rather ductile material over a wide range of angles. Such material would be difficult to perforate under high obliquity unless temper brittleness prevents the plate to deform plastically.
Tests conducted on soviet Medium hardness RHA and high hardness RHA /CHA after end of ww2 in SU revealed different t/d ratios. F.e. For the 8.8cm pzgr39, the 42s grade 300BHN armor yielded a figure of merit =0.48 Times the 30 deg penetration when striking at 60deg. The same projectile perforated 0.65 times the 30 deg penetration when striking 430 BHN 8s or 52s high hardness RHA. In all cases the 8.8cm was fired from Pak43 at close range, thus high velocity.
The 30 deg performance was lower than the performance of the same projectile striking german RHA (198mm and 168mm, respectively, instead of 203mm vs German RHA), as could be expected due to the higher tensile strength. However, much greater absolute thickness (95mm and 109mm, respectively) could be perforated at high obliquity than vs german plate (88mm pak43 struggling against the PANTHER glacis for a reason).

[Avalancheon]

The amount of armor which can be penetrated at high obliquity varies with the inverse of plate hardness, too. The memos above concern german test RHA, which was rather ductile material over a wide range of angles. Such material would be difficult to perforate under high obliquity unless temper brittleness prevents the plate to deform plastically.

Tests conducted on soviet Medium hardness RHA and high hardness RHA /CHA after end of ww2 in SU revealed different t/d ratios. F.e. For the 8.8cm pzgr39, the 42s grade 300BHN armor yielded a figure of merit =0.48 Times the 30 deg penetration when striking at 60deg. The same projectile perforated 0.65 times the 30 deg penetration when striking 430 BHN 8s or 52s high hardness RHA. In all cases the 8.8cm was fired from Pak43 at close range, thus high velocity.
The 30 deg performance was lower than the performance of the same projectile striking german RHA (198mm and 168mm, respectively, instead of 203mm vs German RHA), as could be expected due to the higher tensile strength. However, much greater absolute thickness (95mm and 109mm, respectively) could be perforated at high obliquity than vs german plate (88mm pak43 struggling against the PANTHER glacis for a reason).

Okay, so these tests involve 88mm Pzgr 39 fired at high velocity. When the target plates have their obliquity increased from 30 to 60 degrees, then we see a change in the kindof resistance they offer. At these angles, the high hardness armor offers less resistance than the medium hardness armor. Is that a proper summary of what happened, CM?

BTW, do you know when and where these Soviet tests were conducted? Could the results have influenced the decision to change the armor composition of their tanks?

[critical mass]

Yes.

Similar observations were made within the US ballistic research using US domestic AP.
However, as with normal armor tests, this was not a linear relationship. There is a sweet spot of optimum resistance which is not very soft. 260/70 rather than 220/230 BHN, at least for 1.0 cal T/D.
Soviet medium hardness 42s was softened twice. Once by late ww2, in order to reduce the Bad cracking of is2 and T44 plates (adding molybdenum to reduce temper brittleness) and once in the late 1940s, to improve ballistic performance of highly sloped plate (T54B glacis- but not side plates). Only the tempering treatment was changed in the latter, not the armor composition. 42sm (by that time), had plenty of hardenability and through hardenability for cross sections up to 120mm.

The problem partially was with high hardness cast turret armor, which was relatively inferior if attacked obliquely by capped high quality AP. Beeing cast, it would not have the benefits of grain orientation typical with RHA, but it also was not ductile enough. In order to work, it needed high t/d overmatch ratios, which were costly in weight, of course.

[Mobius]

Miles Krogfus interpretation on this table has turned out to be entriely correct. The penetration figures in this table are CALCULATED with De Marre K=2400 (Расчетная при K=2400).
they do not represent actual test data -at all-

I'm not so sure. I did a spot check of the 76.2mm APBC and it turns out this is the data for the BR-350 APBC (in captured and translated firing table). If using K-2400 I get the numbers in blue below. Reverse calculating the K-factor from striking velocity and penetration the K vary from 2459-2502.

76,2mmvsdemarre.jpg

Aside note the 76.2mm APCR penetration in the first table is higher than the late war values. I'm not sure why. It may be a miscalculation.

[Stiltzkin]

I'm not so sure.

The Soviet figures are or appear to be based on test data, the German/foreign are not.

[critical mass]

It writtem in the front page that it’s calculated penetration using k=2400.

[Mobius]

It writtem in the front page that it’s calculated penetration using k=2400.

That's what it says, but that's not what it does.

[critical mass]

It writtem in the front page that it’s calculated penetration using k=2400.

That's what it says, but that's not what it does.

I am not sure. For one part, the weight of the projectile is not given. Thus, You need to make an unverified prior assumption in regard to the projectile weight. The tabulated data match K=2400 for the 76.2mm BR350A (=6.1Kg) relatively well (648m/s = 75mm; 561m/s = 61mm).
Higher K´s, on the other hand, seem to match well with higher projectile weight of the BR-350B (6.3Kg)...

Not really surprising. Variances still exist but given the involved uncertainities here in regard of terminal velocity, its nothing which really falls out of the ballpark.

[Mobius]

The PDF can be downloaded from here:
https://digitalcommons.unl.edu/dodmilintel/66/
On page IV the weight is given as 14.3 lb. which is 6.486 kg. The Russians give the BR-350 kg as 6.5 and the BR-350A as 6.3 kg. The later BR-350B is also 6.5 kg.
Here's a link to the Model 39 on a Russian site.
https://ru.wikipedia.org/wiki/76-мм_див ... года_(УСВ)

[Miles Krogfus]

Published in 1939, the FT 125 for the Model 39 gun lists these striking velocities using the BR 350A projectile of 6.3 kg. with a MV of 662 m/s:
100 meters 651 m/s
300 meters 631 m/s .
500 meters 611 m/s
1000 meters 564 m/s
1500 meters 519 m/s
2000 meters 478 m/s
2500 meters 440 m/s
3000 meters 404 m/s
3500 meters 372 m/s
4000 meters 345 m/s
Note: I need to know the date of a Russian FT when another person posts it here, then by looking at my collection of the original Soviet 1930's-1940's FT's, I may then find what actual AP round is being used. For example, the BR 350A projectile was changed in design ( its length and shape were different after WWII ) so its performance changed . . .