German 75mm v Allied 75mm (1 Viewer)

Thanks. But You will notice, the list is based upon secondary sources. The following excerpts are from primary source documents detailing the development of armour-piercing capped shell in Germany. The original is entailed in the Lilienthalreport 166 files I quoted above.

[A] State of AP-performance through 1940/41:

Generally, german AP was uncapped APHE, except for 8.8cm and 7.5cm which were APCBC-HE but both with a relatively large explosive filler, which had potential to cause very severe damage but also compromised penetration capability.



The appearence of the Char B1 bis and Matilda tanks caused new and higher requirements for armour penetration. the easiest way to improve performance was to increase striking velocity. However, all german AP of this period would shatter at high velocities and any increase in striking velocity would not result in a similar gain in penetration performance. There were two solutions:

[A] improve the properties of the projectile. This can be done in two ways, first one could get rid of the filler or at least make it smaller or alternatively, introduce sheath hardening as employed by Krupp for the KM naval L.4.4 type APCBC projectiles. Sheath hardened projectiles have the highest hardness at the surface and go softer to the inner cavity of the projectile thus high hardness can be found on all surfaces like a cocnut enveloping a softer core. The decremental hardening is more useful for mass production (hardness is highest in the nose and gradually -in one level from side to side- goes softer towards the base), however and sheath hardening wasn´t employed outside the Navy and experimental prooving grounds.
add an armour piercing cap with an as hard as possible surface hardness gradient. The initial APCBC had soft caps (soft caps don´t work outside very limited parameters of obliquities 0 to 15 deg) but replacing them with hard caps (440 to 560 BHN) resulted in prevention of nose shatter. high striking velocities thus could be exploited for the firt time. The solution is easier for mass production, too. The following graph shows the differences between old and new type 7.5cm APCBC, in mass production and service by 1943:



You will notice how the Pgr39 get´s more penetration by enjoying more velocity while the Pgr rot suffers breakage precluding any higher penetration due to soft cap failure. The Pgr39 is actually a pretty interesting projectile, beeing a composite one with part of the nose detached from the body and acting as an auxilary AP-cap und the thick AP-cap. The prooving ground committee was enthusiastic about the Pgr39 performances in combination with high velocity guns and some very ridiculous thicknesses were trialed to explore the limits of the new projectile. The following is a trial with a 200mm thick RHA armour plate carried out 1942. It was successfully penetrated at 1090m/s velocity by the Pgr.39 on 30 deg obliquity (= 30 deg inclined from the vertical and normal impact -german definition was 60 deg because they considered 90 deg as normal). This is quite interesting, one projectile at 1020m/s just made a bulge and was recovered in front of the plate (intact), the projectile striking at 1070m/s velocity stuck in the plate (penetration according to british and US Army definition and partial penetration according to US Naval Ballistic Limit definition) but the "grenz" definition required that the complete projectile (100%) had to be recovered behind the plate, that´s why the 1090m/s figure also found entry in the
penetration tables.



It appears that the 75mm Pgr39 was the primary study object of the ballistic research dep. and various obliquities were trialed with this projectile covering right angles impact as well as 30 deg, 45 deg and 60 deg high obliquity impact studies. The corresponding results are reproduced below:



Note how difficult it appears to penetrate anything at really high obliquity (45 to 60 deg), and how exceptional the penetration capability at right angle´s impact was. Similar studies have been executed -albeit at a smaller scale- for other projectiles and most of the data have been standartised for 30 deg obliquity impacts, the following graph gives various 1943-period APC performances at 30 deg obliquity:



Late war, much intention was paid to increase the obliquity performance of the projectiles. You can realize that the trend was generally to go for blunter headshapes and flat APcaps in order to improve the biting angle and prevent ricochet.
The 88mm Pgr.39/43 still had a very pointed headshape, 1.47crh (the US M79 AP had 1.67 crh), which allowed for optimum penetration at 15 to 45 deg obliquities. The 100mm APCBC was down to 1.35 crh and the 128mm APC which was issued later further down to 1.29 crh with a completely flat cap and no windscreen. The 75mm was reduced from 1.53crh at around 1940 in the 7.5cmK.gr. rot Pz.Gr. to only 1.1crh in the PGr.39 for 1943 and correspondingly had quite credible high obliquity performance for her period. The further reduction of the point was on schedule with plans for a new model P.Gr.45 88mm and 75mm APCBC with only 0.93 caliber radius headshape combine dwith a thick and flat AP-cap under a very pointed break away aluminium ballistic windscreen.

hope it helps,
delc
 
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Does anyone have data on how the Russian 76mm compared to their German / American equivalents?
There is annecdotal evidence of the impact of the T34 being impervious to AT fire - unless it was the 88mm!
But, I do not recall much about its firepower?
 
I found this on high hardness armor.
Trends in Soviet Metallurgical Developments
Central Intelligence Agency
Washington 25, D, C.
16 April 1953

Watertown Arsenal for metallurgical examination

The very high hardness encountered in most Soviet tank armor has
caused much unnecessary concern regarding the relative ballistic performance
of the hard Soviet armor and the softer American armor. Many
people associate high hardness with high resistance to penetration.
Although this is true, within limits, in the case of attack of armor
by undermatching projectiles (i.e. caliber of shot is less than the
thickness of the armor), articularly at low obliquities of attack, it
is definitely not true when the armor is attacked by larger caliber
shot at higher obliquities of impact. Competitive ballistic trials
which have been conducted at ordnance proving grounds on both very hard
and normally hard domestic armor and Soviet armor have established
beyond question of doubt that in many cases, representative of actual
battlefield attack conditions, very hard armor is distinctly inferior
in resistance to penetration as compared to armor of more conventional
hardnesses (280-320 Brinell).
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Added new information -apologies for 88mm instead of 75mm data.

I have tried to register in tank.net but unfortunately, they generally don´t accept new memberships in the moment.
I am just writing here in order to offer some thoughts about the discussion of IS-2 obr.44 armour vs KWK43 88mm/71 using APCBC. Particularely with regard to the aspect of the upper front plate (glacis plate) versus penetration or its alleged immunity. It has been suggested by Bojan -citing the later Yugo trials on T.54 glacis plates whiches the 88mm APCBC couldn´t penetrate- that the upper front plate of the IS-2 mod. 44 (100/110mm @ 60 deg) was correspondingly also immune to penetration at any range from 88mm firing either APCBC or APCR.
In this context, direct evidence is hard to come by but the IS-3 penetration trials are significant for the question, as it details also include -among others- the penetration of 88mm vs IS2 for both, upper and lower front plate. Being a russian source, it has primary source character and should not be rejected easily as holding no reference to IS-2, people who compiled the report only notified the results but they indeed had access to all results, not just the few we know today. While the document deals primarely with IS3 in order to demonstrate the improvements attained by russian tank design in beating the high performance german 88mm KWK43 /PAK43, it also contains valuable data for the IS2, too and is more likely to make use of the latest APCBC ammo (Pgr 39/43) captured in Germany than earlier documents deteailing penetration of IS85 or T34.

I quote the relevant passage:
Нос корпуса

Выбранная конструкция носовой части корпуса танка ИС-3 аначительно увеличивает снарядостой-кость этого узла по сравнению с носовой частью серийного корпуса ИС-2, за исключением передней наколонной част и крыши корпуса.
Верхние лобовые листы корпуса ИС-3 полностъю зацицают экипаж от 88мм немецких бронебойных снарядов с дистанция 100 метров и более, при любых углах обстрела.
Верхний лобовой лист корпуса ИС-2/Уралмашзавода/ непробивается толъко дистанции 600 метров и далее.
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The bold lines demonstrate that the soviets did considered the glacis of the IS-2 to be proof from 88mm at distances of 600m and more -suggesting that inside this range it was very vulnerable.
A later page detailing the lower front plate says that the IS-3 was able to repell 88mm from larger distances than 300m while the same aspect on the IS-2 could be penetrated at 5100m (!).
This soviet era source points out that the frontal glacis plate of IS-2 can be penetrated by 88mmL71 APCBC inside within 600m while the lower plate can be penetrated in excess of 5000m, the huge difference entailed in both numbers (600m vs 5100m) is a valuable secondary information and in effect rules out the possibility that the IS obr.43 is referred to here in source instead of the IS-2 obr.44. The IS-2 obr. 43 had 30 deg sloped 120mm plate (from the vertical) for the "step" on the glacis and a similarely 30 deg declined lower front plate of 100mm. Thus minimal differences in striking velocity are required to penetrate both plates at similar angle (~680m/s lower front plate vs 750m/s upper front plate for the 88mmL71 using Pgr.39 using official WaPrüf penetration curves for post 1943 APCBC on german RHA). Only the IS-2 obr. 44 had the required difference in upper and lower front plate (110/100mm@ 60 deg glacis vs 100/90mm @ 30 deg lower front plate) to explain the distance discrepiancy mentioned in the soviet prooving ground report.
This doesn´t explain why the T54 glacis wasn´t penetrated at all even from close range. It doesn´t need to. The T54 is a younger, more advanced metallurgical design. It benefitted from all late war research conducted and the data gathered in Germany and abroad which helped to produce the best plates for a specific purpose.


It is also necessary in this context, and I think overdue, to make clear - having compiled sufficient body of evidence- to suggest that plate hardness is not important at all under high obliquity impact. While against vertical plates and even moderately declined ones armour plates of high BHN levels significantly improve the ballistic resistence (sometimes, if the plate is able to shatter some of the older, or weaker capped and all uncapped projectiles by a very considerable margin - shatter yields in a ~1/3 raise in necessary striking velocity to perforate plate), they don´t do it at 60 deg. I have the results of systematic penetration trials conducted with different BHN level good RHA armour plate (even mild steel) at varying obliquities to proove this. Differences in ballistic resistence drop to being negliable against extremely sloped plate. (a gain in 30kg/mm^2 tensile strength equals only about 5% gain in resistence at 60 deg as opposed to 25% for a vertical plate)
More important than hardness is that the armour plate -be it cast or RHA- is entirely free of flaws, has a fine grain size and -orientation and contains no internal laminations and remains ductile under dynamic impact stress. Overly hard armour plates tend to be more brittle and flaw affected than ductile ones in comparison. The decision, f.e. to make the PzIII, PzIV and TIGER-I´s vertical armour plates very hard but the HETZER´s and TIGER-II very oblique sloped frontal plates comparably soft in fact represents a sensible and correct application of ballistic research carried out in Germany in the early 40´s (compare Lilienthal report 166). Soft but highly sloped armour improves toughness under impact and allows the plate to stretch making deflections and ricochet more likely. A harder front plate, as was practice in soviet union still at the end of ww2 would likely even have reduced the level of ballistic protection here, such as mentioned also in US Watertown reports for impact under high obliquity.
Flaws in armour plate may result in roughly 1/3 loss (25% to 40%) in effective thickness and explain why ,f.e. a 28mm s.PzB.41. was able to pierce the lower front plate of an IS-85 (with stepped hull and 100mm cast carmour) in one documented case 1944, a performance which is well beyond the scope of the certified penetration of 67mm@ 30 deg and short range credited to this weapon. Also the failure of lower plate armour at distances of ~5km as cited by russian IS-3 document can be seen as a strong indication of the existence of such flaws in armourplate. Armour plate of the period was hardly ever systematically tested at very high obliquity (60 deg realm) before well after end of ww2. A flawfree, and soft armour plate (~205 to 220 BHN) would probably have eleminated the vulnerability of the glacis of the IS-2 mod.44 against 88mm KWK but as it is, the frontal aspect appears not to be impenetrable from medium and close range while the lower front plate, which makes for roughly 40% exposed frontal hull target area, is virtually penetrable by this gun using late model Pgr 39,1 at any realistic fighting range.

hope it helps,
delc
 
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Some great info posted here, I must say that with all the development of aircraft, aircraft carriers and anti submarine warfare it is lamentable that the allies were so far behind in Tank development, seems the USA and UK/Canada and other allies were always way behind in fire power armour and HP.
 
delcyros said:
This doesn´t explain why the T54 glacis wasn´t penetrated at all even from close range. It doesn´t need to. The T54 is a younger, more advanced metallurgical design. It benefitted from all late war research conducted and the data gathered in Germany and abroad which helped to produce the best plates for a specific purpose.
Click to expand...
The M47 with 102mm@60 was penetrated by the 88mm/L71 out to 250m but it only had 210 BHN armor.
 
Which is entirely possible if that plate contained flaws such as, f.e. laminations.
Remember, it´s not about hardness, it´s about flaws in the plate. While it may be possible to make flawfree plate in this thickness region (around 4"), even treated to relatively high hardness levels, if the attack is obliquily chances increase that the plate doesn´t behave ductile but brittle thus, softer plates are preferable.
However, that doesn´t mean that a soft plate automatically is flaw-free. For me, it appears that the T54 glacis plate was flawfree (still relatively high hardness level), while both the IS-2 plate (600m, harder) a the soft M47 glacis plate (200m, soft) were not.
The relatively closer distance of penetration of the M47 plate vs that of the hard IS 2 glacis can be used as an indication for softer plates being indeed ballistically better in this specific application (against capped AP).
I think that the exact opposite has been claimed instead.
 
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I guess no flaw in that JSII armor.
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Several russian authors noted "bubbles" in cast elements, which are flaws. This has also been rported from israeli studies on JS-3. Not sure about the 100mm (or 90mm?) RHA elements, which replaced the 110mm/100mm cast ones in the final JS-2, though.

One odd thing is that while the front plate lists the BHN as 444 the tensile strength is equivalent to 295 BHN.
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It is possible, that only the surface was hardened, probably by additional heat treatment. It�s then kind of face hardened (but not true, as there is no ski-ramp like change in hardness gradient in the section, just a thin, hardened surface with an abrupt drop to normal plate homogenious hardness levels, which under dynamicly applied impact force is guaranteed to be flawed.
High hardness / high tensile strength doesn�t add materially in ballistic resistence at 60 deg.

It may still have benefit in breaking up projectiles, particularely at high obliquity impact (by projectile damage), which may be desirable.
The basic problem is that ductility corresponding to 295BHN is simply to low to facilitate effective deflection properties (without going into to many details, the problem is worse for smaller calibres than for bigger ones due to the difference in impress exposure size and plastic deformation limits encountered). Rather, it will cause regularely some kind of shatter (even hard AP-caps don�t work in 60 deg obliquity range), which may turn a sharp pointed penetrator into a blunt one, or in other words, a penetrator which is much better able to penetrate obliquily...

f.e. the KT�s 55 deg sloped 150mm glacis plate may also contain flawed elements (difficult to manufacture thick homogenious plates without center plate sections differential cooling issues avoided) but it was thick enough that even with existing flaws it acted minimum like a 100mm unflawed, high quality plate, which at this obliquity is virtually proof against then existing weapons (unless previously damaged).

Nose breakage /nose shatter is a desirable aspect at low impact obliquity, where the required striking velocity goes up in a shattered projectile (by roughly 1/3), but it represents an entirely undesirable feature at high obliquity impact, where nose breakage would reduce necessary striking velocity to perforate armour- and greatly reduce to probability of ricochet in the first place (blunter penetrators have better biting angle).
 
All the science and number-crunching! Good reading, but in the field, only rough ideas. Much of what gets penetrated, or not, can be influenced by compound angles (armor angles AND angle of the target to the angle of the gun as well as the hull angle to the horizontal due to terrain). Add some stowage on the tank, "smart" stowage aka goodies purposefully placed, track section, road wheels, and the like.

T-34. Decent gun but the fighting effectiveness was limited by the vehicle commander also being the gunner. GREAT armor for the wt by late 41 standards. This armor advantage was surpassed by the Germans fielding better and more guns. But what the Germans could field, the Russians were able to field many times over and in places the Germans had little heavy forces to counter with. Better was the T34/85 with the 3-man turret and the more useful 85mm gun. On par with the US 76 and broadly close to the PaK40 in penetration, the 85mm gun had more useful HE round for BREAKTHROUGH.

NOT A SINGLE US Sherman that landed in NORMANDY was armed with the 76mm gun! All were at the depots in England, refused by the Armored Divisions. I do not recall the number present/sitting at the depots, but I seem to recollect 250-400 units. One must ask WHY? The first reason was that the 75mm gun had done well so far. Most of the combat was against infantry or strongpoints where the 75mm HE round was vastly superior to the US 76mm HE round. Another factor was Intel! Part of this was US believing the Panther was of limited production such as the Tiger I. I believe it was not long before D DAY that the alarm as sounded that the Panther was actually is wide-scale production.

Let's talk Sherman Firefly. At Normandy and until late war, only 1 in 4 were Firefly conversions. Then consider the all-powerful 17pdr had some bad ammo lots early-on. Some rather embarrassing results at a test shoot in Normandy. You cannot penetrate what you cannot hit! Once this was sorted out, a very respectable performer. Firefly tanks were a priority German target indeed. The 17pdr AT gun was less successful as the size and weight limited mobility.

Consider the lower rate of fire for some of these big guns and or slow traverse rates. Consider the reliability, mobility, and quantity. Tanks are no good unless they are running and fully-mission-capable. They are not good if stuck in the mud, have a busted final drive, or out of fuel! Must shoot, move, and communicate!!! On a tank, it is the package of all the characteristics. The big wild card is crew quality, training, and tactical sense. From the Operational Commander, down to the BN Cdr, Co Cdrs, PLs, vehicle commanders and the vehicle DRIVER! Taking on the COMBINED ARMS concept, lots of tactical air power and artillery make up for not having great tanks.
 
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I was reading some media articles about D-Day. One of the things that saved the US Army's bacon in Normandy was the fact that the vast majority of German tanks and armoured divsions were concentrated against the Commonwealth forces, to prevent a breakthrough around Caen. This rather fortuitously allowed the US to escape the worst consequences of having such poor AP weapons on their tanks.
 
T-34. Decent gun but the fighting effectiveness was limited by the vehicle commander also being the gunner. GREAT armor for the wt by late 41 standards. This armor advantage was surpassed by the Germans fielding better and more guns.
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High hardness armour plate works best in wrecking up the projectile (even hard capped ones) in the obliquity range 25 to 45 deg. There is still a bonus against shattered projectiles in this obliquity range, albeit much smaller than the 1/3 generated by shatter against vertical impact. Thus, the way, the soviets employed armour on their T34 was effectively canceling the older capped AP-shell used by the germans then. However, the solution was not -as often believed- the introduction of larger or longer barreled guns but instead

[1] a significant improvement of the APCBC shell, which resulted in a virtually indestructable projectile (HE-filler reduced, more blunter noseshape of 1.1crh, composite welded nose, sombrero shaped AP-cap with better biting angle hardened all the way through), and
[2] the adoption of APCR for existing 5cm KWK/PAK.

While [2] was an interim ad hoc solution to the problem, [1] was a necessary precondition before longer barreled guns (5cmL60, then also 7.5cmL43/48) could be employed at all. Raising the striking velocity didn�t improved penetration unless the projectile was improved so much as to cope with the larger dynamic impact forces.
[2] also effectively countered high hardness and face hardened (more like poor quality Harvey hardened akin...) armour plate employed by the SU tanks. The low ductile, high hardness armour plate only works if it succeeds in messing up the projectile (nose shatter or breakage) at this obliquity range, if it fails to achieve this, the lower toughness (less ductile under impact) works against the plate. It was good when introduced but once improved APCBC -which were virtually immune to nose damage in mid obliquity- were issued in 1942/3, the T34 armour turned into comparably poor protection, even against projectiles from guns it previously defeated.
 
I got this from tank.net a few years ago. Aberdeen tests of 88mm/L71.
Compare it to the 88 projectiles for 3000 + velocities vs targets.
8" plate@ 0°:Two complete (NBL) penetrations with projectile passing through plate. 3124mv and 3257 mv.(550y/90y) Two ABL penetrations. Projectiles Intact. 3001 mv 3038 mv. (990y/860y)

6 1/16" plate@ 30°:Two complete penetrations, projectile through plate. Projectile intact 3008 mv 2971 mv (974/1100y)

5 1/8" plate@45°: One complete penetration, projectile through plate. Projectile fractured. One projectile shattered. 3288 mv 3211 mv

3 7/16" plate@ 55°: Two complete penetrations. both projectiles fractured. Projectile fragments passing through plate.
One partial penetration projectile fractured.
3334 mv on two penetrations. 3310 mv on 2.5" deep partial.
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kettbo said:
All the science and number-crunching! Good reading, but in the field, only rough ideas. Much of what gets penetrated, or not, can be influenced by compound angles (armor angles AND angle of the target to the angle of the gun as well as the hull angle to the horizontal due to terrain). Add some stowage on the tank, "smart" stowage aka goodies purposefully placed, track section, road wheels, and the like.

T-34. Decent gun but the fighting effectiveness was limited by the vehicle commander also being the gunner. GREAT armor for the wt by late 41 standards. This armor advantage was surpassed by the Germans fielding better and more guns. But what the Germans could field, the Russians were able to field many times over and in places the Germans had little heavy forces to counter with. Better was the T34/85 with the 3-man turret and the more useful 85mm gun. On par with the US 76 and broadly close to the PaK40 in penetration, the 85mm gun had more useful HE round for BREAKTHROUGH.

NOT A SINGLE US Sherman that landed in NORMANDY was armed with the 76mm gun! All were at the depots in England, refused by the Armored Divisions. I do not recall the number present/sitting at the depots, but I seem to recollect 250-400 units. One must ask WHY? The first reason was that the 75mm gun had done well so far. Most of the combat was against infantry or strongpoints where the 75mm HE round was vastly superior to the US 76mm HE round. Another factor was Intel! Part of this was US believing the Panther was of limited production such as the Tiger I. I believe it was not long before D DAY that the alarm as sounded that the Panther was actually is wide-scale production.

Let's talk Sherman Firefly. At Normandy and until late war, only 1 in 4 were Firefly conversions. Then consider the all-powerful 17pdr had some bad ammo lots early-on. Some rather embarrassing results at a test shoot in Normandy. You cannot penetrate what you cannot hit! Once this was sorted out, a very respectable performer. Firefly tanks were a priority German target indeed. The 17pdr AT gun was less successful as the size and weight limited mobility.

Consider the lower rate of fire for some of these big guns and or slow traverse rates. Consider the reliability, mobility, and quantity. Tanks are no good unless they are running and fully-mission-capable. They are not good if stuck in the mud, have a busted final drive, or out of fuel! Must shoot, move, and communicate!!! On a tank, it is the package of all the characteristics. The big wild card is crew quality, training, and tactical sense. From the Operational Commander, down to the BN Cdr, Co Cdrs, PLs, vehicle commanders and the vehicle DRIVER! Taking on the COMBINED ARMS concept, lots of tactical air power and artillery make up for not having great tanks.
Click to expand...

watching a documentary on Normandy there was a tank veteran who said his group of 150 tanks used 500 during the campaign to maintain that 150 at the front.
 
Mobius said:
I got this from tank.net a few years ago. Aberdeen tests of 88mm/L71.
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Yes, that demonstrates the problem. A projectile encountering it�s critical velocity (each plate-projectile-obliquity combination had it�s own crit velocity) will suffer breakage but here the breakage is increased by trying to establish the limiting velocity for perforation. Under those conditons, where the projectile is subject to severe forces acting on it for a longer time period, damage is typically most severe. Once You increase the impact velocity, projectile breakage again ceases and the penetrator makes it through intactly.
Shatter, however, does not cease. Even with meteoric striking velocity, the projectile won�t make it through intactly.
I have seen (and I believe previously referred to here) pictures of 7.5cm Pgr.39/1 penetrating a 2 2/3 calibre strong homogenious plate (67 kg/mm^2) at 30 deg with virtually no nose damage. These projectiles were optimised for obliquities around 15 to 40 deg, and unlike older projectiles didn�t suffered any damage. The 7.5cm was optimised for even higher obliquities.

Sometimes it is more useful to use relative cal/plate ratio�s instead of absolute figures. The Aberdeen trials for 8.8cm show
that it perforated

2.4 cal homogenious armour at 0 deg and (mean of) 940m/s -intact
1.8 cal homogenious armour at 30 deg and (less than) 918m/s -intact
1.5 cal homogenious armour at 45 deg and (about) 1002m/s -projectile damage starts
1.0 cal homogenious armour at 55 deg and (mean of) 1013m/s -projectile damaged

Using the 7.5cm Pgr.39/1 curves in cal/plate ratio�s we attain the following data:
2.5 cal homogenious armour (70kg/mm^2) at 0 deg and (~) 940m/s -intact
1.9 cal homogenious armour (80 kg/mm^2) at 30 deg (~) 920m/s -intact
1.3 cal homogenious armour (100kg/mm^2) at 45 deg (~) 1000m/s -intact
-no data for 55 deg but at 60 deg:
0.9 cal homogenious armour (110kg/mm^2) at 60 deg (~) 1015 m/s -damaged

As can be seen, the 8.8cm is able to penetrate slightly more armour (relatively spoken) than the 7.5cm, that�s due to a much fine nose shape (1.47 crh as opposed to 1.1crh for the 7.5cm). However, the way these trials are tabulated hide the fact that the 7.5cm is actually better able to negotiate obliquity, particularely softer plates. The blunter nose shape would make penetration at low obliquity against soft armour (70kg/mm^2 plate, see above) more difficult (requires penetration by plugging) than the 8.8cm but soft armour plate would be more easily overcome at high obliquity. I don�t know which plate hardness was used in Aberdeen but I strongly suspect that the 8" plate was harder than 70 kg/mm^2 used in german trials for this thickness range. Unfortunately, the trials then shifted to high hardness plates at higher obliquity (100kg/mm^2-110kg/mm^2, see above) and as long as the projectile stays intact (slight nose damage acceptable), a finer nose shape WILL STILL PERFORATE better by drilling action against hard plate (only, against soft plate, the blunt nosed ones are significantly superior). This is explainable by the failure mode of the armour plate. For homogenious armour it has been established on the prooving ground that higher BHN plates result in larger normalization than softer plates as measured by the perforation channels (at least in smaller calibre trials, 20mm model shot, 37mm and 75mm).

normalization.jpg


The 1.1crh 7.5cm is -however- able to negotiate higher obliquity conditions more intactly than the 1.47 crh 8.8cm Pgr.

Normalization depends on two aspects:
[A] Nose shape and nose coverings of the APCPBC (hood effect, cap effect, nose shape)
hardness of the target plate (the harder, the more normalization, but it�s like the back hardness which counts in face hardened types of plates)

Coming back to the IS-2 obr. 44 glacis:
The 100mm cast high hardness plate is -despite an angle of 60 deg- particularely vulnerable to 8.8cmL71 firing APCBC (and APCR) -narrowly within the performance envelope, even without flaws -around 110 to 120mm would be required here instead to secure immunity.
The 90mm medium hardness RHA plate of final IS2 would be better suited to defeat 88mm, 1.47 crh APCBC (though lower hardness plate would be preferable) and still be sufficiently strong to defeat the 75mm APCBC -unless flawed.

The adoption of blunt nosed 8.8cm projectiles was under consideration in 1945. Krupp reported that 8.8cm APCBC with 0.97 crh nose shape yielded excellent results. Further improvement could be attained by adopting sheath hardeneing technology for the projectile body instead of the cheaper decrementally hardening prefered for manufacturing reasons. Sheath hardening was known and used since the mid 30�s for Krupp�s navy projectiles, like L4.4 20.3cm APCBC. Sheath hardened projectiles could practically get rid of the armour piercing cap and stay intact (APBC) and at 60 deg obliquity would yield an increase in perforation capability vs the 1.47crh Pgr.39/1 which is roughly proportional to the relative mass of the deleted cap.
These modifications would have kept the 88mm effective vs even the improved glacis of IS-2 (final design) and IS3, manufactured 1945.


edit:

I have extracted the relevant graphs from the Lilienthalreport 166 which demonstrate how the plate hardness change ballistic resistence of plates depending on obliquity. For better readability I have deleted out the 3cm L5 projectile and cumulated both original graphs into a single one. These trial plates (50kg/mm^2 mild steel and 145-150kg/mm^2 ultra hard armour steel) were of very high quality and generally flaw-free. Note that while there is -using the 30mm shot example- about 82% benefit in penetration of mild steel compared to specially hardened armour grade steel at vertical impact (90 deg), the difference at 30 deg impact would be entirely canceled out -the specially hardened plate is no better than mild steel!

change_of_quality.jpg


This is important for glacis plates. Attention paid to make very hard plates for glacis plates were entirely wastes of effort. Just extrapolate the curves by mk1 eyeball and guess what happens at 20 deg. Hard plates would then be entirely at ballistic disadvantage compared even against mild construction grade steel. Also, pay attention to the differences between 20mm and 30mm shot. The point of crossover moves. For the 20mm it would be at 27 deg, while for the 30mm it is at exactly 30 deg. I have limited data for 75mm which seem to suggest about 35 deg as point of crossover.
We can call this effect "inverse scaling". I stumbled over the effect when I tried four years ago to verify whether or not scaling is dependent on obliquity in another context.
Right now I can confirm it for various projectile sizes of the same family. 2cm and 3cm AP,
75mm and 88mm Army APCBC; 15cm and 28.3 cm Navy L3.7 APCBC and finally 20.3cm, 28.3cm and 38cm Navy L4.4 APCBC (where the point of crossover is at between 60 and 55 deg, btw).
What is known as scaling effect, the loss of quality of plate when the absolute calibre is increased (but the relative plate/cal relation stays the same) is only true at vertical impact and becomes inverse at very high obliquity.

Consequently, cast high hardness armour grade material for the glacis of IS-2 obr. 44 is likely not offering the degree of mild steel if engaged by 88mm at 30 deg, too.


edit: apologies for the technical -and sometimes not so easy to understand reply. English isn´t my primary language but I hope its possible to follow the reasoning explained herewith.
 
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Is that resistance in terms of any particular type of penetration? And how would it be tested?
 
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