8,8 cm PzGr 39 Performance

[Peasant]

Something I hope will be relevant: a neat way to describe how several different projectile designs interacts with armour with varying characteristics, is to express the projectile performance in therms of energy required to defeat a unit of LOS of armour. Here we can see how the late German PzGr.39 AP design negotiates slopes: the required energy stays mostly the same up to about 50° and then it drops drastically. I believe it's the effect of the projectile nose breaking off somewhere between 50-60° changing the penetration mode from ductile hole enlargement to shearing of the plug from the armour.

[Peasant]

Some information that I didnt know which might prove useful:

The changes made for the new PzGr.39/43 introduced with the new 45 deg acceptance conditions in autumn 1944 included the following:

- improved heat treatment of the penetrator, specifically the nose

- changed ballistic cap & windshield design

In addition to this it appears that the shape of the burster cavity was changed as well, wider at the bottom, narrower at the top. Sadly no original German cut away drawings exist for this round, only the polte drawings for the initial prototype 39/43 design which ended up the stop gap PzGr.39/1 with wider driving bands, not to be confused with the small drive band PzGr.39/1 issued in the beginning, which also shortly had the PzGr.41 designation. It's a confusing mix of projectiles used, esp. due to similar designations, safe from the serial number, making it all the more understandable how the people at the USAPG could mix it up.

Three different PzGr.39's compared

The 8.8cm PzGr. recovered designated PzGr.41:



Source: https://forum.warthunder.com/index.php? ... nt-7790515

[critical mass]

Never heard about a 88mm Pzgr41 in german sources (includes Referat III HWA development tables).

[seppw]

The 7.5cm Pzgr39 had the conical cap eventually, the 12.8cm Pzgr39 had the conical cap eventually, so why didn't this 8.8cm Pzgr39 get it?

[critical mass]

Not necessary for the 88mm pzgr39. Only relevant if target thickness is >200mm for this cal. Under all other conditions, a smaller cap is better (less dead weight wasted needs to be accelerated).
For the 12.8cm/8.8cm Pzgr.39 -TS, which was expected to deal with plates >25cm, the cap was indeed changed towards the thick Army type cap.

[Peasant]

Whelm. do you have any British Critical Velocity charts on the ballistic capped British 6 pdr? It would be interesting to compare it's ballistics with that of the Russian 57mm and US 57mm M86.

BTW, plotting 88mm MV=780 m/s velocities on the British CV of 88mm small capacity yields these data points.
88mmsmallcapcityOB44.jpg

Mobius, I have good news. we can stop pixel-hunting on these charts. These curves were computed using this formula here:





With n = 1,43, as the later paper suggests, the R constant is very close to 3260, for 6pdr AP at normal incidence (SD=27)
Here is the calculated graph from excel overlayed on top of the chart:



To get R not so small as to go into decimal values, I've used Kg for shell weight, cm for diameter and m/s for velocity.
Other charts will have slightly different "R"s, mind you.

[Mobius]

Nice,
I must of missed them but whelm already uploaded firing graphs for other 6 pdrs.

[Peasant]

If anyone wants it, I've packed this chart into an Excel function that automatically coverts the thickness at any obliquity into an equivalent at 30°.
Link to the speadsheet:
Edit: new link: https://docs.google.com/spreadsheets/d/ ... sp=sharing

[critical mass]

Thank You.

May I ask for which velocity this tabulation is? I notice that the penetration graph from the Lilienthalreport (I recall its from the paper therein of Herr Baurat Dr. Sitz) does not show a curve at any velocity for 30mm. Is it extrapolated to this thickness and has the change of tensile strength been corrected for?

The 1943 period graph from Dr. Sitz (Sitz was then the chief of Bureau of Ordnance department (WaPrüf), responsible for arms and ammunition production as well as quality controll and improvements in Germany) is a very important primary source document as it demonstrates the performance of mass production specimen of the newly introduced 75mm Pzgr.39 right after the improvements in tempering have been achieved by, and circulated to other manufacturers by WaPrüf following the extensive report of the Sächsische Gussstahlwerke.

This is the early mod. Pzgr.39 performance. The projectile therein was improved to become virtually unbreakable when it attacks the armor between 90* and 60° up to and beyond MV striking velocity (the same paper has a photo of 75mm Pzgr.39 striking a 200mm RHA plate (2.67 cal/d for the 75mm projectile) at normal for velocities between 1000m/s and 1090m/s, where the projectile always stayed intact, whether or not it penetrated, stuck in plate or even rebound).
In other countries during ww2 the R&D was convinced that full calibre steel AP will always break up if striking velocity approaches or exceeds 1000m/s.Both, the 75mm and the 88mm service AP were eventually tested to impact velocities out to >1400m/s in experimental trials and break up did occur, when striking at 30°, at velocities between 1200m/s and 1300m/s, very approximately with a good quality projectile (i.e. mass produced service projectile AP lot but no failure in either proof or physical testing of the lot).

Late war Pzgr.39 were somewhat better, despite leaner alloy composition. The penetration at 90-60° didn´t change, because the projectile also stayed intact (no advantage) but break up was delayed to well beyond 45° obliquity in thick d/T ratios and very high impact velocities (proof changed from 60° to 45°) with an appreciable narrowing of the gap between the 60° and 45° curves as a consequence.

[Stiltzkin]

If anyone wants it, I've packed this chart into an Excel function that automatically coverts the thickness at any obliquity into an equivalent at 30°.
Link to the speadsheet: https://docs.google.com/spreadsheets/d/ ... sp=sharing

Hm, it says that the file is not available.

[Peasant]

I have merely fitted the variables into the Thompson's universal penetration formula to describe the data in the chart. Any values outside those present already in the chart are mathematical extrapolations of the curve and, at this moment, I cannot say how close they are to reality.

May I ask for which velocity this tabulation is?

Sorry, I don't understand what you are asking here?

Late war Pzgr.39 were somewhat better, despite leaner alloy composition. The penetration at 90-60° didn´t change, because the projectile also stayed intact (no advantage) but break up was delayed to well beyond 45° obliquity in thick d/T ratios and very high impact velocities (proof changed from 60° to 45°) with an appreciable narrowing of the gap between the 60° and 45° curves as a consequence.

Are you sure that the latter part, in high T/D region, of the curve for 45° penetration is for broken (grentz) projectile? I'm dubious because, as you can see, it can be described perfectly by the same function for every T/D ratio, with no discontinuity between low and high T/D regions, which, in my experience, would be unmistakable if some drastic change, like breakage, had occurred between those two regions.

[Peasant]

Hm, it says that the file is not available.

Sorry it was so bad I had to remake it from ground up so other people could understand it more easily. I've updated the link with the new version.

[critical mass]

I have merely fitted the variables into the Thompson's universal penetration formula to describe the data in the chart. Any values outside those present already in the chart are mathematical extrapolations of the curve and, at this moment, I cannot say how close they are to reality.

May I ask for which velocity this tabulation is?

Sorry, I don't understand what you are asking here?

The impact velocity is an important variable at high obliquity from many tests and summaries I have read. The higher the velocity, the lower the factor of armor thickness penetrable compared to reference 1.0= thickness "x" at 30°obliquity (60° in german definition) (compare HSc 634, Fig.7b from STEEL AP AND THEORY OF PENETRATION (copied 1946), which was drafted following results with 88mm Pzgr39 not 75mm Pzgr.39).

Are u sure....

Quite so. The curves used by Army are G(D) curves. G(D) is an old penetration definition, which does not include a specification whether or not the projectile stays in intact bursting condition after affecting perforation. In practice, the projectile starts to stay intact until a certain obliquity point and then, at higher obliquities, may also break up (shown graphically by a change from solid lines to dotted or interrupted lines). Relevant here is only that the projectile penetrates 5 times in a row (for 75mm calibre) with no residual velocity after performing complete penetration. The definitions of G(heil) and G(grenz) (used by the Navy at that time) each are much stricter in regard to projectile state.
As far as WaPrüf was concerned, they issued a proof requirement for passing AP lots specifying a G(D) condition with the additional condition that the projectile also had to stay intact at least 2/3 times. Because the official tabulations of penetration for service pamphlets are within the zone of obliquity where the projectile generally stays intact, the service pamphlets are including what would be called "intact penetration" (slightly harder than the US Navy effective limit, f.e.).
Notice the difference to the Navy G(heil) where the projectile would be required to stay intact.

[Peasant]

The impact velocity is an important variable at high obliquity from many tests and summaries I have read. The higher the velocity, the lower the factor of armor thickness penetrable compared to reference 1.0= thickness "x" at 30°obliquity (60° in german definition) (compare HSc 634, Fig.7b from STEEL AP AND THEORY OF PENETRATION (copied 1946), which was drafted following results with 88mm Pzgr39 not 75mm Pzgr.39).

No I didn't make any changes. I assume that whoever made this chart had already taken this into consideration.

Quite so. The curves used by Army are G(D) curves. G(D) is an old penetration definition, which does not include a specification whether or not the projectile stays in intact bursting condition after affecting perforation. In practice, the projectile starts to stay intact until a certain obliquity point and then, at higher obliquities, may also break up (shown graphically by a change from solid lines to dotted or interrupted lines). Relevant here is only that the projectile penetrates 5 times in a row (for 75mm calibre) with no residual velocity after performing complete penetration. The definitions of G(heil) and G(grenz) (used by the Navy at that time) each are much stricter in regard to projectile state.
As far as WaPrüf was concerned, they issued a proof requirement for passing AP lots specifying a G(D) condition with the additional condition that the projectile also had to stay intact at least 2/3 times. Because the official tabulations of penetration for service pamphlets are within the zone of obliquity where the projectile generally stays intact, the service pamphlets are including what would be called "intact penetration" (slightly harder than the US Navy effective limit, f.e.).
Notice the difference to the Navy G(heil) where the projectile would be required to stay intact.

Yes, I believe you're right. Extrapolating from post-war data of US 90mm T50 APC shell, adjusting for the difference in shell absolute size and plate tensile strength and assuming that T50 is similar enough in design to 7.5cm PzGr.39 the ballistic limit for 1,41 T/D (105mm) plate of 96 kg/mm^2 tensile strength at 45° obliquity, I get 914m/s (US Navy), which is quite lower than what the chart shows(1027m/s).
This is also confirmed by how it was able to defeat 5+3/16" (131mm) of 217-230 BHN RHA at 45° in US tests at only 3127 fps.

[critical mass]

Its a good practice to cross verify with high quality post war AP ammunition. It didn´t occur to me before but it makes perfect sense. The way how to approach armor penetration and plate quality is first to look for cases with undeformed penetrators in order to delete one family of variances and then work back to isolate the source of a variance.