Tirpitz plate results...

[marty1]

My post is really a question -- i.e. why broach the subject of lateral confinment constraints relative to hyper-velocity rod penetration when disscussing conventional AP.

Be that as it may...

I did a bit of quick plug and chug assuming 15-inch APC vs. Krupp Cementite. Using The Universal Krupp Equation with the following input parameters:

Projectile Weight = 1938-lbs
Krupp C constant for British 15-inch vs. intact Kc armor = 1689

Using limit velocities of: 1400-fps, 1450-fps and 1500-fps
I get the following associated limit thicknesses:

Impact Velocity -----Obliquity ------ Kc Limit Thickness (inches)
1400-fps ------------------0-deg-----------------15.39-inches
1400-fps-------------------30-deg---------------12.21-inches

1450-fps--------------------0-deg----------------16.07-inches
1450-fps-------------------30-deg---------------12.76-inches

1500-fps--------------------0-deg---------------16.77-inches
1500-fps-------------------30-deg---------------13.31-inches

The number of significant figures used is sort of ridiculous on my part, but what the heck. I figure anyone interested can round to more reasonable levels of accuracy. I came up with the obliquity effects based upon 90mm M82 APC BL(N) data vs Face Hardened test plates. I did the obliquity effect in this manner as my previous questions to this forum regarding obliquity effects on the Universal Krupp Equation drew nothing but blank stares ;o)

Anyway, at t/d ratios (or e/d if you like) between about 0.78 to 1.4 -- the obliquity effects for M82 APC vs FH at 30-deg are a relatively uniform 0.806 to 0.80. In other words the limit thickness at 30-deg is about 80% of the limit thickness at 0-deg.

[marty1]

Sorry .. while reading through the Admiralty report I noticed there was a reference to cast armor on the Tirpitz. Were any of the cast plates subjected to ballistic trials? Maybe I missed this part of the report? Were there any limit velocities quoted for the cast armor?

[George Elder]

With regard to your FH armor extrapolations, what is the presumed face thickness and hardness you are using? I'm not at all sure you can extrapolate up based on the test you are using because the material properties of the FH armor in that sample and the KC may be quite different. You can understand my concern here.

George

[George Elder]

... I have yet to find those data, but would sure like to. Have you seen the 14" data vs the Tirpitz plates. Drop me an e-mail at ghe101@aol.com, and I'll send the files -- thank you Neil Stirling. You will find these are peculier.

George

[marty1]

I understand what your saying, and see that the thickness of the face hardening is discussed in the Admiralty report. But its effect seems in question. One set of British estimates place the German armor being better than an equivelent thickness of British FHA, and another indicates the British armor is better. Or have I misread the report?

My sense is that the Krupp formula was developed by the Germans based upon their understanding of how their own armor functioned ballistically. That's why I used it instead of DeMarre, or Milne. One would assume that the Krupp EQ was developed by slapping a regression equation onto German armor ballistic test data. Therefore FH thickness effect on limit velocity is by default already present in the estimation format. And it does appear that my back of the envelope calcs are relatively close to the experimental results inspite of using American M82 APC to back out the obliquity term. I would have thought the M82 would have been the weak link in my estimates ;o) But hey, that's just me.

I would be interested to see you elaborate a bit more on how you would model the thickness of face hardening within the Krupp equation or even the DeMarre equation. What sort of contrasts in limit velocities should we expect as a function of the face hardened thickness – is it 1%, is it 5%, is it 20%? And what are the boundry conditions we are talking about. Moreover does the effect of face hardened thickness vary with t/d. Does it vary with angle of attack?

[marty1]

Here it is from Nathan Okun’s web page: http://www.combinedfleet.com/okun_biz.htm

“The original KC a/A had a thinner 33% face layer but this was less able to damage projectiles so Krupp increased the layer to 41% in KC n/A. Other manufacturers greatly changed this both ways, with some face-hardened plate types having very thin faces (U.S. 1921-23 Bethlehem "Thin Chill" Class 'A' armor with its 15% face, for example) or very thick faces (U.S. average post-1935 "Thick Chill" Class 'A' armor with its 55% face, for example). The thicker the face, the more effects scaling (making both the plate and projectile larger in step with one-another, but keeping the other properties the same) had on the plate and the weaker the resistance against larger projectiles, though a thicker face may cause more projectile damage and thus offset this in some cases. The change from 33% face to 41% face in Krupp KC armor had a small scaling penalty, but the generally higher steel quality more than made up for this.

British post-1935 "Cemented Armor" (CA) seems to have been the most resistant of all types when used in battleship thicknesses (10 inches (254 mm) and up). The British thinned down the face to only 30% of the plate and softened the cemented layer to only circa 600-Brinell, so that maximum plate toughness and reduced scaling effects worked together to increase resistance, and they did this without any noticeable loss in the armor's ability to break projectiles. Since only British battleships or battle-cruisers used face-hardened armor after WWI, thin face-hardened plates of this type were rare (4" (101.6mm) aircraft carrier deck armor was the only other major use).”

The Krupp C value I employed for intact 15” APC attack was apparently based upon the newer form of Kc armor with the thicker face layer – 41% of total plate thickness. The 41% seems to be in the ball park of the Admiralty reported face thickness estimate for the Tirpitz armor, i.e. 45% to 50%.

[George Elder]

Well, you would have a most difficult time replicating the 14" shell trials with this methodology. If you do not have these data, drop me a line and I will send you the study. I strongly siggest that the results will not be in line with predictions. Either the 14" shell is very very good, or the plate samples are not up to par. or a combination of both factors is in play. Do you have this 14" study?

George

[George Elder]

My understanding of this is that shell head shape and the material properties of both the projectile and plate are important factors here. For example, as obliquity increase, the more rounded head shapes are less prone to suffering negative obluity effects, such is the case when a more finly pointed projectile begins to suffer nose damage. This is a rather well Known effect, and I presume your methodology deals with this.

George

[marty1]

Well I love to tinker with this stuff, and no I have not seen the 14-inch APC data, but would like to take you up on your very kind offer. I dont know if you have the material in scanned form or pdf or what, but if it isnt a huge amount of trouble you can email it to me at:

martygrace@hotmail.com

The email address can take up to 1-meg file attachments/per email. Total storage is about 10-meg worth of attachments.

Thanks a bunch.

[marty1]

My understanding of this is that shell head shape and the material properties of both the projectile and plate are important factors here. For example, as obliquity increase, the more rounded head shapes are less prone to suffering negative obluity effects, such is the case when a more finly pointed projectile begins to suffer nose damage. This is a rather well Known effect, and I presume your methodology deals with this.

George

Blunt projectiles perform differently than ogival shapes. Capped projectiles perform differently from uncapped, obliquity affects differ with projectile type and armor type, etc etc, etc. When employing any penetration prediction equation one needs to be aware of its unique constraints and applicability to the specific problem being examined. I probably wouldn’t use Thermodynamics to predict the effect of a uniform load on a simply supported beam. I make no claims as to what the Krupp formula can and can not do. As I assume you already know, much can be hidden in the constant. It’s sort of like the form factor in exterior ballistics. The common notation for the form factor is “i”. A rather famous ballistician at Aberdeen used to say the form factor “i” stood for the idiocy factor.

My set of predictions simply touches on those elements which smarter guys than myself have determined will most directly effect the penetration prediction. Projectile weight, projectile diameter, projectile impact velocity, projectile type, target type and obliquity effect. I presume you are familiar with the workings of the Krupp Equation and therefore know what it does and does not do?

Other bits and pieces can also affect the results, but the above described elements are those that are typically considered of most significance. I suppose one should always keep in mind that any limit velocity prediction is simply a probability. Be it V50 or V20, or V80 – or some other odd ball criteria I have not yet heard of. There is always statistical noise in any set of ballistic test data. This makes predicting very subtle influences of say a 41% FH face thickness relative to say a 48% FH face thickness difficult to pin down. Or whether an RHA plate of BHN=250 is better than a plate of BHN=245. Does that 7% increase in FH face thickness account for a 1% increase in limit velocity between two sets of trial data? Or is that 1% increases in limit velocity simply a subtle difference in plate quality, or test cartridge propellant temperature, or projectile yaw, or some other systematic error. The equations are just meant to put you in the ball park. Once you’re in the ball park you have to pick the right bat – don’t expect to hit a home run with a tennis racket – unless of course you have been pumping up on steroids.

[George Elder]

There is a rather more modern approach described by Goldsmith (1999) in a lengthy article on Non-Ideal Impacts that he wrote for the International Journal of Impact Engineering, No. 22., IIRC. I highly recommend this piece if you don't have it. As for the "I" factor, it found its way into most "quick & dirty" solutions. Given the inherant variations in plate and shell quality, evolving any absolutely principled set of terminal ballistic criteria is problematic. Modern testing has advanced the ball a ot, but this is often done with homogeneous plates. Of course, there are those complex laminates... and here we see abstraction that boogle my tiny brain, as in "I" factors with wings.
It seems to me that the old tools fit the needs of thier users rather well, but they were fairly crude. They could provide general clues that would allow one to determine if a given batch of armor was more-or-less up to specs, but that's about it. I'm of the general position that their was such a great qualitive varience in production that one almost has to do a case by case study to dimly determine how a given batch would perform. We have lots of data on this, and CYA letters that seek to protect the testers when bad results were found. It's great stuff to read. But they were between a rock and a hard place. The plates were needed pronto, regardless of proving ground tests. This is a great uintold story... but I have much to do at the moment.

George

[George Elder]

Hi Marty:

I tried sending two pages of this through but each was 650 kb, so your buffer may have gotten dusted. Perhaps I should sned this to Jose and he can put the material in the archive.

George

[marty1]

I am familier with Goldsmith's 1999 for IJIE. However I have only read snippets.

I received both of the 14-inch vs Tirpitz attachments you sent. Thanks. My server gives me about 10-meg worth of storage. The catch is that no single attachment can be bigger than about a meg. So you can send me ten emails each with a 1-meg attachment. You just cant send one email with a 10-meg attachment.

I'll try to look these over today during my lunch hour.

[Bill Jurens]

MIght I get a copy of these reports as well? I probably have them already, but do want to make sure that we are all looking at the same documentation.

e-mail is bjurens@shaw.ca

Thanks in advance...

Bill Jurens

[José M. Rico]

George, if possible send me a copy via e-mail of this report too.
Thanks-