1921 firing trials against Baden

[Bill Jurens]

I've read the entire document carefully many times, and so far as I can see, GKdos100 nowhere specifies the exact type of British projectile at all used in their computations at all.

How could it?

The Germans who wrote GKdos100 did not know the exact weight of the British 15" shell, and were not even confident of their figures for the initial velocity of the British 15" gun. Although they may have had some very sketchy information on the external shape of the projectile, they would have had no information at all on things like cap weight, etc. This is clear from the document itself, where the German authors go out of their way to emphasise that their information on foreign guns -- all foreign guns -- was very very sketchy. Basically guesswork, though educated guesswork to be sure.

The immunity zone bars in GKdos100 were computed on the basis of some sort of generic British shell that was assumed to be about the same quality as the best of German shells of the time -- the document is explicit about this -- fired from their best estimate of what the current performance of the British 15" rifle was likely to be. The armor penetration curves per se were based upon German projectiles vs German armor, and do not reflect the performance of British projectiles at all. The Germans would have had little-to-no first hand knowledge of British projectile performance against any armor when writing GKdos100.


Bill Jurens.

[marty1]

FWIW, The British 6 crh 15" projectile was introduced in 1937. The V drop is obviously much less compared to the 4 crh models, as the range is increased about 10% at porpotional gun elevations. As The GkDOS 100 materials are from the 40's it could well be an V-drop estimate of then current British projectiles, and be mislabeled as a MKI projectile. Much data in GKdos100 and others are Generic for appoximating, and may not lend it's self to such exact specific models

Hi Dave:

This is what I suggested earlier on the thread. But personally I don’t think the KDOS velocity drop curve represents anything more than the Germans best guess on British 15" APC at the time the document was produced.



The yellow and blue curves represent data from Campbell on 6-crh, 1938-lbs APC -- yellow being based upon the higher muzzle velocity from super charges. As one would expect the two curves parallel each other. Same projectile weight and geometry – different muzzle velocity.

On the other hand the KDOS estimate, represented by the white curve, has a fairly obvious contrast in slope relative to the yellow and blue curves. The CRH is not specified in the KDOS manual and the projectile weight is a bit odd.

This is a good example of the potential error entailed in guesstimating at velocity drop. I’d guess the German ballisticians involved had access to photos or line drawings of the projectile, but may have had to make an educated guess about projectile weight (875-kg) and muzzle velocity (745m/s). But there is something else going on here as well. Assuming the Germans did estimate nose length correctly from a photo (not too challenging) – why would a lighter projectile with the same nose length and same diameter have a lower Vdrop? Note how the KDOS curve converges toward the tail end of the Super Charged curve. We should expect the opposite. So even in skilled hands, recomputing velocity drop is not quite so simple.

[José M. Rico]

Bill is correct. GKdos100 does not specify the type of British shells used. It seems that the penetration curves of the foreign guns are based on what the Germans thought about those shells at that time. Nelson's 930-kg AP shells for example are given a muzzle velocity of 840 mps when actually this figure was below 800 mps.

Perhaps they even increased some figures on purpose to have some margin of security. If in real battle it turned out that their assumptions were overestimated, well, so much the better for them.

[Dave Saxton]

...................

Perhaps they even increased some figures on purpose to have some margin of security. If in real battle it turned out that their assumptions were overestimated, well, so much the better for them. .............

This type of thing is done all the time by various military and Itel organizations, and I agree that most likely it was being done by the German Navy here. A safty margin gets built in, should for example, the performance of a particular batch of Wh be at the least acceptable level. I find in most German scientific and engineering documents from the period, that this type of thing was done. An example, is rating the elongation of a steel at, say 21%, when the actual destructive test data averaged around 30%.

[marty1]

Regarding the 16” MkII APC it is also interesting to look at the German Estimates for velocity vs. Range. Again we have the Germans underestimating the projectiles weight – by a considerable amount in this case. Let’s presume for a moment the Germans had intelligence on the projectiles geometry. Here again we are seeing that a much lighter projectile is holding on to its velocity much more efficiently than a heavier projectile with the same diameter and same nose length. Makes me wonder what sort of drag function the Germans were employing for their estimates.

[José M. Rico]

Hi Marty,

Actually, the Nelson Class mounted the 16"/45 (40.6 cm) Mark I with a shell weight of 2,048 lbs. (929 kg). GKdos100 has the correct weight (930 kg). It is the muzzle velocity what was for some reason overestimated.

[marty1]

My bad. In that case the Germans KDOS velocity drop estimate is very good. The two curves for 16” MkI APC (white for KDOS and Blue for the J.Campbell Data) are almost parallel. In other words, if the Germans had had accurate information on muzzle velocity the white and the blue curves would just about plot on top of each other.

[Dave Saxton]

As originaly designed, the Nelson 16" had a MV of 2,700 f/s. This was later reduced to avoid un-acceptable levels of bore errosion, when using shorter than normal projectiles at such MV.

[marty1]

As originaly designed, the Nelson 16" had a MV of 2,700 f/s. This was later reduced to avoid un-acceptable levels of bore errosion, when using shorter than normal projectiles at such MV.

That's an interesting factoid. This might be taken to mean the Germans had very accurate intelligence on the Nelson's 16" APC (perhaps even British firing tables for Mv=2700-fps). Accurate that is up to the point the British decided dropped muzzle velocity to reduce barrel ware.

[Bill Jurens]

I ran a few trajectories which showed that the German KDos data vs the British data seemed to be behaving properly. I was going to write a memo explaining this observation, but I see others have 'beat me to it', and the apparent difficulty has been identified, understood, and corrected. The data is, in fact, quite well behaved and conventional.


So far as drag function selection is concerned, I have once again looked at the British projectile profiles posted here earlier with a view to assigning a proper drag function to them. Unfortunately, it does not appear that this particular projectile fits any of the 'standards' very well -- I routinely select from a list of about twenty of them. In this case the effective head length appears to be in the vicinity of 1.5 calibers, and we have a tangent ogive design. I hate to use a form factor, but in this case the first best guess might be to select a form factor in the vicinity of 0.83 against projectile type 1, which (as a first cut) I would increase to about 0.90 in order to account for the additional length of the body.

I could do more complex calculations later on, if required or desired, but this should serve as a reasonably close jumping-off point.

Hope this helps...

Bill Jurens.

[marty1]

The data is, in fact, quite well behaved and conventional.

I disagree in the case of the KDOS estimate for 15" APC for the reasons I've already stated. If we assume the same drag function, A lighter projectile with the same nose length and same diameter will not loose velocity at a slower rate than a heavier projectile with the same diameter and same nose length. So either the Germans are assuming a different drag function, or they are assuming a different nose length. I doubt the issue is the Germans having trouble measuring nose length from an intelligence photo -- so the contrast has to be their drag assumptions.

[Bill Jurens]

I'm not sure exactly what reasons you are referring to, but if you don't believe me, try to run a few trajectories and you will see that if you use the same drag function (i.e. identical projectile geometries) and and compare relatively light high velocity projectiles against relatively heavy low velocity projectiles, the (slightly curved) lines of velocity drop vs range will remain almost parallel to one another, especially if the range is short.

Honest. Try it.

Told you a lot of this was counter-intuitive...

Bill Jurens.

[marty1]

Hi Bill

Sorry...but I still disagree . The lighter projectile will decelerate faster. Its quite intuitive as it has less inertia -- easier to get going but easier to stop once it gets going. The heavier shell has more inertia -- its harder to get going and harder to stop once it gets going. Your probably assuming the same ballistic coefficient between the two. If I hold diameter constant and reduce weight obviously the sectional density will decrease and therefore BC will go down with the lighter projectile.

[marty1]

Has anyone come across a line drawing or even a photo 15" Greenboy?

[Bill Jurens]

Hi Marty:

All I can suggest is that you try to numerically integrate a few trajectories and see how they behave. Your observation is correct, of course, certainly in principle, but it appears that the practical effect of the weight change on projectiles this big is much smaller than one might at first assume, or that some other effect is cancelling out the effect you expect.

Bill Jurens