1921 firing trials against Baden

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marty1
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Re: Ballistic coefficients etc. (to Marty1)

Postby marty1 » Fri Mar 18, 2005 3:56 pm

Bill Jurens wrote:Hi Marty:

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


Hi Bill:

This is precisely my point. Look again at the slope contrast between the KDOS estimate for 15" APC and the J.Campbell data. For the very small contrast in the German weight estimate for 15" APC we should see a resultant velocity drop vs. range that just about parallels the British 6-crh curves (the white curve should parallel the blue and yellow curves -- ala the 16-inch example). If anything the German velocity estimate should decrease at a higher rate than the British estimate. But it doesn’t. It has a very pronounced slope differential between the 6-crh British estimates and is actually losing velocity at a much slower rate than the British estimates.

George Elder
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Interesting, and why do you...

Postby George Elder » Fri Mar 18, 2005 5:28 pm

... supect this might be the case? With regard to intelligence as it relates to shell weights and initial MVs, these data were published in Brassey's and other journals, and most of the data is fairly reliable. Indeed, even in the 1920s they were putting in cross-sectional diagrams of some shells, etc., IIRC. But this focus on drag vs cross-sectional area and weight only tells part of the story. The flight characteristics of the shell itself can generate a good deal of added resisitance via its influence on the relative cross-sectional area of the shell. As you know, even spin-stabilized shells wobble a bit depending on their mass distribution relative to their length, width, and form. This yaw, although small, can effect drag and V losses -- or so I am advised. My understanding is that the Germans did a lot of wind-tunnel research in this area, and it would not overly surpise me if they found a way of maximizing flight "balence" in ways that mitigated losses in V over distance. Here we particularly think of flight effects in the last half of the range. I have seldome seen this issue addressed, and find myself wondering if your current set of formulae take the wobble/yaw effect into consideration. I shall forward this to Dr. Don C, who teaches ballistics, if it is deemed of importance. He doesn't have a heck of a lot of time, but this very interesting -- at least to me. Yes, you have got me very curious about all this. Some interesting notions and possibilities here.

George
Last edited by George Elder on Fri Mar 18, 2005 5:49 pm, edited 1 time in total.

Tiornu
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Postby Tiornu » Fri Mar 18, 2005 5:48 pm

"Has anyone come across a line drawing or even a photo 15" Greenboy?"
have you tried TREATISE ON AMMUNITION AND ORDNANCE?
http://web.ukonline.co.uk/stephen.johnson/arms/

marty1
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Postby marty1 » Fri Mar 18, 2005 7:44 pm

Tiornu wrote:"Has anyone come across a line drawing or even a photo 15" Greenboy?"
have you tried TREATISE ON AMMUNITION AND ORDNANCE?
http://web.ukonline.co.uk/stephen.johnson/arms/


That was the first place I looked several days back. Great web page. However the line drawings of 15" APC cover only the MkXII and MkXXII. No sign of MkIII.

marty1
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Re: Interesting, and why do you...

Postby marty1 » Sun Mar 20, 2005 6:07 pm

George Elder wrote:... supect this might be the case? With regard to intelligence as it relates to shell weights and initial MVs, these data were published in Brassey's and other journals, and most of the data is fairly reliable. Indeed, even in the 1920s they were putting in cross-sectional diagrams of some shells, etc., IIRC. But this focus on drag vs cross-sectional area and weight only tells part of the story. The flight characteristics of the shell itself can generate a good deal of added resisitance via its influence on the relative cross-sectional area of the shell. As you know, even spin-stabilized shells wobble a bit depending on their mass distribution relative to their length, width, and form. This yaw, although small, can effect drag and V losses -- or so I am advised. My understanding is that the Germans did a lot of wind-tunnel research in this area, and it would not overly surpise me if they found a way of maximizing flight "balence" in ways that mitigated losses in V over distance. Here we particularly think of flight effects in the last half of the range. I have seldome seen this issue addressed, and find myself wondering if your current set of formulae take the wobble/yaw effect into consideration. I shall forward this to Dr. Don C, who teaches ballistics, if it is deemed of importance. He doesn't have a heck of a lot of time, but this very interesting -- at least to me. Yes, you have got me very curious about all this. Some interesting notions and possibilities here.

George


This is sort of what I am driving at although I wasn’t thinking specifically along the lines of this sort of physical causation -- i.e. in-flight wobble etc. The British had already developed a correction factor for determining a projectiles ballistic coefficient for in-flight stability well before World War I. The so-called "coefficient of steadiness". So the concept was understood and adjustments were made to a projectiles BC accordingly.

But you can’t separate pesky diameters and weights and mathematics from an understanding of exterior ballistics. No one after all could physically measure in-flight velocity during this period when a projectile was 10,000-yards down range and a mile up in the air – well maybe Baron Von Munchausen could. But Joe Firing Table Maker Guy would have access to some empirical information from range testing – chronograph data, yaw card data etc etc . But this may be limited to range determined information which covers only several hundred yards from the muzzle. You may also have resistance\retardation firing data for a specific projectile over a number of velocities – adjusting initial charge accordingly. A ballistician of the period would than have to extrapolate this into a full blown set of range tables using common drag functions that might extend out to 25,000 or 30,000-yrds or more. A well written set of firing tables of this period will generally detail what was assumed for a drag function, what was assumed for a form factor, and what was assumed for a projectiles ballistic coefficient -- or some combination thereof. Of course modern military proving grounds don’t use common drag functions to develop FTs anymore. But they did in WWI and WWII.

Sadly, in a large percentage of cases, official range tables remain classified, inaccurate, or frankly non-existent.

George Elder
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The totality of all variables...

Postby George Elder » Sun Mar 20, 2005 6:57 pm

Weight, diameter, "wobble" and air density all play roles. The range tables also have to consider such nicities as proplellent temperature, etc., and thus it was very easy to make mistakes on the tables. We were tracing down faults in the US range tables for the 16"/45 and 16"/50 tables, and found part of the problem was a 30 degree temperature assessment error for the propellent. It seems this was not the only problem, but it appears the cams, etc., in the mechanical FC computor had been generated while this error was not recognized. These sorts of things show how delicate FC is, and how an error in one variable can impair the general performance of an entire system. Well, we have a lot of research in this area that still needs to be done. but it does illustrate the problems we can have in a system were in the reliability of external ballistics is influenced by the viability of internal ballistic calculations. Every step along the way must be well thought out and accurate -- which makes it a small wonder that any target could be hit at 25,000 meters -- let alone even greater ranges.

George

marty1
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Postby marty1 » Sun Mar 20, 2005 9:26 pm

Hi George

Change in initial propellant temperature equates to change in muzzle velocity. An adjustment for this sort of thing is typically accounted for in firing tables. A ballistic computer cam is not particularly complex. It puts the appropriate super elevation on the gun. You can either rewrite your firing tables to account for the this, or manufacture a new cam. To what specific projectile do you refer in the 16” example you have cited, and which specific set of 16” firing tables includes this error – date of tables?

Regards
MKS

Bill Jurens
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"Problems" with USN 16" guns, etc.

Postby Bill Jurens » Sun Mar 20, 2005 10:39 pm

I am, in conjunction with another author, currently preparing a detailed paper -- an extension of my previous paper on the "Evolution of Battleship Gunnery in the US Navy" -- which will provide comprehensive coverage of the WWII gunnery experiences of the 'fast battleships' and address most of these issues -- including the notorious 'overshooting' experiences, in detail.

Bill Jurens.

marty1
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Re: Velocity drop curves etc.

Postby marty1 » Sun Mar 20, 2005 10:56 pm

Bill Jurens wrote: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.
Bill Jurens.


I ran the numbers using both form factors. The result is attached.

Image

I have also inluded J.Campbell's data for 15" APC (4-crh & 1920-lbs) as a gut check. The J.Campbell data is represented by the thick purple curve. Normally -- given all other critical imput parameters being held constant -- a 3-CRH projectile will lose velocity faster than a 4-CRH projectile. So using either i=0.9 or i=0.83 for the Jutlannd 15" APC with 3-CRH makes sense intuitively. Moreover both sets of curves are below the 4-CRH curve. My tendency would be to run with i-0.83 for ranges of 20,000-yrds or less. This is more based upon British theory at the time which indicated the form factor for 3-CRH projectiles was about 0.83.

Anyone know what circa-1916 British 15" CPC looked like?

Bill Jurens
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huh?

Postby Bill Jurens » Mon Mar 21, 2005 2:28 am

I hate to appear to be thick, but I still cannot for the life of me figure out exactly what you are trying to demonstrate by these calculations and graphs, so cannot really respond coherently. If you read back in the thread, you will see that I've really never been able to grasp what you are trying to get at.

If you could explain again, please, perhaps I could help futher.

Sorry...

Bill Jurens

marty1
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Postby marty1 » Mon Mar 21, 2005 6:45 am

Hi Bill

No great mystery. Sorry if this is confusing for you. I want a handle on 15" APC velocity drop with range for the Jutland version of 15" APC -- potential impact velocities. I dont have firing tables for this projectile. So I asked for your opinion about potential form factors. I crunched the numbers based upon the form factors you provided and posted the results.

Regards

George Elder
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Write me off line...

Postby George Elder » Mon Mar 21, 2005 1:17 pm

Hi Marty:

The problem wasn't even fully quantified untill 1944 firing trials, although one suspects that adjustments were made to the firing tables earlier that were based on observations. The cam fix wasn't made untill late 1945, and it isn't that simple a procedure. This is just an example of how difficult it is to make accurate range tables. All the variables need to be properly considered and weighted as to how they influence one another. Of course, this has little to do with your present exercise.

George

Bill Jurens
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Range table

Postby Bill Jurens » Mon Mar 21, 2005 3:20 pm

Thanks, Marty1.

Your objective is perfectly clear now. Please give me a bit of time to go through my files -- and make a few inquiries -- to see what I can find regarding 15" range tables. I have four or five of these now in my files now, but most, as I recall, were 'corrected' to later dates, and so may not reflect the performance of the guns at Jutland.

I suppose that I 'lost the bubble' when this all seems to have become somehow entangled with GKdos100.

Bill Jurens

marty1
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Postby marty1 » Mon Mar 21, 2005 7:43 pm

Thanks Bill.

If you come across anything detailing 15" CPC used at Jutland I would be interested in seeing this material as well -- A projectile line drawing or an FT.

After playing about with the Jutland 15" APC figures a bit more, and looking over various sets of range data presented by J.Campbell for a number of projectiles in this same caliber range, I am inclined to think the transition from i1=0.83 to i1=0.9 is occurring very subtly below about Mach No. 1.4 or 1.5. My best guess for velocity vs. range is shown on the amended graph. I am guessing that the velocity curve vs. range follows the i1=0.83 initially, than transitions along the white dashed line and finally merges into the i1=0.9 curve. It didn’t seemingly make sense that the 3-CRH curve would begin to converge so rapidly upon the more ballistically efficient 4-CRH at ranges in excess of about 16,000 or 17,000-yrds.

Image
Last edited by marty1 on Tue Mar 22, 2005 12:29 am, edited 2 times in total.

marty1
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Re: Write me off line...

Postby marty1 » Mon Mar 21, 2005 8:12 pm

George Elder wrote:Of course, this has little to do with your present exercise.


No it doesnt, but like the KDOS digression it's interesting material.


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