38 cm shells

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Re: 38 cm shells

Post by alecsandros » Sat Nov 03, 2018 11:21 am

I don't know exactly what text from David attracted your attention.

I'll try to put down a few pieces of information:

" I also heard that the the lighter shell would wobble more lose accuracy over range."

German testing in the mid to late 1930s were done for SK C34 guns 380mm caliber, as well as for other types of guns.
testing ranges were up to 30.000 meters.

The grouping of the shots was good, and the pattern sized for 4-gun shots were, from what I remember , around 250 to 320meters, at that range.

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Re: 38 cm shells

Post by spicmart » Sat Nov 03, 2018 2:16 pm

Dave once wrote that the light german 38 cm shell was in fact a better deck penetrator than other heavier shells, contrary to the believes of many.

It would decelerate faster ,because of its lightness, and thus descend at a steeper angle at long range allowing it to penetrate deck armor better.

So I was wondering why the US built the superheavy shells in the first place when their greater weight wouldn't give them greater advantage against horizontal armor according to Dave's logic. The advantage would only kick in at very long range.

It's a bit contradictory and I don't quite understand.

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Re: 38 cm shells

Post by Dave Saxton » Sat Nov 03, 2018 8:39 pm

Hi, I only saw your question today.

All other things being equal, such as the angle of fall, the terminal velocity, the shape of the projectile head…., the heavier projectile will penetrate more deck armor. However, it’s virtually impossible to have all the factors except weight be the same. For example, if we take two projectiles of the same caliber and of the Approx. the same initial velocity of 2500 f/s, but one weighs 2700lbs and the other weighs 2240lbs, once the projectile arrives at 30, 000 yards down range the 2700lbs projectile has a velocity of 1565 f/s and the 2240lb projectile has a velocity of 1470 f/s. The heavier projectile has lost less velocity by the time it reached 30,000 yards down range.

What is really interesting is that deck penetration at that range is almost exactly the same, within 0.1” of each other, despite the greater weight and greater striking velocity of the 2700lb projectile. Moreover, the head shape is the same. What’s the difference? One difference is the angle of fall is greater for the lighter projectile at that range. This is mainly because the lower mass projectile has a lower velocity by the time it reaches that range. (one notes that the angle of departure to reach that range between the two will probably not be exactly the same but it will be not greatly different).

Another interesting aspect is that among similar caliber projectiles, with blunter head shapes, the amount of deck penetration is virtually the same per angle of fall, despite disparate weights and striking velocities. Given an angle of fall of about 30 degrees, the deck penetration of a 2700lb 16” projectile, a 2240lb 16” projectile, and a 1764lb 15” projectile, are all about 5”.

Comparing the German 15” to the Italian 15” is instructive as well. The Italian gun has higher muzzle velocity and a heavier projectile. It, therefore, will retain a greater velocity by the time it reaches 30,000 meters range. At that range its angle of fall is only about 25 degrees compared to the other 15” shell’s 31 degrees. It therefore doesn’t penetrate as much deck armor at 30km despite being more massive. The range must be out to near 35,000 yards before it can penetrate 5”. In practice, the deck penetration by the Italian projectile was less than German shell per angle of fall as well, because its head shape was less favorable.
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Re: 38 cm shells

Post by Bill Jurens » Sun Nov 04, 2018 1:45 am

regarding 'light' vs 'heavy' shells and deck penetrations, as is usual It's very difficult to generalize. Really, each situation needs to be analysed on a case-by-case basis.

There is certainly little-to-no justification to assume that lighter projectiles will in some way 'wobble' more as they travel downrange. Provided the gun tube is designed properly, it should be no more difficult to achieve adequate stability with a 'light' projectile than with a 'heavy' one.

Regarding terminal characteristics and performance, a great deal depends upon whether on not one makes a correction for changes in initial velocity. All things being equal, a lighter projectile will leave the muzzle with increased velocity, so assuming constant initial velocities may (or may not) lead to misleading conclusions.

If one keeps propellant charge constant and -- at least for the moment -- ignores the related effects on maximum bore pressures, for battleship-caliber projectiles, a 10% increase in projectile weight will generally be associated with about a 3% decrease in range, and about a 15% increase in resolved vertical kinetic energy. And vice-versa. These differentials can be -- albeit roughly -- associated with inverse ratios of resolved horizontal kinetic energy, i.e. energy delivered for penetrations of the belt. These relationships quoted above will, of course, vary somewhat with projectile types and ranges, etc., but seem to remain relatively constant over ranges from 25000 to 40000 yards. In some cases direct comparisons cannot actually be made insofar as a 10% heavier projectile -- using the same initial propellant charge -- will not range as far as a a lighter projectile will.

Different results will result if one considers initial velocity independent of changes in projectile weight. In such situations, as might be expected, maintaining a constant initial velocity will generally result in a (perhaps unacceptable) increase in bore pressures as well.

As before, generalization is risky, and careful analysis on a case-by-case basis is required to answer the question definitively.

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Re: 38 cm shells

Post by spicmart » Sun Nov 04, 2018 1:43 pm

Dave Saxton wrote:
Sat Nov 03, 2018 8:39 pm
Another interesting aspect is that among similar caliber projectiles, with blunter head shapes, the amount of deck penetration is virtually the same per angle of fall, despite disparate weights and striking velocities. Given an angle of fall of about 30 degrees, the deck penetration of a 2700lb 16” projectile, a 2240lb 16” projectile, and a 1764lb 15” projectile, are all about 5”.
Yes, but why would US designers then accept a 50% weight penalty for the 16" superheavy compared to german 15" shell when deck penetration is about equal anyway? Is it an advantage in belt penetration at longer ranges? The US designed there ships to deliver more effective plunging fire afaik.

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Re: 38 cm shells

Post by Dave Saxton » Sun Nov 04, 2018 3:53 pm

spicmart wrote:
Sun Nov 04, 2018 1:43 pm


Is it an advantage in belt penetration at longer ranges?
Yes, it could be. Mr Okun has stated on this forum that striking velocity is one of the more important factors in the penetration of face hardened armor. More so than weight. The super heavy's ability to retain more velocity down range is a function of what is called its ballistic coefficient. The ballistic coefficient can be defined as the projectile's ability to lose less velocity as it reaches down range. All shells lose velocity as soon as they leave the muzzle, but they all don't lose velocity at the same rate. For example, the German 15" had a velocity of 460 m/s at 30,000 meters range from a start velocity of 820 m/s. The Iowa class 16" has a velocity of 475 m/s at 30,000 meters range from a start velocity of 760 m/sec. BC also affects the maximum range the projectile can obtain.
The US designed there ships to deliver more effective plunging fire afaik.
Did they or is that a popular modern interpretation we have made post ergo?

I'm sure they considered it an important factor in their planning, but getting a more than adequate maximum range seems to have been a factor mentioned in the records of the General Board, at least concerning the Iowa class. Once the range exceeds 34,000 yards it becomes very difficult to build enough deck protection into any design, against any 14" -18" gun, because the increasing angle of fall causes the deck penetration to increase at an exponential rate. Deck penetration becomes a default capability in most cases.

Additionally, designers considered the 16"/56 firing the 2700 lb projectile for the Iowa class but rejected it because it mainly offered additional maximum range compared to the 16"/50, which was more than adequate, and of course would have been a much heavier installation. They also briefly considered an 18"/48 but only consisting of six guns.

Looking at the 16"/45 firing the 2700 lb projectile at 200 f/s less muzzle velocity compared to the 16"/50, we see improved deck penetration before 30,000 yards with 5" deck penetration reached by 27,000 yards. It also gets improved deck penetration compared the older 16"/45 firing the 2240 lb projectile but at ~2500 f/s. The trade off for the heavier projectile at lesser muzzle velocity is less vertical armor penetration. As Mr. Jurens pointed out, there were many complex trade offs to consider. There are a various ways of obtaining particular design goals. One approach is not necessarily better than another.
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Re: 38 cm shells

Post by spicmart » Sun Nov 04, 2018 4:45 pm

Mr. Saxton, Mr. Jurens. Thank you for the enlightening answers.

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Re: 38 cm shells

Post by Bill Jurens » Sun Nov 04, 2018 5:41 pm

This is really a repost of something I wrote several hours ago, which appears to have disappeared into cyber space. In the interim several other posters have submitted memos, but I have thought it still useful to re-post this one anyway.

For typical big projectiles -- and again assuming reasonable corrections for variations that might be expected in initial velocities, i.e. a constant propellant charge -- angles of fall are closely linked to range, i.e. at a given range the angle of fall tends to be about the same regardless of changes in projectile weight. Flight times are not affected much either, at least at the lower end of the range bands.

What DOES change is striking energy. In general, at any given range one might expect a 10% increase in projectile weight to result in a 15% increase in striking energy. Because the angle of fall at any given range is nearly a constant, this translates into a 15% increase in the energy delivered to the deck armor as well.

How this change might be translated into predictions of armor penetration remains the subject of a different sort of study. In simple terms, however, the extra kinetic energy must go somewhere, and as the angle of obliquity should remain about constant one might reasonably expect that increase in delivered kinetic energy to likely result in a related increase in armor penetration.

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Re: 38 cm shells

Post by pgollin » Mon Nov 05, 2018 1:40 pm

.

I would merely note that whilst Mr Jurens compares different shells using equal propellent charges, the RN compared proposed shells using equal muzzle energies.

.

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Re: 38 cm shells

Post by Thorsten Wahl » Mon Nov 05, 2018 2:06 pm

its the same.

but sometimes if you use muzzle energy, it may be confound with muzzle velocity and
weight of propellant charge is more intuitive to energy I would say.
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Re: 38 cm shells

Post by pgollin » Mon Nov 12, 2018 12:16 pm

.

No, they aren't the same.

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Re: 38 cm shells

Post by Bill Jurens » Mon Nov 12, 2018 4:29 pm

@pgollin:

Although I'd agree that careful interior ballistic analysis would likely not equate propellant weight and muzzle energy EXACTLY, considering the general nature of the discussion at hand, I'd certainly consider them as essentially interchangeable. Might you explain in more detail how they are significantly different, and perhaps provide an example?

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Re: 38 cm shells

Post by RobertsonN » Sun Nov 18, 2018 5:28 pm

The 38 cm SKC/34 was the most accurate German heavy gun: it had the lowest dispersion by some margin. In gkdos100, vol. a, there is a graph at the end showing 50% longitudinal dispersion curves versus gun elevation (less than the angle of fall) for weapons from 15 cm upwards. Over 7 deg the 38 cm SKC/34 (initial velocity 820 m/s) had the lowest dispersion, at little over a half those of the other weapons at higher elevations. Below 7 deg elevation the 15 cm SKC/25 (i.v. 960 m/s) had the lowest dispersion and it had the second lowest below about 18 deg. The 28 cm SKC/28 (i.v. 910 m/s) had the greatest dispersion over all the elevations given for it (4 deg up to nearly 40 deg). The curve next to that for the 28 cm SKC/28 is that for the 28 cm SKC34 (890 m/s) up to about 23 deg. The curve for the 20.3 cm SKC/34 (i.v. 925 m/s) is a little below that for the 28 cm SKC/34 at all elevations.

From this it will be seen that although there was a tendency for guns with lower initial velocity to have lower dispersion that there were plenty exceptions to this rule. The 28 cm SKC/28 was the least accurate German gun while, at all but the lowest elevations, the 38 cm SKC/34 was the most accurate. The 15 cm SKC/25 had excellent accuracy considering that it gave the highest initial velocity. Also the 20.3 cm SKC/34 was a little more accurate than the 20.3 cm SKC/34 despite having 35 m/s higher initial velocity. Despite these figures, the main gunnery performance of the Graf Spee at the River Plate was probably superior to that of the Bismarck during her cruise, possibly due to the crew of the earlier ship being at a higher state of fighting efficiency,

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Re: 38 cm shells

Post by RF » Mon Nov 19, 2018 9:17 am

Neil, I am a little puzzled at the last sentence of your post.

Whilst you do mention the 28 cm as the least accurate of the Gernam heavy guns I don't find the AGS gunnery at the RP battle that impressive. It was a much longer action than DS and AGS was able to knock out the Exeter (but not sink her) with initial good gunnery at long range but then was unable to deal with the two smaller cruisers with either the 28 cm or 15 cm guns. Now the AGS was constantly changing course etc but so were the British cruisers and AGS took most of the hits.
The other point is that while AGS might be more ''run in'' than Bismarck her crew had no actual combat experience until 13 December 1939.
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Re: 38 cm shells

Post by alecsandros » Mon Nov 19, 2018 9:39 am

IIRC Graf Spee had both fire control positions damaged in the first 20 minutes of the action - therefore the degradation of own gunnery in the following minutes / hours is largely attributable to this.

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