by Nathan Okun
Updated 21 January 2013
French APC designs of the World War II era
French WWII APC projectiles were 5 calibers long, making them the longest gun projectiles used on any warship ever (at least against surface targets -- there may have been some WWII AA projectiles of longer form factor), to my knowledge (4.6 is the longest, otherwise, with 4-4.5 being the norm in WWII). This is taking ballistic drag reduction to the extreme! I am not sure how much this buys compared to the fact that the spin rate of the projectile has to be increased somewhat to keep such a projectile moving nose-first in a stable manner and the problems with handling them in the ship.
Trying various drag shapes in Bill Jurens's and
my EXTERIOR BALLISTICS 6.0G program, the best fit was with the Minimum
Drag Shape using only its external physical shape to get this minimum,
without any special add-ons like the modern base-bleed anti-base-drag tail
heating units. This is a kind of reversed tear drop going point-first
with a sliced-square highly-tapered rear end instead of rounded and the
hull curve never getting hollow -- that is, it is always drawn with its
arc centered inside the ship or on the far side -- though it gets almost
conical near the tip of the long, sharply-pointed nose. It gave a
Form Factor of 1.6884 when matched to the given Range of 21,872 yards in
the French table at 10.9 degrees Gun Elevation and 2723 ft/sec Muzzle Velocity.
That is, the average drag over the entire trajectory in lbs/square inch
(or any other units) due to pressure on the front, friction on the sides,
and suction at the base on the French shell was 168.84% what the actual
Minimum Drag Shape had. This gave a Striking Velocity of 1795 ft/sec
(10 ft/sec higher than the French
table) and an Angle of Fall of 14.5 degrees (0.5-degree higher).
Using other shapes and their optimizing Form Factors to match this French Range Table range entry gave slightly better angles of fall, but much higher Striking Velocities, when they were matched to the given range for the gun at 10.9 degrees and 2723 ft/sec.
It seems that the French shell did indeed optimize the drag to the minimum possible for a real projectile requiring a major portion of its middle and lower body to be cylindrical to allow the driving bands to be used; only the super-long-nosed and base-drag-eliminating modern Extended Range Base-Bleed projectile design had less average drag.
French 380 mm Mark 1 AP Projectile made by Crucible Steel Company for Richelieu
These US designed-and-manufactured APC projectiles were externally identical to the French design and weighed the same, with the exact same cavity shape and percentage. The base fuze was the US Mark 21 BDF. The filler was Explosive "D", not TNT. The base plug was the standard US Navy design, as was its threaded sides and other details.
I also assume that the standard US Navy WWII-AP-projectile "Sheath Hardening" hardness gradient technique was used -- hard on the surface almost to the driving band, but getting soft as you go toward the centerline, starting near the lower nose (most of the nose was a constant high hardness all the way through like the surface hardness was down through the middle body). The centerline of the projectile completely surrounding the explosive cavity and reaching into the lower nose and the surface from just above the driving band to the base was the same optimum-toughness, medium-hardness level. This kept the projectile insides and lower body tough against internal cracking and base distortion as it penetrated, but hard in its upper area to resist nose and upper-body bending or compression from any direction, optimizing the shells for maximum penetration ability, giving minimum deformation and breakage as it tried to make a hole in the plate, and yet tough and just deformable enough to remain more-or-less intact against moderately-oblique impact against the thickest face-hardened armor, as the shell tried to force itself through the hole it had just made (these were way above most foreign specifications requirements).
The biggest visual difference in the blueprints between the US and original French APC projectiles was that the AP cap and nose shape was that of the US Navy 14" Mark 16 Mod 8 AP projectile: Oval nose under the cap and a round-tipped-cone-faced moderately thick, moderately hard (circa 555 Brinell maximum) AP cap with the windscreen threaded to near its softened (circa 225 Brinell) lower skirt edge just above the forward bourrelet, not at the maximum-hardness upper-face edge as with most foreign and later US Navy AP shells (even the 14" Mark 16 MOD 10 AP shells had the new-model, short-windscreen AP caps late in WWII). This odd-ball late-1930's US Navy standard cap and windscreen design allowed the windscreen-holding threads to be cut into softer metal -- less expensive -- and made the windscreen several inches longer than later designs (also slightly heavier, of course); there was a narrow gap between the inside of the lower windscreen and the slightly-narrowed AP cap side above the threaded area. The caps were soldered on with a ring of 8 (I think) shallow pits in the nose at the bottom edge of the cap having the cap edge bent into them (forming "dimples"), reinforcing the solder; identical to the British AP cap crimped-pit attachment method -- used as the only attachment method in some pre-WWI and WWI British APC shells prior to always requiring soldering (“sweating”) too in its new post-Jutland ‘GREENBOY’ APC shells. The windscreen might have had the plugged cut-outs for an internal dye bag used in US WWII large-caliber AP projectile to allow water to ram though the windscreen on water impact and dye the splash, but I am not certain; it most certainly did not use the French "K" dye-bag design (see below).
The nose shape and cap details of the US manufactured 380 mm APC projectiles look more like the 14” Mk 16 MOD 4 (though the nose under the AP cap was somewhat blunter and rounder than the picture given in the discussion shows) because the weights and balance had to be matched with the original French projectile, including the bigger French cavity length and volume, but the date that the shell was made is during the time that the MOD 8 was being made for the US Navy, so I do not think that they would go back to the older projectile manufacturing processes when they had a running production line for the new 14” MOD 8 shells which could be adjusted easily for the new French-designed, American-made shells. Now the AP caps might have been taken off of decommissioned MOD 4 projectiles and re-used for the French shells -- this would save time and money -- requiring that the nose shape match the MOD 4 exactly, but to my knowledge, the AP caps of all of these 14” shells (prior to the end-of-WWII modernized MOD 10, that is) were about the same in hardness (constant 555 Brinell hardness for most of its thickness except just around where it touched the projectile nose and near its lower edge, where it abruptly went down to about 250 Brinell or so) and other details; it was the projectile nose and middle/lower body that were improved as to toughness to get the MOD 8’s superior performance.
The original French projectiles had pointed, hardened AP caps contoured to the bluntly-pointed nose under the cap, getting gradually thicker from just above the bourrelet to about the same level as the tip of the nose (33 cm gun design) or somewhat higher (last 38 cm gun design), at which level they abruptly became much blunter, though still thickening to a blunt point in the center. The lower edge of the cap was softened and was forced into a shallow groove ring cut into the projectile's lower nose all around, not in the spaced dimples of the US design. I assume that solder was used, too, since I know of no other nation that did not use solder after WWI, no matter what else they did (the Germans and Japanese used only solder).
Interestingly, other than details as to how thick the cap was (these caps are thicker than they used to be) and the cap hardness contour, this AP cap design is EXACTLY the same as on WWI French APC projectiles!!. They were noticeably thinner than almost any foreign WWII AP caps, except at the very center over the tip of the projectile's nose, where they were about as thick as the average foreign WWII AP cap, but a lot thinner than some US caps!!
The long French windscreen design was reinforced to hold the K spotting dye-bag charge (see next paragraph) and had a much wider attachment threaded area to the AP cap's upper side to more firmly hold the windscreen in place. Also, this reinforcement had a wide flat "floor" braced against the upper face of the AP cap, so that when the windscreen's K filler explosion removed most of the windscreen, this flat area would remain, allowing the shell to dive underwater exactly like a Japanese Type 91 diving shell with minimum drag or projectile path distortion. (Did the Japanese get this idea from the French, who had worked on this concept from before WWI, or did they develop it on their own?) I do not know how the base fuze was designed, though, so I do not know if it allowed a long underwater trajectory, like the Japanese 0.4-second fuze delay in its large-caliber APC shells did (the fuze was set off on water impact).
The Crucible shells had virtually identical face-hardened armor penetration quality to the US Navy shells they were based on when tested against French KC-type armor after WWII by the French themselves in comparison to their own 1936 APC shells ("38 cm Obus de Perforation (RC) K Modele 1936" -- the K being used to indicate a shell fitted with the explosive-distributed, nose-fuzed, windscreen-loaded, colored-dye-bag spotting concept -- worked at night and if a direct hit was made, which no other system did -- that the French introduced in the mid-1930's and the British copied in the middle of WWII). In fact, my FACEHARD program gives the EXACT same test result value (all four digits!!) to the one the French calculated to these tests with the US design when I use the "generic" WWII KC armor and the US 14" Mark 16 MOD 8 projectile choice (duplicated in the French Projectile Table, of course) -- and I never used this test in developing FACEHARD, either!!! Surprised me, since I knew I was good, but that was amazing!!
My data is from US Navy Drawing #430517 (Ammunition #389053) Revision “C” dated 10 July 1943 for the “380mm A.P. PROJECTILE MOD 1 -- ASSEMBLY AND DETAILS” manufactured by the Crucible Steel Company of America.
Italian 320 mm APC Projectile
The Naval Museum in Venice, Italy, has a 320 mm
APC projectile on display (Inventory #4402). Using a photo of the
projectile, which has a VERY long secant-ogive windscreen, I have a close
estimate that the length is about 4.5 calibers. Since the 381 mm
is 4.46 calibers long and they are proportionately exactly the same in
weight (close to WWII British APC), it seems reasonable that this is also
the length in calibers of the 320 mm projectile, too. That is how
most Navies did things: They made their projectiles scale models
unless introducing entirely new designs, of course, which was not being
done here between the 320 mm and the 381 mm of nearly the exact same age;
they were both of a single brand new design. I would be willing to
bet that they had the same filler type and weight percentage and
even had the exact same AP cap design and base fuze.
05 November 2008 - Benchmark
19 December 2012 - US Navy drawing reference
21 January 2012 - Corrected French AP nose tip shape