Rosselsprung 1942 actual combat

[Dave Saxton]

... What is not certain is IF the 5 + 8cm arrangement would stop the projectile from entering the machinery spaces (intact or not).

No, that is certain. With the German system 5cm +8cm does indeed = at least 13cm. The effective thickness is not ambiguous. There will likely be no penetrations, broken or not, until the range exceeds 25km.

[alecsandros]

... What is not certain is IF the 5 + 8cm arrangement would stop the projectile from entering the machinery spaces (intact or not).

No, that is certain. With the German system 5cm +8cm does indeed = at least 13cm. The effective thickness is not ambiguous. There will likely be no penetrations, broken or not, until the range exceeds 25km.

... The terminal energy of a 1227kg shell striking at 470m/s would be greater than any of the shells tested against German armor array, so this should be kept opened. 16" shells were well thermically treated, and, except the fuze problems from 1942/1943 (but which did not became manifest at all against Kirishima and Jean Bart - on the contrary), performed remarkably well.

[tommy303]

We do not know how many hits on the Kirishima were duds, although two of the five on Jean Bart were. Those, however were due to oblique impacts which caused base slap and the loss of the fuze plugs and fillers. It should be noted, however, that the US fuzes were not optimal for oblique impacts and worked best at normal to low obliquity. In both cases, I believe, the shells in the magazines of the ships in question had been only recently fuzed and stored aboard, and corrosion due to fumes from the ammonium picrate filler had likely not had time to take effect to such an extent as to cause malfunctions.

[tommy303]

I should add that it was not appreciated at the time how corrosive the fumes from the Explosive D were to fuze mechanisms. The new Mk 19, 20 and 21 fuzes had tighter tolerances in the moving parts than had earlier fuzes and very complex arming mechanisms to enhance bore safety over the previous Mk13 (which was found to cause an unacceptable number of bore prematures). Even very minor corrosion or build up of metal oxides would cause the fuze to fail. Generally, fumes from the explosive D would take about 6 months to render fuze operation questionable, and in the case of USS Massachusetts it is unlikely that her AP shells had reached the 'shelf life' limit at the time she engaged the Jean Bart.

[Steve Crandell]

I should add that it was not appreciated at the time how corrosive the fumes from the Explosive D were to fuze mechanisms. The new Mk 19, 20 and 21 fuzes had tighter tolerances in the moving parts than had earlier fuzes and very complex arming mechanisms to enhance bore safety over the previous Mk13 (which was found to cause an unacceptable number of bore prematures). Even very minor corrosion or build up of metal oxides would cause the fuze to fail. Generally, fumes from the explosive D would take about 6 months to render fuze operation questionable, and in the case of USS Massachusetts it is unlikely that her AP shells had reached the 'shelf life' limit at the time she engaged the Jean Bart.

What do you mean by "at the time"?

[tommy303]

I meant, that in 1940-42, the corrosive properties of the small amount of fumes given off by the Explosive D filler was not fully appreciated, particularly as fuzes were manufactured of zinc alloys containing no copper or lead (it was known at the time that Explosive D did react with lead and copper alloys to form sensitive explosive salts). Previous fuzes did not have the tight tolerances between moving parts and such corrosion or formation of oxides as occurred had little effect on fuze function. Further more, in peacetime, most shells carried aboard USN warships had travel plugs instead of fuzes, and were only fuzed when the need arose. In wartime, instant readiness for action required war munitions to be fuzed and ready for action all the time and it was not realized at first that fumes from the explosive could render the moving parts of the fuze inoperable in as little as 6 months. The new fuzes, Mks 19--21, did have extremely close tolerances and multiple safeties which had to function flawlessly for the fuze to arm itself. Even a little oxidation was sufficient to prevent one or more of the safeties from operating properly, and the failure of any one component meant the failure of the entire fuze. It was not until examples of fuzes which failed to function were examined, as well as fuzes installed long term in shells filled with Explosive D, that the causes of the failures became apparent. The cure for the problem was an easy one, fortunately, and consisted of simply sealing the fuze with bakelite resin to prevent fumes from entering the fuze interiors. Fuzes treated in this fashion were marked Mod 1 and had become standard in AP, Common, and SAP shells by 1943.

[Steve Crandell]

Ah, thanks. It was the date that I was asking about, but the further info was helpful as well. I'd read that at some point, but I don't remember things as well as I used to.

[Thorsten Wahl]

...The British found post war that de-capping is a very effective type of deck protection.

Both yaw and de-capping increase the effective thickness, or the relative quality, depending on how you look at it, of the main armoured deck, resulting in an effective thickness equal to or exceeding the sum thickness.

there was a american test with 3 inch projectiles i know about that shows the possible ineffectivenes of yaw under certain conditions.
PENETRATION OF HOMOGENEOUS PLATE BY 3 INCH FLAT NOSED PROJECTILES

"A comparative test of M79 and flat nosed projectiles(with and without cap) was carried out against a divided armor structure(comment model of deck structure of US BBs) consisting of a 3/8", 1,5" an 1/4" STS spaced 2-feet apart at 30° obliquity"


used STS plates: UTS 116,000 psi to 127,000 psi
mass of M79 15lb
mass of flat nosed projectiles (without cap) 15 lb

hardness of projectiles (WD 4150 steel) heat threated to a uniform hardness 55-60 RC except for a base draw to about 40 RC (standard distribution of M79)

divided structure total thickness 2,125" (t/d~0,7)
limit velocity
M79 1130 f/s
flatnosed projectile (without cap) 880 f/s

comparative performance single plate 2,0"
M79 1176 f/s projectile deformed
flatnosed projectile (without cap) 1188 f/s projectile deformed (seemingly a outlier)


single plate 1,5"
M79 1082 f/s projectile deformed
flatnosed projectile (without cap) 791 f/s projectile deformed

Based on this data the result seem to indicate a ineffectivenes of this structure (at the given impact conditions) to provide additional protection compared to a single plate.
Additionally one has to consider that the projectiles basically perform worse, as they deform on impact against these plates at a moderatly low obliquity.

[Dave Saxton]

This test system is very different from the German deck system. It is an arraingment of three thin ballistic plates spaced only 2 feet apart. The German system is an arraingment of two plates (the battery deck is only 6mm and considered non-ballistic) of specified thickness ratio and hardness, with sufficient space for yaw to become fully manifest.

The test array is actually much like a scaled down incremental system one may find on the Hood or an R-class battleship. (Or a West Virginia class battleship as built: 38mm STS+38mm mild steel + a thin spinter deck) Krupp's square root of each plate squared = the effective thickness calculation would apply and yields similar results.

Also 30* from the normal would be rather extreme angle of fall vs a deck system.

When was this test done?

[Thorsten Wahl]

Report Date : 19 APR 1943
http://www.dtic.mil/docs/citations/ADA955288

[Steve Crandell]

This test system is very different from the German deck system. It is an arraingment of three thin ballistic plates spaced only 2 feet apart. The German system is an arraingment of two plates (the battery deck is only 6mm and considered non-ballistic) of specified thickness ratio and hardness, with sufficient space for yaw to become fully manifest.

The test array is actually much like a scaled down incremental system one may find on the Hood or an R-class battleship. (Or a West Virginia class battleship as built: 38mm STS+38mm mild steel + a thin spinter deck) Krupp's square root of each plate squared = the effective thickness calculation would apply and yields similar results.

Also 30* from the normal would be rather extreme angle of fall vs a deck system.

When was this test done?

Maybe it's not relevant to do this, but the test projectile was 3", which is one fifth of 15". If you multiply the two foot plate separation by the same ratio, you get 10 feet. That would imply the whole system scaled up would have a 20 foot depth. Isn't that about the same as the German system, except that the middle deck isn't armored?

[alecsandros]

... The German system had varying plate declinations, which made it harder for the shell to perforate.

Belt: 7-17* next to magazines, slope 60 - 68*, TDS 0*.

Decks: 2-3* upper deck forward and aft, 0* MAD, 60-68* slopes.

[Steve Crandell]

... The German system had varying plate declinations, which made it harder for the shell to perforate.

Belt: 7-17* next to magazines, slope 60 - 68*, TDS 0*.

Decks: 2-3* upper deck forward and aft, 0* MAD, 60-68* slopes.

I don't believe the comparison was supposed to have anything to do with the belt or slopes. Purely the horizontal deck armor including the weather deck and MAD, which are what would be hit from long range prior to entering the engineering spaces.

[alecsandros]

... The German system had varying plate declinations, which made it harder for the shell to perforate.

Belt: 7-17* next to magazines, slope 60 - 68*, TDS 0*.

Decks: 2-3* upper deck forward and aft, 0* MAD, 60-68* slopes.

I don't believe the comparison was supposed to have anything to do with the belt or slopes. Purely the horizontal deck armor including the weather deck and MAD, which are what would be hit from long range prior to entering the engineering spaces.

it would depend on trajectory. some shells may go through weather deck + flat MAD. other shells through weather deck + slope.
In Bismarck and Tirpitz, the MAD was a turtle shell design.

[Dave Saxton]

Report Date : 19 APR 1943
http://www.dtic.mil/docs/citations/ADA955288

Maybe it's not relevant to do this, but the test projectile was 3", which is one fifth of 15". If you multiply the two foot plate separation by the same ratio, you get 10 feet. That would imply the whole system scaled up would have a 20 foot depth. Isn't that about the same as the German system, except that the middle deck isn't armored?

"A comparative test of M79 and flat nosed projectiles(with and without cap) was carried out against a divided armor structure(comment model of deck structure of US BBs) consisting of a 3/8", 1,5" an 1/4" STS spaced 2-feet apart at 30° obliquity"

Okay, scaled up it looks like it is the USS Montana deck protection design vs 16" at about 40,000 meters range. The 1943 date would certainly indicate this as well. The main problem with yaw not being fully effective is the distance between the upper deck and the MAD is too small for procession to become manifest.

Isn't that about the same as the German system, except that the middle deck isn't armored?

No it is not about the same as the German system. The German system has ample space between upper deck and panzer deck for procession to become fully manifest. This is key. If procession becomes manifest then it meets Dr Gercke's condition for spaced armour effective thickness to =, or exceed the sum thickness, because the AP shell will require a significantly greater velocity to attain penetration than it normally would. This is tantamount to either increasing the effective thickness of the panzer deck or increasing its relative quality. Additionally, if the tensile strength of the panzer deck is more than 80kg/mm2 a de-capped shell (or uncapped AP) will require greater velocity, further enhancing the effect. The STS of the middle plate in this case just isn't hard enough. A de-capped shell will also have its mass reduced, further requiring additional velocity, which was a key point of the British post war findings. The extreme angle of fall will also reduce the amount of energy consumed by the upper deck compared to lesser angles of fall, and the upper plate would be unlikely to de-cap with the striking angle only 30* from the normal.

One of the interesting things about the historical Tirpitz AP bomb penetration is that it should have penetrated about 130mm single plate, consuming all of the energy. The plates penetrated before it came to stop were 50mm + 80mm= 130mm. The bomb was broken and could not function.