Thorsten Wahl wrote:
lwd wrote:I noticed in the file gsap_088 (part of the zip package linked by Thorsten) it states something to the effect that a plate thickness of .2 D required to decap a projectile. Now some latter infromation shows that this is very conservative but it implies that the the deck if it was designed to decap was designed to decap 250mm and over the magazines only 400mm projectiles. Rather brings to question whether or not decapping was the purpose.
0.2 D in the line of flight if i understand the meaning right
so 50 mm thickness at 60° obliquity should be equivalent to 100 mm in flight direction
against 1600 lbs AP bombs at 300m/s and 20° obliquity no existing horizontal protection scheme should be able to defeat such a piece
Indeed, the striking angle is most important to most of these discussions. I looked up some data today , rather than trust my increasingly feable memory
, on the required decapping plate thickness per the striking angle. It appears that ~13% D (50mm for a 38cm) by the point of 30* obliquity is relative to 26% D at the normal. But there was much more that I had forgotten. Another factor is kenetic energy. It was found that with higher levels of kenetic energy, that slightly less thickness was required to effect de-capping. Next the material strength and hardness of the de-capping plate mattered. It needed to be armour grade material for the 20% D rule, or much more was required. Another factor was the caliber radius of the projectile's head as previously mentioned. An additional factor was the interspace distance. If the interspace was close, then much more was required to insure de-capping. APP30 mentions that it was found that the great interspace distance of the Tirpitz type arrays would mean that a capped projectile would be de-capped by the time it reached the panzer deck. In the British test arrays the interspace was always very small, well less than 1 caliber for a large caliber shell. It is thought that this was a primary factor in inconsistent results. But overall the amount of de-capping plate thickness required was mostly a function of the relative striking angle.
A 1600lb projectile, only 20* from the normal, and at 300 M/s, can defeat more than 200mm of both types of armour according to the curves. Cemented armour would probably be better against a projectile striking that close to the normal.
The British commented that a spaced array should be constructed with a cemented main plate if the typical striking obliquity is less than 30* -40* from the normal, but this proves counter productive against more accutelty oblique striking angles, and for more accutely oblique striking angles a spaced array should use homogenous armour instead. Indeed the US Nat Lab tests that yielded +100% effective thickness used RHA throughout, as these test were for accutely oblique striking angles. RHA is used mostly today anyway. Most of the British 1940's era test arrays were for face plates or belts in mind, and so used cemented resistance plates. One series of vintage British tests investigating which armour type was better for a spaced array deck system, provided no comparable data, because the test arrays which used cemented, and thoses which used homogenous resistance plates, both rejected the projectiles.