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How Exactly Does Sloped Armor Work, Anyway?

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Ok, let's go with say, a 30 degree slope, which a vast assortment of shells in WW2 were tested against, basically, when a plate is facing completely perpendicular, It's exactly as "thick" as it will be on paper performance wise,  however, when you lay out out at 30 degrees, suddenly the projectile has to penetrate it at a diagonal, so, even though the plate didn't get thicker, it effectively still has to penetrate more of the plate then it would if it were facing perfectly upright because it now has to get through said diagonal angle as opposed to a perfectly straight path.


However, when a slope gets to around 60 degrees or more, this is where many projectiles really struggle, because, at least in the days prior to modern long rods and improved fuzes, projectiles require a good "bite" angle or there's a large risk of the projectile, particularly a higher velocity one simply deflecting due to the fact it can't keep that bite on the target and smash it's way through, or, even worse, a strike at a steep angle will also subject a projectile body to severe uneven distribution of impact stress which can cause it to shatter outright, a projectile that shatters will basically lose almost any chance it has of penetrating due to the loss of the strength of the projectile and the energy now being disspated everywhere in several smaller fragments instead of focused in one large one.


Ironically, as I've pointed out before on how this affects smaller, higher velocity rounds worse due to them being more likely to deflect or shatter at such high speeds, this is why, even though APDS and APCR from guns such as the 20 pounder and various 90mms the US used late and post war have similar or higher penetration "on paper" then the slower 120mm AP round of the M58 gun, the former would bounce off something like the front of a T-54/55 or IS-3 even at point blank range, whereas the M103 could penetrate them at a pretty good distance.


As for HEAT, It's not that the shells themselves couldn't penetrate, infact, 90mm M348 HEAT could penetrate even the front of an IS-3 or potentially an IS-4, the problem is, mainly with earlier HEAT designs, they had terrible fuzing, and with the M348, which is noted here because it's one of the most notorious cases due to it's particularly flawed design, it was based solely on the nose striking at a good enough angle to reliably detonate the fuze while facing the armor plate (the fact the M348 was designed without a well designed standoff didn't exactly help this), however, if the slope is so steep the round glances off before the fuze can be triggered, well.... It's basically useless.

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This is the chart from Technology of Tanks showing the "effective" thickness vs. the LOS thickness of sloped RHA vs. APDS type projectiles:




The solid line tracks the ratio of LOS thickness to normal thickness, while the dotted line tracks equivalent protection vs. normal thickness.


This is why you had weird shit in tests like the 20 pounder being able to penetrate the T-54's turret, but not its hull.

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