Sloped armour

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The Soviet T-34 tank has sloped armour on all sides of its hull.

Sloped armour for tanks and other armoured fighting vehicles greatly increases protection, in comparison to perpendicular armour plates of the same thickness.

The idea behind sloped vehicle armour is that shots will be deflected causing them to move through a longer stretch of armour. At sixty degrees to the vertical the effect against light and short AP shells is about fifty percent in terms of plate thickness equivalence, depending on shell shape. Consequently, less steel is needed per tank, and the tank will be tougher for the same weight or more manoeuverable for the same protection. Up to 45 degrees the effect is negligable, as the impact point also acts as a friction point and at more perpendicular impact angles the projectile tends to cantilever turning into the plate. At steeper slopes it quickly becomes more important, reaching against traditional "bullet shaped" rounds the highest differential increment at about 67 degrees to the vertical; making the slope steeper deflects the projectile even better, but the relative gain diminishes.

This effect should not be confused with another effect of sloping armour: that the thickness of the armour measured along a horizontal line increases — this gives no direct weight advantage because the sloped armour plate must be larger. This horizontal line is called the Line of Sight or "LOS". Although there are no direct weight effects by changes in slope in relation to a single LOS, it is often possible to design for a much more favourable internal space volume - armour weight ratio, simply by cutting off "unused corners".

An illustration of sloped armour. A comparison of a (vertical) slab of armour to the left and a 45 degree sloped armour section to the right. The horizontal distance through the armour is the same (black arrow) but the actual thickness of the sloped armour (green arrow) is less. It can be seen that the actual cross-sectional area of armour, and hence the weight of metal, is the same in each case.

This other effect can be calculated by the cosine rule. At thirty degrees to the horizontal this increase is exactly one hundred percent. Both effects must be multiplied, so a 100 mm armour plate reclined at sixty degrees from the vertical will render a protection equivalent to about 300 mm of vertical plate against lighter traditional AP rounds. The armour thickness figures given for armoured vehicles are often the equivalent to this value, taking both thickness and angle into consideration. When armour thickness or rolled homogeneous armour equivalency (RHAe) values for AFVs are provided without the angle of the armour, the figure provided generally takes into account the effects of the slope, while when the value is in the format of "x millimeters at y degrees", the effects of the slope are not taken into account.

The principle of sloped armour is complicated by the "backplate effect": sloped armour plates thinner than 25% of the projectile diameter tend to catastrophically fail because their tensile strength is insufficient to resist the pressure generated by the deflection of the projectile, so their protective value is less than would be predicted.

The sloped armour effect is also a function of the penetrator length-width ratio. When the penetrator is very elongated — and since the late seventies all are of the "long rod" type — it will on impact deform so that it turns into the sloped plate. For this reason at about sixty degrees to the vertical not only the deflection effect is negated but the backplate effect leads to a lesser protective value than LOS thickness: the bended penetrator tip acts like a large diameter projectile, overloading the remaining armour plate.

Sloped armour can also cause projectiles to ricochet, but this phenomenon is much more complicated and not fully predictable. Traditional rounds ricochet more easily. High rod density, impact velocity, and length-to-diameter ratio are factors that contribute to a high critical ricochet angle (the angle at which ricochet is expected to onset) for a long rod projectile, but different formulae may predict different critical ricochet angles for the same situation. Modern penetrators ricochet only at extreme angles at above eighty degrees or more to the vertical. Above seventy degrees their deformation becomes a negative effect; also they then tend to break.

Research into the effects of sloping armour plate was first conducted in the 1930s by the French SOMUA, and by the Soviet tank design team of the Kharkov Locomotive Factory. It was a technological response to the more effective anti-tank guns being put into service at this time.

The principle itself was well known of old and had been in use on warships and partially implemented on the first French tank, the Schneider CA1 in the First World War, but the first tanks to be completely fitted with sloped armour were the French SOMUA S35 and other contemporary French tanks like the Renault R35, which had fully cast hulls and turrets. It was also used to a greater effect on the famous Soviet T-34 battle tank. Sloping armour robs a vehicle of internal space, so the sharpest angles are usually seen on the frontal glacis plate. After the war the principle became very much the fashion, its most pure expression being perhaps the British Chieftain.

However, the latest main battle tanks use perforated and composite armour, which attempts to deform and abrade a penetrator rather than deflecting it, as deflecting a long rod penetrator is so difficult. These tanks have a more blocky appearance. Examples include the Leopard 2 and M1 Abrams. The resistance of sloped targets made of a single layer of Chobham armour is less than the line-of-sight thickness would suggest, because much of the protective value of ceramic tiles is generated by "face defeat", i.e. surface resistance.

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