Sloped armour

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

Sloped armour is armor that is mounted at a non-vertical and non-horizontal angle, typically on tanks and other armoured fighting vehicles. For a given normal to the surface of the armour, increasing armour slope improves the armour's level of protection, while for a given area density of the armour the protection can be either increased or reduced by sloping, depending on the armour materials used. Sloping armour reduces a vehicle's internal volume, so the sharpest angles are usually seen on the frontal glacis plate. On the other hand, armour can be sloped to remove internal volume that would go unused, thereby reducing the vehicle's mass.

An illustration of sloped armour. A comparison of a vertical slab of armour on the left and a section of armour sloped at an angle of 45 degrees on the right. The horizontal distance through the armour (black arrows) is the same, but the normal thickness of the sloped armour (green arrow) is less. It can be seen that the actual cross-sectional area of armour, and hence its mass, is the same in each case.

The cause for the increased protection at a given normal thickness is the increased line-of-sight (LOS) thickness of the armour, which is the thickness along a line parallel to the oncoming projectile's general direction of travel (horizontal or vertical). The LOS thickness is equal to the armour's normal thickness times the quantity one divided by the cosine of the armour's inclination from perpendicularity to the projectile's travel. For example, armour sloped sixty degrees back from the vertical presents to a projectile travelling horizontally a line-of-sight thickness twice the armour's normal thickness. When armour thickness or rolled homogeneous armour equivalency (RHAe) values for AFVs are provided without the slope of the armour, the figure provided generally takes into account this effect of the slope, while when the value is in the format of "x units at y degrees", the effects of the slope are not taken into account.

Beyond the change in line-of-sight thickness, sloping armour has other effects on the level of protection it provides. Sloping armour can cause additional protection-enhancing effects such as shattering of a brittle kinetic energy penetrator or a deflection of that penetrator away from the surface normal, but in thick homogeneous plates a long-rod penetrator will, after initial penetration into the armour's LOS thickness, bend toward the armour's normal thickness and take a path with a length between the armour's LOS and normal thicknesses. When the latter occurs, better protection would be provided by normally-mounted armour of the same area density. At any given area density, ceramic armour is also best when mounted normally, as maintaining the same area density requires the armour be thinned as it is sloped and the ceramic fractures earlier because of its reduced normal thickness. ^[1]

Sloped armour can also cause projectiles to ricochet, but this phenomenon is much more complicated and not fully predictable. 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^[2], but different formulae may predict different critical ricochet angles for the same situation.

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.

Sloped armour on the front of Soviet T-54 tank, here cut open for demonstration purposes

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. Sloped armour became very much the fashion after World War II, although not every tank designed since the war makes much use of it.

[edit] Notes

1. ^ Yaziv, D.; Chocron, S.; Anderson, Jr., C.E.; Grosch, D.J.. "Oblique Penetration in Ceramic Targets". Proceedings of the 19^th International Symposium on Ballistics IBS 2001, Interlaken, Switzerland: 1257-1264. 
2. ^ Tate, A (1979). "A simple estimate of the minimum target obliquity required for the ricochet of a high speed long rod projectile". J. Phys. D: Appl. Phys. 12: 1825–1829.