Rolled homogeneous armour

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Rolled homogeneous armour (RHA) is a type of vehicle armour made of a single steel composition hot-rolled to improve its material characteristics, as opposed to layered or cemented armour. Its first common application was in tanks. After World War II, it began to fall out of use on main battle tanks and other armoured fighting vehicles intended to see front-line combat as new anti-tank weapon technologies were developed which were capable of relatively easily penetrating rolled homogeneous armour plating even of significant thickness.

Today, the term is primarily used as a unit of measurement of the protection offered by armour on a vehicle (often composed of materials that may not actually contain steel, or even contain any metals) in equivalent millimetres of RHA, referring to the thickness of RHA that would provide the same protection. Typically, modern composite armour can provide the same amount of protection with much thinner and lighter construction than its protective equivalence in RHA. Likewise, the term is also used as a unit of measurement of penetration capability of armour-piercing weaponry, in terms of the millimetres of RHA that the weapon system can reliably penetrate.