Rolled Homogeneous Armour
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RHA stands for Rolled Homogeneous Armour. Through the end of World War 2, the armour for almost all tanks and other armoured vehicles was sheets of steel. Increasing the protection on a vehicle meant adding thicker sheets of steel, increasing the vehicle's weight and reducing its mobility. Since then, other forms of armour, incorporating empty spaces and materials such as ceramics or depleted uranium in addition to steel, have been developed.
The term is used currently as RHAe (Rolled Homogeneous Armour equivalency) to give a rough estimate of either the penetrative capability of a projectile or the protective capability of a type of armour which may or may not be steel.
The usefulness of RHAe in comparing different armour has been described thus: "Using RHA equivalencies to measure the protective capacity of armour is a lot like weighing a pig in Iowa. You lay a board over a fence, and you put the pig on one side, and a load of bricks on the other. Then, you guess the weight of the bricks."