RHA is a TLA for 'Rolled Homogenous Armor.' It is the measuring stick against which anti-tank weapons are tested. Tank armor effectiveness is measured and expressed in millimeters of RHA, even though the actual thickness of the armor may be quite different; it is said to perform in the same fashion as a sheet of RHA that so many mm thick.

RHA is steel, essentially. It is homogenous in that the steel has the same properties throughout, unlike earlier armor which was usually hardened on the outside face to resist high-velocity shot, and left soft on the interior layers to allow for deformation rather than brittle cracking. It is rolled rather than cast into its required shape in order to increase the strength of the plate against penetration by stimulating long-chain crystal growth along the armor plane.

Note that although armor is measured in RHA, as are the penetrating capabilities of weapons, this is usually taken to mean equivalent RHA. This is because anti-tank weapons rarely strike their targets at an angle normal to the armor plate. Rather, they strike at an angle less than ninety degrees, and as a result, they are facing more than the thickness of the armor as shown by the following formula:

Equiv. RHA = (Armor Thickness) * Sin(angle of incidence)

Alternatives to plain RHA include Chobham, Reactive Armor, spaced armor and ablative armor.

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