An element of explosive reactive armor (ERA) consists of a slab of high explosive sandwiched between two metal plates, called the reactive or dynamic elements. On attack by a penetrating weapon, the explosive detonates, forcibly driving the metal plates apart to damage the penetrator. Against a shaped charge, the projected plates disrupt the metallic jet penetrator, effectively providing a greater path-length of material to be penetrated. Against a long rod penetrator, the projected plates serve to deflect and break up the rod.
The disruption is attributed to two mechanisms. First, the moving plates change the effective velocity and angle of impact of the shaped charge jet, reducing the angle of incidence and increasing the effective jet velocity versus the plate element. Second, since the plates are angled compared to the usual impact direction of shaped charge warheads, as the plates move outwards the impact point on the plate moves over time, requiring the jet to cut through fresh plate material. This second effect significantly increases the effective plate thickness during the impact. To be effective against kinetic energy projectiles, explosive reactive armor must use much thicker and heavier plates and a correspondingly thicker explosive layer. Such "heavy ERA," such as the Soviet-developed Kontakt-5, can break apart a penetrating rod that is longer than the ERA is deep, again significantly reducing penetration capability.
Explosive reactive armor has been valued by the Soviet Union and its now-independent component states since the 1980s, and almost every tank in the eastern-European military inventory today has either been manufactured to use ERA or had ERA tiles added to it, including even the T-55 and T-62 tanks built forty to fifty years ago, but still used today by reserve units.
Explosive Reactive Armor Documentary Video