Reactive armour is a type of vehicle armour that reacts in some way to the impact of a weapon to reduce the damage done to the vehicle being protected. It is most effective in protecting against shaped charges and specially hardened long rod penetrators. The most common type is explosive reactive armour (ERA), but variants include self-limiting explosive reactive armour (SLERA), non-energetic reactive armour (NERA), non-explosive reactive armour (NxRA), and electric reactive armour. Unlike ERA and SLERA, NERA and NxRA modules can withstand multiple hits, but a second hit in exactly the same location will still penetrate.
Essentially all anti-tank munitions (with the exception of HESH) work by piercing the armour and killing the crew inside, disabling vital mechanical systems, or both. Reactive armour can be defeated with multiple hits in the same place, as by tandem-charge weapons, which fire two or more shaped charges in rapid succession. Without tandem charges, hitting the same spot twice is much more difficult.
The idea of counterexplosion (kontrvzryv in Russian) in armour was first proposed by the Scientific Research Institute of Steel (NII Stali) in 1949 in the USSR by academician Bogdan Vjacheslavovich Voitsekhovsky (1922–99). The first pre-production models were produced during the 1960s. However, insufficient theoretical analysis during one of the tests resulted in all of the prototype elements being blown up. For a number of reasons, including the accident, as well as a belief that Soviet tanks had sufficient armour, the research was ended. No more research was conducted until 1974 when the Ministry of the Defensive Industry announced a contest to find the best tank protection project.
Similar work was carried out by a West German researcher, Manfred Held in 1967–69. For the first time, reactive armour, created on the basis of the German experience, was installed on Israeli tanks during the Arab-Israeli Conflict in 1982 and was judged effective.
Explosive reactive armour
An element of explosive reactive armour consists of a sheet or slab of high explosive sandwiched between two plates, typically metal, 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.
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