Reactive armour
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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 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 work by piercing the armour and killing the crew inside. 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. Lacking these weapons, emulating this effect is difficult as it requires either precision artillery, luck, or close-quarter use of either shoulder-launched anti-tank weapons or adhesive explosives such as satchel charges.
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[edit] 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 increases the effective plate thickness during the impact significantly.
To be effective against kinetic energy projectiles, ERA 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.
The effectiveness against shaped charge warheads was recognized in 1967–68 by a German researcher, Manfred Held, working in Israel. He and his team were testing shells against tanks wrecked in the Six-Day War. They observed that tanks that still contained live munitions could disrupt a shaped charge by the explosion of the ordnance, the basis of ERA. The device was patented in 1970.
Explosive reactive armour 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.
ERA tiles are used as add-on (or "appliqué") armour to the portions of an armoured fighting vehicle that are most likely to be hit, typically the front (glacis) of the hull and the front and sides of the turret. Their use requires that the vehicle itself be fairly heavily armoured to protect the vehicle and its crew from the exploding ERA; usually, ERA cannot be mounted on the less heavily armoured sides or rear of a vehicle.
A further complication to the use of ERA is the inherent danger to anybody near the tank when a plate detonates. Although ERA plates are intended only to bulge following detonation, the combined energy of the ERA explosive, coupled with the kinetic or explosive energy of the projectile, will frequently explosively fragment the plate. The explosion of an ERA plate creates a significant amount of shrapnel, and bystanders are in grave danger of serious or fatal injury. As a result, ERA cannot be used on vehicles deployed as combined arms with infantry.
[edit] Non-explosive and non-energetic reactive armour
NERA and NxRA operate similarly to explosive reactive armour, but without the explosive liner. Two metal plates sandwich an inert liner, such as rubber. When struck by a shaped charge's metal jet, some of the impact energy is dissipated into the inert liner layer, and the resulting high pressure causes a localized bending or bulging of the plates in the area of the impact. As the plates bulge, the point of jet impact shifts with the plate bulging, increasing the effective thickness of the armour. This is almost the same mechanism as the second mechanism that explosive reactive armour uses, but it uses energy from the shaped charge jet rather than an explosive.
Since the inner liner is not explosive itself, the bulging is less energetic than on explosive reactive armour. However, NERA and NxRA are lighter and completely safe to handle (and safe for nearby infantry), and can be packaged in multiple spaced-out layers if necessary.
[edit] Electric reactive armour
A new technology of electric reactive armour is in development, where the armour is made up of two or more conductive plates separated by some space or by an insulating material, creating a high-power capacitor. In operation, a high-voltage power source charges the armour. When an incoming body penetrates the plates, it closes the circuit to discharge the capacitor, dumping a great deal of energy into the penetrator, which may vaporize it or even turn it into a plasma, significantly diffusing the attack. It is not public knowledge whether this is supposed to function against both KE-penetrators and shaped charge jets, or only the latter. This technology has not yet been introduced on any operational platform. This concept was borrowed by the producers of Star Trek: Enterprise as the "polarized hull plating" of Enterprise, a predecessor to deflector shields.
