Reactive armor or explosive reactive armour (ERA), is a type of armour used primarily on tanks to lessen the damage from explosions caused from missile warheads, exploding shells, grenades, or dropped bombs.

Essentially all anti-tank munitions work by piercing the armor and killing the crew inside. Reactive armor's protective mechanism involves producing an explosion or other such reaction when it is impacted by a weapon, actively "pushing back" against it. This is particularly effective against shaped charge warheads, in which the warhead directs a focused jet of molten metal against the armor; reactive armor's reaction disrupts the jet before it reaches the armor's surface.

The effect was discovered in 1967–68 by a German researcher, Manfred Held, working in Israel. He and his team were using the large quantities of wrecked tanks from the Six Day War to test shells. They accidentally discovered that tanks that still contained live ordinance could disrupt a shaped charge by the explosion of the shells, the basis of ERA. The concept was patented in 1970.

Modern ERA such as the Russian Kontakt-5 is made up of "bricks" or "tiles" of explosive sandwiched between two metal plates. The plates are arranged in such a way as to move sideways rapidly when the explosive detonates. According to the designers, this will force an incoming KE-penetrator or shaped charge jet to cut through more armour than the thickness of the plating itself, since "new" plating is constantly fed into the penetrating body. A KE-penetrator will also be subjected to powerful sideways forces, which might be large enough to cut the rod into two or more pieces. This would significantly reduce the penetrating capabilities of the penetrator, since the penetrating force will be dissipated over a larger volume of armour.

Reactive armour has found great favor in the former Soviet Union since the 1980s, and almost every tank in eastern military inventory today has either been manufactured to use reactive armour or had reactive armour tiles added to it, even the very old T-55 and T-62 tanks from forty and fifty years ago, used today by reserve units.

ERA tiles are used as add-on armour to the most vulnerable portions of an armoured vehicle or tank, typically the front of the hull and the front and sides of the turret. They require fairly heavy armour on the vehicle itself, since the exploding ERA would otherwise damage the vehicle and injure or kill the personnel inside. Usually, ERA is not mounted on the sides or rear of a vehicle, since the underlying armour is not as heavy on those parts. Exploding ERA also poses a danger to friendly troops in close proximity to the vehicle. Though it was once quite common for a dozen or so infantrymen to ride on the outside of a tank's hull, this is not done with ERA plated vehicles—for obvious reasons.

Recent research has produced the idea of Electric Reactive Armour, where the armour is made up of two electrically charged plates separated by an insulator. When an incoming body penetrates the two plates and closes the circuit, a high current and voltage will flow through the penetrator, and tend to vaporize it, and significantly reduce the resulting penetration. It is not public knowledge whether this is supposed to function against both KE-penetrators and shaped charge jets, or only shaped charge jets. This technology has not been introduced on any operational platform.

Reactive armour can be defeated with multiple hits in the same place, which is employed in tandem-charge weapons, which use two shaped charge explosions in rapid succession. Lacking these, however, it is usually difficult to do as it requires either precision artillery, luck, or close-quarter use of shoulder-launched anti-tank weapons.

Compare with Chobham armor a.k.a. Composite armour.

External links:

• Defense Update: Reactive Armor Suits http://www.defense-update.com/features/du-1-04/reactive-armor.htm
• Defense Update: Advanced Protection for Modern Armored Vehicles http://www.defense-update.com/features/du-1-04/feature-armor-protection.htm