February 13, 2006: The U.S. Army is equipping 150 of its Stryker wheeled armored vehicles with ERA (Explosive Reactive Armor.) Invented by the Israelis in the 1970s, ERA consists of explosives packaged between layers of sheet metal. When the hot gas jet produced by a shaped charge (of an RPG or missile warhead) hits the ERA explosives, the gas jet is disrupted and rendered harmless by the ERA explosion. Many American M-2 and AAV-7 armored vehicles in Iraq are already protected by ERA kits (which cost over $100,000 each). The Stryker ERA will cost nearly $300,000 per vehicle, and add 3.5 tons of weight (compared to 2.5 tons for the current slat armor it will replace.)

There are two shortcomings with ERA. One is that, once a section of it explodes, that section is obviously, no longer protecting the vehicle. Also, it takes 50 or more pieces of ERA to protect a vehicle. The other problem is that, when ERA explodes, it expels some metal fragments that can injure nearby friendly troops. While the army did not release details, it's possible that the Strykers might eventually get some new ERA technologies. For example, CLARA ERA uses several layers of composites (non-metal material) and explosives which, when they disrupt a shaped charge gas jet, only create lightweight chaff. CLARA is much lighter than standard ERA, weighing about an ounce per square foot, versus several pounds per square foot for standard ERA. There is also SLERA (Self Limiting ERA), which uses smaller amounts of explosives to disrupt the gas jet, and less destruction to the section of ERA itself. This also results in lighter ERA. Both these new types of ERA are more expensive, and lack any combat experience.