The U.S. M1 Abrams tank has again proved, this time in Iraq, that it is a very effective, and largely invulnerable, combat vehicle. However, during fighting in Iraq, several have been damaged, mainly through hits in known areas of vulnerability. These are on the side and rear of the tank, where hits are relatively rare, and too much additional weight would result if thicker armor were installed there. The M1A2 tank already weighs 70 tons. The disabling hits were in the engine exhaust. The power plant is a gas turbine, and there's a lot of hot air that has to be pumped out. Iraqis drove up behind an M1 and got off a shot in the exhaust grill with an RPG. This damaged the engine and stopped the tank. Several other hits have been scored, from the side, in the ammunition storage area at the rear of the turret. This part of the turret is designed (quite successfully) to take a hit that would penetrate the thin armor, detonate the ammo (the shell propellant) and leave the crew unharmed. Blow out panels allow the force of the explosion to vent away from the forward part of the turret, where the crew is. This system has worked successfully several times. It is possible to protect the rear and side portions of the tank by using Explosive Reactive Armor (ERA.) But tests with ERA have shown it to be a questionable advantage. When ERA goes off to defeat a shell or missile, the people inside the tank hears a loud sound similar to being inside a bell when it is rung. The crew is disoriented for a while, and the exploding ERA can injure any infantry close to the tank and damage equipment on the outside of the tank. So, for the moment, the losses are considered "acceptable."