The Merkava is a main battle tank used by the Israel Defence Forces. The tank began development in 1973 and entered active service in 1978. Four main versions of the tank have been deployed. It was first used extensively in the 1982 Lebanon War. The "Merkava" name was derived from the IDFs initial development program name. It is designed for rapid repair of battle damage, survivability, cost-effectiveness and off-road performance. Following the model of contemporary self-propelled howitzers, the turret assembly is located nearer the rear than in most main battle tanks. This gives additional protection against a frontal attack. This arrangement also creates more space in the rear of the tank that allows increased storage capacity, as well as a rear entrance to the main crew compartment allowing easy access under enemy fire. This allows the tank to be used as a platform for medical disembarkation, a forward command and control station, and an armoured personnel carrier. The rear entrances clamshell-style doors provide overhead protection when off- and on-loading cargo and personnel.