The Merkava is the main battle tank of the Israel Defense Forces. Since the early 1980s, four main versions have been deployed. The "Merkava" name was derived from the IDF's development program name. It is optimized for crew survival and rapid repair of battle damage. Through the use of spaced-armor techniques and quick-replacement modular designs, the design team was able to incorporate composite armour, a derivative of rolled homogeneous armour (RHA) and Chobham armour. Additionally, the space between the inner and outer hulls is filled with diesel an economical storage method and a means of defeating HEAT rounds. Following the model of contemporary self-propelled howitzers, the turret assembly is located nearer the rear than in most main battle tanks. This gives the crew additional protection against a frontal attack by putting the engine between them and the 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 even under enemy fire.