The Chieftain was the British Army's Main Battle Tank for twenty years, first entering service with the 11th Hussars in 1966. One of the first true Main Battle Tanks, it was designed to replace both medium (Centurion) and heavy (Conqueror) tanks in front line service and provided the backbone of the British Army's heavy armour at the height of the Cold War. It incorporated a lot of revolutionary design features, although some did not work as well as expected. For example, in order to reduce height the driver lay in a reclining position and changed gear with his foot. The L11 120mm gun used a self-combusting bagged charge instead of a brass cartridge case (the gun proved to be highly successful); and the tank was powered by a multi-fuel engine. The Chieftain was continually upgraded during its service life: later marks were fitted with a laser rangefinder; a computerised fire-control system (IFCS), thermal sights (TOGS) and additional compound armour on the front of the turret and around the driver's hatch (so-called Stillbrew armour). The L60 engine was continually modified and improved to increase its output and reliability. The end result of all these changes was a formidable AFV. The Chieftain hull was also used for a range of specialised AFVs, including armoured recovery vehicles (ARV and ARRV), bridge layers (AVLB) and combat engineering tanks (AVRE). A total of more than 2,200 Chieftains of all types were manufactured by the Royal Ordnance Factories and Vickers Ltd. Some 900 of these served with the British Army while the remaining 1,300 tanks were exported to Iran, Jordan, Kuwait and the Oman. Chieftains saw their share of combat with the Iranian Army in the 1980s when they engaged Iraqi tanks during the Iran-Iraq War, while Kuwaiti Chieftains fought the Iraqi Army during the invasion of Kuwait in 1990. Chieftain gun tanks were withdrawn from front-line service with the British Army during the late 1980s and early 1990s, but Chieftain recovery and combat engineering vehicles served with the British Army during the operations to liberate Kuwait in 1991 (Desert Storm).