Despite the Centurion's dominance, the Cold War battlefield was an ever-evolving chessboard of technology-driven pieces and this produced ever more efficient weaponry in the process. As such, the impressive nature of once-formidable tanks such as the Centurion soon began to give way to new anti-armor measures being developed and fielded primarily by the Soviet Union - to which many of these weapons would inevitably be passed on to allies and satellite states within time. Work therefore began on finding the "next British Army main battle tank" and this led to a Leyland-designed tracked vehicle mounting the new and powerful L11 120mm series rifled main gun. Whereas historical approaches to British armor valued speed and battlefield mobility above all else, the Leyland submission decidedly focused on a heavily armored vehicle with firepower to boot. By 1959, a pilot vehicle was ready for formal review leading to the evaluation of six more prototypes spanning from 1961 to 1962. The vehicle - known as the "FV 4201" - was adopted for service in May of 1963 with service beginning in 1965. Production was managed along two separate assembly lines and eventually included the Leyland and Vickers brand labels.
Outwardly, the Chieftain was of a conventional design approach with a four-man crew, 360-traversing turret mounting the main armament and a rear-mounted engine. Like other vehicles of this class, the Chieftain made use of a heavily-sloped, near-horizontal glacis plate leading up to a shallow hull roof. The driver was seated at the center-front of the hull just ahead of the turret ring. The remaining three crew - the tank commander, gunner and loader - held positions within the turret assembly. The commander and gunner were grouped, seated in tandem with the commander rear, along the right side of the turret in a traditional fashion with the loader to the left side of the turret. The cast turret itself was well-formed and heavily sloped with a low profile, the main gun barrel situated along the front facing with noticeable turret overhang (bustle rack) at the rear. The commander and loader were each afforded an access hatch along the turret roof as was the driver with a hatch in the forward hull. The driver drove in a near-horizontal reclined position when the tank was "buttoned down". The running gear consisted of six road wheels to a track side with the drive sprocket at the rear and the track idler at the front. Track return rollers guided the upper track sections. The upper regions of track and hull were generally protected over in armor skirts which concealed the return rollers and most of each road wheel - this feature a now-well accepted protective measure first implemented on a large scale in combat tanks during World War 2. Night driving was aided by a pair of headlights at the front of the hull. Stowage boxes were set over each fender for personal crew effects and general mission gear and storage baskets and boxes were also present along the turret sides.
In later marks, an NBC (Nuclear-Biological-Chemical) protection system was installed for the entire crew (the unit attached externally to the rear of the turret) and the commander, gunner and driver were all given night vision devices for improved low-light level performance.
Despite its excellent qualities elsewhere, the true "heart and soul" of the Chieftain system was undoubtedly its rifled 120mm L11 series main gun. The weapon was mated to a very accurate fire control system that provided precision firing at distance and "on-the-move". Comparatively, the earlier Centurion was given the smaller-caliber, though still excellent, L7 105mm series main gun. The weapon would soon run its course on the modern battlefields as Western tanks would now pitted against ever more powerful Soviet types. The L11 main gun was given an elevation range of -10 to +20 and fired a variety of cleared ammunition types. These included Armor-Piercing Fin Stabilized Discarding Sabot rounds (APFSDS), Armor-Piercing Discarding Sabot (APDS) rounds, High-Explosive Squash Head (HESH) rounds and basic Smoke munitions. The Chieftain crew could therefore engage both "hard" and "soft" targets as required. The fire control system (FCS) was fully-digital and the main gun fully-stabilized. In addition to the main gun, there was a 7.62mm L7 (essentially a British copy of the Belgian FN MAG) coaxial machine gun fitting in the turret and intended for keeping enemy infantry at bay. A second 7.62mm machine gun was fitted at the commander's cupola for point defense against both infantry and low-flying aircraft threats. To cover the tank's offensive or defensive movements from enemy gunners, the Chieftain was also afforded twelve electrically-actuated smoke grenade dischargers fitted as two banks of six, one bank mounted to each turret front side. There were 64 rounds of 120mm ammunition stored about the Chieftain's turret and hull as well as 6,000 rounds of 7.62mm ammunition (early Chieftain production tanks also fielded a 12.7mm "ranging" machine gun and these were given 300 rounds of 12.7mm ammunition). The 120mm ammunition was housed in water-tight holds containing both bag charge and loading ammunition to help prevent inadvertent explosions from direct hits or internal fires.
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