Chieftain Tank - Design

Design

The Chieftain design included a heavily sloped hull and turret which greatly increased the effective thickness of the frontal armour - 388 mm (15.3 in) on the glacis (from an actual thickness of 120 mm (4.7 in)), and 390 mm (15.4 in) on the turret (from 195 mm (7.7 in)). It had a mantleless turret, in order to take full advantage of reclining the vehicle up to ten degrees in a hull-down position.

The driver lay semi-recumbent in the hull when his hatch was closed down, which helped to reduce overall height. The commander, gunner and loader were situated in the turret. To the left side of the turret was a large infra-red searchlight in an armoured housing.

The Leyland L60 engine is a two-stroke opposed piston design intended for multi-fuel use so that it could run on petrol or diesel or anything in between. In practice the engine did not deliver the expected power, and was unreliable, estimated to have a 90% breakdown rate, but improvements were introduced to address this. Primary problems included, cylinder liner failure, fan drive problems and perpetual leaks due to vibration and badly routed pipework. However, as the engine power improved the tank itself became heavier.

The tank was steered by conventional tillers hydraulically actuating onto external brake discs. The discs worked via the epicyclic gearbox providing "regenerative" steering. In reality the discs and pads became soaked in oil and diesel and the steering became difficult. The gearbox was operated motorcycle-style with a kick up/kick down "peg" on the left which actuated electro-hydraulic units in the gearbox; the accelerator was cable operated by the right foot. In the turret the loader was on the left and the gunner on the right of the gun with the commander behind the gunner. The suspension was of the Horstmann bogie type, with large side plates to protect the tracks and provide stand-off protection from hollow charge attack.

The main armament was the 120 mm L11A5 rifled gun. This differed from most contemporary main tank armament as it used projectiles and charges which were loaded separately, as opposed to a single fixed round. The charges were encased in combustible bags. Other tank guns, such as on the Conqueror, needed to store the spent shell cartridges or eject them outside. The combustible charges were stored in 36 recesses surrounded by a water/glycol mixture - so-called "wet-stowage". In the event of a hit which penetrated the fighting compartment, the jacket would rupture, soaking the charges and preventing a catastrophic propellant explosion.

The gun could fire a wide range of ammunition, but the most commonly loaded types were high explosive squash head (HESH), armour-piercing discarding sabot (APDS), or practice round equivalents for both types. The Chieftain could store up to 62 projectiles (though a maximum of 36 APDS, limited by the propellant stowage). The gun was fully stabilised with a fully computerized integrated control system. The secondary armament consisted of a coaxial L8A1 7.62 mm machine gun, and another 7.62 mm machine gun mounted on the commander's cupola.

The Chieftain had an NBC protection system, which the Centurion lacked.

The initial Fire-control system (FCS) was the Marconi FV/GCE Mk 4. A .50-cal (12.7 mm) ranging gun was mounted above the main gun (with 300 rounds available). This fired ranging shots out to a maximum of 2,600 yards (2,400 m), at which point the tracer in the ranging rounds burned out. The tank commander had a rotating cupola with nine vision blocks and a periscope, plus the 7.62 mm machine-gun and an infrared (IR) projector coaxial with the weapon. The aiming systems were provided for both gunner and tank commander; they had 1x or 8x selectable magnification power, and they were replaceable with IR vision systems for the night operations (3x magnification power). The left side of the turret had a large white light/infra-red searchlight inside an armoured box. It had a relatively long range for this kind of system – up to 1–1.5 kilometres (0.62–0.93 mi).

From the beginning of the '70s, the Mk 3/3 version replaced the ranging gun with a Barr and Stroud LF-2 laser rangefinder with a 10 km (6.2 mi) range. This allowed engagements at much longer ranges, and also could be linked to the fire control system, allowing more rapid engagements and changes of target.

From the Mk 5 onwards, fire control was provided by the Marconi IFCS (Improved Fire Control System), using a digital ballistic computer. The upgrade was not finished until the end of 1980, when some examples (but not the majority) had the IR searchlight replaced with TOGS, the Thermal imaging system already fitted to the Challenger. Many examples had Stillbrew armour as well, meant to face Soviet 125 mm tank guns and heavy anti-tank missiles. These became the Mark 13 version.