The Chieftain was a development of the legendary Centurion, which introduced the world to the MBT concept in 1945, dominated the battlefield in the Middle East and imposed its main gun as “the” NATO standard during the Cold War. However, the fast pace of Soviet advance in ammunitions triggered a study for a new main battle tank aimed at exceeding all expectations and setting up a new reference for the Cold War. And in 1966, when it entered service, the Chieftain was, indeed, the most formidable main battle tank in the world. The Chieftain’s rifled Royal Ordnance L11A5 120 mm (4.72 in) gun was specifically tailored for it and also became the new NATO standard calibre. Its cross-country speed was better than that of the Centurion, and it could maintain it longer than the lighter Leopard I. The Chieftain also had the best protection of the time. The Chobham armour became a milestone in tank protection development.

 

Development

 

For at least twenty years the “magic triangle” speed-armour-armament was not achieved as well as the Centurion had, because the new engine did not fill all expectations and the sheer weight of armour was not met by an adequate power-to-weight ratio. Although having excellent mobility, the Chieftain was the slowest of the three Cold War British MBTs. The Centurion before and especially the Challenger after, were faster.

Design of the Fv4201 started in 1958, and the first prototype was built in 1959. Six other prototypes and a pre-production series of 40 tanks followed from 1961 to 1963. It was eventually accepted for service in May 1963, officially designated the Chieftain Mark V MBT, accompanied by an order for the production of 770. In 1966 the first Mk.Is entered fully active service with the tank units.

 

Design

 

The Chieftain emerged from a brand new hull and turret design. Apart from the tracks and some elements of the wheel train and some mechanical parts linked to the new engine, nothing was shared with the Centurion. The initial design combined some unique features, including a mantle-less turret, allowing superior depression angles. The turret was well sloped and roomy, allowing the loader, commander and gunner to be comfortably housed.

A large infra-red searchlight was installed on the left side of the turret. Smoke grenade launchers were mounted on the front of the turret, while the rear received a large gallery to house spare parts and magazines, acting as extra protection. Large storage bins were also mounted on the track covers, and after its introduction into service in 1966, side skirts were added to protect the main part of the tracks.

 

Mobility

 

The Mk.I was equipped with the Leyland opposed-piston two-stroke diesel L60, an ingenious design for multi-fuel use. But this engine proved unreliable and not powerful enough to allow speeds even close to those achieved by the first Centurion, with an average 19-22 mph (30-35 km/h).

 

The particular feature of this engine was that, for good combustion, it needed the ignition delay be reduced, thus requiring the internal temperature inside the cylinder after compression to be higher than usual. Engine research started in 1952 and led, in 1956, to the adoption of a family of 6-cylinder opposed-piston engines, like the two-stroke truck diesel, the Rootes TS-3. The latter inspired the LM L60, but with a different mechanical layout. Whereas in the TS-3 pistons were connected by rocker levers to a single crankshaft, the L60 took the path of the 1930s Junkers Jumo diesel, with two crankshafts geared together. But this formula experienced substantial thermal stress problems.

 

The technical solution adopted for the diesel proved unfit for multi fuel use and prevented it from reaching the planned output of 700 hp without consequences for the piston and cylinder lining. These issues and the overall weight resulted in a catastrophic 90% breakdown rate in exercises for the Mark I series, never really completely solved, but the output was raised in 1967. Later, additional armour and equipment added their own weight to the problem. In 1974, the newly introduced Belzona variant gave 850 hp, which was a great improvement compared to the original BL with 450 bhp. The final speed was around 48 km/h (30 mph) on road, but this was still below the Challenger performances and dictated a specific tactical use when operating in combination with the latter. With years of tactical exercises and well-understood limitations, the Chieftain proved a formidable asset in the British Army arsenal.

 

This power plant’s output was passed to the rear drive sprockets, served by a supine driver’s “hot-shift” epicyclical gearbox. The weight was distributed among the six doubled road wheels, three double return rollers and one idler per side. However, the suspension was the same Horstmann type as that of the Centurion, but poorly suited for the L-60 two-stroke diesel. The latter was specially developed for it, as an ideal engine to meet the contemporary demand for multi-fuel types, because of its opposed piston configuration. But this choice made it inherently troublesome, a fact which would appear later.

The Leyland L.60 diesel also mirrored the adoption, on the other side of the Channel, of the Hispano-Suiza HS 110 diesel for the contemporary French AMX-30. These engines helped achieve the 1957 NATO objective to run fuels ranging from light diesel to aviation high octane aviation gasoline (AVTAG/JP-4). But, in this case, refuelling operations and training needed extra care, maintenance was an issue and combustion was sometimes difficult to achieve without delays, especially for 80 octane combat gasoline. The AMX-30 multi fuel engine was equally troublesome in this respect.

 

Firepower

 

The muzzle velocity and accuracy of the new main Royal Ordnance L11A5 rifled gun easily compensated for the early lack of mobility. It fired separated ammunition with fully combustible bagged propellant charges. The turret was fitted, since the beginning, with a coaxial L7 and later L8A1 7.62 mm (0.30 in) machine-gun, with a second one in commander’s cupola and a Marconi FV/GCE Mk.IV A cal.50 (12.7 mm) ranging gun was mounted over the main gun, capable of ranges up to 2400 meters (1.49 mi), but 1800 m (1.2 mi) was more usual. But at 2000+ meters machine guns cannot be used practically, and the gunner had to rely on visual magnification, his graduated sight and own skills. Ammunition comprised HESH (high explosive squash head) and APDS (armour-piercing discarding-sabot), with 62 rounds carried. It took more time to engage successive targets with it than with the contemporary fire control system of Leopard 1, which was based on an optical rangefinder.

All this and the development of other, more accurate means of determining the range of targets, confined the use of ranging machine guns to only three major types of tanks. The first of them was the Centurion, the improved 105 mm (4.13 in) gun versions having begun the use ranging machine guns in 1962. By then the 12.7 mm (0.5 in) ranging machine gun had also been adopted for the Chieftain, which was produced with it until the mid-1970s.

 

Protection

 

The Chieftain introduced a supine (reclining backward) driver position, allowing a heavily sloped front hull -and thus better protection- while remaining lower. The 1990s Chieftain incorporated the “Stillbrew” armour named after Colonel Still and John Brewer from the Military Vehicles and Engineering Establishment (MVEE), the Improved Fire Control System (IFCS) and the Thermal Observation Gunnery Sight (TOGS). The NBC lining was continuously improved. The front armour, made of steel, had an equivalent thickness of 220 mm (8.66 in). Alternative armour versions were also tried, like the ROMOR-A armour kit, and the Varma passive/reactive explosive armour bricks. Major addition was the Stillbrew add-on package, covering a few areas of the turret, made of concrete with rubber blocks sealed with a plastic coating.

 

Turret features

 

The large rotating commander’s cupola had nine openings, a periscope, an infra-red projector with up to x8 magnification power and an alternative IR vision system. The commander’s sight had a unit power channel and the binocular periscope had x10 magnification (later x12 on the mark II and x15 on the Mark V). The turret’s main infrared range-finder was coupled with an armoured searchlight with a maximum range of 0.93 miles (1.5 km). The interior was lined with a complete NBC system. Nine prismatic blocs/periscopes formed a completely separated ring mounted on ball bearings. The principal advantage of this was a substantial mass reduction of the cupola, which could scan the horizon independently of the turret.

 

Targeting

 

A “low-tech”, more economical method of ballistic ranging was first developed for the Centurion and was also used by Chieftain gunners. It involved a modified 12.7 mm (0.5 in) machine gun to find the range by firing successive bursts in a precise pattern to calculate the trajectory. The gunner had to pick the corresponding mark appropriate to the type of gun ammunition to be fired, laying it on the target. The range of the target was so determined with a high degree of accuracy, also having the advantage of considering trunnion tilt and crosswinds.

However, it could disclose the position of the tank prematurely. Although simple and robust, it took a substantial time and precious time. This was not such a disadvantage at that time, as the T-54/55 and especially the T-62 had long reload and traverse times. In the 1980s it appeared as a “B-plan” in case of a failure of the other systems, as much faster and more precise electronic ballistics computers prevailed. Both the gunner and commander had a firing switch for the gun, while the commander had an override.

 

FV 4201 MBT specifications

 

Dimensions (L-W-H)

35’4″ (24’6″ without gun) x 11’5″ x 9’5″ ft.in
(10.79m (7.51m) x 3.5m x 2.89m)

Total weight, battle ready

55 tons (11000 Ibs)

 

Crew

 

4 (commander, driver, gunner, loader).

Propulsion

British Leyland diesel BL 40, 450-650 bhp, later BL 60, 695 bhp

Speed

48/30 km/h road/cross-country (29.82/18.64 mph)

Range/consumption

500 km (310.68 mi)

 

Armament

 

One L11A1 120 mm (4.7in) with Marconi cal.50 gun
One coaxial 7,62 mm L8A1 (0.3 NATO) machine-gun
One cupola mounted AA L37A1 7,62 (0.3 NATO) machine-gun

 

Armour

 

turret front 7.6in, glacis 4.72in, sides 1.37in (195/120/35 mm)

Ammunition used

Anti personal HESH, armour-piercing APDS.

Total production

900 for Great Britain alone, up to 1381 export variants