Chieftain Mark V MBT Part II ➡️
Chieftain Mk.V 4th RTR
Chieftain Mark 5 BAOR of the British Army stationed in Belgium, 1979.
1/35 Tamiya British Chieftain Mk 5 Tank
While the British Army was still equipping itself with Centurions armed with the 83.8mm gun, work started on a successor to them, the FV 4201. Design of it began in earnest in 1958 and the first prototype was built in 1959. Six other prototypes and a pre-production series of 40 followed between 1961 and 1963, when the FV 4201 or Chieftain, battle tank was accepted for service. In May 1963, the British Army accepted the new tank, officially designated the Chieftain Mark V MBT. An order was then also placed for the production of 770 and in 1966 the first Chieftains entered service with British tank units.
At the time of its introduction into service the Chieftain was the most powerfully armed and the most heavily armoured battle tank. Its main armament consisted of a new, 120mm L11 rifled gun, which fired separated ammunition with fully combustible bagged propellant charges. Its armour was significantly thicker than that of the Centurion and it incorporated such novel features as a supine driver’s ‘hot-shift’ epicyclic gearbox. However, its suspension was of much the same Horstmann type as that of the Centurion and its automotive characteristics were marred by the poor performance of its L-60 two-stroke diesel. The latter was specially developed for it as an engine ideally suited, in theory, to meet the contemporary demand for multi-fuel engines because of its opposed piston configuration. But this configuration also made the L-60 inherently troublesome – a fact ignored when the decision was taken to adopt it.
The Chieftain’s cupola incorporates a commander’s sight with a unit power channel and a binocular periscope with x10 magnification in the Mark 2 and x15 magnification in the Mark 5 versions. There are also the nine unit power periscopes mentioned earlier but they form a ring which does not rotate in relation to the turret. The principal advantage of this arrangement over that of the Centurion is a reduction in the mass of the cupola which has to be rotated to scan with the commander’s sight independently of the turret.
A much more economical method of ballistic ranging was adopted for the British Centurion and early Chieftain tanks. It involved the use of a modified 12.7mm machine gun to find the range by firing short bursts aimed by laying on the target successive dots or marks on the range scale for the machine gun in the graticule of the sight and then observing which burst hits the target and, therefore, which mark gives the correct range. The gunner then picks the corresponding mark on the range scale appropriate to the type of gun ammunition which is to be fired and lays it on the target. Alternatively, he may use the same mark if the range scale of the machine gun and of a particular type of gun ammunition happen to be the same, which has been the case with the HESH ammunition of the Chieftain’s 120mm gun.
The use of a machine gun for ranging is obviously far less demanding of the volume of the ammunition carried in a tank than the use of the main armament for the same purpose. At the same time it enables the range of the target to be determined with a reasonably high degree of accuracy, the standard deviation of ranging errors being about 50m at 1000m. Its use also has the advantage of taking into account trunnion tilt and crosswind. However, the ranges to which it can be used are inevitably limited by the performance of the machine gun ammunition. In the case of the Centurions the maximum range to which the ranging machine gun could be used was actually 1800m. In the case of the Chieftain the maximum range was extended to more than 2000m but, nevertheless, machine guns can not be used when ranges are long and when range information is needed most. The use of ranging machine guns does not produce any of the obscuration associated with the firing of tank guns but it is not entirely unobtrusive and can therefore disclose the position of a tank prematurely. It was also found that, although the ranging machine gun system was basically simple and robust 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 105mm gun versions of which began to use ranging machine guns in 1962. By then the 12.7mm ranging machine gun had also been adopted for the Chieftain, which was produced with it until the mid-1970s.
Depending on the model, the Chieftain weighed between 54 and 56 tons, which was not much more than the final version of the Centurion. Nevertheless, it was criticised at the time of its introduction as being unduly heavy and was compared unfavourably in this respect with the lighter battle tanks being developed in Germany and France. However, the lighter weight of the German and French tanks implied less armour protection and, although no amount of armour could make a tank immune to the anti-tank guided missiles which were then coming into use, the Arab-Israeli war of 1967 demonstrated that heavy armour was still worth having.
The Ley land L.60 diesel ‘multi-fuel engine’ which was adopted at the time for the British Chieftain; another was the Hispano-Suiza HS 110 diesel which was adopted for the French AMX-30. Engines such as these were developed in keeping with the policy adopted by NATO in 1957 that fighting vehicles should be powered by multi-fuel engines which, in practice, meant diesels adapted to run on fuels ranging from light diesel oil to aviation gas turbine gasoline (AVTAG or JP 4) and 80 octane gasoline.
Although they were capable of running on different fuels, the operation of diesels on some of them has had its problems. This has been particularly true of their operation on high octane gasoline, combustion of which is difficult to achieve in compression-ignition engines without undue delay. The difficulty of burning even 80 octane combat gasoline is indicated by the fact that its cetane number, the measure of the suitability of fuels for diesel engines, is only of the order of 15 to 20, whereas that of a typical diesel fuel is 47.
Leyland L.60 opposed-piston two-stroke diesel of the British Chieftain tank. (Leyland Motors)
To achieve satisfactory combustion of gasoline ignition delay needs to be reduced, which requires the temperature of the air in the cylinder after compression to be as high as possible. This requirement is well met by two-stroke opposed-piston diesels in which combustion takes place between pairs of hot piston crowns. As a result, investigations began in Britain around 1952 into the running of diesels on gasoline as well as other fuels led the Fighting Vehicles Research and Development Establishment to opt in 1958 for a family of 6-cylinder, opposed-piston two-stroke engines, one of which became the Leyland L.60. The decision was backed by about two years’ running on a range of fuels of an existing, opposed-piston, two-stroke truck diesel, the Rootes TS-3. However, the mechanical layout of the L.60 did not follow that of the TS-3, in which the pistons were connected by rocker levers to a single crankshaft. Instead, it followed the design of the Junkers Jumo diesel produced in Germany during the 1930s for aircraft, which had two crankshafts geared together and which had long enjoyed a reputation for a high thermal efficiency. Unfortunately, this type of engine also suffered from thermal stress problems of its complicated cylinder liners and from the high thermal loading of its pistons. In consequence, the L.60 was afflicted for many years with piston and cylinder liner as well as other problems, which adversely affected its reliability and prevented it for some time from attaining its intended output of 700hp. In fact, the choice of an opposed-piston engine proved to be unwise, in spite of its merits from the point of view of running on gasoline. Other, more conventional types of diesels could also be run on a range of fuels and they were less troublesome.
It remained in front-line British service until 1996. The Chieftain weighed some 121,000 pounds, had a four-man crew, a 750-hp engine, a speed of 30 mph, and mounted a 120mm main gun and three machine guns. Armor protection is classified. The Chieftain went through 12 different marks, along with additional submarks. Modifications included a new power train and engine, improved fire-control systems, and NBC packs. The Chieftain mounted an infrared/white light searchlight that remained online with the gun for target illumination. Later models utilized laser ranging as well as the Thermal Observation and Gunnery Sight, also on the Challenger I MBT.
