Chobham Armour - Development and Application

Development and Application

The concept of ceramic armour goes back to 1918, when Major Neville Monroe Hopkins discovered that a plate of ballistic steel was much more resistant to penetration if covered with a thin (1–2 millimetres) layer of enamel.

Since the early sixties there were, in the US, extensive research programmes ongoing aimed at investigating the prospects of employing composite ceramic materials as vehicle armour. This research mainly focused on the use of an aluminium metal matrix composite reinforced by silicon carbide whiskers, to be produced in the form of large sheets. The reinforced light metal sheets were to be sandwiched between steel layers. This arrangement had the advantage of having a good multiple-hit capability and of being able to be curved, allowing the main armour to benefit from a sloped armour effect. However, this composite with a high metal content was primarily intended to increase the protection against KE-penetrators for a given armour weight; its performance against shaped charge attack was mediocre and would have to be improved by means of a laminate spaced armour effect, as researched by the Germans within the joint MBT-70 project.

An alternative technology developed in the US was based on the use of glass modules to be inserted into the main armour; although this arrangement offered a better shaped charge protection, its multiple hit capability was poor. A similar system using glass inserts in the main steel armour was from the late fifties researched for the Soviet Obiekt 430 prototype of the T-64; this was later developed into the "Combination-K" type, having a ceramic compound mixed with the silicon oxide inserts, which offered about 50% better protection against both shaped charge and KE-penetrator threats, compared with a steel armour of the same weight. It was, later in several improved forms, incorporated into all subsequent Soviet main battle tank designs. After an initial period of speculation in the West as to its true nature, the characteristics of this type were disclosed when the dissolution of the Soviet Union in 1991 and the introduction of a market system forced the Russian industries to find new customers by highlighting its good qualities; it is today rarely referred to as Chobham armour.

In the United Kingdom another line of ceramic armour development had been started in the early 1960s, meant to improve the existing cast turret configuration of the Chieftain that already offered excellent heavy penetrator protection; the research by a team headed by Gilbert Harvey of the Fighting Vehicles Research and Development Establishment (FVRDE), therefore was strongly oriented at optimising the ceramic composite system for defeating shaped charge attack. The British system consisted of a honeycomb matrix with ceramic tiles backed by ballistic nylon, placed on top of the cast main armour. In July 1973 an American delegation, in search of a new armour type for the XM815 tank prototype, now that the MBT-70 project had failed, visited Chobham Common to be informed about the British system, the development of which had then cost about ₤6,000,000; earlier information had already been divulged to the US in 1965 and 1968. It was very impressed by the excellent shaped-charge protection combined with the penetrator impact damage limitation, inherent to the principle of using tiles. The Ballistic Research Laboratory at the Aberdeen Proving Ground that year initiated the development of a version, named Burlington, adapted to the specific American situation, characterised by a much higher projected tank production run and the use of a thinner rolled steel main armour. The increased threat posed by a new generation of Soviet guided missiles armed with a shaped charge warhead—as demonstrated in the Yom Kippur War of October 1973, when even older-generation missiles caused considerable tank losses on the Israeli side—made Burlington the preferred choice for the armour configuration of the XM1 (the renamed XM815) prototype.

However, on 11 December 1974 a Memorandum of Understanding was signed between the Federal Republic of Germany and the US about the common future production of a main battle tank; this made any application of Chobham armour dependent on the eventual choice for a tank type. Earlier in 1974 the Americans had asked the Germans to redesign the existing Leopard 2 prototypes, considered by them too lightly armoured, and had suggested adoption of Burlington for this purpose, of which type the Germans had already been informed in March 1970; the Germans however in response in 1974 initiated a new armour development programme of their own. Having already designed a system that in their opinion offered satisfactory protection against shaped charges, consisting of multiple-laminate spaced armour with the spaces filled with ceramic polystyrene foam as fitted to the Leopard 1A3, they put a clear emphasis on improving KE-penetrator protection, reworking the system into a perforated metal module armour. A version with added Burlington was considered, including ceramic inserts in the various spaces, but rejected as it would push vehicle weight well over sixty metric tonnes, a weight then seen as prohibitive by both armies. The US Army in the summer of 1974 faced the choice between the German system and their own Burlington, a decision made more difficult by the fact that Burlington offered, compared with steel armour, no weight advantage against KE-penetrators: the total armour system would have a RHA equivalence against them of about 350 mm (compared to about 700 mm against shaped charges). No consensus developing, General Creighton Abrams himself decided the issue in favour of Burlington. Eventually each army procured its own national tank design, the project of a common tank failing in 1976. In February 1978 the first tanks protected by Burlington left the factory when the first of eleven pilot M1 tanks were delivered to the US Army.

Beside these state projects, private enterprise in the US during the seventies also developed ceramic armour types, like the Noroc armour made by the Protective Products Division of the Norton Company, consisting of boron carbide sheets backed by resin-bonded glass cloth.

In the United Kingdom application of Chobham armour was delayed by the failure of several advanced tank projects: first that of a joint German-British main battle tank; then the purely British MBT-80 programme. A first directive to prepare Chobham armour technology for application in 1975 was already given in 1969. It was determined by a study of a possible Chobham-armour protected MICV that a completely new design using only Chobham armour for the most vulnerable front and side sectors (thus without an underlying steel main armour) could be 10% lighter for the same level of protection against KE-ammunition, but to limit costs it was decided to base the first design on the conventional Chieftain. The prototype, FV 4211 or the "Aluminium Chieftain", was fitted with a welded aluminium add-on armour, in essence a box on the front hull and front and side turret to contain the ceramic modules, of which box the fifty millimetre thick inner wall due to its relative softness could serve as their backing plate. The extra weight of the aluminium was limited to less than two tonnes and it was shown that it was not overly susceptible to cracking, as first feared. Ten test vehicles were ordered but only the original one had been built when the project was cancelled in favour of the more advanced programmes. However, the Iranian government ordered 1,225 vehicles of an upgraded Chieftain type, the Shir-2 (FV 4030/3), using the same technology of adding Chobham armour to the main cast armour, bringing total weight to 62 metric tonnes. When this order was cancelled in February 1979 because of the Iranian Revolution, the British government, under pressure to modernise its tank fleet to maintain a qualitative superiority relative to the Soviet tank forces, decided to use the sudden surplus production capacity to procure a number of vehicles very close in design to the Shir-2, called the Challenger 1. On 12 April 1983 the first British tank protected by Chobham armour was delivered to the Royal Hussars.

In France from 1966 GIAT Industries performed experiments aimed at developing a light vehicle ceramic armour, in 1970 resulting in the CERALU-system consisting of aluminium-backed alumina weldable to the vehicle, offering a 50% increase in weight-efficiency against ballistic threats compared to steel plate. An improved version was later applied in helicopter seats.

The latest version of Chobham armour is used on the Challenger 2 (called Dorchester armour), and (though the composition most probably differs) the M1 Abrams series of tanks, which according to official sources is currently protected by silicon carbide tiles. Given the publicly stated protection level for the earliest M1: 350 mm steel equivalence against KE-penetrators (APFSDS), it seems to have been equipped with alumina tiles.

Though it is often claimed to be otherwise, the original production model of the Leopard 2 did not use Chobham armour, but a combined spaced armour and perforated armour configuration, cheaper in terms of procurement, maintenance and replacement than a ceramic armour system. For many modern tanks, such the Italian Ariete, it is yet unknown which type is used. There was a general trend in the eighties away from ceramic armour towards perforated armour, but even many tanks from the seventies like the Leopard 1A3 and A4, the French AMX 32 and AMX 40 prototypes used the latter system; the Leclerc has an improved version.