Stillbrew armour

During the 1973 Arab-Israeli War, the Israeli Defence Forces (IDF) lost many tanks, mainly to hollow or shaped charge projectiles. Among the losses were British-supplied Centurions. The Royal Armoured Corps (RAC), through organisations such as the Military Vehicles and Engineering Establishment (MVEE) and the Royal Armament Research and Development Establishment (RARDE), began conducting research into how to improve the effectiveness of the armour used in its armoured fighting vehicles (AFVs).[4]

With the outbreak of the Iran-Iraq War in 1980, research accelerated after many Iranian Chieftains were destroyed or severely disabled by Iraqi T-62s, and forensic examination of captured Iranian Chieftains supplied by Iraq was undertaken.[4] The British assessment of these vehicles concluded that the front of Chieftain's hull and turret, originally designed to provide protection against all types of ammunition from the Soviet 100mm main gun fitted to the older Soviet T-54/T-55 tanks, protected inadequately against armour-piercing fin-stabilized discarding sabot (APFSDS) and high-explosive anti-tank (HEAT) ammunition from the newer T-62's larger 115mm main gun, nor other similar current or near-future tank ammunition. Consequently, a program to up-armour the Chieftain commenced at the MVEE, which resulted in the Stillbrew armour package.[4]

Chieftain Mark 11C at The Tank Museum, Bovington. Stillbrew armour is visible on the turret front.

Stillbrew armour was a passive (i.e., non-explosive, non-reactive) appliqué armour designed to provide added protection against HEAT ammunition and hollow and shaped charge weapons such as RPG-7 rockets and AT-3 Sagger anti-tank guided missiles, by deflecting the explosive jet, and increased protection against ballistic and kinetic projectiles such as APFSDS ammunition by increased armour thickness.[2][4]

The armour was designed as modular blocks consisting of six layers of steel plate spaced with thick rubber, which were added to the front of the turret and on the hull around the driver's position,[1] which served to increase protection to the turret ring. The blocks were fastened to the vehicle by stainless steel bolts, and a final layer of sheet steel was welded over the block covering the bolt heads and providing a flush surface; this also provided protection from water and chemical, biological, radiological, and nuclear (CBRN) agents from getting under the blocks and causing corrosion and contamination of the vehicle's surfaces.[2]

When struck by a shaped charge, one or more layers of the block would shift or shear away, effectively providing a new surface the explosive jet had to penetrate, thus further deflecting or depleting the jet with every successive layer it encountered. The layers and the spacing was also found to be effective in countering high-explosive squash head (HESH) ammunition.[5] When struck by a kinetic projectile, the armour blocks effectively increased the vehicle's armour path by a minimum of 50% (this figure was more than just the added thickness of the armour block; when struck by a ballistic penetrator, the kinetic energy would cause the layers, particularly the rubber layers, to expand), minimising or removing the chances of penetration of the vehicle's hull or turret.[2] Also, it was discovered that armour blocks were of sufficient strength to render medium-calibre projectile (up to 25mm conventional or 20mm tungsten armour-piercing ammunition (AP) 100% ineffective and did not damage the blocks.[5]

While Stillbrew armour was later found to also be effective against HEAT rounds from the 125mm main gun of a T-72, it was insufficient to stop tungsten and depleted uranium APFSDS rounds from this weapon, and an added minimum increase of 30% effectiveness of the original design would be required. However, further development was cancelled due to the emergence of the superior Chobham armour.[2][5]

Testing of the prototype armour began in 1984, with firing and mobility trials being conducted until 1985. The design was deemed successful with no changes needed. Production of the armour began in 1985 at the Royal Ordnance Factory Leeds (ROF Leeds) and fitting began in 1986. Vehicles located in the UK were fitted with the armour package by the Royal Electrical and Mechanical Engineers (REME) in base workshops across the country, while those vehicles deployed to West Germany were fitted at 23 REME Base Workshop in Wetter in the Ruhr.[4]

The armour was first fitted to the Mark 10 Chieftain in 1986, and was also later fitted to the Marks 11 and 12. It was also due to be fitted to the Mark 13, which was to be the final model Chieftain, but this variant was cancelled with the introduction of the Challenger 1.[6]

Stillbrew armour was rendered obsolete by the introduction of the more effective Chobham armour, first fitted to the experimental Chieftain 800 and 900 (and which features on the Challenger 1).[2][4]

No other armoured fighting vehicles nor Chieftains sold to foreign armies (including exports of the Mark 10A), were fitted with Stillbrew armour.[6]

• World Register of Surviving Historic Armoured Vehicles - FV4201 Chieftain Tank