Monday, 18 January 2016

‘Stillbrew’ for Chieftain

Author: Vollketten

Many people have heard of ‘Stillbrew’ armour for the Chieftain and its inclusion as an option within the Armored Warfare game has brought it a little more attention. During the Iran-Iraq War (1980-1988) the Iranian forces who fielded a quantity of Chieftain tanks and faced a variety of mainly Soviet Iraqi tanks suffered a number of losses. The British assessment of these losses concluded that the turret front was no longer sufficiently protected against modern or near-future tank ammunition. The turret had originally been designed to be protective against the 100mm Sovet APDS shell like the Conqueror tank fielded before it. By the 1980’s though this was no longer a sufficient level of protection. Something else was required.

Stillbrew Chieftain in Berlin Brigade service. 

As already mentioned the Chieftain turret was designed to protect against the Soviet 100 APDS rounds (Soviet 3UBM6 APDS round from 1966 can achieve about 190mm) and various numbers have been given for the thickness from different sources or from attempts to measure it. Richard Ogorkiewiecz gives a figure of 195mm for the turret front and some well quoted Russian sources show schematics with 125mm @ 60 degree which is 250mm Effective Armour. (250mm line of sight thickness at that angle)

From Swedish sources (some of which are estimated) from the 1960’s give values for the effective armour being between 183mm and 366mm based on 120 to 150mm @ 30 up to 300mm @ 55 degrees. A German military analysis of the armour of a Chieftain (probably a prototype) yields the turret front as 222mm @ 58 degrees = 262 mm EA.
Recent tests of an example at Bovington tank museum have yielded numbers which WG is using on their latest game model of the tank but more definatively is a recently uncovered document from the UK Ministry of Defence on the subject about the vulnerability of the turret front which says the following:

Chieftain turret is vulnerable over all angles across the front from:
Soviet 115mm APFSDS Tungsten round (T-62)
Soviet 125mm APFSDS Steel round (T-64/T-72)
Under certain circumstances or at point blank range it is also borderline vulnerable to Soviet 115mm APFSDS Steel round (T-62)

Iranian Chieftain hit by at least two shells with at least one penetration.

Not from that report but some estimates for the performance of those and some other Soviet rounds are:
Soviet 115mm APFSDS Tungsten round (T-62) --- 384mm
Soviet 125mm APFSDS Steel round (T-64/T-72) --- 384mm
Soviet 115mm APFSDS Steel round (T-62) --- 228mm (3UBM5) to 290mm (3VBM5)
Soviet 125mm APFSDS Tungsten --- 450 to 480mm
Soviet 125mm APFSDS DU --- 568mm


These estimates would back up that vulnerability assessment of the turret front.

The Chieftain replacement programme was cancelled in the 1980’s and the in-service life of the tank expected to continue past 1995 so it was felt that Chieftains would face KE (Kinetic Energy - APFSDS basically) threats of 360 to 480mm performance at zero range. The proposal to improve the turret front armour must therefore have 360mm EA as the absolute minimum line of sight armor thickness. Again, this assessment is very much in line for the estimated performance of those Soviet rounds.

This improvement would increase the turret armour path weight by about 50% and would protect against T-62 weapon systems and the 125mm APFSDS Steel round from T-64 and T-72, although even at 360mm effective armour it was felt that its utility would be limited by changes in Soviet policy. This was because the estimated performance of the new 125mm APFSDS Tungsten round was 480+mm RHA; a figure borne out by Russian figures later for that shell (see above). The report concluded that “Protection of Chieftain turret against this level of attack would require an increase in armour path weight of about 130%”

The assessment was clear that over the frontal arc each side of the turret was “maximum and minimum line of sight thicknesses of 360mm and 232mm on the right hand side and 360mm and 228mm on the left hand side” [bear in mind the report says that cast armour has to be 10% thicker to provide an equivalent RHA protection - RHAe (Rolled Homogenous Armour Equivalent)] It concluded that any applique added to the turret would have to add between 36mm and 150mm RHAe to provide the required protection. A 130% path weight increase would have to add about 300mm RHAe.

Armouring options for the Chieftain:

Armour option 1: 
Turret casting 232mm (RHAe 211mm)
Rubber Interlayer 20mm (RHAe 0mm)
Steel Applique RHA 150 mm
= Total thickness 402mm, RHAe 361mm

Armour option 2: 
Turret casting 232mm (RHAe 211mm)
Rubber Interlayer 20mm (RHAe 0mm)
Applique Cast Steel 167mm, (RHAe 150mm) [given that examples of Stillbrew have casting numbers it would appear the cast version was the one produced]
= Total thickness 419mm, RHAe 361mm

For the World of Tanks game WG does not factor in armour quality or hardness but just thickness. Therefore for WoT the Chieftain turret should be considered as a minimum of 232 mm thick LoS. WoT isn’t going to be getting Stillbrew armour for the Chieftain but the Armored Warfare game already has it.
Cross section from the patent that was used (British Patent 2191278)

The pieces are self-explanatory here with 4 as the RHA block, 5 as the interlayer and 6 as the bolt. The bolts are made from stainless steel and tighten down that outer armoured section to the vehicle compressing the rubber interlayer between the two layers of armour. The outer block is thick enough that even medium caliber rounds (up to 20mm) will be ineffective in damaging it (unlike modern NERA). On impact though from HEAT or KE shells the bolts will shear off and the rubber will expand pushing the outer block forwards towards the incoming shell. There are 4 interlayers shown, but looking at real examples shows 6 layers. This is because multiple thinner layers actually increase the effectiveness as the expansion wave effect in the rubber moves faster reaching the surface more quickly. That interlayer will also act as cushioning in the same manner, rendering HE and HESH type rounds far less useful, too.
So as the outer plate is hit, the bolts shear off, the rubber sheets expands and pushes the outer block away from the vehicle. Due to the angle of movement this not only disrupts the path of the jet or penetrator but in simple terms, provides continuously new armour material to penetrate because the initial hole is always moving sideways to the penetrator, substantially increasing the effectiveness of the armour.

Bare Stillbrew on a range showing sheared bolts and slab and rubber interlayers. 

Fitment of the rubber and armoured blocks on the turret.

Once bolted on they look like this and blend nicely to the turret lines. There are also some extra tensioning screw visible, fitted within the bolt. You don’t normally see these bolt heads as the Stillbrew is supposed to be covered with sheet steel welded on top. This prevented water and NBC agents getting in where they’d cause problems with decontamination and corrosion. This outer layer was held off the main Stillbrew applique blocks at a set distance by small rubber blocks along the bottom half of the blocks where they are thinner.

Rubber spacer

Without NERA effects:
Using this information we can see that the 50% path option, without any NERA effect added in, provided immunity against the Soviet 115mm Tungsten APFSDS 3UBM5 and 3VBM5 rounds as required by the original plan. On the other hand, it would not have achieved the required protection against the 125mm Tungsten APFSDS round, thus the 130% path option would have to have been used.

With NERA effects:
However, adding a rather conservative value of between 1.3 and 2.5 for the effectiveness of NERA against KE penetrators, the 50% path option is the absolute minimum for protection against the 125mm Tungsten APFSDS rounds from the T-72. It would not provide any sufficient protection against the DU round. The 130% path option however would have to be used to achieve that level of protection.

So, having looked at the available data rather conservatively as best I can and in the absence of some actual test data, they obviously have to be taken with a pinch of salt. That being said, the British Stillbrew study gives us an interesting insight into a novel solution to the problem of these new powerful Soviet shells.
The original requirement was protection against the Tungsten 125mm APFSDS round and it appears that the British adopted the 50% path increase option for Stillbrew and that the armour performs at or slightly above the 2.5 ratio for effectiveness of this NERA. ‘Stillbrew’ therefore offered sufficient protection against the Soviet 125mm APFSDS Tungsten shells.
An additional increase of 30% was technically possible which could have raised the level of protection to the Soviet 125mm AFSDS DU rounds.

Certainly then for what was a short-term solution Stillbrew armour shows us how protected the hull down Chieftain was and extended the life of the Chieftain in British service until the Challenger 1 could be put into service.

And finally, if we wanted to improve Stillbrew even more we could just add ERA panels to it but that would just be overkill for AW …… or wouldn't it?

Chieftain - Rob Griffen
Feasibility Study of Improving Chieftain turret protection- UK MOD
Chieftain - Simon Dunstan
Various Swedish documents from the 1960’s from Renhanxue/Sp15
German Army Chieftain Armour analysis circa 1960
Richard Ogorkiewicz, Cold War, Hot Science (2002)
Chieftain and Leopard Parts 1 and 2 - AFV Profile Series
Jane’s Armour and Artillery 1985
and various ex-forces sites discussing service and upgrades and modifications done to Chieftains.
Soldat und Technik 1/90

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