HCR plastic armour: Technically, the first composite armour?

Composite armour is no stranger to anyone who knows modern tanks, or tanks at all. It's usually told that the T95 experimental medium tank was the first to feature it. However, reportedly in August 1943, with the headaches of shaped charges, namely the Panzerfaust and Panzershreck anti-tank weaponry that were introduced in 1942 and 1943 respectively.

At the time, the Panzerschreck produced, in German and Finnish tests, around 230 mm of penetration against standard RHA steel at 90 degrees, which lowered to 160 mm at 60 degrees, then 95 mm at 30 degrees. This was impressive, even for around 1 or 2 years after World War 2, where HEAT weaponry was beginning to be mastered by all nations. In Finnish tests, FHA steel was used, where at 30 degrees, the Panzershreck penetrated 100 mm of FHA.

For comparison, the M47 Patton had around ~180 mm of CHA (LOS thickness), dead-centre on the hull which was angled at around 56 to 60 degrees. 

In post-war US tests of captured enemy weaponry, the Panzerschreck was put up against FHA, where it penetrated 216 mm of FHA steel that was angled at 90 degrees.

This is where the issue of shaped charges is introduced. The M4A1 Sherman has only around 51 mm of CHA steel on the front angled at 37 to 55 degrees and only 38 mm of CHA on the sides that are unangled. The late-production M4A2 and mid-production M4A3 have marginally better side, rear, top and bottom protection (despite the armour thickness being unchanged) thanks to the incorporation of welds that made it a must for RHA plates to be used. The front protection however, cannot be considered much but it also cannot be considered as a slight improvement. 

The M4A2 and M4A3 both have 63 mm of RHA angled at 47 degrees on the front. This equates to around 85 - 90 mm of RHA steel (LOS), but it is still not enough to protect against shaped charges from the front, worse still from the sides or rear.

The 76 mm versions of the Sherman did not receive better protection too, so most of the job was up to the crews themselves. Shermans can actually be seen in photos often with welded appliqué armour, namely sandbags (which worked somewhat well against shaped charges) or welded track segments.

M4A1 (76)W with sandbags as armour

Above is a production M4 tank of the 8th Armored Brigade in Remagen on March 4th, 1945. The picture has quite a lot of history to it but it is unrelated.

However, all these upgrades were made on the field and with whatever materiel was available. Such upgrades were not actually always available to inexperienced crews or with tanks at areas with busy mechanics that could not help them apply the armour. Even worse for the crews that used tracks was that, historically, track armour often does not help to defend against shaped charges. It is often recited that, in around 1943, Wehrmacht tank crews were notified that the track armour they used on their tanks would have an adverse effect on the protection on their tanks as the tracks actually helped the shaped charges focus the penetrator jet stream better, however it was never removed due to the psychological effect and fear factor that would have risen had the tracks been removed. It made them feel "safer".

Again, in 1943, the iniative took place. The one that stands out and appeals as more an interesting topic to discuss is the HCR plastic armour designed by Flintkote. One thing to note however, the HCR plastic armour series' most likely takes place later in the war, namely 1944 - 1945.

According to the above image, the HCR armour was actually a separate module from the mounting; "HCR" is the mixture.

There are two variants of the HCR plastic armour, HCR1 and HCR2.

HCR1 contained:
50% aluminium fillings
40% asphalt (occasionally supplemented by pitch)
10% wood flour

Meanwhile, HCR2 used: 
80% gravel
5% wood flour
15% asphalt (occasionally supplemented by pitch)

Unfortunately, only the HCR2 composition's impacts on shaped charges were available. HCR1 was never actually recorded for any effects on shaped charges. 

HCR2 (and most likely HCR1 too) followed an arrangement of 25.4 mm (1 inch) of aluminium followed by 254 mm (10 inches) of HCR2 filling. The armour boxes were held over the sponsons and turret with 12.7 mm (0.5 inches) of RHA.

The HCR2 plastic armour kit increased the weight of an M4A3 of around 8 - 12 (for the Panzerfaust 100) tonnes. 39 (39.0089) kg

The designs later underwent tests in the start of the fall (September to December) of 1945 at Aberdeen Proving Ground. The image above was taken on September of 1945. Tests of the armour were done, both with the kit having been spaced from the main armour panels and on top of the Sherman's armour. Reportedly, tests were also done on M26 Pershing tanks but this was never revealed.

The Panzerfaust 100 and Panzerschreck were successfully defeated in tests, although there were some minor perforations in the armour. It could also somewhat impact the penetration of a 76.2 mm APCR (HVAP) round, though it did not provide as much protection as conventional steel armour.

For the M26 Pershing, the tests and details were never confirmed, however, it only needed 7.1 tonnes of the HCR2 composition and had a front plate of 38.1 mm (1.5 inches) and a backing plate of 50.8 mm (2 inches), all made of aluminium (reportedly 21ST).

Below is a collection of images detailing all photos taken of the M4A3 at Aberdeen Proving Ground (not including images shown on the page above).

However, there also exists a photo of another M4A2/M4A3 Sherman, this time with the HCR composites, however only on the sides of the hull. The front and turret are fitted with tracks as armour:

At the end, it was found in other tests that, when they began to use steel spikes that were 25.4 mm (1 inch) in diametre with three configurations for spike length, namely 190.5 mm (7.5 inches), 203.2 mm (8 inches) and 215.9 mm (8.5 inches). How the spikes worked were that, upon contact, the spikes would immediately begin to perforate the hollow shaped charge and disrupt the forming of the penetrator, thus nearly destroying the entire shaped charge, nulifying its effects or significantly reducing penetration.

This kit wighed only 4.1 tonnes, nearly half the weight of the HCR plastic armour kit. Both developments later extended into the postwar era, though their fate is a mystery.

Sources/links: T95 medium tank, German and FInnish tests of Panzerschreck, M47 Patton, US tests of captured enemy weaponry (.pdf file, page 357), M4A1 Sherman, late-production M4A2, mid-production M4A3, appliqué armour, Aberdeen Proving Ground, Wikipedia (Beware! Wikipedia is known to be notoriously unreliable as a source and I have only used it for the M26 Pershing and not as fact but more as hearsay)