[Vespers War]

One interesting concept that was trialed but never entered service was the US Army's siliceous cored armor. It was a sandwich with two layers of steel around a layer of fused silica. Now-declassified reports state that it replaced an inch of steel in the original armor layout with four inches of silica. A four inch thick plate of silica was the same weight as a one inch plate of steel, so the only weight growth was because thicker armor meant slightly larger plates were needed. Against kinetic rounds, the silica was roughly equal to steel on a mass basis, so the four inches of silica were equal to the one inch of steel they replaced. Conversely, silica was about 50% more effective against chemical warheads (HE/HEAT) on a thickness basis, so the four inches of silica were equivalent to replacing one inch of steel with six inches.

The siliceous cored armor was trialed on the T95 medium tank of 1955 and the XM60 prototypes. The XM60's glacis and turret front were found to be completely immune to 120mm HEAT rounds.

However, there were downsides. The armor was expensive and difficult to produce, although that likely could have been improved over time - experimental armors are nearly always expensive and difficult to produce at first. More concerningly, an AP round that struck the armor but didn't penetrate could still pulverize the silica core, leaving the tank vulnerable to follow-up shots from either AP or HE/HEAT rounds. This did not hold for HE or HEAT rounds, where the silica's "elastic rebound" meant that armor would expand back into the hole made by the explosive (this would also happen as the HEAT jet was penetrating, which was the source of its ability to smother HEAT attacks). Unless perfectly aligned to fire down the same path as the first shell, a second chemical warhead hitting the armor would have no penetration advantage.

The easiest way to simulate an M60 with this armor is probably just to add Cp to the armor figures for HF and TF, and add some weight to account for the increased volume of armor. I haven't seen any estimates of how much weight the armor added, but 0.5 tonnes seems reasonable. This ignores the pulverization issue and simplifies the effect against HE/HEAT, but has the upside of being easy to implement.