Source: Yuri Pasholok’s blog Hello everyone, Yuri Pasholok posted on his blog some quite interesting pictures. Let’s have a look at them. Here, we can see the turret schematics for the “big” IS turret – this turret was used by two tanks: specifically the KV-1S, producing the KV-85 tank (Object 239) and the IS itself. These schematics came to be as a part of the tests, that were held in order to determine, whether arming this turret with the D-5 85mm gun was viable, when it came to ergonomy. The dimensions were calculated in order to see, whether the gunner and loader would be able to operate such a gun. The distances are obviously in milimeters. The schematic in the lower part of the picture show a side-cut of the turret. That big rectangle in the middle is the breech, with the small “objects” below it being the gunner’s seats. Above the breech you can see the “PT-4 aiming device” in aiming position. On the picture above, the breech is the big rectangular block in the middle – you can see that there is enough space for it. The rear of the turret is occupied by a rear ammo rack. The smaller circle is the position of the tank commander with commander’s copula and the two even smaller circles on each side of the breech are loader and gunner positions (gunner is on the left, loader on the right). As you can see, this schematic is completely viable and worked in real life too, producing the KV-85 variant and the first IS model. However, same “space” tests were conducted for arming the Object 238 (early KV-1S model attempt to arm KV-1S with a 85mm gun from July 1943, designated KV-1G). As you can see, that didn’t go very well, as the original KV turret is simply too small for the 85mm S-31 gun: In the end, the Object 238 never entered service, but Object 239 did, with 148 being built.

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