1. The weight of 55 tons specified in the tactical-technical characteristics combined with the requested armour is impossible to achieve for the following reasons:
- According to the tactical-technical requirements, the SPG will use the transmission and suspension of the KV. The weight of these components is 17,400 kg (including the engine, fuel system and cooling system), and cannot be reduced further.
- The Br-2 system used in the SPG weighs 17,600 kg with the mount and ammunition.
- The machineguns, machinegun ammunition, observation cupola, seats, radio, fuel, crew, tools, etc. weighs 3,000 kg.
- If the mass of the SPG is limited at 55,000 kg, then the mass of the hull and the (immobile) turret will be 17,000 kg.
Due to the size of the system, the following dimensions have to be maintained:
- Length: 7900 mm
- Width: 1920 mm
- Height (from bottom): 2570 mm
Building a hull that weighs 17,000 kg with these dimensions is impossible. Reducing the size is also impossible.
As an example, take the SMK-1 hull that had the following armour: 60 mm (sides, front, turret) 20-30 mm floor and 30 mm roof weighed 31 tons (including turrets). The requirements for the SPG are:
- Front: 75 mm
- Sides: 60 mm
- Turret: 60 mm
- Roof: 30 mm
- Floor: 40-30 mm
It is not possible to design a 55 ton SPG with a turret and hull weighing 17 tons. During the design process, we used the following armour thicknesses:
- Sides: 60 mm
- Front: 60 mm (at 30 degrees)
- Lower front: 50 mm (at 45 degrees)
- Lower rear: 50 mm
- Turret: 60 mm (at 10 degrees)
- Floor: 30 mm front, 20 mm rear
- Roof: 20 mm
Without fuel, ammunition and crew, the weight of this design is 60 tons. The combat weight is 65 tons. At 65 tons, the ground pressure (without sinking) is 0.83 kg/cm^2.
- The speed of aiming has been reduced by 33% vertically and 10% horizontally compared to the Br-2 field gun. The effort required to turn the elevation flywheel is up to 10 kg and the effort required for the horizontal flywheel is up to 8 kg.
- With the current return mechanisms, you cannot fire at an angle of over 30 degrees.
- The horizontal traverse is 4 degrees to each side, same as the field gun.
- The ammunition capacity is 47 shells. There is no loading crane, only a loading tray like on the M-10 system in the KV.
- The SPG fits into the "O" railway dimensions and is approaching the limits of the "O" dimensions.
SKB-2 Chief, Military Engineer 1st Grade, Kotin
SKB-4 Chief, Military Engineer 2nd Grade, Fedorov
Senior Engineer Goldburt"
The mysterious "SPG" referred in this document later obtained a factory index of "212", and is more commonly known as Object 212, despite never carrying that designation.