Wednesday 27 May 2020

Object 704

"To GBTU TU chief, Engineer-Colonel comrade Blagonravov

I report that the Kirov factory design bureau is completing the development of an SPG on the chassis of the Kirovets-1 tank.

The hull is characterized by the following:
  • Lower front plate: 120 mm at 55 degrees
  • Cheek plates: 120 mm at 50 degrees
  • Upper side plates: 90 mm at 45 degrees
  • Upper rear plate: 60 mm at 40 degrees
  • Lower side plates: 75  mm at 60 degrees
  • Vertical side plates: 90 mm
  • Floor and roof: 20 mm

The overall height of the SPG is 140 mm lower than the SPG on the IS chassis.

To allow for normal working conditions, the driver's compartment is moved out of the front of the vehicle and to the left front part of the fighting compartment, raised by 600-700 mm, which will be supported by a floor for the driver that will hold the levers, pedals, and the driver's seat.

The driver has two positions:
  • Battle: the driver will look through a MK-4 mirror periscope.
  • Driving: the driver will look out of the hatch installed in the roof.
The seat will have two positions to allow this.

Due to the driver's station being moved to a new place, the control linkages will have an additional stage to connect the levers and pedals on the raised floor and the linkages on the floor of the vehicle.

The transmission, engine compartment, and running gear are the same as the Kirovets-1 blueprints.

Production of the first prototype of the Kirovets-1 hull began at the factory on February 1st.

Representative of the GBTU, Engineer-Lieutenant Colonel Markin.
February 10th, 1945"


  1. This was for the IS-3 version of the ISU-152?

    I can understand some of the practical difficulties, but why did the ISU-152/122 insist on retaining 5 crewmen when 4 were adequate for the IS tanks? I also wonder why it was so difficult to make an ISU with a one-piece sloped front angled at say 50 + degrees when it was doable for the T-34 (SU-100, etc) even if you kept the armor thickness at 90 mm.

    The biggest problem if the ISUs was that their armor wasn't that great.

    1. Yes, this is the vehicle. The IS-2 didn't need a breech operator, so it could get away with 4.

    2. There's also the fact the 152 mm shells were chonky as Hell - 40+ kg for the shell itself (the AP was almost 50!) and IIRC something like 30+ for the separate propellant charge. You definitely wanted more than one lad wrangling those things if at all possible.

    3. I'll admit fatigue is an issue, but so is a relatively weak front plate that was an increasingly easy target for most countries' AT weaponry at least medium ranges and even long ranges as the ISUs continued in service well past 1945.

      If one could even make the front composed of single upper plate, even keeping the original 90 mm thickness at say, 55 degrees, similar to the SU-100, then you make the ISUs fairly resistant to long-range fire at least. They didn't need to be resistant to close-range AT fire as its tactical employment was at medium ranges and longer.

      I'm also curious why some of the high-powered AT gun ISU-projects were not done. The long 152s I can see, but there were plans for an ISU-130 and that gun would be more potent but not much bulkier than the 122 mm D-25.

    4. "I'm also curious why some of the high-powered AT gun ISU-projects were not done."

      They were unnecessary. The D-25S and ML-20S were quite good enough against anything they were likely to face, and excessively long guns create problems for vehicle maneuverability in confined spaces and weight distribution.