Thus, Version II’s design was the most suitable for the OBM-50 project. The breech end with a sliding wedge breechblock stipulated in the specifications for the gun was favored. However, a requirement to increase the spacing between the barrel gibs or modify them was expressed, because the existing design could result in excessive pitching or vibration. The designers were also required to simplify the breechblock because it required a great deal of machining, and reduce its width to 420–450 mm. They were also required to simplify the breech mechanism, which needed to be equipped with a former-type semiautomatic system.
Modifying the gun was not OKB-172’s only task. The last SU-152’s that the gun was designed for were rolling off the assembly line as the finding on the OBM-50 at the Chelyabinsk Kirov Factory was being signed. Therefore, the SP gun needed to be changed.
1. Instruct OKB-172 to submit to the Artillery Committee an engineering design of a 122 mm heavy self-propelled gun that incorporates all of the remarks made in the “Technical Analysis” and “Conclusions” sections.
2. Make provision for mounting the gun in the IS-152 vehicle. It is hereby requested that the People’s Commissariat of the Tank Industry (NKTP) assign the design for mounting the gun in the vehicle to NKTP Factory No. 100.
3. In developing the engineering design, OKB-172 is to coordinate the positioning coordinates and overall dimensions of the gun with Factory No. 100.
4. It is hereby requested that the People’s Commissariat of Arms manufacture the heavy 122 mm gun at Factory No. 172 during the first 10 days of January 1944.
5. It is hereby requested that the NKTP manufacture the high-power 122 mm gun and mount it on a vehicle by January 25, 1944.{14}
The engineering drawings for the modified system were reviewed on December 4, and OKB-172 submitted the design documentation for mounting the OBM-50 on the ISU-152 by the 14th. In addition to the heavy 122 mm gun, the project also included mounting the OBM-43 152 mm system on the ISU-152. The self-propelled version of that gun was assigned the designation OBM-53.
And what was Factory No. 172’s design bureau doing at the time? According to documentation, the conceptual design of the heavy 122 mm gun designated the M21 was finished by October 9, 1943. But something strange happened: according to the same documentation, the chairman of GAU’s Artillery Committee received the conceptual design from the Technical Council of the People’s Commissariat of Arms only on November 12; that is, on the same day the decision on the OBM-50 was reached. Why the documentation sat, going nowhere, for over a month is unclear.
Work on the M21 was led by Factory No. 172’s artillery design bureau chief V. A. Ilyin (who was also serving as acting chief designer at the same time). The design took the elevation and traversing mechanisms, cradle armor, and sights from the ML-20S. It also incorporated the frame and cradle, with some modifications. The barrel was 7747 mm (63.5 calibers) in length. According to calculations, the M21 could penetrate 208.4 mm of armor from a range of 1000 m, and it could penetrate 169.8 mm at a 30° angle of incidence.
As required by the gun’s specifications, the barrel was equipped with a single-chamber muzzle brake. According to the calculations, it absorbed up to 54% of the energy from a shot. Also in accordance with the specifications, the M21 used a sliding wedge breechblock design copied from the M75 107 mm antitank gun. Minor modifications to some parts were the only differences between the new gun’s breech mechanism and that of the M75. The firing mechanism design was similar to that of the ZIS-3. The cradle differed only in that the recoil mechanism included a counter-rod that provided a shorter recoil length, and the trigger mechanism was modified for use of the sliding wedge breechblock. No changes were planned to the design of the SU-152’s superstructure; only the storage racks needed modifying for the new rounds.
There was no further correspondence after the GAU’s Artillery Committee received the M21 documentation. The delay in reviewing the project in the Technical Council of the People’s Commissariat of Arms took its tolclass="underline" by mid-November, there was no longer a need for another heavy gun, especially since the design would have to be modified for mounting on the ISU-152.
CHAPTER 9.
Exposed to the Elements
The idea of using a KV tank chassis as the base for an open heavy artillery system was first expressed in the fall of 1942. Eng. Col. Afonin, chief of the GABTU’s Armor Directorate, received a letter from Factory No. 92’s director, A. S. Yelyan, on November 26:
Comrade P. F. Muravyev, our senior design engineer, spoke with you while he was in Moscow about developing a 400 mm self-propelled gun based on the KV tank. We have now developed the idea to the point that we can discuss it in more detail.
We have developed a conceptual design of the system, which we will send you after verifying some information on the KV tank, and the T-34, which is completely unknown to us, but about which we hereby request information. We are particularly interested in the following data:
1. The gross weight of the KV-1 tank produced in 1941 (before the war with Germany) and the weight of its turret with the basic load for the 76.2 mm gun and the gun itself.
2. The gross weight of the KV-1 tank manufactured in 1942 whose weight was increased as a result of an upgrade, and the weight of its turret with the basic load for the 76.2 mm gun and the gun itself.
3. The gross weight of the KV-1S tank produced in 1942 after its weight was reduced, and the weight of its turret with the basic load for the 76.2 mm gun and the gun itself. Whether the overall length and width, the area of track in contact with the ground, the engine power, the engine and driver location, and the strength of the suspension and hull components were modified in order to reduce the tank’s weight. Whether the suspension and hull could withstand a load from firing of 100 tonnes acting as shown in the attached diagram, and how the tank’s engine and running gear would perform on the road if the weight of our new system, 20 tonnes, is added to its total weight (less the turret, gun, and basic load).
4. We are also working to mount the system on the T-34 tank. Therefore, we also request that you inform us how all running gear components, the hull, and the engine will perform on the road and under the loads shown in the attached diagram, because Factory No. 112 was unable to give us complete answers to these questions, citing their lack of strength calculations for the T-34 tank.
5. We are most interested in the KV-1S tank for installation of our system. According to information in our possession, its gross weight is 37 tonnes. If the turret, the gun, and the gun’s basic load weigh 7 tonnes, the entire vehicle will weigh 50 tonnes, i.e., the weight of the KV-1 tank manufactured in 1941 (before the war with Germany). That means that if the KV-1S’s engine, track, and suspension durability are the same as those of the KV-1 manufactured in 1941, the new vehicle’s road performance will be quite satisfactory. If these assumptions about the new KV-1S tank are reasonable, we request that an order be sent to Factory No. 92 for a set of drawings of the KV-1S hull and suspension and that Factory No. 92 be informed of the rationale regarding opportunities and ways of obtaining this tank for purposes of mounting a prototype of the system.