Soviet SPG development ceased in the late 1930s, but started up again in early 1940. Initially, the focus was on bunker busters, but projects with other purposes started up in mid-1940. Among them were tank destroyers on the T-34 medium tank chassis. The main feature of these vehicles was a rotating turret.
Medium Tank Destroyer
The need for a medium tank destroyer was raised in June of 1940. During the discussion of armament of Soviet tanks and SPGs, the 85 mm mod. 1939 AA gun (52-K) was mentioned for the first time. According to calculations, an AP shell fired from such a gun would penetrate 88 mm of sloped armour at a range of one kilometer, but with the caveat that an armour piercing shell for this gun did not exist yet.
According to a summary of experimental tank, SPG, and towed guns, the 85 mm gun was meant for the KV tank with a small turret (KV-1). This vehicle appears as #3 in the document dated June 21st, 1940. #7 is even more interesting: "85 mm gun on the T-34 chassis" (tank destroyer). This SPG would weigh 26 tons and have a maximum speed of 40 kph. Its gun would be able to rotate 360 degrees.
The 85 mm gun, indexed F-30, was developed at factory #92 under the supervision of V.G. Grabin. In September of 1940, the gun was tested in a T-28 tank, where it showed satisfactory results. However, it did not fit into the KV-1 turret, so the Kirov factory began working on an enlarged turret that was later installed on the T-220 (KV-220) tank. As for the tank destroyer, it was assigned to factory #8 in Kaliningrad (modern day Korolev). Factory #183 sent two tanks without turrets to serve as the foundation for prototypes, but work did not progress past this point.
The idea of a tank destroyer on the T-34 chassis was revisited in spring of 1941. Based on intelligence reports of new German heavy tanks, work on Soviet heavy tanks and tank destroyers resumed. On May 21st, 1941, requirements for a new tank destroyer were approved. In this case, it was not exactly a tank destroyer on the T-34 chassis, but on the chassis of the A-42, an artillery tractor based on the T-34. The gun, covered by a shield, would be placed in the rear of the tank. The design resembled the semi-armoured SdKfz 8/1 tank destroyer built on the SdKfz 8 halftrack.
The same armament was planned for installation on the Voroshilovets heavy tractor, but the idea was rejected. The tank destroyer, indexed A-46, would be developed at factory #183, with a prototype due by October 1st, 1941. Factory #8 would develop the gun, and production would be organized at Kuybishev factory in Kolomna. 1500 such vehicles were planned for 1942. However, the project of a tank destroyer on the A-42 chassis died for a simple reason: work on the A-42 did not progress past a prototype.
Lastly, a fun fact about TDs on the T-34 chassis. On June 17th, 1941, tactical-technical requirements for the SU-34 were finalized. Its mass would be 23.5-24 tons, achieved by reducing the armour to 25-30 mm. An analogous vehicle on the T-34M (A-43) chassis was also described. Its mass would be 19-20 tons.
The requirements stated that a turret was optional, but a roof to protect the crew against strafing planes was necessary. The horizontal range must be at least 15 degrees in each direction. Due to a large amount of workload, work on these vehicles did not progress past the requirements stage.
In the fall of 1941, a large amount of organizations and design bureaus were evacuated eastward. Factory #183 ended up in Nizhniy Tagil, and Kuybishev factory ended up in Kirov to form factory #38. At first the factory assembled T-30, T-60, and T-70 tanks, but by fall of 1942, it was the leading development center for light SPGs. As for factory #8, it was evacuated to Sverdlovsk. F.F. Petrov headed its design bureau at the new location. In February of 1942, artillery production from UZTM was transferred here. In March, the design bureau presented its first design, the ZiK-1. This project was directly connected to factory #8 pre-war designs.
A thematic plan for SPGs was developed in early December of 1941. Among the planned vehicles was an 85 mm tank destroyer on the T-34 chassis, to be developed by factory #8. As factory #8 was just setting up on its new grounds, the development was transferred to UZTM. At this time, Uralmash was known as Izhor factory, as the factory with that name was also evacuated to Sverdlovsk.
On January 3rd, 1942, a technical meeting was held at Izhor factory to investigate plans to install the 85 mm AA gun on the T-34 chassis. F.F. Petrov was the supervisor of this project. According to requirements, the tank destroyer had to have 360 degree horizontal range, and vertical range from -8 to +30 degrees. The ammunition capacity had to be 30-40 shells. One of the requirements was that the T-34 chassis remain unchanged.
The designers proposed two options. One contained many changes to the gun, using components from the M-30 122 mm howitzer and 85 mm U-10 divisional gun. The second was equipped with an 85 mm mod. 1939 AA gun with minimal changes, but with a much bigger turret. In both cases, the gun crew was increased to 3 men. The commission decided that both projects satisfy the requirements. The UZTM design bureau considered it most reasonable to put the engine in the front of the hull, but since it was forbidden to make any changes to the chassis, the proposed variants were considered acceptable. The commission proposed that one of each variant should be built.
On January 14th, 1942, project documents for both designs were sent to the Main Artillery Directorate (GAU). The version with a more involved conversion of the gun was indexed U-20, the version with less changes of the AA gun was indexed U-20-II. Both projects were slightly altered after approval by the commission in early January.
The first variant had higher priority according to UZTM, and combined all experience of Sverdlovsk engineers from the start of the war. Instead of the AA gun, the U-10 divisional gun, designed at UZTM in October of 1941, was used. Initiated by UZTM, this project consisted of a barrel from the 52-K AA gun on an M-30 howitzer mount. The appearance of the U-10 was very logical: the M-30 was produced at UZTM and AA gun production also started up there. A small batch of these guns was built at the end of 1941, but the process did not continue. Despite such a finale, the U-10 could have been a decent artillery system. It was much more suitable for the new tank destroyer than the AA gun. The divisional gun was much smaller, and thus required a smaller turret.
A new turret was installed on the T-34, keeping the same 1420 mm turret ring. The turret, called a platform in the technical description, had room for three. It had an open top and partially open rear. The total mass of the turret was 3800 kg. The curved front was 45 mm thick, sides were 20 mm thick and rear 12 mm thick. The U-10 gun, muzzle brake removed, had a vertical range from -5 to +27 degrees.
Due to the large gun, the rear of the turret had an opening for the breech to enter during recoil. Due to this, loading the gun was not a simple task. The loader had to grab a shell, enter the turret bustle, and load it. The ammunition was stored in the front of the turret, which allowed for a decent rate of fire. Most of the ammunition, 56 shells in total, was held in the hull, where it was harder for the loader to operate. This is where the assistant loader came in handy.
The second variant, indexed U-20-II, was developed with the idea of using as many existing components and assemblies as possible. The overhang of the barrel was also reduced as much as possible, resulting in an odd design. The 52-K gun was installed with almost no changed, barring the removal of a muzzle brake. As a result, the turret bay was almost two meters long.
In order to compensate for the weight, the front plate was 75 mm thick and sides were 40 mm thick. This design desperately needed its assistant loader, as one a gymnast could load the gun alone. To simplify loading, the ready rack was located inside the turret, same as with the U-20.
The GAU Artillery Committee held off, returning to these designs only in April of 1942. The idea of a turreted SPG was deemed correct, but the condition of the loaders was harshly and justly criticized. The idea of using the U-10 was also criticized, as it would complicate production. The U-20-II was deemed preferable, even though its loaders would have to be acrobats.
Anyway, both U-20 projects were deemed unsatisfactory by the Artillery Committee. This conclusion raises many questions. It would appear that the artillerymen did not know what they wanted. The conclusions had contradictory points. ArtKom itself created the requirements, then called the projects that satisfied them incorrect. This story is far from unique. For example, in the case of the U-19, the client was also horrified with what was shown to him.
Thankfully, just as the 85 mm tank destroyer idea died, another medium SPG was approved. It consisted of a captured StuG with a 122 mm M-30 howitzer installed. This vehicle, indexed 122-SG (later SG-122) was the first mass produced Soviet medium SPG. The concept of a front casemate later became a staple of Soviet wartime medium and heavy SPG designs.
Finally, let me mention one more curious fact. Just as the ArtKom buried the Soviet turreted SPG, the T35 GMC arrived at the Aberdeen proving grounds. Its development began in November of 1941. The concept of the T35 was similar to that of the U-20: a new turret with a converted 76 mm AA gun was installed on the chassis of an M4 medium tank. The improved T35 turned into the M10 GMC, the most numerous American tank destroyer of the war.
The M10 GMC ended up in the Soviet Union in the fall of 1943, where it was given a positive evaluation. However, tankers were evaluating it, since all self propelled artillery was transferred under GBTU's control since the start of 1943. Who knows what the fate of the U-20 would have been if it was initially ordered by the GBTU.