U-20: Soviet tank destroyers with circular firing arc

Translated by Vlad.
The development of tank destroyers, which was about to stop at the end of the ’30s, gained momentum in the beginning of 1940. Firstly, SPG’s were designed which should destroy enemy fortifications, but already in mid-1940, the development branched out in other directions. Among the vehicles were tank destroyers based on the T-34. And the main feature of these became a gun mounted into a rotating turret.
Tank destroyer on a medium basis
It was in June 1940 when the question of a SPG on a medium tank basis came up. In discussions about the armament the proposal of the 85mm anti-aircraft gun mod. 1939 (52-K) came up. Based on calculations, the gun should be able to penetrate up to 88mm of armor at a distance of 1km. With one reservation however – there were no AP shells developed for this gun at that time.
According to the journal on tank, SPG and anti-tank systems pending development, the 85mm gun was first proposed to be installed on the KV tank with the small turret (KV-1). In this document, dated on the 21st July 1940, this system was featured as the third point. Even more interesting however is the point No. 7 of this journal. It is stated as “85mm self-propelled gun on T-34 chassis”. This SPG should weigh 26 tonnes and accelerate up to 40km/h. The horizontal firing arc should be 360°.
Self-propelled gun U-20, factory sketch
The 85mm gun system, indexed F-30, was developed by a collective from the factory No. 92 under the guidance of V.G. Grabin. In September 1940 the system was tested on the T-28 tank on which it displayed satisfactory results. However, the F-30 was unable to be mounted into the KV-1 turret, so the Kirov factory started development of a bigger turret, which was later on installed on the KV-220. As for the tank destroyer on the T-34 chassis, its’ development was handled by a collective of the factory No. 8 in Kaliningrad (city near Moscow). Two T-34 chassis without turrets were delivered from factory No. 183 which should be used as bases for prototype vehicles of these tank destroyers. However, the case did not move further from this task.
The idea of an SPG on T-34 basis returned in the spring of 1941. On the background of intelligence about the German development of heavy tanks, work on the creation of domestic heavy tanks and tank destroyers started. On the 27th May 1941, technical characteristics for a 85mm SPG were outlined. In this case it was not quite about a SPG on T-34 basis – the artillery tractor A-42 was chosen, which was developed on basis of the T-34. The gun system, protected by a shield, was planned to be mounted in the rear part of the vehicle. The construction somewhat resembled the German Sd.Kfz. 8/1, based on the halftrack Sd.Kfz. 8.
A vehicle with similar armament was planned to be developed on basis of the artillery tractor “Voroshilovets”, but this idea was scrapped. It was assumed that the tank destroyer on the A-42 chassis, indexed as A-46, would be developed at the factory No. 183 with due date for the first prototype on 1st October 1941. The gun system was to be developed by factory No. 8. Production of the A-46 should be handled by the Kolomensk machine-building plant. For 1942, it was expected to produce about 1500 such vehicles. However, the story of tank destroyers on the A-42 chassis did not move farther because of a simple reason: the development of the artillery tractor itself stopped in the prototype phase.
85mm divisional gun U-10, which was the proposed armament for the U-20
Another fact in the development of the SPG on T-34 chassis is worth mentioning: on 17th June 1941, technical characteristics for a SU-34 SPG were specified. According to these, the vehicle should have 23.5-24 tonnes of mass. The mass reduction was achieved by reducing the armor to about 25-30mm. Also, analogous requirements were made for a vehicle on basis of the modernized T-34-M (A-43). Its mass should be 19-20 tonnes.

The demands stated that an installation of the gun system without a turret is possible, but that the crew should be protected from attacks of diving planes. In this case, the horizontal firing arc should be +/-15°. In light of emergency workload of the designers, the project did not move farther than the technical specifications.
Sverdlovsk opening
In fall of 1941 a whole line of enterprises and design bureaus were evacuated to the east. The factory No. 183 found itself in Nizhniy Tagil, and the Kolomensk machine-building plant was transferred to Kirov, where the factory No. 38 was created on its basis. The factory first engaged in construction of T-30, T-60 and T-70 tanks, but after the fall of 1942 it became the main place of development for Soviet light SPG’s. The factory No. 8 was moved to Sverdlovsk. Its design bureau was led by F.F. Petrov. In February 1942, the artillery production of the Uralmash factory was given to factory No. 8. The collective presented it’s first work already in March – the 85mm tank gun ZIK-1.
General views of the U-20
In the beginning of December 1941, a thematic plan about work on SPG’s was prepared. Among the vehicles was also the tank destroyer on T-34 basis, whose executor was to be the factory No. 8. Because the factory No.8 was just unfolding on it’s new location, the actual development was assigned to Uralmash factory. At this time, Uralmash was called the Izhorsk factory.
On the 3rd January 1941, a technical meeting took place at the Izhorsk factory, in which technical projects about the installation of a 85mm gun on a T-34 chassis were reviewed. F.F. Petrov spoke as the head developer. According to the requirements, the system should have a horizontal firing arc of 360° and vertical aiming angles of -8°/+30°. Ammo storage had to contain 30 to 40 rounds. One of the requirements was also the prohibition of modifications of the T-34 chassis.
The design team had two variants of the tank destroyer. The first assumed many modifications on the gun, using developments of the 122mm M-30 howitzer and the 85mm U-10 gun. The second variant was equipped with the 85mm anti-aircraft gun mod. 1939 with minimal modifications, however in this case, the turret became visibly bigger. Both projects proposed to increase the gun crew to 3 people. Having considered the project, the commision came to the conclusion that both proposals match the technical specifications. The design bureau considered it to be more useful to place the engine compartment in the front, but this was hindered by the modification prohibition. The commission proposed to create one prototype of both variants.
Lengthwise cut of the first U-20 SPG variant
On the 14th January 1942, the project documentation was sent to the Artillery commitee of the Main Artillery Directorate of the Red Army. The variant with the maximum gun modifications recieved the index U-20, the one with the minimal ones – U-20-II. At that moment, both projects had already received some necessary modifications made by the commission in the beginning of January.
Self-propelled gun U-20-II, factory sketch
The first variant contained the experience of Sverdlovsk’s designers collected during the first year of the war. Instead of the anti-aircraft gun, the U-10 gun developed by Uralmash in October 1941 should be installed. It represented the barrel of the 85mm 52-K gun on the gun carriage on the 122mm M-30 howitzer. This made quite some sense because the M-30 was produced at Uralmash, and the production of anti-aircraft guns was also handled there. In the end of 1941, a trial batch of these guns was made, however the U-10 didn’t make it further after this stage, which was somewhat disappointing because the gun was more fitting for a SPG than the anti-aircraft one. The Sverdlovsk gun was more compact and thus required a smaller turret.
General views of the U-20-II. The rear niche gives the turret some resemblance with the xenomorph from “Alien”
Instead of the T-34 turret, a new one was installed which kept a mount diameter of 1420mm. It was called a platform in the technical description and housed 3 people. The turret had an open top and, partially, rear, and should have a mass of 3800kg. The armour was as follows: front: 45mm, sides: 20mm, rear: 12mm. The U-10 gun without a muzzle brake had vertical aiming angles of -5°/+27°.
Due to the large measurements of the gun, the rear of the turret had an aperture where the gun withdrew to during the recoil. Considering this, the loader’s job in this vehicle would’ve been not easy: get the shell, get to the rear niche, load the gun. The first few rounds were located behind the frontal armor of the turret, which allowed to maintain a good firing rate. The rest of the ammunition was located in the vehicle’s body, which was harder to reach for the loader. But he should be helped by the second loader.
Lengthwise cut of the U-20-II
The second variant of the tank destroyer, the U-20-II, was developed accounting for the maximum reusability of already existing components. During the creation of this vehicle, an important aspect was the maximum shortening of the visible barrel part coming out of the turret. The result was a quite weird construction: the 52-K gun was installed as-is, only removing the muzzle brake. This extended the length of the rear niche to 2 metres.
To compensate the mass of this “nape”, the thickness of the frontal armor was increased to 75mm, the sides – to 40mm. In this SPG, a second loader was vital, because to load this gun alone, you would have to be an equilibrist. For the ease of the reloading, the first few rounds were also placed inside the turret.
Weird conclusions
The Artillery committee paused until April 1942, when the U-20 projects were considered again. The idea itself of a turreted SPG was considered correct, but the working conditions of the loaders were subjected to sharp criticism. Moreover, the idea of using the U-10 gun on the first project was also criticized, pointing out that this would complicate production. The U-20-II project was in higher regard, despite the “acrobatic” work of the loaders.
Model of the U-20 in 1:35 scale, made by using factory designs. Author – Alexander Kalashnik, Omsk
Regardless, both projects were considered not fulfilling the demands of the Artillery committee. This conclusion causes a whole load of questions. The impression is created that the artillery comrades didn’t know themselves what they wanted. The conclusion contained mutually excluding paragraphs. The Artillery committee itself formulated the demands and called their realization in projects untrue. This situation was not individual: a similar story happened with the U-19.
Thankfully, another SPG project was approved after the burial of the 85mm project: the installation of the 122mm M-30 howitzer on captured StuG III German SPG’s. This was designated as 122-SG (later on SG-122) and became the first Soviet serial medium SPG. The concept of a self-propelled system with a frontally-mounted gun would become typical for Soviet medium and heavy wartime SPG’s.
Another fun fact: at the same time when the Artillery committee buried the turreted SPG, a prototype vehicle under the designation of T35 GMC arrived at the Aberdeen proving ground in the USA. It’s development began in November 1941, and the concept resembled the U-20 in many ways: on the chassis of the medium tank M4, a new turret was installed and a modified 76mm anti-aircraft gun was mounted. This vehicle became the M10 Wolverine, the most produced American anti-tank SPG during the Second World War. In the fall of 1943, the M10 was being trialed in the USSR, where it recieved positive feedback, however it was rated by tanksmen, not by a commitee.
Author – Yuri Pasholok
Central archive of the Ministry of Defense of the Russian Federation (ZAMO RF)
Russian State Military Archive