It was clear by the spring of 1942 that work on a medium SPG on the T-34 chassis with an 85 mm gun in a rotating turret hit a dead end. The result of this work, which started back in the summer of 1940, was the U-20, which the military considered unsatisfactory. The project didn't leave the drawing board. Later, development of Soviet SPGs took a different path. A significant influence was the study of a captured StuG III Ausf. B. Later, factory #592 built a Soviet version on its chassis, called SG-122. It was clear, however, that converting foreign vehicles was not the end.
From tank destroyer to assault gun
One can't say that Soviet engineers decided to copy a German SPG. Soviet and German opinions regarding the tasks for medium SPGs were too different. The StuG III was more of a budget support tank variant, built, primarily, for direct fire. The self propelled version of the 75 mm gun, initially meant for the B.W. tank (future PzIV), had fairly limited firepower. It was suitable for fighting infantry and MG nests, but even light concrete fortifications were too tough for it.
The Soviet military envisioned a much wider spectrum of applications for a medium SPG. It had to fire not only directly, but indirectly, with the ability to confidently defeat light fortifications and dugouts. The M-30 122 mm mod. 1938 howitzer was ideal for this task. By 1942, the Germans also came to the conclusion that the capabilities of the 75 mm gun were limited. This resulted in the StuH 42, armed with a 105 mm leFH howitzer. However, the Germans retained the primary purpose of direct fire, while the Soviet SG-122 had a maximum elevation of 40 degrees.
M-30 122 mm howitzer, the workhorse of Soviet divisional artillery.
On April 14th, 1942, a plenum of the Artillery Committee of the Main Artillery Directorate of the Red Army (GAU KA) was held, at which a development plan for Soviet self propelled artillery was devised. Among other decisions, the plenum decided to develop a "self propelled howitzer to combat dugouts and concentrations of enemy personnel". Work on creating a "M-30 122 mm howitzer on the T-34 chassis" was given to two organizations: factory #183 (Nizhniy Tagil) and factory #8 (Sverdlovsk). At the same time, a decision was made to cancel the U-20.
Even though this decision did not cancel the SG-122, priority was given to the vehicle on a domestically produced chassis. The decision was a correct one. The amount of foreign chassis was limited, and what little could be acquired came in a heavily worn state. As a result, SPGs on the T-34 chassis ended up in service before the SG-122. Nevertheless, the road to the future SU-122 was far from an easy one.
The choice of factory #8 as the developer of the "M-30 122 mm howitzer on the T-34 tank chassis" was a logical decision. This was the developer of the initial SPG on the T-34 chassis, and many of its staff worked on the U-20. Development of the U-20 was headed by F.F. Petrov, who became the head of the factory #8 design bureau, extracted from the UZTM design bureau, in February of 1942. It was Petrov who developed the M-30 howitzer in the first place.Overall direction of the work was set by T.A. Sandler, the chief designer at factory #8. Two design bureaus ended up being housed in the same building at Uralmash.
Cutaway diagram of the ZIK-10 SPG, showing off the advantages of the U-11 gun.
Meanwhile, despite the Artkom's decision, development of the new SPG was not easy. Factory #183 effectively quit the development program. It's hard to blame the factory's management. Nizhniy Tagil was ramping up production of T-34 tanks in the spring of 1942. 75 tanks were delivered in January, but by April, 380 tanks were delivered, and 500 in May. The idea of saddling the factory with production of an SPG on top of production of tanks, which continued to grow, was a poor idea, both from the point of view of the factory and the GABTU. The army needed T-34s like the air it breathed. It's not surprising that the prospective SPG was left without a chassis.
There are some issues with the layout of the fighting compartment. As you can see, there is hardly any room to the right of the gun.
Sverdlovsk was also having significant issues with the prospective SPG. When the decision to begin development was made, factory #8's design bureau was already loaded with other work. Development of the 85 mm ZIK-1 tank gun began in March, and the development of the 45 mm ZIK-4 anti-tank gun began around that same time. In the spring of 1942, the design bureau, under Petrov's supervision, began working on a gun mount for the SPG made with T-60 components. The 76 mm assault gun received the index ZIK-7, and it was being designed in two variants. In addition, a 25 mm SPAAG, the ZIK-5, was developed.
The first documentation for the ZIK-7 project appeared in May of 1942, before work on the medium SPG had even began. The reason was simple: while it was not certain when the T-34 chassis would be available, the light T-60 chassis was being designed right in Sverdlovsk. As a result, work on the medium SPG began only in the summer of 1942.
Perpendicular cutaway of the ZIK-10.
Let us also discuss some liberties taken by factory #8 design bureau staff. Initially, the Artillery Committee received a tank, instead of an SPG. Instead of an SPG on the T-34 chassis, Petrov's bureau designed an artillery tank, equipped with the U-11 howitzer. The reworked system was indexed U-22. Factory #183's design bureau designed a reworked turret to fit it. The T-34 equipped with a U-22 gun was mistakenly classified as an SPG by some researchers, but it is very much a tank.
The U-22 was designed on the factory's own initiative, and caused mixed feelings in the Artillery Committee. On July 20th, 1942, a response was received at factory #8. The project was deemed nonviable, and factory #8 was instructed to run ideas like these past the GAU, to avoid wasting time.
U-11 gun, redesigned for installation into the ZIK-10. A tray has been added to make loading easier.
The creativity of factory #8's design bureau did not end there. Petrov couldn't come to terms with the fact that the U-11 was unused. Work on the KV-9 hit a dead end, and the future of the gun was not promising. The factory already produced a batch of ten U-11s. There were other, better reasons to use the gun. The M-30 required a large pedestal mount, which took up space. The StuG III and SG-122 suffered from the same drawback, as the mount ate up a lot of space in the already cramped fighting compartment. The U-11, designed for installation in a turret, was viewed by Petrov as a superior alternative.
Ammunition layout diagram. It was stored in the same way on the ZIK-11.
After the failure with their artillery tank, factory #8's design bureau focused on the SPG. Or, rather, SPGs. Considering that the U-11 variant will be superior, Petrov gave priority to that variant, which was indexed ZIK-10. The design philosophy was to make as few changes as possible to the T-34 chassis. The project was reminiscent of the KV-7, where the goal was to make as few changes as possible to the KV-1 chassis.
The demand to minimally alter the chassis of an SPG came from the military, and often caused issues. With the ZIK-10, the factory decided on several alterations. For example, the size of the fighting compartment was increased due to reducing the slope of the sides. The U-11, mounted on a frame, was located inside. This solution allows the removal of the bulky pedestal mount, which made the fighting compartment roomier.
Overall view of the ZIK-11 SPG.
Following the requirement to alter the T-34 chassis as little as possible, the designers became its prisoners. The requirement was met, but introduced issues. For example, the front of the hull had a characteristic "step", with the upper part having a slope of only 15 degrees, a questionable decision from the point of view of shell resistance. The hatches in the sides of the casemate were also questionable. The casemate had no roof, which improved ventilation, but introduced a number of other issues.
The ammunition capacity was also low: only 32 rounds.
The addition of a pedestal mount for the M-30 hardly made the fighting compartment any roomier.
Similar issues could be found in the design bureau's other project, known as the ZIK-11. Unlike the ZIK-10, it used the 122 mm M-30 howitzer, as set in the tactical-technical requirements. There were almost no changes to the system compared to the production version. The recoil mechanisms were covered in armour, a new travel lock was added, and a new lower mount, which still used as many parts from the M-30 as possible.
Strangely enough, there was now more room to the right of the gun, but still a risk to be injured, as there was no guard rail.
Even though a pedestal mount was necessary, it was less massive than the one on the StuG III or SG-122. The aim to use the unaltered M-30 meant that the gun had no guard rails. In a combat situation, this would have caused accidents.
The cutaway shows that there is still not much room to the right of the gun.
The design of the casemate was similar to the one developed for the ZIK-10. The layout of the ZIK-11 casemate was more rational. The "step" in the front of the hull had a higher slope. The ammunition capacity was the same as on the ZIK-10, but it was better laid out. The height of the vehicle was 2100 mm, compared to the ZIK-11's 2270 mm. The opening in the roof was also smaller.
Reconstruction of the ZIK-11 in 1:35th scale, made according to factory blueprints by Aleksandr Kalashnik, Omsk.
Both projects were sent to the Technical Council of the People's Commissariat of Armament (NKV), which, for obvious reasons, picked the ZIK-11. Even this vehicle raised a number of questions. The NKV Technical Council composed a list of issues which had to be corrected before the vehicle would enter production. However, development never reached that stage. At that point, Petrov's bureau was working on four SPGs at once. Work on the heavy ZIK-20 progressed the furthest, reaching the stage of a full scale model. As for the ZIK-11, a different fate awaited it.
While factory #8's design bureau was working on the ZIK-10 and ZIK-11, an important event happened in Sverdlovsk. On July 28th, 1942, GKO decree #2120 "On organization of T-34 production at Uralmash and factory #37" was published. Factory #37 became a part of UZTM, and was renamed to factory #50. Development of light tanks and SPGs at the factory ceased. On September 29th, the factory delivered its first T-34 tanks. Sverdlovsk finally gained an SPG chassis.
Center of mass calculations of the U-35, November 1942
The delays in medium SPG development did not go unnoticed. The deal began to affect organizations outside the GAU and NKV, the customers and operators of self propelled artillery. According to initial plans, factory #8 was supposed to present its project on June 25th, 1942. The date was then moved to August 10th. In reality, the ZIK-10 and ZIK-11 were only presented on September 8th.
The slipping of the deadlines attracted the attention of the Deputy People's Commissar of Defense, Colonel-General of Artillery, N.N. Voronov. A torrent of correspondence erupted between the NKO and the GAU. Voronov reported to Stalin about what was happening. On October 16th, Voronov held a meeting, at which tactical-technical requirements for new SPGs were discussed.
Cutaway of the U-35. Overall, the vehicle was similar to the ZIK-11, but with many improvements.
On October 19th, 1942, GKO decree #2429ss "On production of experimental prototypes of self propelled artillery". The future SU-122 was first on the list.
"The People's Commissariat of Tank Production (comrade Zaltsmann) and People's Commissariat of Armament (comrade Ustinov) must urgently build prototypes of SPGs with a 122 mm howitzer on the T-34 tank chassis, and present them for proving grounds trials, following these deadlines: NKV: November 10th, NKTP: November 20th".
Tactical-technical characteristics for a "122 mm self propelled howitzer on the T-34 chassis" were approved on the same day. They permitted alteration of the side plates, moving the driver, and moving the fuel tanks. On October 20th, by Deputy People's Commissar of Tank Production Kotin's initiative, a meeting of the Uralmash SKB took place. The ZIK-11 project was examined once more. The project was approved in general, but a decision was made to rework it based on the new requirements. The biggest change was that the development moved from the factory #8 design bureau to UZTM. A 16 man group was selected from the design department, headed by L.I. Gorlitskiy.
Interestingly enough, the process of transferring ZIK-11 documentation began on October 19th. This was done without permission from the NKV, on the initiative of the People's Commissar of Tank Production. Further work was overseen by Kotin. Naturally, the NKV was hardly pleased with this course of events, but there was no other way to speed up development.
The layout of the fighting compartment improved. Nevertheless, the width was kept to the T-34's dimensions, which made it cramped.
The decision to move the work of factory #8 to UZTM can be called Gorlitskiy's revenge. Factory #8 managed to force the 152 mm SPG on the KV-1 chassis away from UZTM, and now there was a chance to strike back. However, factory #8 was overloaded with work. In addition, on October 30th, it was split up into two organizations: factory #8, which was tasked with development of AA artillery, and factory #9, which worked on howitzers and SPGs. Petrov was transferred to factory #9. In this situation, everyone understood that transferring the work on the ZIK-11 to UZTM was the right decision.
U-35 on trials, December 1942
The reworked ZIK-11 received the index U-35. The design group tasked with its development consisted of specialists that had experience with SPG design. They included N.V. Kurin, G.F. Ksyunin, and K.N. Ilyin, which worked on the weapons for the KV-7 assault tank. Even though GKO decree #2429 set a very strict timeline, the U-35's design was late.
The overall layout was finalized towards the end of November. Some of its elements were borrowed from the ZIK-11, but, overall, Gorlitskiy's SPG was significantly altered - and improved. The idea of a semi-enclosed fighting compartment was discarded. The fighting compartment received a roof with two hatches, as well as a commander's cupola, analogous to the one that factory #183 designed for the three-man T-34 turret. The SPG still had a "step" in the front hull, but the machineguns in the front plate were deleted.
The thickness of the upper front plate increased to 60 mm. The armour of the recoil mechanism was improved, and now allowed turning the gun 10 degrees to the left and right. The bustle that held ammunition in the ZIK-11 was deleted. The gun was also reworked. Most importantly, a guard rail and loading tray, like on the SG-122, were added.
The Sverdlovsk SPG successfully passed mobility trials.
On October 29th, 1942, the GAU Artillery Committee approved the program of trials for an experimental prototype of the SPG. In practice, the deadline for the prototype production was very optimistic. As of November 28th, the vehicle was still under construction. Completion was awaited in early December. The GAU considered this situation caused by "a lack of necessary attention from the factory's management". In reality, UZTM and its subcontractors were overloaded with work. Factory #592, which was building an analogous SG-2 SPG, was also running late.
The same vehicle from the left.
Assembly of the first prototype was finished on November 30th. Trials at the factory proving grounds took place on the same day. The vehicle arrived incomplete: observation devices, the panoramic sight, seats, and ammunition racks were not installed. This was the result of a rush. The aiming mechanisms were not tuned, as a result of which, the flywheels were hard to turn.
The trials started poorly. On the first attempt to load the gun, the loading tray broke. It was removed, so it wouldn't get in the way. The firing mechanism cable broke on the first shot, and the rest were made by pulling on a cord. There were issues with elevating the gun all the way, and with horizontal aiming. When the gun was turned 10 degrees to the right, it was impossible to use the right friction clutch. The installation of the gun also reduced the driver's visibility. After gunnery trials, a significant part of the issues were resolved.
Reconstruction of the U-35 by Aleksandr Kalashnik.
Experimental prototypes of the U-35 and SG-2 were taken to the Gorohovets Scientific Research Experimental Proving Grounds (ANIOP). The U-35 had an advantage, since the SG-2 was built using a refurbished tank, which turned out to be defective. Even at this stage, some issues were found. The mass of the U-35 was higher by 700 kg, and its ammunition capacity was 34 rounds, instead of the required 40. More issues were discovered when the fighting compartment was examined. The overall conclusion says it all:
"The dimensions of the fighting compartment, the placement of ammunition, and crew positions do not only make normal service of the howitzer impossible, but also the safe operation of the gun."
The practical traverse angle to the right was less than 0.5 degrees. The rate of fire was only 5 RPM, instead of the required 10. The commander's seat was uncomfortable, and the visibility through the cupola was limited. The gunner, whose pose during firing was called "unnatural", didn't fare any better. It was also hard to aim. The loader was in danger of being hit by the howitzer during firing, and the breech operator's shoulder was right up against the commander's feet. It was also hard to work the elevation flywheel. The loading was effectively done by only one man, and it was hard to extract ammunition from the racks. The hatches were uncomfortable, and the ventilator did not do its job. Overall, the commission composed a list of 15 items for improvement. The conclusions were as follows:
"The fighting compartment of the Uralmash SPG in this configuration is unacceptable, and must be radically redesigned to correct the aforementioned defects."
As a result of trials, the U-35 was changed. The model shows how cramped the vehicle ended up.
Considering that the UZTM design was much better than the ZIK-11, one can only imagine the verdict that the product of the factory #8 design bureau would have received. The commission concluded that the SG-2 was better than the U-35. There were also issues with it, but largely with the chassis, which, as mentioned above, was defective.
The issue was that, on December 2nd, GKO decree #2559 was published, turning over factory #592 to the NKTP and forming factory #40. Instead of the SG-122, it began producing light T-70 tanks (later replaced with the T-80). This decree meant that it would be the U-35, referred to as the SU-35 in the decree, that went into production. The commission demanded that the U-35 be altered, using the SG-2 fighting compartment layout. Sverdlovsk got to work, and by the end of December, a completely new vehicle was ready, which retained very little from the U-35. Its story deserves a separate article.