Saturday 23 September 2017

Light Paper SPGs

As with many other Soviet SPGs, the path to the SU-76 was not easy. Initially, the two-turreted T-26 tank would be used as the chassis for an infantry support vehicle. Later, the T-50 joined in the plans. The situation after the start of the war forced the concept of the SPG to change urgently. Instead of a light infantry support SPG, the ZIS-30 appeared, a tank destroyer on the Komsomolets tractor chassis. Designers only returned to the topic of a multipurpose SPG towards the end of 1941. The SU-12, the first production variant of the SU-76, did not come about on the first try. This article tells the story of vehicles that were dead ends, without even being produced in metal.

Bringing in scientific centers

According to surviving correspondence, the first attempt to design a light SPG with a similar layout to the future SU-12 was undertaken in November of 1941. The Technical Council of the People's Commissariat of Armament (NKV) initiated the work. The developer was the combat vehicles department of the Bauman Moscow Mechanical Machinebuilding Institute (modern day Bauman MSTU). 

The light SPG was just one of a series of projects. The situation in November of 1941 put the possibility of mechanizing artillery at risk. Instead of tractors, factories were building tanks. The combat vehicles department, under the direction of G.I. Zaichik, the dean of the "O" faculty, received tasks to mechanize the 203 mm high power B-4 howitzer, 122 mm A-19 corps gun, 122 mm M-30 howitzer, and 76 mm F-22 USV divisional gun. Unfortunately, there are no details about these projects. At the same time, some authors connect this work with the "universal chassis" made with T-60 components, a project curated by S.A. Ginzburg.

On December 4th, 1941, the NKV defined the developers of their self propelled artillery project. Stalin factory #92 was responsible for the weapons of the experimental "moto-mechanized unit support assault guns:. As for the aforementioned universal chassis, the tactical-technical requirements developed by Ginzburg were distributed by the People's Commissariat of Tank Production. Ginzburg, at the time, was the deputy chief of the 2nd Department of the People's Commissariat of Tank Production. According to those requirements, the armament would be either the F-22 USV or the ZIS-5 tank gun.

Self propelled artillery was the responsibility of another organization: the Main Artillery Directorate of the Red Army (GAU KA). On January 31st, 1942, this organization approved the requirements for a "76 mm assault SPG". According to these requirements, a completely different gun was used: the 76 mm ZIS-3 divisional gun. This SPG would not use the universal chassis. A number of requirements, such as the mass, make this clear. The universal chassis would weigh 7.5-8 tons, while the GAU's SPG would weigh no more than 6.5 tons. The ammunition capacity was also different: instead of 20 rounds, it would be 30 rounds.

Requirements for self propelled AA guns (SPAAGs) appeared a little later. In the first scenario, the 25 mm 72-K AA autocannon would be used, in the second, the 37 mm 61-K autocannon. Interestingly enough, the variant with the 72-K was discarded, as its production stopped, and resumed only in 1943.

The aforementioned topics were moved to factory #37 in February of 1942. As for the light SPG from the Bauman institute, it didn't make it past the preliminary draft stage. Even though the order was given in November of 1941, work on the vehicle, indexed SU-USV, dragged on. The bottleneck was poor availability of materials regarding the oscillating part of the weapon, which slowed down work. The materials were requested in February of 1942, but only arrived in April. Because of these delays, the project was only ready in June.

According to a decision made by the NKV Technical Council on June 17th, 1942, the SU-USV SPG was to be converted to use a new artillery system. The 57 mm I-13-52 anti-tank gun developed by NII-13 was proposed. The name of the vehicle also changed: now it was called SU-I-13-53. Unlike the SU-USV, the tactical-technical requirements for this vehicle survived, which disprove any connection between work done by the Bauman institute and Ginzburg's "universal chassis". The chassis for this SPG was a seriously redesigned STZ-5 tractor. This was not the predecessor of the SU-12, but a parallel branch, and a dead end at that.

In addition, the project was redone yet again after a meeting on September 22nd, 1942. Instead of the I-13-52, a "high power 45 mm gun" was used, and the STZ-5 chassis was replaced by T-70 components with the ZIS-5 engine. This project went through several stages before being closed due to not longer being needed.

The combat vehicles department of the Bauman institute was not the only scientific facility involved with development of light SPGs. After a plenum of the GAU Artillery Committee approved the direction of SPG development on April 15th, 1942, the Scientific Research Tractor Institute (NATI) became involved. On May 4th, 1942, the T-133-B, T-133-V, and T-133-Zh projects were presented to the Artillery Committee. The first two vehicles were 37 mm SPAAGs, and the T-133-Zh was an SPG that carried two mortars. The 7500 kg vehicles would use components from T-60 and T-70 tanks. The T-133-B was equipped with two GAZ AA engines placed in parallel. The T-133-B and T-133-Zh used ZIS-5 engines, installed at an angle.

The T-133-B and T-133-Zh chassis were deemed poor, since they needed the manufacture of new components, but the T-133-B was approved. The chassis was to be used in the creation of a 76 mm assault SPG, SPAAG with 25 mm and 37 mm autocannons, and an SPG with a pair of 120 mm mortars. The experimental prototypes were due in August of 1942, but they were never built.

NATI returned to the idea of building a similar SPG in 1943, but that is a completely different story. The T-133-B project was also connected to the NATI-D tractor, which was launched into production under the name Ya-11. 

Light alternative from Sverdlovsk

The story of light SPG development in Sverdlovsk is far from simple. According to a decree by the Artillery Committee plenum, the prospective chassis must use components of the far more promising T-70, instead of the T-60. The 76 mm ZIS-5 gun also vanished from the list of weapons. Instead, the F-22 USV or the ZIS-3 had to be used. In practice, Sverdlovsk deviated from the Artillery Committee requirements, both in the chassis and the armament.

Cutaway diagram of the ZIK-7-II SPG.

The factory #37 design bureau chose the chassis developed using T-60 components as its priority. The reason for the attention to the chassis, called "Chassis 31" or "Object 31" is simple. Factory #37 was building T-60 tanks, and resisted the implementation of the T-70. Ginzburg supported the factory. This kind of stubbornness had a good reason behind it: the factory had constant issues with GAZ-202 engine supplies, a pair of which was used in the T-70, under the name GAZ-203. This was the reason why T-70 production in Sverdlovsk was postponed.

Nobody forgot the requirements for a chassis that used T-70 components. It received the index "Chassis 32" or "Object 32". On the other hand, its priority was low. The design bureau of the Ural Heavy Machinebuilding Factory (UZTM) also considered it low priority. The ZIS-3, 25, and 37 mm gun mounts were designed with the "Chassis 31" in mind.

The gun mount used as many components of the mass produced ZIS-5 gun as possible.

"Chassis 32" was given away to a second artillery design bureau at UZTM. This was the factory #8 design bureau, headed by F.F. Petrov. Here, the ZIS-3 was not considered as an option for armament. Instead of Grabin's gun, the choice was made to use the ZIS-5 tank gun.

The reason for using the gun, initially designed for the KV-1 heavy tank, was simple. Management in Sverdlovsk was afraid of the situation with tank guns from the summer-fall of 1941 repeating itself. That "artillery famine" caused the development of the U-11 tank gun, based on the 122 mm M-30 howitzer. Some explain this with the need to find a more potent HE round, but the cause is even simpler than that: the M-30 was the only gun that was being built in Sverdlovsk.

The reason for using the ZIS-5 was the same. In early 1942, factory #8 began building ZIS-5 guns, resolving the issue of KV-1 armament in Chelyabinsk. The ballistics of the F-22 USV, ZIS-3, and ZIS-5 were identical. Factory #8's work served as a backup for UZTM, in case of a shortage of ZIS-3 guns.

The gun mantlet was very similar to the one used on the KV-1.

The development of a weapon for the "Chassis 32" assault gun, indexed BGS-5, was given the index ZIK-7 at the factory #8 design bureau. Work began in April, and project documentation was largely ready towards the end of May. M.Ye. Bezusov was in charge of the ZIK-7 project. He also worked on the ZIK-10 and ZIK-11 SPG on the T-34 chassis.

Some technical solutions were similar on the light and medium SPGs. For instance, the casemate of the ZIK-7 was open from the top, to ensure proper ventilation. The configuration of the front of the casemate was similar to that of the BGS-5, but the sides and rear were sloped. The rear plate had a hatch in it.

The casemates of the ZIK-7 and ZIK-7-II were as similar as possible.

Factory #8's design bureau had a very liberal way of using the ZIS-5 gun. The cradle and trigger mechanism were seriously altered, the upper and lower mount were designed from scratch. The gun mantlet was also completely new. It was cast into a very complicated shape. A counterweight was attached to the front of the cradle. This was the biggest issue in the realization of the ZIK-7 project.

While the SU-32 chassis was ready in early June of 1942, work on the armament stalled. B.A. Fradkin, the director of factory #8, refused to produce parts for the ZIK-7. Letters from the GAU and NKV sent in June-July of 1942 were of no help. The design bureau did their work on time, a full set of blueprints was ready, but the project remained on paper.

The ZIK-7 gin mount had noticeable differences.

By early August, it was clear that the situation would not change. Factory #37 was tasked with producing ZIK-7 parts, but, on July 27th, 1942, it was included into UZTM and began preparations for T-34 production. In this situation, the only possible way out was to start working on a simplified version of the SPG, indexed ZIK-7-II. The overall layout of the fighting compartment was unchanged, but the design of the artillery system was seriously altered. Now, the cradle had minimal changes, the sight was taken from the 76 mm mod. 1927 regimental gun, the elevation and traverse mechanisms were taken from the ZIS-5. The gun mantlet also became simpler, partially identical to the one on the ZIS-5 on the KV-1.

As you can see, the engineers who were designing the ZIK-7 took no shortcuts.

The project documentation for the ZIK-7-II was ready by mid-August of 1942. The materials were sent to the GAU Artillery Committee, but the answer that came back on October 15th inspired little optimism. Factory #8's design bureau was late, considering that UZTM already designed the SU-32 SPG with the ZIS-3 gun, and factory #37 had already built it. Another gun was unnecessary. The SU-32 showed itself worse in trials than the SU-31. The Artillery Committee proposed that the design bureau should convert the ZIK-7-II to be more like the SU-31. It's no surprise that the SPG remained on paper.

The ZIK-5 was as similar as possible to the SU-31 SPAAG.

While work on the ZIK-7-II was in full swing, factory #8's design bureau began working on another artillery system. This time, the "Chassis 31" was used. The system was indexed ZIK-5. Instead of an assault gun, it was a SPAAG. It may seem that there was competition between the UZTM design bureau and the factory #8 design bureau, but that was not the case. UZTM was designing an SPG with the 37 mm 61-K autocannon, while the ZIK-5 used a different weapon: the 25 mm 72-K autocannon.

The same vehicle from the top.

Overall, the ZIK-5 project was based on SU-31 development. The fighting compartment armour scheme was similar, the main differences came from the dimensions of the 72-K. The AA gun was installed without any serious changes, minor alterations were made only when absolutely necessary. The fighting compartment contained two gunners and one loader.

The loader's work was a little more complicated, but the ammunition was better arranged.

In September of 1942, the project was sent to the Artillery Committee for approval. The chairman of the NKV Technical Council, Satel, had high hopes for the ZIK-5. However, with one important caveat: "in the event that 25 mm 72-K production resumes". The production of this AA gun ceased when factory #4 was evacuated, and resumed only in 1943. That fact meant that the ZIK-5 is destined for the archives.

The Artillery Committee also approved the 25 mm SPAAG project, proposing a minor redesign. The gun shield was altered to be like on the SU-31, as was the ammunition storage. Since renewal of 72-K production was not in the books at the end of 1942, the ZIK-5 ended up in the same place as all other factory #8 SPG designs. Later, Soviet SPAAGs were developed with the 61-K in mind.

Assault gun done quickly

On October 19th, 1942, GKO decree #2429 "On production of experimental prototypes of SPGs" was published. According to the decree, Molotov GAZ and factory #38 were tasked with developing and producing a universal chassis that used T-70 tank components. A lesser known fact is that a third competitor was involved.

Factory #92's design bureau had a successful history of SPG development. the ZIS-30, the first Soviet mass produced light tank destroyer, was designed here. Later, work went along the lines of using the ZIS-22M halftrack (ZIS-42), creating the ZIS-41 tank destroyer. The problem was that the ZIS-22M was not at all what Grabin hoped for. The halftrack was a more stable platform than the Komsomolets, but it overheated quickly, and had a number of other drawbacks. 

By the fall of 1942, the ZIS-41 was in jeopardy. It's not surprising that factory #92's design bureau began looking for an alternative platform. They did not have to look long: the solution was the T-70 tank, mass produced at the Molotov GAZ factory. The T-70 had superior stability to the Komsomolets, and its layout was better suited for an SPG on its chassis. The fact that factory #92 and GAZ were in the same city was icing on the cake.

IS-10 SPG, November 1942.

On November 19th, 1942, the NKV Technical Council received a draft project of the IS-10 SPG, designed by factory #92's design bureau. The work was directed by P.F. Muravyov. The SPG was designed to change the T-70 chassis as little as possible. This was achieved, for the most part: the T-70's layout was preserved, and the casemate was installed instead of the turret and turret ring. However, the casemate "stepped up" on the engine deck slightly.

The objective of minimal changes to the T-70 chassis resulted in a very unusual SPG. Factory #92 proposed turning the two-man T-70 into a three-man SPG with a ZIS-3 gun. On paper, the proposed SPG took an interesting form. According to the design, the internal layout allowed the placement of 40 rounds of ammunition for the ZIS-3, and the mass of the IS-10 was only 500 kg higher than that of the T-70. The gun had satisfactory aiming angles. The ZIS-3 oscillating part was minimally altered. The biggest change was the installation of a telescopic sight. In addition, the gun mount had a coaxial DT machinegun.

The view from above shows how hard the gun crew's life would be.

In reality, the IS-10 was not as ideal as described. One look at the draft blueprints is enough to feel compassion for those who were going to fight in this SPG. The casemate overlapped with the engine deck, and removing the engine would not have been an easy task. This was, however, far from the biggest issue. There is a distinct feeling that factory #92 was thinking of anything but crew comfort in 1942.

For example, theoretically, the 85 mm ZIS-25 gun fit into the KV-1 turret, but the loading process would have been a wonder of gymnastics. Compared to the work that the IS-10 loader would have had to do, it was trivial. The poor loader would have had to put in enormous effort to not be clipped by the gun while it was shooting. Even railings didn't help much, as there was simply not that much room for the loader. Reloading the coaxial machinegun would also have been a sight to behold. Such minutia as seats for the loader or the commander weren't even worth mentioning. According to the diagram, the commander stood on top of the ammo rack. Presumably, this made loading easier. Ventilation of the fighting compartment also raised a lot of questions. Judging by the diagram, the designers didn't think about it at all.

The same vehicle from the front. This idea was, wisely, refused.

On November 26th, another letter arrived from factory #92. Factory director A.S. Elyan asked for a T-70 tank, in order to build an IS-10 SPG by December 15th. The answer from the NKV Technical Council was brief. Since GKO decree #2559 "On organization of SPG production at Uralmash and factory #38" was published on December 2nd, 1942, there was no need for another SPG. Since the IS-10 offered no advantages, the project was declined.

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