Saturday 22 July 2017

SG-122: Assault Gun on a Foreign Chassis

Work on SPGs, especially heavy ones, stopped in the USSR after the start of the Great Patriotic War. This was largely caused by the fact that the factories were busy with other orders. In addition, many factories were evacuated eastward. Only light SPGs were put into production at the start of the war, and these were largely improvised.

Meanwhile, due to the number of factories that switched from making artillery tractors to tanks, the artillery branch was forced to revisit SPGs towards the end of 1941. Experience gained in battle and a number of other factors meant that the new generation of SPGs that was built in 1942 was radically different from pre-war designs. This is especially true for medium SPGs, which became assault guns instead of tank destroyers. The SG-122 was one such SPG, built on the chassis of a captured German SPG.

First, there was Artshturm

In early December of 1941, the GAU prepared a plan for work on SPGs. The plan included an 85 mm tank destroyer on the T-34 chassis, which was to be assigned to factory #8. This project, known as U-20, became a victim of its own requirements. According to them, the factory could not make changes to the T-34 chassis. The designers managed to build such an SPG, but it did not match other requirements.

Column of captured "Artshturms", spring of 1942.

Meanwhile, the Red Army began receiving medium SPGs with short 75 mm guns: captured German StuG IIIs, which Soviet forces captured as early as the summer of 1941. These vehicles, built to support infantry and tanks, were initially called "German T-3 medium tank with an immobile turret". Soon after, they were dubbed "Artshturm", "artillery assault tank". This is how the SPGs were referred to in many Soviet documents.

Initially, the trophies were used haphazardly. Later, centralized repairs of these vehicles were organized. In late 1941, repair factory #82 was created at the Podyemnik factory (modern day OAO MoZAL, Moscow, 14 Podyemnaya St.). Thanks to its proximity to a railroad and a large amount of lifting equipment, the factory became one of the largest tank repair organizations in Moscow.

The organization received a large amount of captured tanks, as well as tanks from the Western Allies. Until the end of the war, Lend Lease and captured tanks became the specialty of repair factory #82. Aside from repairs, the factory performed conversions of tanks into special vehicles, for example repair tanks. StuG III SPGs were also repaired here.

Repair factory #82, spring of 1942.

The captured SPG was well received by our military. "Artshturms" were used widely by the Red Army. However, our specialists had a number of complaints about the vehicle even in late 1941. Most importantly, ammunition was hard to come by, for obvious reasons.

There were also other questions. For example, the StuG III chassis was suitable for more powerful and more effective guns than the 75 mm short barreled gun. The fighting compartment of the German SPG was also too low.

In February of 1942, the issue of rearming captured tanks and SPGs to use domestic guns was raised. According to archive documents, the GAU Artillery Committee tasked E.V. Sinilshikov and S.G. Pererushev with this task. They worked at factory #592, which was organized at the Mytishchi machinebuilding factory in Mytishchi, Moscow oblast.

Formally, the factory housed artillery design bureau OKB-16, but in reality, factory #592 worked on many subjects, including design and production of heavy armored sleds. Sinilshikov and Pererushev used to work not at OKB-16, but the Bolshevik factory.

Factory #592 was tasked with rearming the Pz38(t) with the 20 mm TNSh autocannon or 45 mm gun, the PzIII with a 45 mm gun, and the PzIV with the 76 mm F-34 gun. As for the StuG III, the GAU wanted to install the 122 mm M-30 howitzer.

In reality, the last item on the list was the only one worked on.

Draft of the 122-SG, April 1942.

On March 17th, 1942, the NKV Technical Council gave the StuG rearmament project a green light. Sinilshikov was appointed as the manager and Pererushev as the chief designer. The draft project, indexed 122-SG (122 mm self propelled howitzer on the Artshturm chassis), was completed on April 3rd, and presented to the Artkom on the 6th.

Overall, the 122-SG differed little in purpose from its German predecessor, but the firepower of the 122 mm howitzer was much greater than of the 7.5 cm StuK. It was enough to defeat any German tank, although the main target of the 122-SG was, of course, light fortifications and strongholds.

Installation of the 122 mm M-30 howitzer into the 122-SG.

The fighting compartment had to be redesigned to fit the much larger gun. The roof was raised by 100 mm, increasing the height of the fighting compartment to 1620 mm. The SPG's crewmen, especially short ones, could now stand, which improved working conditions. The overall height grew to 2050 mm; the vehicle was still quite low.

The front of the casemate received applique armour, which increased the armour thickness to 70 mm. The ammunition capacity of the SPG was 50 rounds, more than of the StuG. The number of crewmen increased to 5, but their working conditions were no worse than those of the German crew.

The unaltered M-30 oscillating part was installed on a pedestal mount, covered with 10 mm thick plates from the sides. The recoil mechanisms were protected with 30 mm of armour. According to calculations, this would increase the mass of the SPG by 2.4 tons, which was acceptable.

The SPG from the front.

After inspection of the 122-SG project materials, the 2nd Department of the Artkom approved them as the foundation for a technical project. At the same time, the deadline for producing an experimental prototype of the 122-SG and mass rearmament of the Artshturm was set.

On April 15th, 1942, a Plenum of the Artillery Committee was held, which cemented the direction of Soviet self propelled artillery. Even though the T-34 tank destroyer with an 85 mm gun was still listed in the plans, the overall development took a different route. A decision was made to abandon the concept, and pursue a medium assault gun on the T-34 chassis instead. For the short term, production of the SPG on the StuG III chassis, the name of which was changed to SG-122, was approved.

Chassis shortage

The Plenum's demand that an analogous SPG be developed on the T-34 chassis had a good reason behind it. The use of a captured tank as a base had a number of problems. First of all, it was necessary to capture a required number of SPGs in a functional state. However, even the capture of a suitable vehicle did not mean that it would be sent to conversion. Captured vehicles were used to make up for losses. Repair factory #82 alone, the biggest source of chassis for the SG-122, sent 8 of the 27 StuG IIIs that arrived back to the front lines. 15 captured SPGs arrived in March, giving hope for large volumes in the future. Unfortunately, shipments as large as the one in March of 1942 never happened again.

122-SG from the top. As you can see, there is enough room for five crewmen.

In late April of 1942, tactical-technical characteristics for the SG-122 were developed. Work on the technical project for the SPG began at around the same time. A number of authors state that G.I. Kashtanov was the chief designer on this project, but that is incorrect. All documents, including factory blueprints, list Sinilshikov as the chief designer. In June of 1942, factory #592's design bureau was reinforced with engineers who formerly worked at the Bolshevik factory. Many engineers came from factory #8.

Experimental prototype of the SG-122 on trials, August of 1942.

The SG-122 technical project was completed by the middle of June of 1942. Production of a prototype began at that time. During development, factory #592 introduced a number of changes into the initial design. For starters, the height of the fighting compartment was further increased, letting crewmen up to 170 cm tall work without bending down. The applique armour for the front of the casemate was changed, and the slope in the back of the casemate was removed, increasing the volume of the fighting compartment. The overall height of the SG-122 increased to 2250 mm.

On June 15th, the test program of the SG-122 was prepared, signed by chief designer Sinilshikov and chief engineer of factory #592 Lomakhin. According to the program, the SPG would fire 40 shots and drive for 100 km. The outskirts of Mytishchi were chosen as the proving grounds, and the firing would take place at the factory #8 shooting range, located nearby. On June 25th, the program was approved by the GAU.

This is what the SG-122 looked like in production. The fighting compartment is visibly roomier thanks to the raised roof.

The trials schedule for the prototype SG-122, which was only finished in late June, was very optimistic. Trials began on July 25th and ended on August 16th. The SPG was was delivered for trials had a large number of changes compared to the initial project.

In addition to the aforementioned casemate, the changes touched many other components. The M-30 received new armour for its recoil brake. Additional springs were installed to help balance the gun. A guard rail, trigger pedal, and a loading assistance tray were added.

After all of these changes, the mass of the SPG reached 23.3 tons, which was acceptable. To compare, the StuG 40 Ausf. F with a 7.5 cm StuK 40 with a 43 caliber barrel weighed 23.3 tons, and the StuH 42 weighed 24 tons.

The top speed achieved during trials was 50 kph on a highway and 30 kph off-road. During trials, the SG-122 drove for 50 km, mostly off-road. The average speed was 15 kph, and fuel consumption was 300 L per 100 km. The vehicle survived the off-road portion of the trials, but the crew noted that it was hard to drive, since the center of mass shifted towards the front. The friction clutch caught fire. In addition, the road wheels wore down faster.

Gunnery trials were performed in much larger volumes than anticipated. 234 shots were fired, almost 5 times as many as planned. The sight shifted by 1-2 divisions with every shot, but the precision was judged to be satisfactory. The rate of fire ranged from 5 to 11-12 RPM, depending on the angle of the gun. The gun was comfortable enough to service, and the location of ammunition was satisfactory. It took 20-25 seconds to switch from travel to battle mode. There was enough room for the crew, but only the driver had a seat.

Overall, the trials were deemed a success. Factory #592's design bureau was instructed to add seats for the rest of the crew, add armour for the panoramic sight, and make the trigger guard sturdier.

The layout of the fighting compartment was largely the same.

The fate of the SG-122 could have been sealed before the trials even began. The GABTU was against the rearmament of the StuG, claiming that there was a shortage of materiel as is. This is similar to the story with the Matilda and Valentine, where the vehicles passed trials, but production never began.

However, there was a difference between the Lend Lease tanks and captured SPG, which didn't let the GABTU kill the SG-122. Self propelled artillery was under the jurisdiction of the GAU, and the tankers weren't allowed to touch its projects.

Work on the SG-122M, which would have used the T-34 chassis, stalled for the same reason: the GABTU didn't give them any tanks, even though the first request was made on June 30th, 1942. It took until August 20th, when Molotov personally gave his permission to use two T-34s that returned from repairs. This strange behaviour from the GABTU nearly left the Red Army without any medium SPGs.

Despite these dramatic events, a contract between factory #592 and the GAU was signed on September 7th, 1942, for the first five SG-122s. The cost of converting an Artshturm into an SG-122 was 45,000 rubles. 

Perpendicular cross-section of the SG-122.

The final word on the issue of rearming the Artshturm was up to Stalin personally, who signed GKO decree #2429ss "On production of experimental prototypes of SPGs" on October 19th, 1942.

"10. The GABTU (comrade Fedorenko) must provide factory #592 with 120 captured German Artshturm SPGs or T-3 tanks according to the following schedule: October: 20, by November 15th: 10, by December 1st: 20, by December 15th: 20, by January 1st: 25, by January 15th: 10, by February 1st: 15.
11. The People's Commissariat of Armament (comrade Ustinov) must produce and deliver to the Chief of Artillery (comrade Voronov) 120 SPGs with the M-30 122 mm howitzer, like those that passed GAU trials, according to the following schedule: by October 25th: 10, by November 15th: 10, by December 1st: 10, by December 15th: 10, by January 1st: 15, by January 15th: 20, by February 15th: 20."

In reality, repair factory #82 could only deliver 19 StuG IIIs and 20 PzIIIs in all of 1942. No order could change this situation, and real production was nothing like the GKO expected it to be. Even involving other repairs factories did not change the situation significantly. As of December 5th, 1942, the factory could have theoretically received 47 captured tanks for conversion.

Production issues added to that, since factory #592 had never produced anything en masse before. Judging by correspondence, the factory tried to blame the GABTU for some production issues, but the attempt was unsuccessful.

The factory managed to produce seven SG-122s in October, two more in November, and none at all in December. Six vehicles were delivered: two each for the 15th Reserve Training SPG Regiment, 2nd Kiev Artillery Academy, and the 2nd Rostov Artillery Academy.

By that point, the fate of the SG-122 was sealed. GKO decree #2559 issued on December 2nd, 1942, "On organization of production of SPGs at the Uralmash factory and factory #38" removed the responsibility of T-70 production from factory #38. Instead, decree #2661 issued on December 27th, 1942, tasked factory #40, which factory #592 was reorganized into, with production of the light T-80 tank.

Nevertheless, factory #592 was still expected to build 10 SG-122s. In reality, the factory built 12 more SPGs in January. The total production was 21 units, 20 of which were sent to the army. These vehicles had serial numbers 1001-1021.

Brief battle career

The 14 SG-122s that were delivered in January were sent to the Moscow Artillery Center, from where they were transferred to the 1435th Self Propelled Artillery Regiment (SAP), formed on January 1st, 1943. On January 5th, the unit began receiving SU-12 (SU-76) SPGs, followed by the SU-35 (SU-122). On January 28th, the SU-35 were transferred to the 1433rd and 1434 SAPs, which were replaced by SG-122s in early February. The unit's documents periodically referred to them as SU-35.

As of February 2nd, the regiment had five rearmed Artshturms, and by the 4th, three of them already needed repairs. Documents state that delivery of SPGs from factory #40 was slow, and several of the SPGs arrived with defects. Some of them were so severe, that they needed repairs at the factory.

By February 8th, the number of SG-122s reached seven, five of which were in repairs on the next day. Finally, on February 15th, the number of SG-122s reached sixteen (14 from the factory and two from the reserve training regiment), five of which were in repairs.

The only known photo of the SG-122 at a repair base. SPG with serial number 1002 is seen on the right of the photograph.

The 1435th SAP was sent to the front on February 20th, by which time the number of SG-122s dropped to 12. Four vehicles were given to the SPG Training Center. The 1435th SAP unloaded at the Dabuzha station. SG-122 1002 and 1011 were left along the way. By March 3rd, only eight SG-122s remained combat capable out of 12.

The SPGs trial by combat happened on March 7th, 1943, in battle for the Verkhnyaya Akimovka village. The SPGs supported an attack by the 248th Tank Brigade of the 9th Tank Corps. Seven SU-12s and seven SG-122s went into battle. Three of them were knocked out, and two burned up. The SPGs' fire destroyed three anti-tank guns, two machinegun nests, and one tank.

On the next day, SG-122 #1003 burned up, #1016 and #1025 were knocked out. On March 9th, the battle for Verkhnyaya Akimovka continued. There were no losses that day, and the SPG crews claimed one anti-tank gun and two MG nests. Lieutenant Koval and gunner Yurin excelled in these battles, claiming two guns, four dugouts, four MG nests, and one car.

The 1435th SAP went into battle on March 14th for the last time. Three SU-12 and four SG-122 SPGs were used to attack the Yasenok village. Two vehicles returned from the fight with insignificant damage, two burned up, and three were knocked out.

The fate of the SG-122 was unfortunate. Nevertheless, the SPG left its mark on Soviet SPG building history. Many solutions developed by factory #592 during its design were later used to design medium SPGs, creating the SU-122. The idea of a loading tray was later used on the SU-152.

The SG-122 was not the last SPG for Sinilshikov. On November 5th, 1942, GKO decree #2477ss organized the Central Artillery Design Bureau, headed by V.G. Grabin. Sinilshikov was included into its staff. The TsKB's first project was the S-1 SPG, a 76 mm F-34 gun on the PzIII chassis. This vehicle was built in large numbers and deserves its own story.

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