Saturday 9 May 2020

A Captured Chassis

The overall concept of Soviet SPGs was fixed by the start of 1943. The light SU-12 (SU-76) and medium SU-35 (SU-122) were already in production, and work on the heavy KV-14 (SU-152) began in early January. Another, fourth SPG appeared in the spring. Its mass put it in the medium class, but its armament was more appropriate for the light. It was also based on a foreign chassis. This was the SU-76I, an unusual vehicle for Soviet tank building.

The TsAKB's self propelled debut

Unlike the Germans, which built hundreds of SPGs on captured chassis, the USSR was wary of such development. First of all, the tanks and SPGs captured from the enemy were hardly in new-in-box condition. As a rule, these vehicles were heavily worn and required repairs. The absence of a manufacturing base in Soviet territory meant that spare parts would be hard to come by. Thirdly, the number of trophies that were suitable for further use was typically low.

These three factors had an influence on the production of the SG-122, the first Soviet medium SPG to be put into production. A lack of StuG III and Pz.Kpfw.III chassis in the USSR, and later their poor technical condition, led to only 21 vehicles of this type being built. The SG-122 also suffered from technical issues and had to be slowly massaged into a functional state. On January 9th, 1943, GAU chief Colonel General Yakovlev petitioned Stalin to cease production of the SG-122.

An improved draft of the S-1 SPG project. February 20th, 1943.

Considering the questionable results of the SG-122 program, this could have been the first and last experience with using a captured chassis. However, it was not. A number of publications touching on the SU-76I claim that it was developed due to the issues that arose with the SU-12. In reality, work on the new SPG began long before numerous complaints about the SU-12's gearbox defects. The real cause was different.

The Red Army captured a large amount of German tanks as a result of successful operations. After factory #592 of the People's Commissariat of Armament was transformed into factory #40 of the People's Commissariat of Tank Production, Moscow gained yet another factory where tanks could be produced. This was the territory of factory #37, which was evacuated to Sverdlovsk in the fall of 1941. A tank repair workshop was organized in its place. Later, a branch of factory #37 was set up here. When factory #37 was rolled into the Ural Heavy Machinebuilding Factory (UZTM) in the summer of 1942, the Moscow branch once again became the Ordzhonikidze factory #37. Until the start of 1943 the factory's main task was repairing T-60 and T-70 tanks. M.Ya. Zelikson was assigned as the factory's director and L.T. Dombrovskiy became its chief designer. Design department #22 was restored at the factory, headed by A.I. Taleysnik. Repair workshop #82 set up nearby supplied factory #592 with captured tank chassis.

This draft as a number of differences from the prototype. The PTK periscope was set too far back and had to be brought forward.

The GAU and NKTP initiated the work to create an SPG on a captured chassis. Experience with the SG-122 showed that the M-30 was too heavy for this chassis, and the idea of a deeper rebuilding of the Pz.Kpfw.III and StuG III came up in mid-January. A converted 76 mm F-34 gun could be installed instead of a 122 mm howitzer. This gun was much lighter, was built in sufficient numbers, and did not require a large pedestal mount. Yakovlev, People's Commissar of Tank Production Zaltsmann, and the Chief of the Tank Directorate (BTU) of the GBTU Korobkov petitioned Stalin to let them produce the "SU-76 on the chassis of the captured Artshrutm and T-3 tanks" on January 17th.

Stalin agreed, and GKO decree #2758ss "On the organization of production of SU-76 SPGs on the chassis of Artshturm and T-3 captured tanks with a 76 mm F-34 gun" was signed on January 18th. The SPG did not even exist on paper yet. The design was supposed to be finalized by January 25th, and the first prototype built by February 15th. There were no tactical-technical requirements either. They were supposed to be developed by January 20th. The first 15 SPGs on a captured chassis were expected in March of 1943, 35 would be built in April, 40 in May, and 45 in June. The existing SG-122 parts were moved from factory #40 to factory #37. 

The first SU-76 prototype on a Pz.Kpfw.III chassis. Sofrino proving grounds, March 1943.

The GAU Artillery Committee was the first to finish their urgent task. Tactical-technical requirements for a "76 mm SU-76 assault SPG" were ready on January 19th, ahead of schedule. The F-34 was supposed to be equipped with the panoramic sight from the 76 mm ZIS-3. The basic chassis did not change during conversion. The vehicle's mass was supposed to remain at the level of the Pz.Kpfw.III or StuG III. The ammunition capacity was 75 rounds for the F-34. The crew would consist of 4 men. Two PPSh SMGs served as auxiliary armament.

A German Pz.Kpfw.III Ausf.J medium tank was used as a chassis.

The design of the SPG began right after GKO decree #2758ss was signed. The factory #592 design group headed by G.I. Kashtanov is often credited with the design, but that is not so. Kashtanov only had a connection with the production SG-122. Another man played the key role in the creation of the SU-76I: Ye.V. Sinilshikov,. the creator of the SG-122. He did not work at factory #592 by early 1943, having already moved to Grabin's Central Artillery Design Bureau (TsAKB). Here, in the role of the head of the 3rd department of the TsAKB, he began to work on the vehicle that would be the design bureau's debut. This topic was indexed S-1/

Work at factory #37 also began, headed by the chief of technical department #22 A.I. Taleysnik. Blueprints of the SG-122 were sent to factory #37 on January 19th. Even earlier, on January 18th, blueprints of the ZIS-3, F-34, M-30, and the draft of the ZIK-7-II SPG were sent to the Department of the Chief Designer of the NKTP (OGK), which was headed by S.A. Ginzburg. The ZIK-7-II design used the ZIS-5 gun, but overall was similar to what was supposed to be built on the captured chassis.

The creation of the new vehicle was performed by three organizations: the TsAKB (gun mount), factory #37 department #22 (chassis) and the NKTP OGK (general direction.

View from behind, rear hatch is closed.

The joint work of the TsAKB and factory #37 was done in time. The S-1 project was reviewed at a meeting at factory #37 on January 25th, attended by representatives from the GAU, NKV, NKTP, and factory #37 management. A number of changes were requested and the project was returned for improvements.

At the same time, factories #112 and #92 began to work on the S-1 gun and its mount. According to GKO decree #2812 issued on February 3rd, 1943, the production of guns and their mantlets was being transferred from the responsibility of the NKV to the NKTP.

Delays led to the deadline for building the first prototype slipping to March 1st by early February. Nevertheless, the vehicle continued to take shape. Detailed blueprints and a model were prepared by February 15th. The improved variant of the S-1, or SU-76 as the vehicle was called, was not a significant alteration of the Pz.Kpfw.III. The tank kept not only its chassis, but a part of the turret platform. Only the roof was removed, replaced with a prism shaped casemate. The driver's observation device was moved to the left.

This decision made the conversion simpler. Minimal changes were also made to the S-1 gun. It differed from the F-34 by the absence of the coaxial machine gun and a frame that the gun turned in when aimed in the horizontal axis. Even the gun mantlet was very similar to the F-34.

The escape hatch could also be used for ventilation.

The casemate was very compact. The roof of the S-1 was only 2200 mm high, only 40 mm taller than the StuG 40 Ausf.G. The height of the fighting compartment was about 1800 mm, and the crew could work while standing. The StuG 40's crew could only dream of such comfort. Unlike the SG-122, the new SPG had a crew of four, as there was no need for a breech operator. This allowed for a substantial increase of the ammunition capacity. It included 95 76 mm rounds and 1000 PPSh rounds.

The front armour was 45-60 mm thick, the sides were 25-30 mm thick, the rear and roof were 25 and 10 mm thick respectively. The rear of the casemate had a two flap hatch with another flap on the roof. The roof also had a hatch for a panoramic sight and a hatch for wiping down the commander's sight. The crew layout was similar to the one later used on the SU-85. The driver, gunner, and loader were positioned one behind another on the left. The commander and the radio station were located in the front right corner. 

Inside the fighting compartment. The commander's station is in the upper right corner. 

The experimental SU-76 with the serial number 3001 was ready by March 7th, 1943. The characteristics of the vehicle differed both from those specified by the requirements and stated in the project documentation. The number of rounds for the F-34 was increased to 96, and amount of PPSh ammunition to 1095. The thickness of the armour plates changed slightly. The mass of the SPG was 23.2 tons, similar to that of the StuG 40 Ausf.F/8.

Factory trials were completed by March 10th. A list of 22 desired improvements was made. These were generally minor. The biggest changes were in the commander's station. His PTK periscope had to be moved as far forward as possible to make working with it easier. Gunnery trials in the volume of 119 shots were held on March 9th at the Sofrino proving grounds. The vehicle also drove for 145 km as a part of mobility trials.

The driver, gunner, and loader were located in a line on the left side of the fighting compartment, just like in the StuG III.

Proving grounds trials took place between March 13th and 20th, 1943, at the Sofrino proving grounds. The vehicle arrived on its own, having travelled for 62 km. 280 more kilometers were covered: 93 on a highway and 187 on dirt roads. The mobility trials started off poorly, with a breakdown of the drive sprocket crown after the vehicle hit a ditch. The road wheels heated up and peeled in motion, and one track link broke. This was the result of driving at high speeds, which was forbidden for the Pz.Kpfw.III tank. The SPG showed a top speed of 50 kph on straightaways, the exact speed when German tanks began having problems with road wheel tires. The average highway speed was 34 kph and the average speed on dirt roads was 22 kph.

Another issue, typical of the Pz.Kpfw.III, was poor performance in snow. A half-meter deep snowbank could be negotiated at a speed of 6-7 kph, and a slope of 12 degrees was an impassable obstacle. The situation improved when V-shaped spurs taken from a captured StuG were installed, but then the highway driving characteristics were reduced.

Mobility trials in snow.

434 shots were made at the shooting range, 212 of which were supercharged. The maximum rate of fire was 19-20 rounds per minute, the maximum aimed rate of fire was 12 rounds per minute. Attempts to fire on the move did not give satisfactory results, but the vehicle was not designed for this anyway. Firing from short stops was more effective.

A list of 14 improvements and 9 additional notes was composed as a result of the trials. These included a request for extra fuel tanks, reduction in gaps between the gun shield and mantlet, and alteration of the tow hooks. Nevertheless, the commission deemed the SU-76 on a captured chassis to have passed trials. This allowed the vehicle to enter mass production. To avoid confusion, the index SU-76I was assigned in April of 1943. Variations such as SU-76-I and SU-76i were also used.

Dependence on a foreign chassis

The rates of production required by GKO decree #2758ss were not met. Only 5 partially done gun mounts, 3 casemates, and 5 guns were present at the factory by the end of March. Delays with trials were only a part of the problem, production issues had a greater scope than this. There were not enough workers and equipment, other issues were rampant, especially with subcontractors. Far more than just final assembly took place in Moscow. Factory #37 also had to ensure the proper condition of their chassis, and was dependent on these supplies as well as supplies of guns.

The right track slipped off and the right drive sprocket crown was bent while driving on a dirt road. The first production SU-76I were nearly identical to the prototype.

The requirements for the first batch of SU-76I was approved on March 27th, 1943. These vehicles would be accepted up to April 20th. 20 changes to improve crew working conditions were made before production began, although most of them cannot be seen from the outside. Externally, the SU-76 prototype and the first production vehicles look nearly the same. 

Bigger changes were introduced into the second batch, built as of April 20th. The requirements for this batch were approved on April 1st and included 12 changes. The biggest change was the installation of a big extension to the gun shield (item #6). Additional fuel tanks were also installed (item #8), the sight opening was altered (item #7), as well as the PPSh firing port on the rear hatch (#11) and protective glass on the vision slits (#12). This was the mass production SU-76I. Its mass was slightly reduced at 22.78 tons.

A mass production SU-76I, September 1943. This vehicle had the serial number 3155.

A contract was signed with factory #37 for 135 SU-76I on April 16th. Each vehicle cost 50,000 rubles. This contract was unrealistic from the beginning: it included 15 SPGs built in March, even though March was already over and not a single vehicle had been produced. Most likely the shortfall would be made up in later months.

Whoever drew up the contract did not take into account the experience with the SG-122. Factory #37 could not overachieve as much as they wanted to. There was a shortage of chassis and casemates, only the guns were in sufficient supply. There were 37 of them in stock by April 25th.

The idea to convert the 6 SG-122 that were stranded at the factory into SU-76I came up in mid-April. These SPGs were in need of repairs, but factory #37 refused to do it. It looks like these were the first SU-76I to be built.

The factory received 84 chassis by late April, 45 of which were taken apart, 23 were repaired, and 15 were converted into the SU-76I. 11 were accepted by the customer. 25 vehicles were built in April in total. The rate of production slowed down in May with only 17 built and 15 accepted. None had been accepted by May 25th out of the planned 35. This was caused by a lack of spare parts for the chassis and delays in shipments of S-1 gun mantlets by factory #112.

25 SU-76I were sent to the Self Propelled Artillery Training Center on June 2nd, 1943. Of those 25, 8 (serial numbers 3002 through 3009) had no extra fuel tanks or ammunition rack covers, meaning that they belonged to the first batch.

One of the most distinctive features of the production SU-76I was the extra plate on the gun shield.

The spare parts situation continued to plague factory #37. The factory was supposed to deliver 45 SU-76I in June, but the problems with the chassis left their mark. The gun mantlets were also a limiting factor. Factory #112 had sent only 14 sets by June 20th. The factory only delivered 13 vehicles before then.

There were plans to begin production of components such as road wheels and magnetos at the factory, but there were few domestic components installed on the German chassis: only the lights and air filters. The latter were installed due to the poor performance of German filters.

The factory delivered 20 SU-76I in June. The plan for Q3 was corrected: instead of 45 vehicles per month, only 25 were due. This helped factory #37 turn from slackers to Stakhanovites: 26 vehicles were delivered in June, even greater than the quota. The same rate kept up in August and September of 1943.

Extra fuel tanks on the rear plate and the firing port for the PPSh in the left hatch flap can be seen.

Experience in battle revealed an issue with the commander's station. Practice showed that one PTK sight was not enough. The idea to install a commander's cupola that would radically improve the commander's vision came up in August of 1943. In addition to the UZTM, factory #37's department #22 began to develop this design. A draft of a modernized SU-76I was prepared by August 11th, 1943. An SPG with a cupola was also built in August. Department #22 didn't try and reinvent the wheel, simply taking a Pz.Kpfw.III cupola that they already had in stock. A bulge had to be added to install it. The modernized SPG was accepted into service on September 6th. It is not know how many of these SPGs were built. There are almost no photos of the SU-76I in combat. However, there is no doubt that the modernized SU-76I was mass produced.

Draft of the SU-76I with a commander's cupola, August 1943.

Another SU-76I modernization project had a vastly different fate. The SU-85I was developed under Taleysnik's direction in mid-September 1943. It was based on the SU-76I with a commander's cupola. No images of the SU-85I survive, but a description is enough to imagine what it may have looked like. Instead of the S-1, it was equipped with the D-5S-85, with a similar gun mantlet to the SU-85. The commander's cupola was shifted back by 145 mm. According to calculations, the mass of the SU-85I would have been 21.98 tons, even lower than the SU-76I. This paradox is explained by the mass of the gun: the S-1 with armour weighed 2900 kg, while the D-5S-85 weighed only 2100. The 58 rounds of 85 mm ammunition also weighed less. The project was very promising, but only drafts were made. 

A SU-76I with a commander's cupola in the factory #37 courtyard.

The SU-85I never saw the light of day chiefly due to the situation with SU-76I production. Three factories began to produce a domestic alternative, the SU-76M. The captured chassis turned out to be very demanding, which delayed their departure for the front lines.

The SU-76I was only produced in small numbers. 30 vehicles were planned for October, 31 were delivered. 85 chassis out of the stock of 160 were completed parted out. The plan for 30 vehicles in November was once again overfulfilled, but this was only the swan song of the SU-76I. People's Commissar of Tank Production Malyshev ordered the production to cease in November and shift to producing diesel engines in December.

201 SU-76I were built in total: 1 prototype and 200 production. Moscow had 210 Pz.Kpfw.III and StuG III chassis left, including 91 at factory #37, after production was complete. More than a month was needed to clear this scrap.

A trophy's brief service

As mentioned above, the SU-76I were sent to the army as of early June 1943. The vehicles were sent to the Self Propelled Artillery Training Center and issued from there. The first such unit was the 1902nd Self Propelled Artillery Regiment (SAP) formed in early July 1943. The regiment received 15 SU-76I that were spread out between three batteries of five vehicles each. The regiment had its debut on August 14th, 1943. By this time it had been reinforced with a battery of five SU-122.

The 1902nd SAP was assigned to the 5th Guards Army and attacked height 202.4 near Lozovaya-Krysino. The Germans counterattacked at night and retook the height, but the 1902nd SAP took it again. N.P. Alekseyev's crew excelled, destroying one medium tank, one anti-tank gun, and up to a platoon of infantry. One SU-76I was lost.

The regiment successfully fought alongside the 33rd Guards Rifle Corps and the 57th Tank Regiment. Even though the vehicles were prone to mechanical trouble, the 1902nd SAP remained on the front lines until November 1943. It took a very active part in the liberation of Krememchug.

One of the biggest issues with the SU-76I was lack of a user's manual. One was only published in 1944, when the vehicle's career was coming to an end.

The 7th Mechanized Corps received 70 SU-76I SPGs in August 1943. Later, in September, it received another 35 vehicles of this type. The 58th, 84th, and 177th Independent Tank Regiments received 35 of these vehicles each. Serious issues with reliability delayed their departure for the front. 34 out of 70, nearly half, had various mechanical defects as of September 14th, 1943. There were also complaints about a lack of spare parts.

The 7th MC departed for the front lines with two regiments, the 177th and 84th. The SU-76I did not leave with them, as the 5th Guards Army sounded the alarm on December 14th, 1943: out of 90 SU-76I 40 had mechanical issues. Engine components and assemblies broke down, there were seven cases of gearbox issues. By that point, 33 vehicles were lost in battle.

At the time the Germans found out about the SU-76I. At least one such vehicle was used by the German 23rd Tank Division. It was captured in late December 1943 and served until at least the spring of 1944.

The only known photographs of a SU-76I on the front lines show it in German hands, where it was a trophy of the 23rd Tank Division.

Due to incomplete information from factory #37, it is not possible to establish a full list of SU-76I users. One of the last regiments to receive the SU-76I was the 1456th SAP.

The fate of the 58th Tank Regiment, which did not leave for the front with two other regiments of the 7th MC, was interesting. It remained in reserve until early 1944, when it was transferred to the 2nd Guards Cavalry Division. The 58th TR had its debut on February 28th, 1944, when it burst into the Volyanka village in support of the 288th Rifle Regiment. Two tanks, five machine gun nests, and up to 100 Germans were destroyedin that battle. The regiment later assaulted the German positions in Rozhishe.

The 58th TR continued to use the SU-76I intensively until April 9th, 1944, when it was ordered into reserves. The regiment kept its SU-76I until June of 1944. 11 of these vehicles were irreparably lost. One of these vehicles survived to this day. On February 3rd, 1944, even before it saw battle, the SU-76I with serial number 3038 slipped off the bridge across the Sluch river near Sarny and fell into the river. Its crew was killed. 30 years later it was raised and installed as a monument. Another SU-76I, a former range target, was restored and put on display in Victory Park.

Despite the small amount of vehicles produced, the SU-76I had a notable career. However, experience with production and use just shows that it's best to build vehicles based on a domestic chassis.

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