The SU-12 was accepted into service with the Red Army on December 2nd, 1942. It was a good design with a fatal flaw: the parallel placement of its engines. This layout was the SPG's Achilles' heel. The amount of defects linked to the engine and transmission was very high. Many SU-12s never made it to the front lines. The flawed components had to be drastically redesigned. This is how the SU-15 was born, better known as the SU-76M. This was the Red Army's second most numerous AFV, beaten only by the T-34. This article covers the development and mass production of the SU-76M from the summer of 1943 to the spring of 1945.
T-70 without a roof
Initially, the SU-12's designer S.A. Ginzburg considered the issues with the gearboxes to be caused by low production quality. Factory #38's design bureau had a different opinion. Director K.K. Yakovlev, chief engineer L.L. Terentyev, and chief designer M.N. Schukin prepared a letter describing their opinion of the defects by March 11th, 1943. They blamed not the quality of the gearbox, but its design. The letter proposed some solutions, but a radical change was needed to fully solve the issue. The engine and gearbox from the T-70 tank had to be used.
The military representative at the factory and the Main Artillery Directorate (GAU) supported this decision. State Committee of Defense (GKO) decree #3184ss "On the SU-76 SPG" was published on April 14th, 1943. According to this decree, factory #38 was tasked with production of two SU-76 4-gun batteries by May 1st, 1943. The first battery would be built on the T-70B tank chassis, the second on the SU-12 chassis but with the T-70B engine group.
The first SU-15 prototype, Gorohovets ANIOP, June 1943.
This paragraph of the decree kicked off the work on SU-38 and SU-16 SPGs. Their design had a lot in common with the IS-10 project designed by the factory #92 design bureau. The development of another light SPG family, the SU-74, also began. These vehicles were developed by the GAZ factory design bureau. These SPGs would use ZIS-16 engines. Work on three vehicles in parallel is explained by the importance of the work. The low reliability of the SU-76 threatened to leave the army without a light SPG. There had to be a backup plan.
The gun at maximum depression.
The vehicle with a T-70B transmission group was indexed SU-15 at the factory. Even though the only requirement was to adapt the vehicle for a new transmission and engine, the fighting compartment also changed. This was because the commander and loader's workspaces weren't roomy enough. A large bulge appeared in the fighting compartment that housed the radiator and cooling system air ducts. Since the engine was now only occupying the right side, the air intake was also only on the right. An access hatch to the engine starter was located in the right side of the central front plate. The maintenance hatch for the clutch was located on the right side of the upper plate. A compartment with fuel tanks storing 412 L of fuel in total took its place to the left of the driver.
Maximum gun elevation.
The casemate was widened, which caused an overhang to the left. The gun mount also shifted to the left. It changed slightly and resembled the ZIS-8 mount designed at factory #92. A special beam with a cast foundation held up the upper gun mount. The beam was attached to the front plate in the middle. This design allowed the designers to get rid of the shortened gun trails that took up a lot of space in the fighting compartment.
The fighting compartment kept its SU-12 style roof. The total mass of the vehicle was 11.2 tons.
Like the SU-12, the SU-15 had a closed fighting compartment.
There are two myths regarding this vehicle that migrate from one source to another. One was that N.A. Astrov had something to do with this design. In reality, Nikolai Aleksandrovich was developing the SU-15's competitor, the SU-74. The second myth had to do with the production of this vehicle. No single SU-15 prototype was built. The GKO decree requested a 4-gun battery, and thus that was the amount of vehicles made. They were built in April of 1943 and received serial numbers 15-601 through 15-604.
The SU-38 and SU-74 showed serious defects during trials. The vehicles had to be radically changed as a result.
Mobility trials showed that the SPG had to be lightened.
The first SU-15 began trials on April 21st, 1943. The vehicle was supposed to travel for 1000 km, 300 of which were by highway. The trials act stated that assembly of vehicle 15-601 was rushed. It was built with a refurbished engine, a used clutch, and an old idler with a rubber rim.
The vehicle drove for 804 km by April 30th. Instead of a cobblestone highway the only road available was a muddy dirt road. The average speed in difficult conditions was 15.2 kph. The SPG used up 280 L of fuel per 100 km. A T-70B tank was sent for comparative trials. The SU-15 ran a little cooler, both vehicles drove equally smoothly. Gunnery trials showed that the crew was positioned comfortably.
One of the pilot SU-15M vehicles, August 1943.
SU-15 15-601 travelled for 960 km by mid-May of 1943. The other three vehicles drove for 50-100 km by then. In general, the design was good, but the commission had a number of complaints. High wear of the running gear was among them.
A SU-15 and three SU-38 were sent to the Gorohovets ANIOP for trials held between June 4th and 16th. The vehicles passed gunnery trials but showed the same running gear issues. High wear of tracks and other components was recorded.
The Red Army's Main Armoured Vehicle Directorate (GBTU), the organization responsible for SPGs as of late April of 1943, considered the cause to be excess weight. In addition, some ideas for improvement came up after studying captured German equivalents of the SU-15, the Marder II and Marder III. A decision was made to reduce the armour thickness to 25 mm in front and 13-15 mm on the sides. This was enough to protect the crew from small arms fire. The GBTU also decided to remove the roof and armour from a portion of the sides and the rear. This reduced the weight to 10.5 tons. The SPG received a tarp to protect it from precipitation. These changes as well as the limit of the top speed to 30 kph were approved by GKO decree #3760ss published on July 17th, 1943.
A SU-15M that went through trials at the NIBT proving grounds in August of 1943.
The SPG converted in this way was designated SU-15M. The SU-38 was changed in a similar way and renamed SU-16. The SU-74 underwent even larger changes, but by early June it was clear that the SU-15M was more promising. Its fighting compartment was better laid out and even the first prototype with many issues turned out more reliable than the SU-12 and on par with the SU-38. GKO decree #3703ss dated July 8th, 1943, ordered factory #38 to produce a pilot batch of 25 SU-15M vehicles. Five SU-15Ms were being assembled on July 24th and the whole batch was ready by August 1st. There was not a word about any second SU-15 prototype described in a number of publications. People's Commissariat of Tank Production (NKTP) order #360 issued on July 9th, 1943, planned production of 225 SU-15M in the 3rd quarter of 1943.
The fighting compartment of the SU-15M had no roof.
Two SU-15M and one SU-16 arrived at Kubinka in early August. The SU-15M was also referred to by its better known name at this point: SU-76M. Trials at the NIBT proving grounds took place from August 10th to August 20th, even though the SU-15M was already in production for over a month.
Factory #38 didn't just reduce the mass during modernization. A number of changes were made to the engine-transmission group and running gear. The driver's hatch received a vane sight for rough aiming. A vision port with a shutter and bulletproof glass was added in the front plate, which improved the commander's vision. The testers noted that the observation periscope mounts were flimsy and the periscopes themselves were poorly produced. Issues with prisms were common among Soviet tanks and SPGs at the time.
A tarp protected the crew from precipitation. It was easier to install than on German SPGs.
Thanks to the lightened design, the difference in power to weight ratio between the SU-15M and the SU-16 was not high. The SU-16 had a smaller fighting compartment. During trials SU-15M with serial number 57402 travelled for 1016 km (of those 309 on a highway and 707 on a dirt road), 57406 travelled for 953 km (304 on a highway, 649 on a dirt road). The first had an average movement speed of 26 kph, the second 34.1 kph. The top speed was 43 kph, higher than required. The average driving speed on dirt roads was 17.5 kph and 21.5 kph respectively. Trials showed that the highest grade this vehicle could climb was 28 degrees and the highest tilt it could drive at was 30 degrees.
Trials at the NIBT Proving Grounds, August 1943.
The SU-15M and SU-16 both passed trials. Issues with the design common with the T-70B were observed. The SU-15M was preferable. It was equally reliable but more comfortable for the crew, had less load on road wheels, and a smoother ride. The vehicle also had a high rate of fire (at least 10 RPM). The SU-15M's victory was confirmed by trials. Production was already underway at factory #38. 47 SU-76M out of a planned 45 were delivered in August.
Replacement for light tanks
The summer of 1943 marked some serious changes for Soviet tank building. The Battle of Kursk gave the GBTU a lot to think about. Doubts about the effectiveness of Soviet light tanks began to creep in. The T-70B light tank had little chance of success in battle with not just enemy heavy tanks, but medium ones as well. The effectiveness of the T-80 light tank that was just entering production at factory #40 with great difficulty was also in question. Even if the long barrelled VT-43 gun was installed on it, its future in battle against a German tank looked grim.
SU-76M produced at factory #38 in August of 1943.
On August 21st, 1943, Stalin signed GKO decree #3964ss "On production of 76 mm SPGs at the People's Commissariat of Medium Machinebuilding Gorkiy Automobile Factory and factory #40. This document killed the T-80 tank which had yet to begin true mass production anyway. The factory would instead deliver the first 15 SU-76M SPGs in September, 50 in October, 100 in November, and 150 in December. As for the GAZ, the first 15 SU-76M were due in October, after which the quota increased drastically: 200 vehicles were due in November of 1943, 300 in December. The Gorkiy factory had the means to achieve this. The SU-76M would become a truly mass production vehicle.
The first SU-76M SPGs driving out of the GAZ Molotov factory, October 1943.
As of August 25th, 1943, factory #38 had 58 SU-76M SPGs with gearbox defects. This time the problem wasn't with the layout, but the quality of gearboxed assembled in Miass and the ZIS factory in Moscow. These gearbox defects were also encountered on the T-70. The defective gearboxes had to be urgently replaced. There were also issues with engines, for example those revealed by warranty trials of SU-76M SPGs assembled by factory #40 in September.
Gearbox defects composed 37.6% of overall defects for 1943, electrical defects made up 19%, engines - 11.8%, and the main clutch was responsible for 10%. Even with these defects, the SPGs was more reliable than the SU-12.
SU-76M produced in October 1943 by factory #40 with experimental track extenders.
Factory #40 began assembling SU-76Ms in late September of 1943. The delay was caused by lengthy setup of parts production. Nevertheless, the factory managed to deliver 15 vehicles by October 1st. Factory #38 delivered 78 SPGs out of the planned 75. Kirov also overdelivered in October: 102 SPGs instead of 100. The GAZ delivered 16 vehicles instead of 15.
Factory #40 delivered their 50 vehicles in October, but instead of 100 SPGs in November they only delivered 75. 70 were delivered in December. The issue was with a lack of backlog and a large amount of components and assemblies rejected by QA.
Rear-mounted travel lock. This design was used until the spring of 1944.
Issues with subcontractors (especially delivery of GAZ-203 engines) plagued other factories too, but the quota at Gorkiy and Kirov was still habitually overfulfilled. GAZ and factory #38 delivered 225 and 126 SPGs respectively, and 360 and 136 in December. This made up for the shortage at factory #40.
There were issues with organization, too. Of the three factories that built the SU-76M one belonged to the NKTP (factory #38) and two to the NKSM (#40 and GAZ). Cooperation was hampered by several issues linked to technical documentation and interchangeability of components. Factory #38 was the lead factory when it came to the SU-76M, but in reality each factory introduced their own changes as necessary. The GBTU wrote a letter to Beria on November 10th, 1943, proposing the transfer of factory #38 to the NKSM, but nothing came of it.
Extra travel clamp added in December of 1943. Different factories had different designs.
In addition to fighting quality issues and ramping up production volume, improvements were implemented. This was mostly done by factory #38. It turned out that the stock travel lock used in the ZIS-8 gun mount was not enough for the gun. An additional travel clamp was developed by December of 1943. It was installed on the upper part of the front plate. Trials showed that it took 23 seconds to switch to fighting mode and 40 seconds to switch to travel mode. The additional clamp was quicker to engage or disengage than the travel lock. A set of light indicators to quickly send signals to the driver was developed in December of 1943, but it was only put into production in 1944.
GAZ SU-76M, March 1944 production.
The gearbox quality issues were so serious that production in Miass nearly ceased in January of 1944. Military QA was hurriedly organized to deal with this issue. Nevertheless, gearbox issues pursued this vehicle for some time. As of February 8th, 1944, the GAZ had 97 SU-76Ms that could not be delivered due to gearbox issues.
Vehicles produced at different factories started to differ visually in early 1944. This was most noticeable in GAZ vehicles. In January-February changes were introduced including addition of a visor to protect the air intakes from water, thicker rear plate, altered suspension arm mounts, changes to mounting points of various items. There was also work done on fully isolating the engine, new rear tow hooks, improved ammunition racks, and a new latch for the rear door. The aforementioned travel clamp was also different.
The most noticeable change was introduced in Mytishi: a perpendicular rod to make installation of the tarp easier.
The same vehicle from the left.
The SU-76M was built in large numbers despite a shortage of engines. Factory #40 delivered its quota of 75 vehicles in January of 1944, factories #38 and GAZ overfulfilled the quota again, delivering 141 and 316 vehicles respectively. In February 85, 141, and 341 SPGs came out of Mytishi, Gorkiy, and Kirov, 87, 176, and 365 in March. It took a lot of effort to keep up this rate of production. Issues with gun supplies cropped up in early 1944. Now, instead of factory #92 the guns came from factory #235. There were many complaints about the 12-RT radios and cracks in the hulls.
Vehicles from different factories were visually different by the spring of 1944.
The GBTU noticed the unauthorized changes to the design. A list of 24 changes was composed and approved at meetings in mid-April at factory #38 and GAZ. In addition to the aforementioned changes, the travel lock was changed. The rear lock was removed and the front clamp could now only be disengaged by the driver. A spent brass catcher was added, but it was really only used at factory #38.
The new changes were supposed to be introduced starting in May of 1944, but really they were only added in June. The SU-76M received idlers with rubber rims, new suspension arm buffers, and a three light signalling system. The vehicles from various factories still had their differences. Each factory still had its own travel clamp. Factory #40 introduced its own changes on top of those approved at the GAZ.
SU-76M SPGs produced in Mytishi had a perpendicular beam that helped with installing the tarp as of early 1944.
Factory #40 introduced changes as of the first SPG built in June, factory #38 only started in July. The GAZ introduced some changes as early as April. The new travel clamp was added on April 26th, a portion of the toolboxes and ammunition racks was also changed in April, and the main clutch was improved. A latch for the rear door, light signals, and a rack for an unditching log were added in May. A T-34 type tow hook was installed starting on May 20th. Meanwhile, even GAZ SU-76Ms were built without rubber rimmed idlers.
Left to right: travel clamps used at factory #40, factory #38, and the GAZ. They were visually distinct and introduced at different times.
Interestingly enough, only factory #38 began modernization and improvements without dragging their feet. Reports from factory #40 indicated that the Chief Designer's Department only takes action after pressure from military QA. The situation at the GAZ was even more interesting. Factory management was pressuring the military representatives as of late 1943. Chief Designer A.A. Lipgart habitually rejected new orders. This was linked to plans to organize civilian production. In early 1944, V.A. Grachev left the GAZ after a row with Lipgart. He was irreplaceable when it came to military wheeled vehicle development. There was periodic friction between the chief designer and military QA.
Idler with a rubber rim introduced in May-June of 1944.
Rates of production grew despite issues with gearboxes, engines, and other components. Factory #38, 40, and GAZ delivered 176, 87, and 365 vehicles respectively in April of 1944. In May the factories delivered the same amount, in June - 175, 90, 370. A decision was then made to transfer factory #38 to Kharkov to rebuild factory #75. Initially, a proposal was made to build AT-45 prime movers at this location, then T-44 medium tanks. Nevertheless, factory #38 managed to deliver 141 SU-76M SPGs in July. The role of chief developer was transferred to factory #40.
SU-76M fighting compartment. Even though it was small, the inside was rather roomy.
The load on other factories increased when factory #38 was disbanded. The GAZ delivered 386 SPGs and factory #40 delivered 100 in July. In August this was 430 and 130, in September 450 and 130, 425 and 150 in October, the same number in November, 451 and 150 in December. The monthly production volume reached 600 units once more. In reality, the GAZ finished 470 vehicles in December, but People's Commissar of Medium Machinebuilding Akopov ordered that only 451 be delivered. Quality was slowly increasing as well, although the increased production volume made this difficult.
Spare road wheel installed on GAZ vehicles as of January 1st, 1945.
Factory #40 began development of a new hull in the fall of 1944. This will be covered in a separate article. Both factories continued working on improvements. GAZ vehicles received a hatch linking the engine and fighting compartments, which made evacuation easier for the driver. A new travel clamp was added in January, and the rear plate near the tow hook was reinforced. A spare road wheel on the rear plate was added. The location of the engine shifted slightly in March.
A hub for light SPG development began to grow in Mytishi. It fully formed only after the war when Astrov came to the factory. Introduction of new elements into production at factory #40 lagged behind, for instance the engine compartment door was only introduced after March of 1945. A new travel clamp was introduced in March. Mytishi SU-76Ms didn't get a spare road wheel even in May of 1945 when both factories transitioned to new hulls.
SU-76M vehicles produced at the GAZ received a hatch linking the engine and fighting compartment in December of 1944.
Factory #40 delivered 140 vehicles in January of 1945, the GAZ delivered 435. The same number was delivered in February. In March 150 vehicles came out of Mytishi and 450 from Gorkiy. The rate of 600 vehicles per month continued even after production transitioned to the modernized hull. This variant of the SPG, which will be covered in a later article, can't be called "post-war production" as at least 600 of them were built during the Great Patriotic War.