The history of Soviet light SPGs is inseparable from the city of Gorky, modern day Nizhniy Novgorod. Guns installed on Soviet light SPGs were developed and built here. The ZIS-30, the first Soviet wartime light SPG, was also created here. Gorky housed the main manufacturing base of T-60 and T-70 tanks, which were used as SPG chassis. It's not surprising that the design bureau of the Molotov GAZ factory eventually switched to building SPGs. The GAZ-71 and GAZ-72 SPGs might have become the main light SPGs of the Red Army.
Mandatory competition
Work on SPGs was not considered high priority at the Molotov GAZ. The factory had enough work in its primary field. Production shifted from the T-60 to the superior T-70 in the spring of 1942. This was not the first vehicle created in Gorky: the TM (Molotov Tank) amphibious reconnaissance tank was designed here in 1936 under the direction of V.V. Danilov. This was a very unusual vehicle, equipped with a pair of GAZ AA engines. The TM did not progress past a prototype, but the GAZ-70, aka T-70, was a real ace in the sleeve of the Soviet tank industry and the Red Army. Thanks to this tank, the vacuum left by the cancellation of the T-50 could finally be filled.
Of course, the T-50 was superior to the T-70, but you have to fight with the army you have. The T-50 never made it to mass production, but the T-70 was built to match the capabilities of wartime industry. It's not surprising that this tank was the second most numerous tank to be produced during the war, after the T-34. The chassis of the T-70 was also perfect for the development of SPGs.
Tactical-technical requirements for a 45 mm tank destroyer. This vehicle was to be developed by GAZ on the chassis of the T-70 tank.
The center for medium SPG development in the first half of 1942 was in Sverdlovsk. Factory #37 was evacuated here in late 1941. Department #22, reborn on its new grounds, worked on the creation of light SPGs in addition to their work on the T-30 and T-60. The design bureau worked closely with S.A. Ginzburg, putting his idea of a "universal chassis" on the T-60 platform into fruition. This was the start of the development of the SU-31 and SU-32 SPGs.
One of these vehicles could have gone into production, but fate intervened. GKO decree #2120 "On the organization of T-34 production at Uralmash and factory #37" was published on July 28th, 1942. According to this decree, factory #37 was included into the Ural Heavy Machinebuilding Factory (UZTM), and production of light tanks ceased. Work on light SPGs in Sverdlovsk also stopped. Materials on the SU-31 and SU-32 were passed onto factory #38 in Kirov, where Ginzburg began working closely with the factory design bureau under the direction of M.N. Schukin.
GKO decree #2429 "On the production of experimental SPGs", the starting point for the development of the SU-11, SU-12, SU-71, and SU-72. The SPAAG did not make it into the initial draft, and was written in by Stalin personally.
Trials of the SU-31 and SU-32 continued until September of 1942. As a result, the choice was made in favour of the "31" chassis, with a parallel placement of GAZ-202 engines. This layout was used at factory #38. On the other hand, the GAU and GABTU decided to have a backup plan. Serious delays took place in every light SPG development program, and an idea came up to enlist the help of the Molotov GAZ design bureau. Deputy Chief Designer N.A. Astrov became the head of this project. At the time, the bureau was working on the modernization of the T-70 tank, but did not refuse this urgent order. Work on yet another SPG began. In case Ginzburg and factory #38 failed, it would have become the long-awaited SU-76.
Another way
The tactical-technical requirements for new SPGs were completed by October 16th, 1942. The brass decided to not reinvent the wheel and give largely the same requirements as were given for the SU-31 and SU-32. Even the layout was the same as for the Sverdlovsk SPGs. For instance, the "76 mm assault SPG" was based on the chassis that was built with T-70 components. This meant that it used the GAZ-203 dual engine. This seems interesting, as the GAU rejected the same setup on the SU-32, since it overheated. GAU Chief, Colonel-General N.D. Yakovlev and the Deputy of the People's Commissar of Defense, Colonel-General N.N. Voronov knew about these trials, but signed the requirements anyway.
The 57 mm IS-1 anti-tank gun was considered as an alternative to the ZIS-3. This gun was a reworked version of the ZIS-2 gun, designed at factory #92 under Grabin's direction in the summer-fall of 1942. The same gun would be used on the ZIS-41 halftrack SPG. The requirements stated that a ZIS-3 armed SPG had to carry 60 rounds of ammunition. The weight limit was 10 tons, and the maximum height of the vehicle during travel was 2 meters. The calculated top speed was 45 kph, and the range was 200-250 km.
Cutaway diagram of the GAZ-71. As you can see, the layout of the vehicle is unusual.
The chassis had to allow for a SPAAG to be developed on the same base. However, the requirements for a 37 mm SPAAG were filed separately. This vehicle was nearly identical in its layout to the SU-31, including the parallel placement of GAZ-202 engines. The requirements for the chassis were the same as for the 76 mm assault SPG.
A third vehicle on the chassis of the T-70 appeared. On the same day, Voronov and Yakovlev approved a "45 mm SPG". This vehicle would use the 45 mm M-42 gun, recently accepted for service by the Red Army. The gun would be installed on the T-70 tank. The whole tank, not just the chassis.
The chassis looked even more unusual from above.
Stalin signed GKO decree #2429 on October 19th, 1942, titled "On the production of experimental SPGs". The SPAAG was not included into the original draft, but added later.
"2. The People's Commissar of Tank Production (comrade Zaltsmann) and People's Commissar of Medium Machinebuilding (comrade Akopov) must immediately create prototypes of 76 mm SPGs on the T-70 chassis, delivering them for proving grounds trials by November 15th of this year.
3. The People's Commissar of Medium Machinebuilding (comrade Akopov) must immediately build a prototype of an SPG with a 45 mm gun on the T-70 tank chassis, delivering it for proving grounds trials by November 20th of this year.
4. The People's Commissar of Tank Production (comrade Zaltsmann) and People's Commissar of Medium Machinebuilding (comrade Akopov) must build and deliver for trials samples of SPAAGs with 37 mm guns on the T-70 chassis by December 1st of this year."
All three SPGs were to be designed at the Molotov GAZ factory. The 76 mm assault SPG was indexed GAZ-71. The lead engineer on this project was V.S. Solovyev. The SPAAG received the index GAZ-72, with A.S. Malakov as the lead engineer. Finally, the 45 mm SPG on the T-70 chassis received the index GAZ-73. Major P.F. Solomonov, who oversaw work on SPGs since the fall of 1941, curated the work from GAU's side. According to plans, the GAZ-71 was to be done by November 15th, the GAZ-73 by November 20th, and the GAZ-72 by December 1st, 1942.
Diagram of the transmission of the GAZ-71 and GAZ-72
The Molotov GAZ design bureau treated the requirements rather frivolously, as did the design bureau of factory #38. Primarily, this had to do with the layout of the SPGs. Neither Kirov nor Gorky intended to design a vehicle with GAZ-203 engines. This is a reasonable decision, since, as mentioned above, this engine overheated on the SU-32. It's not surprising that GAZ-202 engines placed in parallel were preferred.
The life of the GAZ-73 project was brief. No imaged of this vehicle remain, but it would generally be similar to the IS-10 SPG, designed at factory #92. GAZ quickly understood that such a vehicle is senseless. Work did not progress past the design stage. In order to place the weapon properly, the vehicle would have to be 20 cm taller. The fighting compartment was still small, and the mobility of fire and rate of fire would be low. Work on the GAZ-73 took a different path by the end of November of 1942: now it was being built on the GAZ-71 chassis. Instead of turbocharged GAZ engines, it would use ZIS-16 engines. The last mention of this vehicle is dated November 29th, 1942. No work was performed after that.
GAZ-71 at the factory, December 1942.
The GAZ-71, also called SU-71 in correspondence, was doing much better. It was not build by November 15th, as GKO decree #2429 required. However, it was built by November 28th, and was being prepared for factory trials. The SPG was rather original. Formally, the SU-71 was based on the T-70B chassis, but many changes were introduced. The drive sprockets and final drives were moved from the front to the rear. The idlers migrated to the front, and also lost their rubber rims. The GAZ MM gearbox and friction clutches migrated under floor of the fighting compartment, on the right side. The fuel tanks were on the left side.
Unlike the SU-31, the gearboxes were not placed separately along the sides of the hull, but placed near each other. The friction clutches were nearby too. The main clutches were positioned in such a way that it was possible to turn them on and off individually, making it possible to drive with only one engine. The engines remained in the front of the SU-71, but they were placed next to each other, shifted to the right. The driver's station was moved to the left.
The same vehicle from the front.
The hull of the SU-71 was also very original. The front was built from two, not three parts. The lower part had hatches to access the engine starters, and the driver's hatch and engine access hatch were on top. The gun mount was also different. only the oscillating part of the ZIS-3 and its upper mount were taken. The upper mount was installed in a hole on top of the front plate. This kind of design was envisioned at factory #37, but was never implemented. This solution made the fighting compartment roomier than on the SU-32. The recoil mechanisms were covered by a complex plate.
The fighting compartment.
The upper sides of the hull and casemate were made in one piece, and were sloped. This gave the SU-71 a roomy fighting compartment. However, the floor was elevated, since the transmission and fuel tanks were underneath. The fighting compartment was accessed through a large two-piece hatch in the upper rear plate of the casemate. The radio was placed to the left of the compartment, but the commander's station and his periscopic observation device were to the right. The ammunition was housed underneath the gun (15 rounds), and in crates along the sides of the fighting compartment (three creates to the right and one to the left, their lids acting as seats during travel). Eight more rounds were held on the rear wall of the compartment. Since the SU-71 had no fenders, most of the tools were kept inside the fighting compartment.
Original, but unreliable
The issues that were discovered during the development of the GAZ-73 were the first, but not the last, misfortune encountered by the Molotov GAZ design bureau. As mentioned above, the SU-71 was being prepared for factory trials on November 28th. Meanwhile, factory #38 had not only already designed its own vehicle, indexed SU-12, but built it, and performed factory trials, which ended on November 27th. It was scheduled to be sent to the Gorohovets proving grounds (ANIOP) for trials on November 30th. Work in Gorky was moving slower, which is why their SPG already fell out of favour by December. GKO decree #2559 "On the organization of SPGs at the Uralmash factory and factory #38" was published on December 2nd, 1942. The Gorky SPG lost even before the comparative trials began.
Gun at maximum elevation.
Despite the decision of the State Committee of Defense regarding SU-12 production, comparative trials of the SU-12 and SU-71 were still proceeding as planned. The SU-12 arrived at the Gorohovets ANIOP on December 5th. By that point, the SPG had driven for 150 km.
As for the SU-71, its delivery was behind schedule. A member of the trials commission, Major Solomonov, was sent to the GAZ on December 3rd. A deadline for the reception of the SU-71 was set in the ensuing negotiations with the head of the commission, Lieutenant-General of Artillery V.G. Tikhonov in attendance: December 6th. The vehicle did not arrive in time once again, and only a second visit by Tikhonov ensured that the SPG was sent to the proving grounds. However, the SPG was turned back due to a defect in the cooling system. As a result, the SU-71 only reached the proving grounds by December 9th. After the factory trials program was completed, it returned to the factory on the next day.
The crew working in the fighting compartment.
The SU-71 returned to trials only on December 15th. The head of the GAZ design bureau, V.A. Dedkov, and Kulikov, the military representative, arrived with it. The SU-71 fired 64 shots and travelled for 350 km by then. Proper trials of the suspension were never performed, as the vehicle was pursued by technical issues. As a result, the SU-71 only went through gunnery trials. A further 235 shots were fired to test the reliability of the gun mount.
GAZ-71 from the rear.
Technical breakdowns notwithstanding, the SPG was not entirely satisfactory from the point of view of requirements. Instead of 10 tons, like the requirements stated, the SPG weighed 11.75. This kind of overloading resulted in engine trouble and other defects. The vehicle was 15 cm taller than required, and the elevation and traverse angles were insufficient. It was not possible to establish the top speed due to technical issues, but there is a sneaking suspicion that it would not have been 45 kph. One of the few positives was the gun mounting method. The overall verdict was not unexpected: the SPG cannot be recommended for service, and attempts to improve the design would be senseless.
GAZ-72 SPAAG, created on the GAZ-71 chassis.
The GAZ-72 SPAAG was lost in the troubles of the GAZ-71/SU-71. Even its exterior is practically unknown. This is because work on the GAZ-72 took even longer. The hull was not ready by November 28th. The factory management optimistically estimated that it would be ready by December 6th, but that was not the case. Overall, the design of the vehicle was the same as the GAZ-71, but the rear housed the oscillating part of the 61-K 37 mm AA autocannon. It differed little from the one used on the SU-31. The rear of the vehicle had to be widened to fit the gun.
The poor performance of the GAZ-71 meant that the SPAAG was never even tested.
After the SU-71 was rejected, interest in the GAZ-72 waned. Since these vehicles shared a chassis, it was obvious that similar problems would be encountered during trials. There were also additional issues with servicing the transmission, as the gun would have to be removed. It is not surprising that the GAZ-72 never made it past factory work.
However, this was not the end of light SPGs at the Molotov GAZ. The GAZ-74 SPG entered trials in May of 1943. This vehicle deserves a separate article.