The T-50 tank was considered the highest priority platform for light SPGs in the pre-war Soviet Union. However, a proposal for an SPG chassis based on the T-40 amphibious reconnaissance tank was made at a meeting on June 9th, 1941. The idea was quickly abandoned, and, a few weeks later, the USSR was too busy for the T-50 SPG. Suddenly, the first wartime light Soviet SPG turned out to be the ZIS-30, which used the chassis of the Komsomolets artillery tractor. Due to the cancellation of artillery tractors, the idea of building an SPG using T-40 components came out of retirement. The result was a small family of experimental vehicles, such as the SU-31 SPAAG and SU-32 SPG.
Universal chassis
Soviet plans to build 3000 ZIS-30 SPGs were crushed by harsh reality. The Komsomolets was removed from production in August of 1941, and the SPG was left with no chassis. The plan was reduced to only 100 vehicles.
This was only the beginning of issues with mechanization of Soviet artillery. In September of 1941, factory #183 was evacuated to Nizhniy Tagil, and production of heavy Voroshilovets tractors ceased as well. In the fall of 1941, Chelyabinsk stopped producing the S-65 tractor and S-2 artillery tractor. The reason for this was the need to increase KV tank production. Finally, the Stalingrad factory began to think about taking the STZ-5 tractor out of production. The reason for this was the same: increasing tank production, this time, the T-34. In the end, the STZ-5 remained in production, but in reduced amounts. The Red Army Main Artillery Directorate (GAU KA) was in danger of losing mechanized artillery movers.
Draft of the "Chassis 31", May 1942. The vehicle that was built in metal had a similar layout.
The most logical solution at the time was the development of SPGs. GAU didn't reinvent the wheel, and decided to resurrect pre-war plans for self propelled artillery. There were a number of alterations, however, especially for light SPGs.
Due to a lack of alternatives, the idea of a T-40-based chassis resurfaced. However, neither the T-40, not its replacement, the T-30, were in production in November of 1941. The new chassis came from the T-60 tank, which used many identical components. An SPG chassis of this type was included in the plan for 1942. S.A. Ginsburg supervised this project.
On January 29th, 1942, a draft set of requirements was developed for a project of a self propelled chassis based on the T-60.
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- The universal chassis is built, primarily, from T-60 components and assemblies, to be used for the following SPGs:
- 76 mm assault gun for support of moto-mechanized forces using the USV or ZIS-5 gun.
- Self propelled 37 mm AA gun mod. 1939.
- A tank with a 37 mm AA gun in a turret, based on Savin's group project.
- A light tank with armour up to 45 mm and a 45 mm gun in a turret with a coaxial machinegun.
- An armoured carrier for infantry, munitions, and other purposes.
- The top speed of the chassis will be 45 kph, according to gear ratios. For some variants, the top speed may be lowered by altering the gear ratios.
- Since the aforementioned designs will be heavier than the T-60, which is already strained for power due to a lack of more powerful engines, a layout must be developed, according to the attached diagram, with two GAZ-202 engines in the front on the sides of the hull, with the driver between the engines and the rear fighting compartment. Each engine works through a gearbox, its own individual clutch, and final drive. The final drives will be joined by a geared or elastic clutch. The latter will be especially developed to withstand shock that may be generated while changing gears. Gas and the gearshift will be joined. The suspension will be the T-60 suspension with five road wheels.
- The main armour must be equal to the armour of the T-60, approved for 1942. Calculations of characteristics of the chassis for use with heavier armour (37, 40, 45 mm) of the main vertical plates must be provided.
- For self-defense at close range, the crew must be armed with a PPD or PPSh submachinegun and grenades, 700 rounds per submachinegun and 5 grenades per person.
- The platform must have a cruising range of 200-250 km."
The initial concept of "Chassis 32" for a 76 mm SPG.
As you can see, the result of the project would be not only SPGs, but a light tank. Ginzburg was gunning to create a replacement for the T-60, the tank that "ate" his brainchild, the T-50, just a month ago. However, the GAZ factory's design bureau, under the supervision of N.A. Astrov, was nearing completion of the GAZ-70, which later became the T-70. The tank was finalized by February 14th, and Ginzburg was too late. Meanwhile, some questions came up regarding the 76 mm SPG.
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- The gun is designed for cooperation with moto-mechanized units in action against tanks, dugouts, fortified settlements, and enemy personnel (with shrapnel).
- The mass of the vehicle will be 7.5-8 tons, and the mass of the special trailer will be 3.5-4 tons.
- The system will have bulletproof and fragment-proof armour from the front, and partially from the sides. The crew, engine, fuel, and ammunition will be armoured in the same way as on the T-60.
- The ammunition capacity must be at least 20 single piece rounds.
- The horizontal range must be at least +/- 7 degrees (as much as possible), with a vertical range of -5 to +30 degrees.
- The design of the system must allow for firing without special rests (trails).
- The crew (including the driver) will be 4-5 men."
The mass and ammunition requirements are puzzling. One can confidently say that it would be impossible to build an SPG with these parameters. The increased number of road wheels per side mean an increased hull length, but further development showed that it would not be enough.
Nevertheless, the first step was made.
Factory #37's courtyard
While work on composing requirements went on, Sverdlovsk was ramping up T-60 production. Factory #37, evacuated from Moscow, initially built tanks out of leftover T-30 parts, building actual T-60s only in February of 1942. On March 9th, 1942, GKO decree #1417 "On organization of production of T-70 tanks at factories #37 and 38" was published. The production of T-60 tanks that was set up with such difficulty had to make way for a new tank.
For obvious reasons, the factory management had a few unkind words with the GABTU. S.A. Ginzburg, who understood what was going on, took the factory's side. On April 12th, 1942, GKO decree #1581 "On production of T-60 tanks at factory #37" was signed. Factory #37 was allowed to produce the T-60 until August.
SU-32 at the factory courtyard, July of 1942.
Meanwhile, in February and March of 1942, consultations and clarifications of requirements for the new light SPG chassis were taking place. Here, the interests of Ginzburg and factory #37 coincided. The factory wanted to preserve production of the T-60, and Ginzburg wanted a platform to bring his developments to life to replace the one he lost when factory #174 was evacuated from Leningrad and T-50 production was cancelled.
The gun at maximum elevation.
Initially, the position of these new allies were very shaky. On April 15th, a plenum of the GAU Artkom took place, dedicated to self propelled artillery. This was the starting point for practical work on Soviet SPGs. Naturally, discussion of light SPGs took place, which was recorded in the plenum minutes.
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- Confirm the correctness of requirements from the GAU Artillery Committee regarding the need for the following SPGs in the Red Army:
- Bunker destroyer SPGs
- Tank destroyer SPGs
- Assault guns to support motorized infantry
- Self propelled AA guns to support tank or motorized forces
- Consider it necessary to introduce a self propelled howitzer into the Red Army to combat dugouts and enemy personnel.
- Consider it necessary to distribute orders to factories to produce the following types of SPGs:
- Assault guns:
- 76 mm USV or ZIS-3 divisional gun on a universal chassis made from T-70 components. Assign this work to factory #37 with aid from factory #92."
Since the T-70 was already accepted into service, the light tank vanished from lists of experimental work. Due to a lack of need, the munitions carrier vanished as well, and a separate drama developed around the AA tank. As for using the T-70 as a universal chassis, the GAU's requirements were interpreted very creatively.
Unlike on the T-70, the driver of the SU-32 sat on the right.
Without rejecting their assignment, factory #37's design bureau began working on two variants of the chassis side by side. Formally, N.A. Popov acted as the chief designer, but blueprints and correspondence bears another man's name: G.S. Surenyan. The same Surenyan who proposed assault guns on the T-40 chassis in August of 1941.
Based on his activity, Surenyan was one of the main proponents of SPGs at factory #37. It's hard to argue with him: the T-60 had a 20 mm gun, and the T-70 had a 45 mm gun. Both weapons were outdated by 1942, which reduced the tank's value on the battlefield. Instead, production of SPGs with the same armour and more powerful armament was a bright idea. The correctness of this idea is confirmed by its further development: by the fall of 1943, the SU-76 entered production in place of light tanks.
Steps were added in the rear to help the crew enter the fighting compartment.
Officially, the factory's design bureau began work in mid-April of 1942. By late May, calculations and drafts were ready. Assault and AA variants were also worked on at the same time, in two variants: one with the 37 mm 61-K gun, and one with the 25 mm 72-K gun. For understandable reasons, the T-60 chassis had priority. It was indexed "Chassis 31" or "Object 31". K.E. Istomin, an engineer-designer from department 030, was the senior engineer on this project.
In many ways, Object 31 matched the requirements developed in January of 1942. Parallel GAZ-202 engines provided power, the cooling system, gearbox, and clutches were taken from the T-60. In an emergency, the steering system allowed the tank to drive with only one working engine. The gearboxes were connected with one long lever. The suspension was taken from the T-60. The length of torsion bars increased from 1718 mm to 2225 mm, and the number of road wheels increased to six per side.
SU-32 fighting compartment, not exactly roomy. Later vehicles had an enlarged fighting compartment.
According to the project, "Chassis 31" could use a 25 mm AA gun mod. 1940 72-K, 37 mm AA gun mod. 1939 61-K, or the 76 mm ZIS-3 mod. 1942. In all cases, the SPG had a backup DT machinegun. The thickness of armour of the AA variant was 10 mm all around. The assault gun had 30 mm in the front and 15 on the sides and rear. The SPAAG with a 25 mm gun was indexed BGS-1, the 37 mm SPAAG was indexed BGS-2, and the assault gun was called BGS-3. Their mass was estimated at 9500 kg, 9700 kg, and 9900 kg respectively.
The SU-32 received three-colour camo by August of 1942.
The second project was called "Chassis 32" or "Object 32", and consisted of a universal chassis made from T-70 components. Engineer-designer N.N. Efimov from department 030 was in charge of this project. The chassis used the GAZ-203 engine, gearbox, cooling system, and final drives from the T-70 tank. The suspension was taken from the T-70, but since the factory didn't produce this tank yet, the T-60 chassis with lengthened torsion bars was used.
Unlike Chassis 31, Object 32 would use the 76 mm ZIS-5 tank gun. The assault variant of Chassis 31 had higher priority, but SPAAGs were also designed on this chassis. Like with Object 31, it had a DT machinegun as secondary armament. The thickness of the front armour of the assault gun was creater than that of Chassis 31: 35 mm. The AA version was indexed BGS-4, and the assault gun was indexed BGS-5. Their mass was 8700 and 10,500 kg, respectively.
SU-32 became the starting point for later Soviet light SPGs.
On May 22nd, 1942, a technical meeting took place at factory #37, at which both chassis were examined. Interestingly enough, the factory's design bureau was driven by requirements from January of 1942. During the discussion, clarifications and changes were made, since the vehicles did not match them. At the same time, a trials program for Chassis 31 and Chassis 32 was composed.
The T-60 variant had priority, an AA gun version, at that. It's possible that the cause was the choice of weapon for the SPG. The mass produced ZIS-3 was a logical choice, but the ZIS-5 was more compact, and also produced in Sverdlovsk.
SU-31 during demonstrations in August of 1942. The vehicle already has three-colour camouflage.
Factory #37 worked on the chassis, while their colleagues from the UZTM design bureau had a direct connection with the armament. The ZIS-5 mount for the BGS-5 carried the index U-31, and the BGS-2 SPAAG's gun mount was called U-32. Work was supervised by L.I. Gorlitskiy, with K.N. Ilyin and A.N. Shlyakov as chief engineers. The vehicle, indexed SU-31, was a combination of three interests: Ginzburg, as the ideologue, factory #37 as the chassis manufacturer, and UZTM as the gun manufacturer.
For obvious reasons, the SU-31 used more parts from the T-60 than not.
The experimental SU-31 was built in June of 1942. The mobility trials took place that same month. Meanwhile, the idea of using the ZIS-5 tank gun did not find its place. For this reason, the BGS-5 project, the initial source of the SU-32 SPG, changed significantly. Factory #37's hopes that the quickly produced SU-31 would have higher priority were not justified. In addition, GKO decree #1958ss "On production of T-34 and T-70 tanks" came out on July 3rd, 1942, ceasing production of T-60 tanks at factory #37. There was no longer a point to using T-60 components. Nevertheless, the trials of the prototypes were still on.
Maximum gun elevation.
The SU-32 was built in July of 1942. Like the SU-31, it was built from mild steel, 15 mm thick all around. The suspension and driver's hatch were taken from the T-70. The casemate was redesigned to be roomier, which allowed the ammunition capacity to grow to 60 rounds. Just like the prototype, the hull was open from the top, but the size of the opening in the rear was decreased by adding rear plates. To help the crew get into the SPG, handholds and footholds were added.
SU-31 fighting compartment. Later, analogous fighting compartments would be used on other Soviet light SPGs.
The 76 mm ZIS-3 gun on a pedestal mount was used. The gun was installed along with elements of the mount and trails, which made it take up a lot of room inside the SPG. However, the gun could be taken straight from the production line with few changes. A welded shield for the recoil mechanisms was added. A DT machinegun in a ball mount, the same as the one used on the KV-1, was placed to the right of the gun.
Futureproof
Officially, mobility trials began at Kubinka on August 21st. Earlier, on August 18th, the vehicles were demonstrated at the Sofrino shooting range. From August 21st to September 3rd, the SU-31 and SU-32 traveled 630 and 524 km respectively. 490 shots from the 61-K and 205 from the ZIS-3 were fired.
The SU-32 was loaded down to match the weight of a fully armoured vehicle. During trials, both vehicles exhibited frequent track tearing. This problem would be solved by modernizing the suspension like on the T-70B (an arbitrary name, since no name was given to the T-70 with a reinforced suspension in the documents). The trials also showed serious issues with overheating on the SU-32 engine. The SU-31 did not have this issue. The SU-32 also had an inconveniently placed ammunition rack, which slowed down the rate of fire compared to the regular ZIS-3. At an elevation of 25 degrees, the gun shield covered up the panoramic sight, and it became impossible to aim. The addition of armour increased the amount of effort required to turn the aiming flywheel.
SU-31 on mobility trials, late September of 1942.
The following conclusions were made after the comparative trials:
"Mobility trials showed that the SU-32 has the following defects, in addition to weak brakes:
- Engine overheating.
- Weak gearbox attachment.
- The aforementioned defects do not allow the vehicle to be recommended for service in the Red Army.
Considering that the layout of the 76 mm on an SPG in general works satisfactorily, the commission considers it reasonable to produce a trial batch of 6-10 76 mm assault guns, in addition to the 37 mm SPAAG, with the parallel engine placement used in the 37 mm SPAAG, for trials in the active army."
Work on the SU-31 and SU-32 at factory #37 did not continue. GKO decree #2120, issued on July 27th, turned factory #37 into a subsidiary of Uralmash, and it began preparations for production of the T-34. All light SPG designs were passed onto factory #38. Overall direction of the work was still Ginzburg's responsibility.
The off-road mobility of the SU-31 was no worse than that of the T-70.
Trials of the SU-32 ended here, but the situation with the SU-31 was different. Ginzburg considered the layout with parallel engines more correct. As trials showed, these engines overheated less. The fact that the vehicle had two gearboxes that had to be operated with an enormous lever was no obstacle. This is the layout that was used on factory #38's prospective SPG.
Also, the NIIBT proving grounds held comparative trials of the off-road mobility of the SU-31 and T-70. A swamp was chosen as the testing area. Trials showed that the mobility was approximately the same. The SU-31 concept was approved. It's unlikely that Ginzburg could guess that this was going to be the greatest mistake of his life.