Sunday 12 February 2017

Self Propelled Artillery on a Tractor Chassis

The idea to use tractors as a chassis for SPG was born in the USSR in the 1930s. The SU-2 and SU-4 prototypes were built, but the projects did not move past the prototype stage. On the other hand, the Germans achieved different results. Taking captured French Renault UE tractors, they created SPGs with 3.7 cm Pak guns. The resulting vehicle wasn't the best, but could be produced in large amounts at a small cost. A year later, the ZIS-30 was built in the USSR, the first mass production SPG of the war.

Anti-tank Ersatz

The USSR first began seriously considering using artillery tractors as a chassis for tank destroyers in the spring of 1941. The first tractor considered for the role was the STZ-5. A more powerful ZIS-16 engine was proposed to increase mobility, as well as a longer chassis to add stability. The 57 mm ZIS-2 anti-tank gun, currently in the pre-production stage at factory #92, was proposed as the armament.

The Voroshilovets heavy artillery tractor was also considered as a chassis for a tank destroyer. An 85 mm mod. 1939 (52-K) AA gun was to be installed in its cargo compartment. Both vehicles would be partially armoured.

Experimental prototype of the ZIS-2 57 mm anti-tank gun, May 1941

Experimental SPG prototypes were discussed on June 9th, 1941. Alongside the tank destroyer on a lengthened STZ-5 chassis, a SPAAG with a 37 mm automatic 61-K AA gun was proposed. However, the idea didn't live long. During the meeting, the concept of SPGs on the STZ-5 or Voroshilovets chassis was rejected due to thin armour, an overloaded suspension, low ammunition capacity, and low range. At the same time, the meeting minutes contain the following phrase: "The 57 mm ZIS-4 gun on a chassis using STZ-5 components can be considered as a self propelled anti-tank gun."

The start of the Great Patriotic War buried pre-war plans for SPGs. Production of tanks had to be ramped up instead of working on prospective prototypes. In addition, production of tractors begin winding down to free up resources for tank production.

The first victim was the light partially armoured Komsomolets tractor. According to a Council of People's Commissars decree issued on June 25th, 1941, Ordzhonikidze factory #37 in Moscow was ordered to cease production by August 1st. This miniature tractor with a GAZ AA truck engine was never even considered as a chassis for an SPG. The GAZ-22 tractor was in development since 1940, making what happened in the summer of 1941 even more surprising.

The initiative to build new types of SPGs came not from the GAU or GABTU, but from the People's Commissar of Armament. People's Commissar D.F. Ustinov issued an order on July 1st, 1941, to design SPGs on tractor and truck chassis with a deadline in two weeks. The task of designing an SPG fell to the designers of the gun: the design bureau of factory #92. The project was headed by P.F. Muravyev with overall guidance from V.G. Grabin.

The choice of chassis for a new SPG was slim. The STZ-5 was not considered due to its slow speed and possibility of overload. All that was left were trucks and the light Komsomolets tractor. A decision was made to concentrate on the GAZ AAA and Komsomolets.

Experimental prototype of the ZIS-30 SPG, late July of 1941. The SPG doesn't have deployable trails or additional floor panels.

The GAZ AAA chassis variant, indexed ZIS-31, looked more like a backup plan. On one hand, the truck chassis was a more stable platform than a tiny tractor. On the other hand, it suffered from the same issues as the STZ-5.

According to requirements, the cabin and engine compartment were armoured, and that created additional load on the chassis, as did the mass of the gun. The mass of the wheeled SPG reached 5 tons, which is about the same as a BA-10. While this was fine for driving on roads, the situation changed when the SPG went off road.

The Komsomolets was a whole different story. The mass of the SPG was the same 5 tons, but the tracked chassis gave it better mobility than the ZIS-31. Unlike the wheeled variant, the conversion of a Komsomolets into a ZIS-30 required very few changes to the base vehicle. A П-shaped structure replaced the crew seats, and the gun was mounted on it. Ammunition racks lined the sides of the vehicle. According to a data sheet from factory #92, the vehicle carried 30 shells (other sources say 20). The gun range was the same as on the ZIS-31: 28 degrees horizontally and from -5 to +15 degrees vertically.

In Support of Tank Brigades

The experimental prototype of a ZIS-30 was ready on July 20th, 1941. The accompanying memo explained that, if necessary, the 76 mm ZIS-3 gun could be installed instead, the prototype of which was finished around the same time. On July 21st, a GKO decree draft titled "On the production of SPGs with 57 mm ZIS-2 anti-tank guns on the Komsomolets tractor and production of 76 mm mod. 1939 (USV) guns on the ZIS-2 chassis" was signed.

The plans were certainly impressive. Production of 3000 ZIS-30s was planned from August to December of 1941. The issue was that the wishes of Grabin and the NKV did not match reality. It was impossible to free up enough Komsomolets tractors, especially since their production ceased in favour of T-30 small tanks. Decree #252ss issued on July 23rd contained much more humble numbers.
  1. Order the NKV, comrade Ustinov, to install the first 100 57 mm anti-tank guns on Komsomolets tractors.
  2. Order the NKSM, comrade Malyshev, to supply factory #92 with 100 Komsomolets tractors before August 10th, 1941.
  3. Order the NKV, comrade Ustinov, to produce towed 57 mm anti-tank guns using the GAZ-61 car as a tractor.
  4. Order comrade Malyshev to supply factory #92 with sufficient GAZ-61 cars to fulfill the quotas for 57 mm anti-tank guns.
  5. Retain the current quota for 57 mm anti-tank guns and 76 mm divisional guns at factory #93.
  6. Reject the proposed installation of a 57 mm anti-tank gun on a GAZ AAA truck platform

 As you can see, the document confirms the use of the GAZ-61-416 car as the main tractor for the ZIS-2. As for the ZIS-30, the situation with producing them was not simple. A finished prototype couldn't immediately be put into production. The Red Army wisely requested proving grounds trials. The trials program was approved on August 10th, 1941, and the trials themselves were finished during the following days.

Some changes were made to the design of the vehicle as a result. The appearance of trails is the most noticeable. This was done to partially compensate for the oscillations after firing, which occurred due to the Komsomolets' small length. There were also extra floor panels that could be flipped out, making life easier for the crew.

Mass production ZIS-30. The deployed floor panels are visible.

Mass production of the SPG was plagued by greater issues. The production of ZIS-2 guns lagged behind schedule and there were issues with procuring enough tractors. Factory #37 stopped production by the end of 1941, so Komsomolets tractors had to be taken from the army.

All of this led to the first ZIS-30s coming out of the factory only by the middle of September. The batch of 100 vehicles was only completed by early October of 1941. Nevertheless, this vehicle was the first true mass produced SPG of the war. It's worth mentioning that all ZIS-30s left the factory with three-colour camouflage.

ZIS-30 in firing mode. The trails are deployed.

Most ZIS-30s were sent to tank brigades. They were distributed as follows:
  • 10th TBr: 4 vehicles, received on September 22nd, 1941
  • 11th TBr: 8 vehicles, received on September 30th, 1941
  • 12th TBr: 4 vehicles, received on September 22nd, 1941
  • 13th TBr: 4 vehicles, received on September 22nd, 1941
  • 14th TBr: 4 vehicles, received on September 22nd, 1941
  • 15th TBr: 4 vehicles, received on September 22nd, 1941
  • 16th TBr: 4 vehicles, received on September 22nd, 1941
  • 18th TBr: 8 vehicles, received on October 3rd, 1941
  • 19th TBr: 8 vehicles, received on October 5th, 1941
  • 20th TBr: 8 vehicles, received on October 7th, 1941
  • 21st TBr: 8 vehicles, received on October 11th, 1941
  • 22nd TBr: 8 vehicles, received on October 7th, 1941
  • 23rd TBr: 8 vehicles, received on October 9th, 1941
  • 27th TBr: 8 vehicles, received on October 13th, 1941
  • 38th Motorcycle Regiment: 4 vehicles on September 22nd, 1941
However, there were other units where ZIS-30 SPGs ended up. However, the main issue with studying their use in combat is connected with the fact that, as SPGs, they fell within the jurisdiction of the GAU. As such, tankers (GABTU) cared little about their use in combat. In correspondence they are often referred to as just anti-tank guns or Komsomolets tractors.

The opinion that the Red Army only used these SPGs in the fall-winter of 1941 does not match reality. The ZIS-30 pop up in documents dated summer-fall of 1942. For example, two such SPGs were used by the 20th Army. Other vehicles survived as late as 1944.

Knocked out ZIS-30, October-November of 1941. The three-colour camouflage can be seen.

The quality of ZIS-30 in battle is described by a report from the Southern Front, composed in April of 1942 based on the use of ZIS-30s by the motorized rifle battalion of the 4th Guards Tank Brigade (former 132nd TBr). Good sights, long effective range (up to 2-2.5 km), and high maneuverability were listed as its advantages. The SPG could be hidden easily, and the presence of a gun shield protected the crew from shrapnel.

A characteristic use of the ZIS-30 was the deflection of an enemy attack on March 17th, 1942. One ZIS-30 fired 13 shots and knocked out three German tanks at a range of two kilometers. The rest turned back. These SPGs were also used on the offensive, accompanying Soviet tanks. Here, their targets were not only tanks, but fortified positions.

ZIS-30 during the Battle for Moscow. The photo is staged, as the trails and floor panels are not deployed.

At the same time, there were complaints about the ZIS-30. The biggest issue with the ZIS-2 was its recoil brake. As for the tracked chassis, the engine was its weak spot. It lacked power off-road, especially in the snow. Weak armour was also listed as a drawback. The last phrase of the report is very telling: "It is desirable to install the gun on a T-60 chassis". 

By coincidence, the GAU and GABTU began preparing requirements for a light SPG on the T-60 chassis at the same time as the report was written.

Personal Initiative

The ZIS-30 was not the only Soviet SPG that was planned on a tractor chassis, although it was the only one to enter production. Others were developed by various design bureaus on personal initiative, usually as a result of the same NKV order that created the ZIS-30.

Tank destroyer on the A-46 tractor chassis, reconstruction by Aleksandr Kalashnik, Omsk.

SPGs designed at factory #183 fall into this category. Ustinov's order from July 1st, 1941, tasked factory #8 to design an SPG with an 85 mm 52-K gun, but factory #183 did most of the work in practice.

A meeting regarding SPG designs was held of August 27th, 1941. An 85 mm SPG on the T-34 was discussed, which was being designed since 1940 and turned into the U-20 project, an 85 mm SPG on the A-42 tractor chassis indexed A-46, and two SPG on the Voroshilovets tractor chassis. The T-34 SPG was not even considered by the attendees. As for the A-46, it was initially considered high priority, but then vanished as the A-42 was never put into production.

The meeting participants had a different idea about and SPG on the Voroshilovets chassis. Initially, the idea was to install an 85 mm 52-K AA gun on the tractor, but factory #183 developed another vehicle in parallel. Sadly, only a text description survives to this day, but it was impressive. A 23 ton vehicle, it would have 30 mm of armour in the front and 20 mm on the sides. Either a 76 mm F-34 or 57 mm ZIS-4 with a coaxial machinegun would be used. The SPG would have a turret with a 360 degree range. The bore axis height was 2300 mm, a little more than that of the T-34. By the time of the meeting, the blueprints and a model of this SPG were complete.

This design was approved with the 76 mm F-34 gun as its armament. The first 25 SPGs would be built in October-November of 1941 on top of the Voroshilovets quota. The plan was to put a prototype through trials and make changes to the rest based on their results. In addition, further development of the SPG by adding an 85 mm gun was planned. This work would be done in cooperation with factory #8 with a due date of September 15th, 1941.

An order was issued by the GAU in early September to urgently produce a prototype with an F-34 gun, but factory #183 had more important things to do. The People's Commissar of Tank Production, I.I. Nosenko, put an end to the vehicle, stating that production of 25 SPGs was impossible due to evacuation of the factory. 

SU-S2, Chelyabinsk, October 1941

At the same time, work began at ChTZ on an SPG on the Stalinets S-2 platform. The characteristics and purpose of this tractor was about the same as the STZ-5, but it was twice as heavy. The fate of this tractor was sad: even the STZ-5, no stranger to complaints from the military, was a better choice.

The SU-S2 from the front poses questions regarding servicing the engine. 

Understanding that the Stalinets S-2 was unsuitable as an SPG chassis in its current state, ChTZ designed a lengthened chassis which only retained the drive sprocket and return rollers from the original. The suspension now used torsion bars and shrunken versions of KV-1 idlers and road wheels were used. A welded hull was mounted on the tractor, preserving the existing crew layout. The passenger seat had a DT machinegun in front of it.

The main armament of the SPG was a 122 mm M-30 howitzer, positioned in the rear. The howitzer was mounted on the chassis along with its gun shield. A fighting compartment roomy enough for the crew and ammunition was build behind it.

It's easy to see that the vehicle turned out very large.

The vehicle, indexed SU-S2, went through factory trials in October of 1941. Its story ends here. The army needed KV tanks, not ersatz SPGs with uncertain futures. ChTZ-65 and S-2 tractors were cancelled in favour of KV production.

Nevertheless, evacuated SKB-2 engineers continued work on various projects. For example, designer N.F. Shashmurin proposed a two-man 2.5 ton tankette called "Rage of the People" with 20-25 mm of armour and two S-65 tractor engines. SKB-2 was also working on a "raid tank", a lighter T-34 with a top speed of 70 kph and increased range. These projects were also cancelled.

152 mm 152-SG SPG on the Komintern artillery tractor chassis, early April of 1942.

Other tractor based SPG projects designed at factory #592 by engineers E.V. Sinilshikov and S.G. Pererushev were much more thought out. During their work on the 122-SG (SG-122) they also designed SPGs on other platforms.

The most powerful among them was the 152-SG (152 mm self propelled howitzer) on the Komintern artillery tractor platform. The SPG received an open top armoured hull with sloped armour. The armour was 15 mm thick, impenetrable to DShK bullets from 200 meters according to calculations. A variant with 30 mm armour was also designed, but bulletproof armour was enough for an indirect fire vehicle.

The 152 mm mod. 1909/30 howitzer was planned as the armament. The mass was estimated to be 18.5 tons with a crew of 5. The project did not move past the draft stage, as there was a lack of Komintern tractors and mod. 1909/30 howitzers were also in deficit.

45-SP light SPG

The 45-SP (45 mm self propelled gun) on the STZ-5 chassis suffered a similar fate. Unlike the HTZ-16 armoured tractor, the gun was shifted to the side and the fighting compartment was semi-open. The thickness of the front armour was 20 mm, and the armour was sloped. The mass of the vehicle was 8.5 tons and its maximum speed was 20-30 kph. These estimates are optimistic, since the HTZ had the top speed of 20 kph at that mass, plus the engine overheated. GABTU didn't need another tractor, especially since the T-70 with the same 45 mm gun was entering production in April of 1942.

A.S. Shitov and P.K. Gedyk tank destroyer, UZTM, June of 1942

One of the last tractor SPG projects was designed in the summer of 1942. Its name was simple and descriptive: "tank destroyer". Its designers were A.S. Shitov and P.K. Gedyk. The project, dated June 29th, 1942, was based on a heavily modified Stalinets S-2 chassis. Some design elements were clearly inspired by the BGS-5 SPG (a precursor to the SU-32), such as the ZIS-5 gun with a cast mantlet on a special mount.

The tank destroyer had a small height, only 1800 mm. Its crew consisted of three men, a driver, a commander/gunner, and a loader. Unlike other SPGs from Sverdlovsk, the fighting compartment was closed. However, the design did not impress GABTU representatives. Since superior SU-31 and SU-32 SPGs were undergoing trials and the Stalinets S-2 has been out of production since November of 1941, its successor never entered production.

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