Saturday 16 September 2017

Big Dreams, Small Chassis

Despite the difficulty with which the Soviet T-40 amphibious reconnaissance tank entered production, designers considered the platform quite promising. The T-40 chassis would be used to produce the GAZ-22 artillery tractor, which would eventually replace the Komsomolets. The fact that the T-40 was also seen as an SPG platform was less well known. Most of the designs remained on paper, but at least one was produced in a small batch.

Small tank destroyer

Designers didn't think much about the T-40 until the summer of 1941. Work on the vehicle headed in two directions: improvement of its armament and design of the GAZ-22 tractor. Both projects reached the practical implementation stage, but it is hard to call the results impressive.

The 23 mm PT-23TB SPG (Tank Gun, 23 mm, Taubin-Baburin), designed by OKB-16, was worked on until May of 1941, when Taubin and Baburin were arrested. This happened, in part, due to their own machinations around this gun, which was installed in the T-40, then removed, and sent to be used on the OKB-50 aerosan. However, it did not perform on the aerosan either, since the gun itself was faulty.

Trials of the GAZ-22 showed that the GAZ-11 engine used in the tank is too weak for this role. A proposal was made to replace it with a more appropriate engine, for example the ZIS-16 (turbocharged version of the ZIS-5 truck engine).

Meanwhile, the waves made by the alleged presence of heavy tanks in the German army reached light tanks. Work began on the development of tank destroyers based on both obsolete light tanks (T-26, BT) and the T-50, production of which was just getting started.

On June 9th, 1941, a meeting was held with representatives of the GABTU and GAU. The Chief of the 3rd Department of the GABTU BTU, Military Engineer 1st Class S.A. Afonin, presented his report. Afonin proposed considering the BT-5, T-26, T-50, and the modified STZ-5 tractor as a chassis for a tank destroyer armed with a 57 mm ZIS-2 gun.

The T-26 and BT-5 were rejected outright, since their use would mean low ammunition capacity and an overloaded chassis. The T-50 was seen as the most promising option. As for the STZ-5, it was unsuitable for the task for a wide number of reasons, even though it was quite possible to build an SPG on its chassis

Unexpectedly, a proposal was made to design an SPG using components of the T-40 tank. Even though this was only a proposal, this meeting can be considered the starting point in the history of the SU-76.

The issue of installing a 25 mm AA autocannon in the the T-40 was also raised, but work did not move past the proposal stage. It turned out to be much easier to place the AA gun on a truck bed.

ShIT-72 SPG design, armed with the 25 mm 72-K AA autocannon.

The T-50 had priority in the 57 mm tank destroyer project. On May 27th, 1941, tactical-technical requirements for a "57 mm self propelled anti-tank gun on the T-50 platform" were approved. According to correspondence of factory #174, this vehicle would be indexed SU-51.

However, work did not move past composition of requirements. The start of the Great Patriotic War put an end to any plans for this SPG. The T-40 was also forgotten as a potential SPG chassis. In addition, according to a Council of Commissars (SNK) decree, issued on June 25th, 1941, factory #37 would cease production of T-20 "Komsomolets" tractors and T-40 tanks by August 1st, 1941. Instead, the factory would begin producing T-50 tanks within two months.

By mid-July, a letter was sent to Stalin, bearing the signatures of designer N.A. Astrov and military representative of factory #37, V.P. Okunev. They made their case for why production of the T-50 at factory #37 was a mistake, and proposed production of a land tank on the T-40 chassis instead. Stalin supported the idea, and GKO decree #179ss "On the production of light T-60 tanks at factory #37" was issued on July 17th, 1941. The T-40 chassis returned as a prospective platform for self propelled artillery.

The same vehicle from the front.

Let's make a pause to explain the structure of the design bureau at factory #37. It carried the name "Department 22" or "22nd Department", and a part of it was called KB-1, headed by N.A. Astrov. This KB designed the T-40, T-30, and T-60 tanks. However, G.S. Surenyan was in charge of Department 22 overall. Astrov, the manager of KB-1, was the deputy manager of Department 22, Surenyan's deputy.

Because of a misunderstanding, some researchers write that Astrov replaced Surenyan at the head of Department 22, but that is not the case. Surenyan kept his post in the fall of 1941, after the evacuation of the factory to Sverdlovsk, and until the spring of 1942. While Astrov concentrated on preparing the T-30 and T-60 for production, Surenyan was concerned with other issues, no less important. For example, he took an active role in the modernization of the small amphibious T-38 tank

ShIT-37 SPG, equipped with the 61-K 37 mm AA autocannon.

In the summer of 1941, Surenyan began to develop an SPG on the T-40 chassis on his own initiative. His project, or rather, projects, took final form by late August. On August 27th, he sent an explanatory memo and drafts of an assault-tank destroyer vehicle: ShIT.

Don't think that Surenyan worked on these projects out of boredom. Similar initiative popped up frequently during the difficult early period of the Great Patriotic War. Recall that the T-30 itself was designed on personal initiative. Most of these projects remained on paper, but a few reached mass production. Surenyan's motives are made clear in his explanatory memo:

"The main components of the vehicle, such as the transmission (main clutch, side clutches, final drives) suspension, the engine and cooling equipment, are all borrowed from the T-40 tank. All aforementioned parts are finalized and carefully brought to a condition suitable for mass production (there is full technical documentation available). 
The GAZ-202 engine used in the vehicle is fast, light, reliable in a tank, and is in mass production at the Gorkiy Automotive Factory.
Elements of the T-26 and T-50 tanks (for the 45 mm gun) can be used in the vehicle.
The armoured hull of the vehicle is as simplified as possible, and does not need significant processing. The armoured parts only need to be cut up and welded. Given a small setup for thermal treatment of armoured parts, hulls can be produced at any boiler factory. Processing of all parts can be done on medium type equipment. Types of steel used in the vehicle don't have expensive or rare additives.
Production of these tanks would go far to quickly supply the front with well armoured and well armed vehicles. The vehicle fully covers this requirement, and is very light. In combination with other vehicles, they can successfully defeat the enemy.
All variants, due to their small size, are easily camouflaged, difficult to hit, and can be easily transported by trucks. 
I attach overall schematics and ask for your permission and cooperation in building such a vehicle. Experimental prototypes can be ready within a month. I think that mass production of these vehicles should be organized immediately."

The ShIT concept envisioned a vehicle with minimal dimensions, which had shell-proof armour and armament capable of defeating modern tanks. It had to use the T-40 chassis. The minimized size of the SPG can be seen from the fact that the largest of the family would only have a 3200 mm long hull. To compare, the hull of the initial vehicle, the T-40, was 4110 mm long.

In all presented variants of the vehicle, the engine was located in the rear. Components were placed as tightly together as possible.

A larger artillery system forced Surenyan to make the casemate larger and not slope the sides.

The lightest and most weakly armed was the ShIT-72 SPG. Its designation came from its armament: the 25 mm mod. 1940 AA gun, 72-K. Even though this was an AA gun, it could effectively combat light ground vehicles. The small size of the vehicle limited the ammunition capacity to only 100 rounds. However, the PzII, which also had a cannon with AA roots, also had a small ammunition capacity, only 120 shells.

At a weight of 5400 kg, the ShIT-72 would have 60 mm of front armour, 45 mm on the sides, 30 mm in the rear, and a 10 mm thick roof. Not every medium tank could brag about having this much armour. According to calculations, the mobility of the ShIT-72 remained at the level of the T-40.

The second SPG with an AA gun was larger and heavier. It was called ShIT-37, and it was armed with a 37 mm mod. 1939 AA autocannon, also known as ZIS-37, also known as 61-K. The amount of ammunition carried was the same: 100 rounds.

To fit into the vehicle, it was mounted upside-down. The larger gun meant that the height of the hull had to be raised from 1350 mm to 1520 mm. The mass of the vehicle increased to 5800 kg. The armour was the same as on the ShIT-72. Surenyan claimed that the increased mass would not significantly affect the mobility. As for firepower, the 61-K's characteristics were, at the very least, equal to that of the 45 mm anti-tank gun, and it could successfully be used against German tanks of the period.

ShIT-45. The dotted line indicates the dimensions of the T-40 tank, for comparison.

Finally, the heaviest variant was the ShIT-45, armed with the 45 mm anti-tank gun. The dimensions of this SPG were the same as those of the ShIT-45, but, due to the heavier gun and ammunition, the mass increased to 6 tons. In order to keep under that limit and retain mobility of the T-40, the front armour was reduced to 45 mm.

The GABTU did not approve of Surenyan's ideas. on September 24th, 1941, a conclusions was prepared, and it was not positive for the "assault-tank destroyer". The military did not like the overly dense layout, which would have limited crew comfort and reload speed. The overhang of the armament was also received poorly, as it would limit the mobility of the vehicle when driving in the forests and on uneven terrain. It would increase pressure on the front road wheels, which would speed up wear.

Table of characteristics of various German and Soviet vehicles, as well as "assault-tank destroyers".
Type, mass, length, width, height, clearance, gun caliber, # heavy machineguns, # light machineguns, maximum armour thickness, engine power, hp/ton, top speed, maximum grade, ground pressure.

Despite such a short and unfortunate history, the assault-tank destroyer became the first SPG on a light tank chassis that was designed during the war. Surenyan returned to the idea of converting light tanks into SPGs, but these were already vehicles on the T-60 chassis. Later, Surenyan's concept of a light tank chassis with a forward fighting compartment was proposed many times by other designers.

Chassis for Katyushas

The idea of using a T-40 chassis for combat vehicles returned in September of 1941. This time, as strange as it sounds, it was connected with rocket launchers.

Initially, the M-13 and M-8 rocket launchers were mounted on ZIS-6 three-axle trucks. Often, the replacement of the chassis is explained by cessation of production of the ZIS-6 and evacuation of the Stalin factory, but that is not the case. Yes, production of the ZIS-6 ended, but it was possible to install the rocket launchers on trucks that were already produced. This is what happened with ZIS-30 tank destroyers, which used the Komsomolets chassis. Production ended on August 1st, 1941, but installation of anti-tank guns began in September.

The real issue was different. Information about issues with the ZIS-6 chassis began coming in August of 1941. Compared to the stock vehicle, the mass of the rocket launcher grew by several tons, which impacted the vehicle's performance on dirt roads.

To solve this issue, the design bureau of the Compressor factory in Moscow designed M-13 launchers for the STZ-5 tractor, ZIS-42 halftrack, and T-40 tank. The latter was, effectively, the T-30 tank, since that is what the T-40 was called as of August 31st, 1941. According to factory #37's report, four T-30 tanks without turrets were sent to the Compressor factory. Three more followed on the 19th. According to shipments, the use of the T-30 chassis was already decided at this point.

On September 14th, 1941, an experimental prototype of the "M-13 launcher on the T-60 chassis" was sent to comparative trials. Don't let the index fool you: the prototype used a chassis that was completely identical to that of the T-40, with the exception of amphibious equipment. M-13 launchers on the T-30 and STZ-5 were declared favourites after the trials. However, the low clearance made itself known: with mobility comparable to that of the ZIS-42, the M-13 on the T-30 chassis had great difficulty in driving over swamps.

Strangely enough, the military chose the ZIS-42 chassis after the trials. However, another important factor got in the way. The chassis was the most vulnerable from a production standpoint, since it had not entered mass production yet. The STZ-5 chassis won out: it was not as quick, but was available in sufficient numbers. 

The M-13 launcher on the STZ-5 chassis, factory index KS-75, was accepted into service with the RKKA by GKO decree #726ss on September 30th, 1941. According to plans, the first vehicles of this type would be delivered in mid-October of 1941.

This was the reason why rocket launchers were no longer installed on ZIS-6 trucks.

Of course, that did not mean that the T-30 chassis would not be used for other tasks. On September 27th, trials of M-8 rocket launchers on the T-30 chassis began at the Sofrino proving grounds. This vehicle, factory index KS-77, was also designed at the Compressor factory. The M-8 launcher changed along with its chassis: the rails were lengthened to 2 meters, and their number decreased to 12. One burst now consisted of 24 RS-82 rockets, instead of 36.

The T-30 chassis received a new turret platform roof. The turret was replaced by the M-8 launcher, which could be aimed horizontally, as well as vertically. Unlike the ZIS-6 mounted launcher, the T-30 was stable enough to fire without additional trails, which shortened the time needed to set up for firing.

The only available image of the M-8 launcher on the T-30 chassis.

Two vehicles took part in trials, with the only difference being in the design of the rails. The first one had box rails, which were attached to a frame. The design used engineer Gorelik's flip-up explosive gears. The second had girder-shaped rails (like on the M-13), and its explosive gears were designed at NII-3.

The rails and explosive gears of the second vehicle turned out to be low quality, which impacted the trials. Out of 24 charges, only 17 worked during the launch, whereas only one charge failed to go off on the first variant. During precision trials, the situation was reversed: due to electrical problems, the first variant could only fire two times. The second one fired twice more.

The conclusions of the trials were that both variants were equivalent, but the second was put into production. The same GKO decree accepted this vehicle and the M-13 on the STZ-5 into service.

According to GKO decree #726, the first regiment armed with M-8 launchers on the T-30 chassis would be formed by October 15th, 1941. 72 vehicles would be produced in October. In reality, the situation progressed differently. In October, factory #37 managed to deliver only 14 T-30 chassis. GKO decree #752ss "On the evacuation of factories #37, KIM, Podolsk, and the Kolomna factory tank plant". The Kompressor factory only received 21 chassis in total, including those sent for experimental work.

It is worth mentioning the production version of the M-8 on the T-30 chassis differed from the experimental vehicles. A cabin for the commander, also the operator of the launcher, was added to the left of the launcher. The Podolsk Ordzhonikidze factory was supposed to install them, but, due to the factory being overloaded with work, the process dragged on. Eventually, Compressor was tasked with the production of the cabins as well.

As of October 10th, 1941, 15 vehicles were in the process of assembly. As a result of delays with several components, only three M-8s were delivered by November 1st. Later, 10 M-8s on the T-40 chassis appear in documents of the milling machine factory in Gorkiy. Don't let this information confuse you: this is where the T-60 chassis produced by the Molotov GAZ factory ended up.

As for Compressor, it moved on to producing M-13 launchers on the STZ-5 chassis. In the factory's reports for January, two more M-8 launchers on the T-60 chassis are mentioned. It's possible that these were two T-40s. One can confidently talk of three experimental and between three and five production vehicles. Aside from a drawing and some correspondence, no information remains.

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