Sunday, 24 November 2013

Soviet Light Tank Destoyers

The Germans were very well known for using obsolete tank chassis to create tank destroyers. The Marder series, the PzJg I, and several other designs gave new lives to obsolete light tanks. The Soviets also considered such a move.

"Report of Military Engineer 1st class comrade Afonin on a potential plan for constructing SPGs.
  1. Install a 57 mm ZiS-4 gun on tanks: BT-5, T-26, T-50 and the STZ-5 tractor (with lengthened base and ZiS-16 engine).
  2. Install the 85 mm AA gun on tractors: A-42 and Voroshilovets.
  3. Install the 76.2 mm gun model 1927/32 on the T-26 tank.
  4. Install a 37 mm automatic AA gun on the T-50, T-26, BT-5, and STZ-5 tractor (with lengthened base and ZiS-16 engine).
  5. Install a 25 mm automatic AA gun on the ZiS-32 and ZiS-36 vehicles, as well as the T-40 tank and STZ-5 tractor (with lengthened base and ZiS-16 engine).
Proposals:
  1. A 57 mm ZiS-4 gun cannot be used on the BT-5, T-26 and STZ-5 as a tank destroyer for the following reasons:
    1. Weak armour.
    2. Overloading the suspension (T-26 and STZ-5).
    3. Low operational range.
    4. Low ammunition capacity.
  2. It is agreed that an SPG made using STZ-5 components and armed with a ZiS-4 57 mm tank gun could be used as a self propelled anti-tank weapon.
  3. An SPG made using T-40 components with a 57 mm anti-tank gun is feasible.
  4. The 85 mm AA gun cannot be installed on the Voroshilovets tractor for the following reasons:
    1. Weak suspension (when armour is added).
    2. Bore axis is too high."
Well, most of these didn't pan out. However, after subtracting the second list from the first, the following vehicles are deemed feasible:
  • T-50 with a 57 mm gun.
  • A-42 tractor with an 85 mm gun.
  • T-26 with a 76.2 mm gun.
  • 25 mm or 37 mm AA autocannons on the T-50, T-26, BT, T-40, and STZ-5, as well as some trucks.
The document is dated June 9th, 1941, so I assume that a lot of these were forgotten when the Germans showed up. However, if you're into alternate history or bashing some model kits together, that should give you plenty of new ideas.

The Soviets never quite got around to making any of those T-26 based tank destroyers, but someone did. Pz.Jag.Abt. 563 converted 10 T-26es into tank destroyers in 1943. In March of 1944, they were replaced with Marders.




As you can see by looking at the gun shields, each vehicle is slightly different. This was clearly a field conversion done out of necessity rather than something done at a factory with actual schematics.

In November of 1941, a plan came out for next year's experimental SPGs. Among the many vehicles is a directive to mount a 76 mm USV or ZiS-3 gun on the T-60. The T-60 couldn't quite manage the extra load, but, next year, a solution came: the T-70. The T-70 was sturdier, and gave the ability to finally mount the ZiS-3 gun.

"Experimental prototype of the SU-76 (SU-12) SPG built by factory #38"
CAMD RF 38-11369-1

"In accordance with the decision of GOKO, factory #38 designed and produced an experimental SU-12 SPG, in October-December of 1942. The SU-12 SPG consisted of a 76 mm ZiS-3 gun on a special chassis composed of T-60 and T-70 elements. The engine assembly and transmission were positioned in parallel. The prototype was tested at Gorohovets during December 6th-16th and December 29th-31st, 1942. 
The SU-12 SPG passed trials, and was recommended by the GAU KA for the Red Army.
GOKO decree #3530 from June 7th, 1943 removed this SPG from production. The 560 SPGs that were built were modernized."
...
"According to GOKO decree #3530 from June 7th, 1943, experimental prototypes of the SU-15 and SU-16 were trialled at Gorohovets in June of 1943 by a commission led by Major-General comrade Pechenikin of the tank engineering service. It is worth noting that the GBTU KA and the commission checked the transmission and motor assembly especially thoroughly, as it performed unsatisfactorily on the SU-76 (SU-12)."

"Experimental prototype of the SU-76M (SU-15) SPG built by factory #38"
CAMD RF 38-11369-1

"Experimental prototype of the SU-16 (SU-38) SPG built by factory #38"
CAMD RF 38-11369-1

The "hard characteristics" of the two SPGs were more or less equal. The SU-76M had a small advantage over its five-wheeled brother: its average off-road speed was 0.5 kph higher and it had 5 degrees more gun elevation. However, comparing tanks is not that easy.

"It was discovered that the SU-15 SPG is more stable than the SU-16 during movement, and has a larger fighting compartment." 

Several defects were discovered in both SPGs, but the SU-76M performed so impressively that it was accepted into production before a second prototype was built. Trials in August of 1943 confirmed that the choice was correct. 

"The SU-76M SPG did not undergo significant changes until today. In 1944, a partial modernization and industry experience raised the quality of the SU-76M to the point where these SPGs can pass trials 3000 km long.
Engineer-Major comrade Lisin's contribution to the trials and improvements cannot be glossed over. Thanks to his determination, a series of defects was resolved in experimental prototypes and pre-production run. 
Factory #38 took four months to get the SU-76M to mass production quality, which is a good figure, especially considering the difference between it and the SU-76 (SU-12). 
The SU-16 SPG was deemed inferior to the SU-15 (SU-76M). Work on this SPG ceased when the SU-76M entered mass production."

The 1944 modernization also equipped the vehicle with a DT machinegun, and ports in the sides from which it could be fired. The MG could be mounted on the rear to fire at airborne targets. 

Even though the SU-76M was already accepted for mass production, development of similar vehicles did not cease. The SU-NATI-TsKB SPG was developed in July-October of 1943, consisting of a custom built chassis using T-80 and BA-64 components and a 76 mm K-3 gun, with ballistics equivalent to the 76 mm S-54 AA gun. 

"Experimental prototype of the SU-NATI-TsKB SPG"
CAMD RF 38-11369-1

The new SPG was very different from the SU-76. It had a fully closed fighting compartment in the front of the hull. The gun was much more powerful. Both gun depression and elevation were improved. It had the same mass as the SU-76, but had a weaker engine (2x54 hp), a lower top speed (35 kph), and worse horizontal gun traverse. The SPG had serious problems with ergonomics, and did not even make it to trials.

Molotov factory (GAZ) had their own horse in the light TD race. 3 horses, even: SU-74, SU-74B (SU-57B), and SU-74D. The SU-74 was similar to the SU-12: a rear gun placement (except with an F-34 instead of a ZiS-3), front ZiS-16 engine. However, the engine failed four times over 198 km of trials in 1943, and further trials were not performed. Work on the vehicle was discontinued. That same year, the factory attempted to design a light tank destroyer once more, with the SU-74B.

SU-74B, despite its similar index, was a radically different design. The fighting compartment was now in the front instead of the back. The engine and transmission were placed in the rear. The engine was the same ZiS-16, turbocharged to 104 hp. However, this engine power was not sufficient, and the TD failed trials in August-September.

"Experimental prototype of the SU-76B SPG, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

Much like the SU-76, the TD weighed 10.5 tons and had a crew of 4, with 25 mm of armour in the front and 15 on the sides. The weak engine could only provide a maximum speed of 28.8 kph. The ZiS-4 57 mm gun could traverse vertically between -5 and +14.33 degrees and horizontally in an 18 degree arc. 

A third attempt in that same year was the SU-76D. It used the same hull, and an American made GMC diesel engine, with a different transmission. The vehicle's drive wheels were in the front. The gun was once again a 76 mm F-34. It ended up being heavier than the SU-76B, at 11.8 tons. In the same August-September trials, the vehicle was deemed acceptable, under the conditions that the engine be replaced with an improved domestic ZiS-16 and mass reduced to 11 tons. However, due to the adoption of the SU-76M, and the fact that Molotov factory could not make the required improvements in time, the SU-76D was not adopted by the Red Army, and work on it ended. The document points out that Astrov's three TDs were perfectly fine designs, but the ZiS-16 car engine could not cope with running at maximum load for a long time.

"Experimental prototype of the SU-76B SPG, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

The 110 hp diesel engine allowed the TD to reach a maximum speed of 35.8 kph, even though the average speed was not much higher than that of the SU-76B (26.6 kph vs 27 kph). The gun range was similar: -5 degrees depression, 14 degrees elevation. Despite the larger weight, the SPG carried the same amount of armour. 

In September of 1943, after the adoption of the SU-76M, Molotov factory redirected their efforts to modifications of that TD. Their first modification was quite impressive: the gun was replaced with a more powerful D-5S, and the front armour increased to a whopping 82 mm. The fighting compartment was now entirely closed. The TD was named "SU-85", with no way to differentiate it from the SU-85 on the T-34 chassis. 

"Experimental prototype of the SU-85 SPG, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

With the same pair of 70 hp engines, the 14 ton TD could reach a top speed of 36.3 kph. The TD passed 1045 km and 152 rounds of trials, but was not recommended for production, as 10 hp/ton was deemed insufficient. 

In May-September of 1944, Astrov made yet another 85 mm TD: the SU-85A. This vehicle was much closer to the mass produced SU-76M, with an 85 mm gun, a TSh-15 sight, and with additional springs on the road wheels. The SU-85A was tested much more thoroughly: 1570 km and 824 rounds at Gorohovets proving grounds, then 1028 km and 234 rounds at the NIBT proving grounds in January of 1945. The verdict was that the SU-85A requires further modification to achieve higher stability after firing and more engine power to improve off-road performance. 

"Experimental prototype of the SU-85A SPG, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

The mass of the SU-85A was reduced to 12.3 tons. That, with the same two engines, allowed it to reach a speed of 39.8 kph. This time, the front armour was the same as the SU-76M, only 25 mm. 

The SU-85A's successor, the SU-85B, was ready for trials in April of 1945. The vehicle was trialled for 1000 km and 800 rounds, and passed the trials. The more powerful engines and LB-2 85 mm gun with a muzzle brake solved previously encountered problems. Crew placement in the new widened fighting compartment allowed a very high rate or fire with the 85 mm gun, up to 10 RPM. 

"Experimental prototype of the SU-85B SPG, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

The vehicle now had a GAZ-203 model 15V engine, with 160 total hp, allowing it to nearly match the top speed of the SU-76M (43.5 kph vs 45 kph), despite its 12.4 ton mass. Unfortunately, problems arose when mass producing the LB-2 gun, leading to delays past the end of the war.

The SU-76M was also modified with 57 mm guns at the Molotov factory. Two prototypes were built, one with the same fighting compartment, and one with an altered fighting compartment, and an armoured roof. After 270 rounds fired from the first prototype and 358 rounds from the second in trials during May-June of 1944, the second prototype was recommended for adoption by the Red Army. 

"Experimental prototype of the SU-57 SPG, second variant, designed by the Molotov Automotive Factory "
CAMD RF 38-11369-1

The vehicle was armed with the longer ZiS-2 57 mm gun, growing in mass by 200 kg, but otherwise unchanged. However, since the primary purpose of the SU-76M was infantry support, GBTU was unsatisfied with reducing the caliber of the gun, as well as reduced accuracy when firing at moving targets and poor ventilation of the fighting compartment. Installation of a roof and a gun with higher armour penetration was deemed undesirable.

Work on light SPGs did not end here. The SU-76M proved itself to be a powerful ally of infantry. Lightened versions of the vehicle were developed for airborne operations, but I will leave that for another article.

8 comments:

  1. "Installation of [..] a gun with higher armour penetration was deemed undesirable."

    Nice fact in an article about Tank Destroyers ;)

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    1. Well tank destroyers are SPGs, so it's more like an article about SPGs. Also these machines with their weedy speeds are clearly intended to support infantry, and as such will most likely be firing HE most of the time. So mounting lower caliber gun is running counter to their primary mission. Carrying around a bulletproof 76 mm cannon.

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    2. Sure thing. The reasoning in the article is clear on that [Plus all these vehicles use the SU prefix, which, IIRC, just means SPG...]. From the whole article the headline should maybe have been "Soviet Light SPGs"? The specific TD-important parts (gun characteristics) have been handled already...

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    3. Aside from the Murricans "tank destroyer" and "assault gun" tended to be pretty interchangeable concepts, and these are obviously rather more of the latter.

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  2. Is... is that a backwards BA-64 hull being used as a casemate?

    HONK HONK

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  3. This comment has been removed by the author.

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  4. Of course there were the 650 or so SU-57. A good if not great SPG.
    7 Brigades total were formed in 1943 each with 60 SU-57 each according to most literature on I have found.

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    Replies
    1. The SU-57 that saw service was the 57mm Gun Motor Carriage T48.

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