SU-76

The SU-76 was a Soviet light self-propelled gun used during and after World War II. The SU-76 was based on a lengthened version of the T-70 light tank chassis and armed with the ZIS-3 mod. 1942 76-mm divisional field gun. Developed under the leadership of chief designer S.A. Ginzburg. Its quite simple construction and multipurpose combat role made it the second most produced Soviet armored fighting vehicle of World War II, after the T-34 medium tank.

History

SU-76

Design of the SU-76 began in June 1942, when the State Defense Committee ordered the construction of infantry support self-propelled guns armed with the ZIS-3 76-mm divisional field gun and the M-30 122-mm howitzer. The T-70 light tank chassis was chosen by chief designer S.A. Ginzburg for mounting the ZIS-3 gun, and basic tank's hull was lengthened, adding one road wheel per side, to facilitate better gun mounting. The gun was installed in the embrasure of the front armored plate of the fixed fully closed armored casemate above the rear of the hull. The power plant consisted of two engines connected in parallel with the transmission. The units of the latter were also paralleled and connected at the level of the main gears. The mechanic-driver sat in the bow of the vehicle, and the gun crew of three men including the commander was located in the casemate. The SU-76 was put into service by a GKO decree of December 2, 1942.
The first batch of SU-76s was manufactured by January 1, 1943, and sent to the self-propelled artillery training center. At the end of January, the first two self-propelled artillery regiments of a mixed organization were sent to the Volkhov Front to participate in breaking the siege of Leningrad. In March 1943, two more regiments were formed - the 1485th and 1487th, which participated in battles on the Western Front.
However, already after 10 days of military operation, most of the SU-76s were out of order due to breakdowns in gearboxes and main shafts. An attempt to correct the situation by strengthening the shafts did not lead to anything. Moreover, such vehicles failed even more often. It became obvious that the transmission of the SU-76 had a fundamental design flaw, which was the parallel installation of two twin engines that worked on a common shaft. Such transmission scheme led to the occurrence of resonant torsional vibrations on the shafts. Moreover, the maximum value of the resonant frequency accounted for the most intense mode of operation of the engines, which led to their rapid failure. Elimination of this defect required time, therefore, on March 21, 1943, the production of the SU-76 was suspended. A total of 560 units were built at Plant No. 38 in Kirov. During the investigation that followed the mass failure of SPGs in the winter of 1943, a commission chaired by I.M. Zaltsman defined that the main culprit was the head of the Department of the Chief Designer of the People's Commissariat of the Tank Industry S.A. Ginzburg, who was removed from his position and sent to the front as the head of the repair service of the 32nd tank brigade belonged to the 29th tank corps. Stalin, having learned about this, did not approve of such a hasty decision, and ordered the talented tank designer to be recalled from the front, but it was too late - Ginzburg was killed in action.

SU-76M

A more reliable vehicle, the SU-15, appeared as a result of a competition announced by the management of the People's Commissariat of the Tank Industry for a light assault SPG armed with a 76-mm divisional gun. GAZ and Plant No. 38 took part in the competition. Tests of the new self-propelled guns took place at the Gorokhovets artillery training ground in July 1943, at the height of the Battle of Kursk. The SU-15 enjoyed the greatest success with the military, and it was recommended for mass production after some improvements. It was necessary to lighten the vehicle, which was done by removing the armored roof over the casemate, at the same time this solved all problems with its ventilation, and also facilitated the boarding and disembarkation of the crew as well as the gun maintenance. In July 1943, the SU-15 under the army designation SU-76M was adopted by the Red Army.
After production of the light SPGs resumed, GAZ and Plant No. 40 in Mytishchi near Moscow joined it in autumn 1943, and as a result 13,732 SU-76Ms were built. More than 9,000 of these SPGs were built solely by Gorky Automobile Plant, which became the main plant for the production of the SU-76M from January 1, 1944. The SU-76M became the second most produced Soviet armored fighting vehicle of World War II, after the T-34 medium tank. Under the leadership of the chief designer N.A. Astrov, since the autumn of 1943, work had been going on at GAZ to improve the SPG and adapt its design to mass production conditions. Changes were made to the design of the SU-76M. Vehicles of later series received a higher rear armor plate of the fighting compartment with two firing ports and a larger door, a tube welded to the right and left sides at the back of a casemate appeared to mount a machine gun for anti-aircraft defense. Firing ports of a new shape began to be used, more adapted for firing from a machine gun, etc. Mass production of the SU-76M ceased in October 1945. In contemporary accounts SU-76Ms are often referred to in texts, public radio and TV broadcasting as SU-76s with the "M" omitted, due to their ubiquity in comparison with the original SU-76s.
The layout of the SU-76M and its chassis remained unchanged compared to the SU-76. But the SU-76M had an armored casemate open at the top and partly behind. Two chief designers at the GAZ, N.A. Astrov and A.A. Lipgart, changed the power plant arrangement to that of the T-70 light tank - two GAZ-202 engines were connected in series and installed on the right hand side of the vehicle. The transmission consisted of a two-disk main clutch of dry friction, a four-speed gearbox of the ZIS-5 type, a main drive, side clutches and side drives.
The SU-76M had a clearance. The SPG could climb a slope of 28^o, overcame a wide trench, a high wall and a deep ford.
The ZIS-3 gun pointing angles ranged from -5^o to +15^o vertically and 15^o left and right horizontally. The rate of fire of the gun with the pointing correction was 10 rounds per minute, with the rapid fire - up to 20 rounds per minute.
The SU-76 was the basis for the first serial Soviet tracked armored anti-aircraft vehicle, the ZSU-37. Mass production of the ZSU-37 was continued after SU-76M production ceased. The majority of SU-76Ms had been withdrawn from the Soviet army service in the beginning of 1950s, although some were retained as training vehicles for tank crews as late as 1960s.

Variants

;OSU-76: Experimental model based on the T-60 light tank chassis and armed with the 76-mm gun ZIS-3, three prototypes were built in summer 1944.
;SU-76 : Based on a lengthened T-70 light tank chassis, with the inferior dual-engine arrangement of earlier T-70s. Only 560 units were produced, and these were quickly withdrawn from front line service because of transmission problem, which was the parallel installation of two twin engines that worked on a common shaft. The casemate of this version had armored roof, but it caused ventilation issues and was sometimes removed in field depots. Combat mass was 11,200 kg.
;SU-76 : Also commonly referred as SU-76M. Main production model. The casemate was open at the top and partly behind. The power plant was taken from the later T-70 light tank and consisted of two GAZ-202 engines connected in series. Combat mass was 10,500 kg.
;SU-85A/SU-85B : The SU-85A and SU-85B were based on the SU-76M with an elongated hull. The SU-85A was armed with the 85-mm D-5S-85A gun and a prototype was tested in 1944, although flaws were found in the prototype such as stability during firing not being adequate. The SU-85B was an improved version of the SU-85A with the 85-mm LB-2 gun and an improved fighting compartment and suspension. The SU-85B prototype was tested in April and May of 1945 and was recommended for production, but did not due to cutbacks on vehicle production after the war.
;ZSU-37: Self-propelled anti-aircraft gun, based on the SU-76M.
In 1978, Institute 111 from Romania designed an armoured personnel carrier based on the SU-76M chassis, equipped with the TAB-71 turret. The vehicle entered service as the MLVM.

Unrelated vehicles

The unrelated SU-76i, first designed and fielded in 1943, was based on captured stocks of German Panzer III and StuG III chassis, a large quantity coming from defeated German troops after the Battle of Stalingrad that year. This partially-modified vehicle was armed with an S-1 76.2 mm tank gun in a casemate superstructure but retained the original German Maybach gasoline engine and its torsion-bar suspension system. Around 200 of these ex-German vehicles were sent for conversion into SU-76is at Factory No. 37 to supplement the existing SU-76. They were issued to tank and self-propelled gun units starting in the fall of 1943. They were eventually withdrawn from the front in early 1944 and then used for training and testing until the end of 1945. Only 2 have survived the war, most having been scrapped after 1945. A similar vehicle called SG-122 existed, which was a similar Panzer III conversion, but armed with 122 mm M-30 howitzer. Only around 20 were converted, as the M-30 was considered an insufficient weapon for infantry support.
The also unrelated SU-76P was based on the T-26 chassis. It was built in Leningrad during the Siege of Leningrad and involved removing the turret from the T-26 and mounting a 76 mm regimental gun M1927 on the engine deck. This was created due to the lack of high-explosive 45 mm ammunition inside Leningrad due to the siege, so some T-26 tanks were rearmed with 37mm or 76mm guns for which a reliable source of ammunition was available. They served until 1944, when the siege was broken. They were originally called "SU-76s", until the SU-76 came into service, upon which it was renamed "SU-76P".