Monday, 30 November 2020

On Water Like On Land

WWII clearly showed the importance of combat and transport vehicles that could cross water hazards. Crossings of rivers, lakes, channels, and reservoirs made planning any military operation more complicated, reduced the rate of offensives, and led to significant losses among attackers. The USSR performed nearly no work on amphibious vehicles for transport or combat. Only T-37, T-38, and T-40 tanks designed for reconnaissance were accepted into service. They were armed only with machine guns and could not effectively support infantry. After the end of the war the USSR began development of a wide variety of amphibious vehicles.

The Krasnoye Sormovo factory in Gorky was tasked with the development of the R-39 amphibious tank and R-40 APC "to improve tactical possibilities of land forces when crossing water hazards and improve resources for landing operations".

PT-76 during state trials.

The order explicitly specified the armament for the new vehicles. The R-39 would have the 76.2 mm LB-70T (later renamed D-56T) gun and a SGMT 7.62 mm machine gun. The APC would have a SGMB machine gun and needed to have the ability to tow any divisional artillery system. Both vehicles would use the newest V-54 engine throttled in power to 300 hp. On water they would be propelled by a propeller controlled by a ship style rudder wheel.

The vehicle ended up quite poor. Only the tank was built in metal. It was too heavy, its buoyancy and stability reserves were low, the armour was inadequate. After trials held in 1948-49 on the Volga the R-39 was deemed unsatisfactory. The second prototype was never finished and the APC was never built at all.

On August 15th, 1949, the Council of Ministers of the USSR signed decree #3472-1444ss "On the amphibious tank". This decree marks the start of the PT-76. It stated that the prototype produced by Krasnoye Sormovo was unsatisfactory and unsuitable for use in the armed forces. Because of this the orders for Krasnoye Sormovo to produce a tank and an APC were rescinded.

Development, production, and delivery of new amphibious combat vehicles was moved to the VNII-100 research institute founded based on the design bureau of experimental factory #100 in Chelyabinsk. Zh.Ya. Kotin was appointed as the chief designer. Mass production was planned to take place at the Stalingrad Tractor Factory.

A PT-76 from the reconnaissance battalion of the 2nd Guards Motorized Rifle Division. The attachments for additional 90 L fuel tanks can be seen.

The decree ordered the creation of a 13-14 ton tank with a 3-man crew armed with a 76.2 mm gun (the same LB-70T gun as on the R-39) capable of crossing water hazards along with forward infantry units and capture footholds on the opposite shore. The tank would use a 220 hp 6-cylinder linear V-6 engine. 35-40 rounds of ammunition for the cannon would be carried on board. The front armour had to protect from shell fragments and 12.7 mm DShK armour piercing bullets from a range of at least 50 meters.

M.S. Passov was tasked with development of the layout. V. Tarotko was also involved, he was responsible for the armour, rigidity of all hull elements, and water tightness. G.A. Osmolovskiy supported the engine: cooling, air filtering, working underwater, etc. F.I. Bazhan was responsible for the water jet propulsion system. A.S. Okunev led the group of shipbuilder engineers. VNII-100 director P.K. Voroshilov was responsible for all the experimental development.

Early PT-76 with the D-56TM gun.

Work split into several directions. All proposals had different methods for moving in water. There were four variants: fixed propellers, folding propellers proposed by L.S. Troyanov, tracks, and water jets proposed by N.F. Shashmurin.

Kotin chose the variant with folding propellers and design work headed in that direction. However, Shashmurin still finished his design and found a sponsor in V.A. Malyshev, then the Minister of Transport Machinebuilding, and deputy chairman Sovmin. Thanks to their influence prior work stopped and development of two new vehicles began: the tank indexed Object 740 and APC indexed Object 750.

PT-76 in Sevastopol, 1953. The tank is being towed at a speed of 5 knots.
 
Shashmurin developed the transmission with a special gearbox that worked both on land and in water. However, due to tight deadlines the T-34 gearbox was used. As work on detailed blueprints began, VNII-100 received materials on the water jet propulsion system for testing it on a model. Trials showed that the tank might have characteristics even better than those stated in the requirements.

To test new solutions developed for the tank, VNII-100 built a model of the M-270 (Object 270) amphibious tank in 1949 weighing 12 tons with a crew of 4. The Object 270 went through a series of trials in 1949-50 which gave enough data to complete development of the new amphibious tank.

PT-76B on trials at the Scientific Research Institute of the Ministry of Defense, Kubinka.

Two prototypes were built in the spring of 1950. Government trials began in the Belorussian Military District, near Polotsk. The commission concluded that both prototypes passed the trials and production of a pilot batch for military trials was the next step. The commission noted that some of the tank's parameters exceeded requirements as well as a desire for increased protection. The pilot tanks would be built at STZ in 1951, meanwhile ChTZ would be working on two tanks with improved armour.

Trials of tanks produced at STZ and ChTZ were conducted in the Kiev Military District in the summer of 1951. The state commission determined that both tanks passed trials and recommended the variant with improved armour for service.

The fifth PT-76B in Kubinka. The hull has greater displacement.

In 1951 trials of five tanks were also held near Sevastopol to determine their characteristics in open seas and suitability for landing operations. Three vehicles were towed by trawling vessels. As a result, the Object 740 was cleared for use in landing operations alongside ships, aircraft, and marines to capture footholds on the shore. The report stated that the tanks could be dropped off from landing ships in sea or directly on land.

In parallel, STZ was starting to master production of the amphibious tanks. S.A. Fedorov was appointed chief designer. Mass production began in 1952 and by 1954 the factory was producing 200 tanks per year.

Firing from water. This tank belongs to the 4th Guards Tank Division.

A Council of Ministers decree issued on August 6th, 1952, accepted the Object 740 into service with the Soviet Army under the index PT-76. Officially, it was described as "a light tracked vehicle designed for reconnaissance and combat within the parameters of crossing water hazards and amphibious landings". The tank was first shown at the May Day parade in 1952. The designers and testers were nominated for the Stalin Prize, but due to various political processes they received neither the Stalin Prize nor the State Prize that replaced it.

A PT-76B with a tall hull designed for marines. An air intake pipe is mounted on the turret ventilation fan.

The tank was constantly modernized during production. The road wheels were reinforced in 1952. In 1957 the D-56T gun was replaced with the D-56TM with a simpler double baffle muzzle brake and bore evacuator. A HEAT shell was introduced in 1956, which required the ammunition racks to be altered. The turret platform was raised by 60 mm in 1957 to prepare the tank for installation of a stabilizer and increase the gun depression when shooting backwards to -4 degrees. The hull armour was also thickened. The TDA thermal smoke system was installed in 1957, the 10RT radio was replaced with the R-113 and the TPU-47 intercom was replaced with the R-120. A third FG-10 headlight was added on the hull, symmetric to the existing headlights. This headlight was used for the driver's night vision device.

The turret handrail was moved up by 150 mm in June of 1957. Reinforcement ribs were added to the floor of the tank in August. Holders for 90 L fuel tanks borrowed from the T-34-85 were installed in January of 1958. These auxiliary tanks were not connected to the fuel system and could only be used for transporting extra fuel. 

PT-67B that went through refurbishment and modernization. Omsk military vehicles exhibition, 2002.

Hooks to tow the tank on water were added to the front of the sides in January of 1958. They were identical to the hooks on the front plate and on the rear, but not for long. It turned out that the tilted hooks result in the tank dipping forward when towed. New lighter straightened hooks were introduced starting in 1961. FG-10 and FG-26 headlights were replaced with the FG-100 in the spring of 1959. They were almost identical visually but the design was more robust. There were three headlights in total: the FG-102 with a blackout device, the FG-100 with an IR filter, and the FG-101 white light mounted on a pintle on top of the turret. The tank lasted like this from 1957 to 1962.

A hull with greater displacement was put into production in October of 1962 to improve mobility in water. The height was increased by 70 mm, the slope of the front plate was reduced from 45 to 35 degrees. This confused many researchers, since official documents state that the height of the tank was increased by 130 mm. This is correct, but it was done in two steps 5 years apart.

Vietnamese PT-76B tanks with DShKM machine guns on the turret roof.

The PT-76 was modernized to fight in irradiated areas in the late 1950s. The tank was equipped with air filters and anti-nuclear protection. The D-56TS gun with a two plane "Zarya" stabilizer was also installed. The resulting tank called PT-76B (Object 740B) was accepted into service in 1962. Two 95 L external fuel tanks were installed. Instead of cylindrical fuel tanks, oil tanks from the T-54 were used.

Existing tanks were modernized as production went on. Early guns were replaced with the D-56TM during refurbishment. PKT machine guns were installed instead of the SGMT as of 1967. As drivers received the TVN-2B night vision device the FG-100 and FG-102 headlights were replaced with the FG-125 and FG-126. R-113 radios were replaced with the R-123 and the R-120 intercoms with the R-124 in the 1960s. New stamped road wheels were installed during refurbishment.

A PT-76B belonging to Indonesian marines during exercises, 2000s.

Production of the PT-76B continued until 1973. 4172 tanks were built, 941 of which were sold abroad.

A large number of special purpose vehicles were designed on the chassis of these tanks over 20 years of production. The Leningrad Kirov Factory alone designed 8, including the "Penguin", TSM, SPU-L, Object 210, Object 211, Object 280, and Object 76.

The PT-76 was very popular with the army. The tank was not just reliable and maneuverable, but ended up light and compact. These advantages ensured wide adoption of the PT-76 by land forced and marines. As of the signing of the Treaty on Conventional Armed Forces in Europe in November of 1990 the USSR still had 602 PT-76 on its European territory. The tank remained in use by interior forces of the MVD and was even used in the Chechen conflict.

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