This week's highlight is the D-56T, taken from the recently published PT-76 article.
D-56T RIFLED GUN
The PT-76 is equipped with the D-56T rifled gun. The D-56T was a completely new design developed in 1949 by the design bureau No.9 factory, where it received the designation LB-62T (not to be confused with the LB-76T designed by the No.92 factory and installed in the Object 101). It was later renamed as the D-56T when the Object 740 prototype officially entered service as the PT-76. The cannon is chambered for the 76.2x385mm cartridge used in the ZiS-3 towed divisional gun and F-34 obr.1940 tank gun, and was designed with identical ballistic characteristics. To that end, the barrel measures 42 calibers (3200mm) in length, identical to the ZiS-3 and F-34. However, the barrel is of a distinct design, featuring a characteristic tapered step at the one-third mark of the tube.
Unlike many of the other tank guns designed in the late 40's, the D-56T has a recoiling mechanism that is reminiscent of more modern guns like the 115mm U-5TS or the 125mm D-81. Instead of having the hydro-pneumatic recoil buffer and hydraulic recuperator mounted on top of the chamber and forward of the breechblock like on the D-10T and D-25T, the recoil buffer is mounted in a slot in the lower right corner of the breech block and the recuperator is mounted in a slot in the lower left corner. This frees up valuable space on the top part of the cannon and permits the installation of the gun in a low profile turret, and also helps to somewhat reduce the amount of interior penetration (the length of the gun extending into the turret). Besides its atypical configuration, the recoiling mechanism is also designed with a long recoil stroke to help reduce the recoil impulse to a manageable level. The maximum recoil stroke length is 550mm and the normal recoil stroke length is 470mm to 545mm, depending on the ammunition fired. The drawing below depicts the D-56TM cannon without its mounting cradle and without the recoil buffers.
The photo below (credit to Stephen Tegner from the scalenews.de walkaround page) offers a better view of the two recoil buffers below the breech block of the D-56T.
There were guns with better anti-armour performance, like the Ch-51 57mm gun, and the newly developed S-60 57mm autocannon may have offered great firepower against hard and soft targets alike, but the official requirements stipulated that a 76mm gun was to be installed in the Red Army's prospective new amphibious light tank. In hindsight, it may seem that not installing the S-60 on the PT-76 was a mistake, seeing as this class of weapon is now in vogue among for light combat vehicles, but it should be understood that the requirements set forth by the Ministry of Defence were not arbitrary. The PT-76 was to provide fire support for troops during amphibious landings, most often to suppress and eliminate fixed defences on the shore like machine gun and recoilless rifle nests while having enough firepower to deal with the majority of armoured targets, and in the immediate post-war period, the majority of armoured targets that the PT-76 was expected to encounter would be light tanks mixed with a small number of WWII era medium tanks. Light tanks could be dealt with from the front at any range, and the water-crossing abilities of the PT-76 opened up new tactical opportunities to engage heavier tanks from the sides. From the front, neither a high velocity 57mm cannon or a medium velocity 76mm cannon would be able to do much against a medium tank. Aside from fixed defences, the PT-76 would have to deal with mechanized transportation for infantry, which most often took the form of unarmoured trucks and thinly armoured half-trucks. Such threats could be effectively suppressed or eliminated solely with the complementary 7.62mm co-axial machine gun, so a rapid-firing autocannon may not have been necessary. In this context, a medium pressure large caliber cannon like the D-56T paired with a 7.62mm co-ax appears to be the more rational choice.
However, the 76.2x385mm cartridge itself was not the first choice. Cannons chambered for this cartridge were generally considered obsolete in the Red Army, which was increasingly relying on larger and larger guns, whether it be towed, self-propelled or tank mounted. For example, the ZiS-3 divisional gun chambered for the same cartridge was declared obsolete and immediately replaced by the 85mm D-44 after the conclusion of WWII, and the 85mm ZiS-S-53 of the T-34-85 was replaced by the D-10T. This trend was not ignored during the conceptualization of the Red Army's new amphibious light tank in 1947. Originally, the plan was to create a 20-ton tank armed with an 85mm cannon and with the same mobility characteristics as a medium tank. It was intended to have a 400 hp engine and swim with the use of foam-filled aluminium floats, but the engine could not be ready on time, and it was realized that the reliance on vulnerable unarmoured floats was not viable for an amphibious light tank, so there was no other choice but to create a tank that could float with the buoyancy of its own hull. The weight of the new concept tank had to be reduced to 15 tons, and because of this, the plans for an 85mm gun had to be scrapped for a 76mm gun.
The D-56TS modification featured a new hydraulic elevation drive piston as an addition to the simple rack-and-pinion geared manual elevation mechanism, and also added brackets underneath the gun cradle to accommodate the hydraulic pump for the elevation drive. This increased the weight of the gun at the breech end, so the thickness of the fume extractor tube was increased from 3mm to 4mm in order to act as a counterweight.
The D-56T has a vertically sliding wedge breech, which is highly unusual for a Soviet tank. According to Soviet engineering manuals, if the bore axis of a tank cannon from the floor of the fighting compartment is lower than 950-1,000mm, a vertically sliding breech should be used, but if the bore axis is higher than that, a horizontally sliding breech should be used. This is because the convenience of ramming a shell into the chamber changes depending on the height of the bore in relation to the height of the average loader (170cm). If the height of the bore axis is 950-1,000mm or more, the chamber will be above the elbow of a half-standing, so a horizontally sliding breech is more convenient for the loader. Under this criteria, having a vertically sliding breech gun in a turreted tank would require a rather tall turret paired with a very short hull (less than 1 m tall) or some other unusual and impractical solution, and for this reason, most tank guns built in the Soviet Union have a horizontally sliding breech, as do many tank guns built abroad, the L7 being a prime example. However, that is not to say that this criteria was strictly enforced or that it made much of a difference. Just as there are plenty of tank guns with horizontally sliding breeches, there are also plenty of tank guns with vertically sliding breeches used in tanks that have the same bore axis height as tanks armed with horizontally sliding breech guns, with examples like the M68, M256, Rh 120 L/44, L11, L30, CN 120-26, and others. In the case of the D-56T, it appears that the decision to use a vertically sliding breech was possibly influenced by the ease of loading from a seated position, since the bore axis height of the D-56T from the floor of the fighting compartment is 1500mm - nowhere close to being below 1 meter.
All of the D-56T variants are fitted with a muzzle brake. A modern muzzle brake is capable of reducing the recoil force of a recoiling rifle or cannon by up to 80%, but they are limited to around 30% for guns and howitzers as an insurance policy, so that the hydraulic recoil system does not fail catastrophically if the cannon is fired with a damaged muzzle brake and experiences an abnormally high recoil force. The main downside to a muzzle brake is that the smoke and hot gasses diverted sideways has the side effect of obscuring the gunner's vision and also revealing the tank's position. You can imagine how much energy is carried by the propellant gasses if a muzzle brake is able to use its power to reduce the recoil force of a cannon by up to 50% - that means that half of the recoil force experienced by a cannon is from the projectile leaving the barrel, and the other half is from the rearward thrust of the propellant gasses alone. For a light reconnaissance tank with only enough armour to stop machine gun fire, these drawbacks are much more serious than they are for a heavily armoured tank intended for open combat. That said, a muzzle brake is absolutely mandatory on the D-56T in order to keep the recoil forces low enough for the turret to handle. The original D-56T is fitted with a multi-slotted muzzle brake of considerable length, but from 1957 onward, new production PT-76 tanks were armed with the newer D-56TM cannon with a new updated muzzle brake with a fume extractor.
The new muzzle brake returned to a double-baffle type like the original ZiS-3, but was of a new style of brakes derived from the muzzle brake of the D-25T designed by the TsAKB design bureau (now referred to as the "TsAKB style" brake, as opposed to the "German style" brake used on earlier D-25T guns). The advantages of slotted-type and baffle-type brakes over each other are debatable and the two types of brakes operate on different principles, but according to "The Basics of Artillery Guns and Ammunition" (Основи Будови Артилерійських Гармат Та Боєприпасiв, А. Й. Дерев’янчук), pages 308-311, double-baffle brakes are typically more efficient and more suitable for tank guns for a variety of reasons. A double-baffle muzzle brake works by placing obstacles of a large surface area (the baffles) in front of the muzzle to impede the forward flow of the escaping propellant gasses, thus absorbing the kinetic energy of the propellant gas particles in the form of pressure, effectively causing the gasses to impart a forward force on the barrel which counteracts the rearward recoil force. This is illustrated in diagram "A" in the drawing below. On the other hand, a slotted muzzle brake works entirely by redirecting the escaping propellant gasses at a rearward angle to produce forward thrust, which counteracts the rearward recoil forces and creates a braking effect. The small size of the slots forces the escaping propellant gasses to exit at an extremely high velocity, but the angle of the slots is inherently restricted by the need to protect the tank and surrounding infantrymen from the same powerful gasses. Because of this restriction, the amount of thrust produced from each individual slot is low, so a large number of slots is needed to achieve a useful level of recoil reduction. All of this is shown in diagram "B" in the drawing below.
For the PT-76 specifically, the advantage of a double-baffle muzzle brake over the earlier slotted one is obvious: the tank was required to ferry up to 20 troops on top of its hull during river crossings, and a slotted muzzle brake would have a much higher chance of harming the tank riders than a double-baffle type. This may also be one of the reasons why baffle-type brakes have been the most common design by far in every application, from custom-made competition rifles to large artillery systems. Slotted muzzle brakes have been used on guns and howitzers of calibers ranging from 152mm on the ML-20 to 57mm on the early production variants of the Ch-51 gun, so clearly there is no caliber limitation, but modern guns are invariably fitted with a baffle-style brake. For example, the early long slotted brake on the Ch-51 was quickly replaced by a much more compact double baffle muzzle brake on the Ch-51M, and all Russian 152mm gun-howitzers produced after the ML-20 - including both towed and self-propelled guns - were fitted with double baffle muzzle brakes as well. Another likely factor behind the popularity of baffle-type brakes over slotted-type brakes may be its compactness, which is important for turreted tanks like the PT-76 as they are sometimes expected to drive into forested areas, where a long muzzle brake may hamper the movement of the tank or prevent it from traversing the turret to aim. It also helps to ensure that the muzzle does not dig into the ground and cause malfunctions when driving across rough terrain.
The recoil guard and shell casing deflector can be folded down by a full 90 degrees, thus creating a spacious passageway between the two crew stations in the turret.