The M53/55 – 203.2 mm of American Freedom
The M53 is an American self propelled gun from the early Cold War, created as a successor to WWII-era vehicles like the M40. It was notable at the time for its use of a fully enclosed, semi-traversable turret that contained all of its crew.
Its fully-enclosed nature permitted the M53, and closely related M55, to be fitted with an NBC (nuclear, biological and chemical) system.
Like many other self propelled guns (SPGs) before and after the M53, it was built on pre-existing chassis, in this case, M46 and M47 Pattons.
Using a tank chassis meant the vehicle could keep up with the mechanised warfare happening around it, providing support to units in the field with short notice.
From the outset, the M53, normally armed with a 155 mm gun, was designed to easily receive a 203 mm gun if necessary.
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WWII saw rapid development of military doctrines, tactics and technologies. When it came to artillery, the US had achieved success with guns mounted on tank chassis, such as the M12.
These were easier to design, cheaper to produce and made logistics simpler, as they used many of the same components as tanks in service. They could also go anywhere the tanks could go.
After the war, the need for self propelled guns continued. The US military issued a requirement for a new 155 mm SPG fit for the modern, atomic era.
As such, it was expected to be NBC protected, and therefore, fully enclosed.
This requirement eventually reached Pacific Car & Foundry before the end of the 1940s, who were awarded a contract to produce a mock-up designed by Detroit Arsenal.
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The pre-existing chassis selected for the vehicles was that of the M46/47 Patton, but this required some major changes to the hulls to make it work.
At the start of the 1950s the design, designated T97, was deemed acceptable, and Pacific Car & Foundry were contracted to build it.
Pacific Car & Foundry were instructed to also produce a version that could carry the massive 203.2 mm M47 howitzer (not the M47 tank – pesky US naming systems!). This version was designated T108.
The T97 and T108 would be renamed to M53 and M55, respectively.
The two vehicles were extremely similar, mainly differing in their armaments. Because of this, they are often referred to collectively as the M53/55.
It proved to be impossible to place the gun in a conventional position on the M46 hull, as the engine and transmission took up the entire rear section and the driver was in the front.
To solve this, the hull was reversed, so the engine compartment became the front, while the driver’s position and crew compartment became the back.
Then, a large turret that contained the entire crew – including the driver – was placed at the rear. This is an odd arrangement, as the driver ideally needs to face the front of the vehicle at all times.
This was not seen as a massive issue though, as the SPG’s turret traverse was limited to 30 degrees left and right, and would rarely need to rotate its turret while driving.
Still, designers dealt with this by placing him at the front left corner of the turret on a gyroscopically stabilised chair that kept him facing forwards at all times.
The turret was protected by a maximum of 25 mm of armor – hardly enough to stop small arms fire and shrapnel.
The running gear was mostly the same as the M46/47, with torsion bar suspension and six road wheels per side. As the hull had been reversed, the sprocket was now at the front and the idler was at the back.
Speaking of the idler, on the M53 it was dropped down to the ground. This increased the track contact area, and improved the shock transfer from the 155 mm gun to the ground. It gives the machine a unique appearance.
At the front was a 29 litre (1791 cu in) AV-1790-5B aircooled V12 petrol engine that produced 800 hp. The components used in the M53/55s closely followed updates to M46, M47 and M48 tanks. Changes made to those tanks were often incorporated into M53/55 production too. This resulted in a few different engines and horsepower levels.
The rear could open up in a typical tailgate fashion, although in combat crews were expected to keep the turret closed.
This engine was good enough to get the 40 ton machine up to a speed of about 30 mph on road. Critically, this allowed the M53 to keep up with armored units which travelled at similar speeds.
The M53 was operated by a crew of six; driver, gunner, commander and three loaders.
At the front of the turret was an 155 mm M46 gun which had been developed from the “Long Tom”. Four recoil compensators helped dampen the massive recoil of this gun, in addition to a large spade at the rear.
It carried immense power, able to throw shells at targets 15 miles (24 km) away. The gun’s crew operated in the relatively large, barn-like turret, which also housed 20 155 mm rounds.
The M53 was designed from the outset to accommodate the 203.2 mm M47 howitzer. This was achieved in part via the guns’ mount, the M86. This mount could accept either gun with rather minimal effort.
To complete the transition from M53 to M55, the ammunition racks had to be changed, as did the equilibrator system.
The larger ammunition for the 203.2 mm gun meant only 10 rounds could be carried internally, and its rate of fire was reduced to a round every two minutes.
Beginning in 1956, most M53s were converted to the M55 standard, being rearmed with the bigger weapon.
Production of the M53 began in 1952, but it would only last until 1955. A total of 550 M53s and M55s were built in all.
They were tremendously powerful vehicles, fully equipped for self-sufficient fighting in an environment polluted by nuclear fallout for short periods of time. The loader’s work was made easier with loading assists, such a ammunition cranes and gun rammers.
The rather specialist M55s saw some small export success, with Belgium, Turkey and Germany purchasing the artillery piece.
Naturally the US was the biggest user of the system, but even they only used it in relatively limited amounts.
It did see service in Vietnam, where it provided heavy, mobile artillery against the enemy. It was eventually replaced in US service with the M107 and M110, which, ironically, were open topped.
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The M55 was retired from the Belgium Army in the 1970s after serving an admirable two decades in service.