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M55 SPH

1952 COLD WAR SELF-PROPELLED ARTILLERY VIETNAM WAR

History

The M55 self-propelled howitzer was a blip in American development in the 1950s. The vehicle was designed around the versatility of auto parts, in line with the existing M46 and M47 Patton series of medium battle tanks that were in stock at the time in the US during the Cold War.

The M47 replaced the M46, but was eventually replaced by the improved M48. The M55 is similar in scope and function to the previous M53 self-propelled assault gun, which was equipped with a 155mm main barrel. However, the M55 switched to the larger 203.2mm M47 howitzer, which had limited overall ammunition due to the more powerful shell. Production of the M55 was taken over by the Pacific Car & Foundry Company in the 1950s.

Many U.S. Army M53 systems were subsequently upgraded to the M55 standard, while the U.S. Marine Corps example retained its M53 design qualities.

On the surface, the M55 is a bit of a clunky, practical machine. Although the chassis, hull and vehicle components were borrowed from the Patton tank, the internal layout was reversed to accommodate the fully enclosed turret position at the rear.

The engine and gearbox are now placed in the front of the fuselage (instead of the rear) and the driver is repositioned (as on the M53) to the front left of the turret. The track wheel system consists of seven twin tire wheels on each side and a drive sprocket mounted on the front of the fuselage. A distinguishing feature of the landing gear is the absence of a chain idler at the rear, which gives the M55's chain linkage system a very recognizable quality. Three track deflection rollers guide the upper track section. The rifle group works under the protection of a closed armored structure that provides safe protection against small arms fire or artillery jets, as the steel armor protection is up to 25mm thick.

The complete crew consists of six people, including the driver, vehicle commander, gunner and at least two loaders.

The main design feature of the M55 is its boxy turret configuration, which is a turret in name only. Lateral was limited to 30 degrees to the left or right, and little else, so the craft itself had to be mostly turned in the direction of the fire. The 203mm M47 howitzer has great strategic value, capable of throwing various shells up to 17,000 meters at enemy positions. However, due to the large projectile, the rate of fire was limited to one round every two minutes, and the loading on the vehicle meant that only 10 rounds could be carried - limiting its tactical value (the M53 could handle 20 x 155mm shells).

The defense was run by Browning. 50 caliber M2HB anti-aircraft/anti-infantry heavy machine gun mounted on the trunnion of the commander's dome.

Power for the M55 vehicle is provided by a Continental AV-1790-5B 12-cylinder gasoline engine rated at 810 hp. This works with an Allison CD-850-4 drivetrain with two forward gears and one reverse. As a result, the vehicle can travel up to 30 miles per hour on ideal surfaces and have a range of up to 160 miles.

Along with improvements to the Button tank line, M53 and M53 vehicles inherited some of these measures, including the AV-1790-7B engine and CD-850-4B transmission. Likewise, the original control arrangement of the joysticks later gave way to a more traditional steering wheel design.

Some M55s served during the US involvement in the Vietnam War (1955-1975), although they were eventually removed from inventory for some time. The M53 and M55 themselves share some parts, being able to use the same ammo racks and M86 gun racks, which is somewhat logistically friendly.

In addition to being used by the US military, the Belgian army is the only other notable operator of M55 vehicles. The last M55 was retired from active duty (with the Belgian army) in the 1970s.