Cadillac Motor Car Division designed a very light and powerful tank destroyer by installing the 90mm M54 gun in an M88 mount on the roof of an aluminum chassis. The prototype was designated “90mm Gun Motor Carriage T-101” and had pneumatic tires and steel cable and 51 cm wide rubber tracks to save weight, as well as a powerful 200 hp engine that gave it excellent mobility since the vehicle weighed only about 6.8 tons. The suspension consisted of torsions bars and tubes set at the first and fourth roadwheels, and the transmission was an Allison CD-150-4 cross-drive.After the relevant tests, the vehicle was approved and entered service in 1953. Between 1953 and 1959, 325 units were built for the US Army, which distributed them among the different airborne units. Within these units, the vehicles were part of the airborne infantry tank companies, which obtained a very significant increase in their anti-tank capacity. The chassis of the M-56 was all-welded and riveted aluminum with the engine installed in the front and the barrel and its mount installed in the central part on the roof of the hull. The tiny rear part served to carry the ammunition. The 4 crew members were installed outside, around the gun, with the only protection of the gun shield of only 5mm thick, that protected them from the blast, but not from enemy fire. This was always one of the biggest drawbacks of the Scorpion, since they were totally exposed to enemy fire and the elements.The production M-56 had the same undercarriage as the prototype, with a final drive sprocket at the front, idler at the rear and four roadwheels with run-flat rubber tires on each side. The vehicle could circulate with some damaged tires, at a slower speed and for a maximum of 25 km. The suspension and tracks are the same as T-101 pilot model and with them achieved a very low ground pressure of 0.29 kg/cm2. The engine was the Continental AOI-402-5, a gasoline air-cooled opposed six cylinder model of 200 hp coupled to an Allison CD-150-4 cross drive transmission with torque converter with two forward and one reverse gears.Although the maximum speed specified was 45 km/h, the M-56 Scorpion easily surpassed it and had such power when starting from a standstill that in an exhibition held at Aberdeen Proving Grounds in the late 50’s it left rubber track marks on the concrete. Unfortunately, the engine consumption was very high and the vehicle only achieved about 230 km of range. It could climb slopes of up to 60% and overcome vertical obstacles up to 0.77 meters, the wading capacity was 1.10 meters.The gun installed on the Scorpion was the 90mm M54, with a similar performance to that installed in the M-36 “Slugger” tank destroyer of the WWII, and post war’s M-47 Patton medium tank. The gun could elevate to 15º and depressed to -10º and could traverse 30º to each side of the center. The gunner had a sight scope with x4 or x8 magnification to aim the gun, which was manually operated. It was such a powerful weapon that despite the muzzle brake, when the M-56 fired, the vehicle recoiled several meters, which made correcting fire very difficult. The vehicle carried 29 rounds and lacked secondary armament and smoke grenade dischargers. It also didn’t have any NBC protection.In 1955 the “90mm Gun Motor Carriage T-101” received some changes before being evaluated by the Armor Board at Fort Knox. Dust shields were installed and the vehicle was fitted with a canvas flap to cover the ammunition, which was carried under the gun breech in front of the loading platform.In the late 1950s the M-56 was also known as “SPAT”, an acronym for “Self Pro-pelled Anti-Tank”. An attempt was made to solve the crew’s exposure to the weather by installing a “winterization package” that included a heater on the right fender and a larger gun shield along with a removable canvas hood. However, this assembly was only seen during some tests and was never installed in the vehicles in service.The Scorpion entered service in 1953 and had its baptism of fire in Vietnam as part of the 173rd Airborne Brigade. Like the M-50 Ontos of the US Marine Corps, it mainly carried out missions of direct support to the infantry, since the role of tank destroyer was transferred to the M-551 Sheridan airborne light tank that offered more protection to its crew. In the mid-1960s, 152 M-56 Scorpions were transferred to Morocco (87), South Korea (60) and Spain (5), and two more units were transferred to Germany for testing, but the vehicle was not approved for service. The Spanish M-56 Scorpion (on the image) were part of the Landing Heavy Weapons Battalion of the Marine Infantry and were in service for some years.
