Staff Writer (Updated: 7/10/2014):
Based on the T101 pilot vehicle, hull construction of the M56 consisted of all-welded and riveted aluminum. Its 90mm M54 anti-tank was closely associated with the main gun armament of the M47 "Patton" Main Battle Tank (MBT). The vehicle allowed for 29 x 90mm projectiles to be carried aboard. Its design was for a standard operating crew of four to include the driver, commander, loader and gunner and power was served from a single Continental A0I-402-5 gasoline-fueled engine of 200 horsepower. This was mated to an Allison CD-150-4 transmission system with two forward gears and a single reverse speed. Suspension was made up of a torsion-tube-over-bar set at the 1st and 4th wheels (four total road wheels to a hull side) while a torsion bar system was set at wheels two and three. Speed was roughly 28 miles per hour on road while range was limited to 140 miles. The vehicle's operating weight was in the vicinity of 14,000lbs to 16,000lbs. Unlike other armored combat vehicles of the period, the M56 was given a pneumatic tire arrangement which further served to lighten its combat weight.
One of the obvious drawbacks of the M56 design was that the crew sat completely exposed to the elements and critically exposed to battlefield dangers with the only protection being a forward fragmentation shield attached as part of the gun mounting. The lack of a proper hull superstructure allowed engineers to keep the M56 a lightweight, transportable design. The loader stood upon folding platform when reloading the gun through the breech at the rear. The main gun held a manual traverse and elevation function for precise ranging. To the left of it was the driver's position with its applicable steering controls. A section of the gun shield was cut out with a windshield so the driver could operate from behind some protection. The radio set and section leader sat to the driver's right. This left the space along the right side of the breech for the gunner and ammunition handler. Ammunition types varied from the usual High-Explosive (HE) types to more advanced Armor-Piercing (AP) types. The ammunition stock sat under the floor of the gun and accessed from a platform at the rear of the vehicle.
Despite the complete lack of crew protection, the system proved itself to be a formidable piece of mobile artillery particularly for those lightly armed paratrooper forces. The vehicle was fielded with the storied 82nd and 101st Airborne Divisions and went on to see combat service during the Vietnam War (1955-1975). Paratroopers were designed with speed and surprise in mind and thusly carried whatever tools of their deadly trade they required, jumping out of aircraft into the fray below ready for action. The M56 allowed such units a fighting chance against armor when on the ground. M56s functioned in the Vietnam theater from 1957 until 1970 to which they were then withdrawn during the American drawdown in Southeast Asia.
Apart from its open-air crew compartment, the M56 was not without other drawbacks. Its main gun held a maximum allowable range of 1,500 meters and the recoil of the tank gun proved the light chassis unsound. During firing, the front of the vehicle would kick up violently, sending the rear portion towards the ground. Not only did this wear on the crew and chassis, it promoted the vehicle's location by kicking up smoke and dust, giving up a once-concealed location.
The M56 was finally replaced by the M551 Sheridan air-droppable reconnaissance light tank which fitted a more potent 152mm main gun in a more suitable turret-and-hull arrangement. Even with the arrival of the M551, the M56 Scorpion was still being fielded in limited numbers by before the end of the Vietnam War though by now relegated to the fire support role as opposed to direct contact with enemy armor. The chassis of the base M56 was also be featured in several other U.S. Army tracked developments of the period including the M50 "Ontos" recoilless rifle carrier.
After the official retirement of both the M56 Scorpion vehicle and M551 Sheridan tank, U.S. airborne divisions were essentially left without a dedicated self-propelled piece in their inventory, a status which remains consistent today (2014). Any remaining M56 pieces went on to be scrapped or enjoy life as museum mainstays.
The M56 was also exported to Morocco, Spain, and South Korea.