90mm Self-Propelled Anti-Tank Gun M56 Scorpion - Tank Encyclopedia (2024)

United States of America (1959)
Self-Propelled Anti-Tank Gun – 325 Built

Introduction

The M56 began life in the heads of an Anti-Tank Panel in Fort Monroe, 1948. They soon developed the idea of a self-propelled, high-velocity small caliber anti-tank vehicle that could be air transportable and deployable.
This idea was put forward to the Army Airborne Panel later the same year, who in turn forwarded the idea to the Ordnance Department. The department didn’t develop the project, under the designation of T101, until 1950. Cadillac was given a contract to build 2 prototypes.
The T101 project ran for 6 years, finally culminating with the 4-crew SPAT (Self-Propelled Anti-Tank) M56 Scorpion.

Development

As the T101/M56 was in development, so was the SSM-A23 Dart Anti-Tank Guided Missile (ATGM). The Continental Army Command did not want to spend the time and money on two projects that effectively fulfilled the same role. This postponed the original 1957 delivery date of the vehicles to troops. A case was argued that the Dart would not be serviceable for another 2 years. Because of this, it was finally agreed that Scorpion would go into production. It finally started being delivered to troops in 1959.
Built by Cadillac Motor Car Division of General Motors for use by US airborne forces, The M56 was designed to be airdropped by heavy assault gliders and cargo aircraft. In later years, it was able to be dropped via helicopter.

This photo of the M56 demonstrates the effect of the recoil. Source: – live.warthunder.com

Design

Due to it being lightweight, it was an extremely maneuverable vehicle on every ground type. It was powered by a Continental AOI-402-5 high-octane gasoline engine. This sent 200 hp through the Allison CD-150-4 transmission to the forward mounted drive wheels, powering the vehicle cross country at a respectable 28 mph (45 km/h). The M56 featured a unique track and suspension. The track was lightweight and rubber connected with metal grousers. It had a torsion bar suspension, connected to all 6 wheels, including the drive wheel and idler to assist with recoil stresses. The road wheels were pneumatic with 7.5×12 tires that could be run even if punctured. Pneumatic road wheels were chosen because they are much lighter compared with the standard solid-steel.
The airborne deployment and weight restrictions associated with it demanded sacrifices, one of which was that the Scorpion was a completely open vehicle. It had nothing that could be considered armor whatsoever, bar a 5 mm gun shield, and reinforcing brush protection bars on the front of the tank. Indeed, the only protection the crew had was the 5 mm gun shield, this only covered the driver and gunner’s positions. Other than that they were completely open to the elements or any fragmenting explosives.
Though the crew probably would’ve enjoyed a bit of armor, the lack thereof wasn’t too much of a downside. The Scorpion, like it’s namesake, was an ambush predator. It was able to fire and scuttle back to cover extremely quickly or engage targets at ranges up to 1000 m. The sting in this Scorpion’s tail was the M54 90 mm gun, which was specially designed for the vehicle. It was originally going to be mounted with the T119 90mm cannon, but it wouldn’t fit onto the tank. Its standard ammo was the M3-18 Armor Piercing round. It could punch through 190 mm of armor at 1000 m. It could also fire the entire range of 90 mm ammunition of the day, including HVAP and APCR-T. Ammunition was stored in a rack at the rear of the vehicle. It carried 29 rounds, in 3 stacked rows, 2 rows of 10, one of 9.
The gun, though it operated and performed as designed, was also somewhat of a problem. The force of the recoil was amplified on the vehicle because it was so light, to the extent that it would lift the vehicle almost 3 feet off the ground. Firing with the gun straight forward was not a problem, bar the intense recoil. However, should the tank need to engage a target to the extreme left or right of the gun’s traverse, it ran the risk of severely injuring either the driver, commander or the gunner himself. Indeed, if the commander stayed in his seat with the gun aimed to the right, he would receive a recoiling breech block to the face. As such, it was recommended by a manual that all unnecessary crew abandon the vehicle when the gun is fired in this way.

Tank Encyclopedia’s own rendition of the M56 Scorpion SPAT by David Bocquelet.
Scorpions operating in Vietnam. Source: – bemil.chosun.com (Korean)

Service life

The M56 saw limited combat service. During the Vietnam War, it was deployed by the 173rd Airborne Brigade, the only brigade to do so. They used it mostly in a supportive role.
The M56 was not popular with the USMC who favored the Recoilless-Rifle equipped M50 Ontos, which was used in the same role but had an armored fighting compartment. The vehicle was effectively replaced in the field by the better armed and armored M551 Sheridan in 1970.
The M56 was exported to The Republic of Korea, Spain and Morocco. Morocco was the only other nation to use the vehicle in anger. It served in combat against Sahrawi rebels during the Western Sahara War.