Anti-tank artillery calibers showed a steady growth in WWII. The US Army entered the war with 37 mm guns and ended it with 76 and 90 mm guns. The increase in caliber meant the increase in the weight of the gun. This was not critical for the infantry, as they could simply introduce more powerful tractors, but the Airborne was not so lucky.
The lessons of the fighting at Arnhem, where British paratroopers had to fight against German tanks, were not ignored by American commanders. The 90 mm T8 anti-tank gun was adopted by American paratroopers in 1945, consisting of a 90 mm M1 AA gun barrel with recoil elements from the 105 mm M2A1 howitzer and a lightened carriage. The result was a 3540 kg cannon, which could be deployed from the C-82 Packet aircraft, but issues began to arise on the ground. The crew could not move the gun around the battlefield. A tractor was needed, which meant that the number of flights required to transport an anti-tank gun battery would have to double.
This problem could be solved by building a compact tank destroyer. This idea was first proposed during a 1948 conference dedicated to anti-tank weapon development at Fort Monroe. In April of next year the potential customer sent out the requirements. The main one was mass: 16,000 lbs (7260 kg), the maximum carrying capacity of the Packet and a heavy glider that was being developed at the time.
The development of an airborne tank destroyer was given to the Cadillac Motor Car corporation, a division of General Motors. The design was based on the solutions that were tried and tested on the M76 Otter amphibious APC. Due to the limited dimensions of the airplane's cargo compartment it was impossible to equip the SPG with a casemate, let alone a roof. Only a small gun shield was installed. The shield was mostly designed to protect the crew from the muzzle blast and could not resist bullets or shrapnel.
The prototype T101, seen from above.
The prototype, indexed T101, was ready in 1953. Two years later the tank successfully passed trials in Fort Knox and was accepted into service under the index M56 Gun, Self Propelled, Anti-Tank. The widely used name "Scorpion" was approved in 1957. The acronym SPAT was less widespread. Production began in December of 1957 and continued until June of 1958. 160 units were produced.
The M56 was a small unarmoured SPG that was equipped for deployment from C-123 Provider and C-119 Flying Boxcar aircraft (as well as larger transport aircraft), or transport by helicopters on an external platform. The hull was made from aluminium alloy. The crew consisted of four men.
The crew of the M56 sat out in the open.
The front engine compartment contained a six cylinder opposite four-stroke carburettor air-cooled Continental AOI-402-5 165 hp engine and a mechanical Allison CD-150-4 (two gears forward, one reverse). The remainder of the hull was occupied by a joint fighting and driving compartment. In the center, the M54 90 mm gun was installed on the M88 mount. The driver sat to the left of the gun (the gun shield had a transparent window with a windscreen wiper for him), and the gunner sat to the right. The commander sat behind the driver and the loader sat behind the gunner. A rack for 29 rounds of ammunition was located in the rear. A collapsible step was installed behind the rack for the loader's convenience.
Second T101 prototype seen from behind. The ammunition rack is visible.
The running gear consisted of four large road wheels per side, sprung on torsion bars, and equipped with pneumatic tires. The tires had special inserts that allowed the vehicle to travel 15 miles (24 km) with a speed of up to 24 kph in case of their rupture. The drive sprocket was located in the front. The track links were rubber-metallic, 510 mm wide. Two tracks composed of rubberized fabric reinforced with steel cables ran per side. The Scorpion's ground pressure was 0.29 kg/cm² (to compare, the M47 and M48 had 1.03 and 0.78 kg/cm² respectively), which gave the vehicle good mobility.
The installed 90 mm M54 L/50 gun was designed based on the M36 gun used in M47 tanks. It was 95 kg lighter than its ancestor. The vertical aiming range was from -10 to +15 degrees, and the horizontal was 30 degrees to the left and to the right. The gun was equipped with a monobloc barrel, a screw-on breech, and a single chamber muzzle brake. The breech was vertical, sliding, semiautomatic. Aiming and loading was done by hand. The gun was equipped with an M186 telescopic sight with variable magnification (4-8x).
The amount of ammunition used was fairly wide and included all types of ammunition usable in M36 and M41 guns. In addition, 90 mm rounds designed by Rheinmetall could be used. For the SPG's main mission, battle with tanks, M82 APC shells, M318 (T33E7), M318A1, and M318A1C shot, subcaliber AP M304, M332, and M332A1, and non-rotating (fin stabilized) HEAT M348 (T108E40), M348A1(T108E46), and M431 (T300E5) shells could be used. In addition, the SPG could fire the M71 HE, M91 HE-T, M336 canister shot, M377 HE-frag, or M313 smoke shells.
The vehicle was equipped with an ultra-shortwave AN/VRC-10 radio that was operated by the commander. The only night vision device available was the driver's helmet-mounted night vision goggles.
An experimental self propelled mortar on the M56 chassis.
Two experimental SPGs were built on the M56 chassis. In 1958, an SPG with an M40 106.7 mm recoilless rifle installed instead of the 90 mm gun was tested at Fort Benning. Since a regular Jeep could carry such a gun, it was not accepted into service. Another vehicle, also not put into production, was equipped with an M30 mortar. Variants armed with SS-10 and ENTAC ATGMs were left on paper.
Service and use in battle
Initial plans armed each of the three American airborne divisions (the 11th, 82nd, and 101st) with a battalion of Scorpions (53 vehicles apiece). However, the adoption of the Scorpion coincided with the reorganization of airborne divisions from the "triple" to the "quintuple" type: instead of three regiments, a division now consisted of five battlegroups, essentially reinforced airborne battalions. As a result, Scorpions were used by the anti-tank platoons of the HQ company of the battlegroups. This platoon included a command section (a lieutenant as a platoon commander, a sergeant as his assistant, and a radio operator on a Jeep with an AN/VRC-18 radio) and three fire sections (8 men and 2 M56 SPGs each). Each platoon counted 27 men, 6 Scorpions, and a Jeep.
An M56 prepared on a parachute platform.
A Scorpion could be transported on the outside of an H-37 Mojave helicopter.
Scorpion platoons were formed in fifteen battlegroups in the first half of 1958. However, in July of 1958 the 11th Airborne was dissolved. Its battlegroups, along with their M56es, were transferred to the 24th Infantry Division, but in January of 1959 were transferred to the 82nd Airborne Division. The latter transferred two of its battlegroups to the 8th Infantry Division. Finally, in June of 1960 one battlegroup from the 82nd Airborne was transferred to the 25th Infantry Division, and one of the battlegroups disbanded in 1958 was restored to refill the 82nd. A number of leftover Scorpions formed battlegroups for the 1st Infantry Division in Germany, as well as for the 1st Cavalry and 7th Infantry Divisions in Korea.
An M56 from the 101st Airborne Division on exercises. Blank rounds can be seen underneath the gun breech.
M56 from the 1st battlegroup of the 16th Infantry Regiment, 1st Infantry Division on exercises in West Germany. February 1961.
The "quintuple" structure was deemed unsuitable for use in non-nuclear conflicts in 1961, and the army began yet another reorganization. Now each airborne division contained three brigade HQs and nine airborne battalions, as well as support units, which included a tank battalion. This battalion was to be equipped with new M551 Sheridan airborne tanks, but as a temporary measure the tank battalions of the 82nd and 101st divisions received 47 Scorpions each in 1964. These vehicles were not only not tanks, but had no armour at all. There were also no resources allocated for crews, so until the Sheridans were issued the tank battalions remained "virtual".
The only armoured unit that used Scorpions in battle was D company of the 16th Tank Regiment (D-16), formed in 1963 as a part of the 173rd Independent Airborne Brigade formed in Okinawa. The company consisted of four platoons of four M56 each, an HQ section (four M113 APCs), and a mortar section (three 106.7 mm self propelled mortars on the M113 chassis).
D-16 SPGs in Vietnam.
The 173rd brigade was transferred to Vietnam in May of 1965. Jungle warfare showed both the good and bad sides of the M56. Good mobility allowed the SPGs to traverse terrain unsuitable for tanks. On the other hand, there were few suitable targets for the 90 mm gun. The main task for the Scorpions was close support of airborne battalions and companies fighting on foot, where the most serious drawback of the M56, the complete lack of armour, came to light. The straw that broke the camel's back was a battle on March 4th, 1968, where the company lost 8 men in one battle. The "tankers" from D-16 replaced their M56es with the much more universal and much better protected M113 APCs.
Scorpions belonging to Spanish marines on parade in Madrid.
After being written off by the US Army, a part of the M56es was sent to warehouses, and a part was given to allies. Spain received five vehicles in 1965. They served in the marine anti-tank platoon until 1970. Morocco received 87 Scorpions in 1966-1967. According to Jane's, Morocco still had 28 M56 SPGs in storage as of 2010.
Moroccan Scorpions during the fighting in Western Sahara.
Two T101 prototypes modernized up to the M56 standard were given to West Germany in 1960. The Germans did not like the unarmoured vehicle, and did not accept it into service. After short trials both vehicles were converted into training vehicles for drivers. The cannons were replaced with glass cabins.
A T101 converted into a driver's training vehicle.
The US Navy acquired a small number of M56es. The vehicles were converted into QM-56 radio controlled targets and used at the Fallon, Warren Grove, and Cherry Point ranges as targets for attack aircraft pilots and gunners in 1966-1970.
The M56 boasted good mobility and powerful armament for its time. HEAT shells fired from its 90 mm gun could defeat any Soviet tank from the early 1960s. At the same time, the gun was too powerful for a seven ton chassis, lifting the front wheels off the ground during firing. In addition, a lack of any kind of armour allowed the SPG to only fight against tanks from ambushes, making the Scorpion unsuitable for supporting airborne troops in offensive operations.
The Scorpion was more than twice as heavy as the comparable Soviet ASU-57 (7.14 vs 3.35 tons). In addition, the ASU-57 was more compact (1.46 m tall vs 2), and was protected from the front and sides with armour (although 4-6 mm could not protect from ordinary rifle bullets at close ranges). A for the armament, the M56 was far superior: its 90 mm gun had 4.57 MJ of muzzle energy, compared to 1.46 MJ of energy from the 57 mm Ch-51 gun on the ASU-57. The mobility (speed and range) of both vehicles were about even.