90mm Self-propelled Gun M561-8

M56: General
Date of first acceptance December 1957 Total acceptances 160
Manufacturer Cadillac Motor Car Division of General Motors Corp. Crew
4 men:
  • Commander-radio operator on hull left rear
  • Gunner on hull right center
  • Driver on hull left center
  • Loader on hull right center
M56: Dimensions
Combat weight 15,750lbs
Height over blast shield 78.9"
Length without gun 179.4"
Gun overhang forward 50.4"
Width over fenders 101.3"
Tread 78.0"
Ground clearance 12.8"
Fire height 66.0"
Ground pressure, zero penetration 4.2psi
M56: Armament
Type Mount Ammunition Traverse Elevation
90mm Gun M54 M88 on hull center 29 rounds 60°
(30° left and right
+15° to -10°
M56: Armor
M56: Automotive
Engine Continental AOI-402-5; 6 cylinder, 4 cycle, opposed, fuel injected gasoline
Horsepower Net: 165@3000rpm
Gross: 200@3000rpm
Torque Net: 325 ft-lb@2200rpm
Gross: 347 ft-lb@2800
Fuel capacity 55gal
Transmission Allison CD-150-4, 2 ranges forward, 1 reverse
Steering Mechanical, steering wheel
Brakes Multiple disc
M56: Suspension
Type Road wheels Track return rollers
Torsion tube over bar at wheels 1 and 4, torsion bar at wheels 2 and 3 4 individually sprung/track with pneumatic tires Flat track
Drive sprockets Idlers Shock absorbers
15-tooth front drive Compensating at rear of track On first and last road wheels/track
M56: Track
Outside guide band type
Width 20"
Pitch 44 long sections;
4 cross bar pitch
110cm long sections;
10cm crossbar pitch
Shoes/track 8 sections/track;
88 cross bars/track
Ground contact length 94"
M56: Performance
Max level road speed 28mph
Max trench 48"
Max grade 60% Max vertical obstacle 30"
Min turning diameter Pivot Max fording depth 42"
Cruising range ~140mi, roads
~230km, roads

The M56, also known as SPAT for Self-Propelled AntiTank, or more colloquially as the Scorpion, was designed as an airborne antitank gun. The 90mm gun M54 was ballistically identical to and used the same ammunition as the M36 90mm gun in the M47 tank, and it was fitted with a blast deflector. The M56 was unarmored, and consisted of little more than a gun mounted on a tracked riveted-aluminum alloy carrier. There was a small blast shield fitted to the weapon, and this had a windscreen in the left side for the driver to look through. Creature comforts were nonexistent as the vehicle was completely open. The commander sat on top of the radio, and the loader's seat was on top of the right fender stowage box, leaving the gunner and driver as the only crewmen "in" the vehicle. Recommended tire pressure was 75psi (5.2bar), but the tires on the M56 could run flat up to 15mi (24km) at up to 15mph (24kph). The ammunition rack was located in the lower rear hull, and the loader was provided with a folding platform from which to feed the gun.

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  1. TM 9-2350-213-10 Operation 90-mm Full Tracked Self-propelled Gun M56. Washington, DC: Headquarters, Department of the Army, 22 May 1958.
  2. TM 9-2350-213-10 C9 Operation 90-mm Full Tracked Self-propelled Gun M56. Washington, DC: Headquarters, Department of the Army, 31 October 1968.
  3. TM 9-2350-213-20 Organizational Maintenance 90-mm Full Tracked Self-propelled Gun M56. Washington, DC: Headquarters, Department of the Army, 19 June 1958.
  4. Hunnicutt, R.P. Sheridan: A History of the American Light Tank, volume 2. Navato, CA: Presidio Press, 1995. Reprinted with permission from Sheridan, R.P. Hunnicutt ©1995, available from Presidio Press, 505B San Martin Drive, Suite 160, Navato, CA 94945.
  5. FM 23-43 90-mm, Full-tracked, Self-propelled Gun, M56. Washington, DC: Headquarters, Department of the Army, 28 July 1959.
  6. Estes, Kenneth W. M50 Ontos and M56 Scorpion 1956-1970: US Tank Destroyers of the Vietnam War. Oxford: Osprey Publishing Ltd., 2016.
  7. Crismon, Fred W. U.S. Military Tracked Vehicles. Osceola, WI: Motorbooks International, 1992.
  8. Hogg, Ian V. The Greenhill Armoured Fighting Vehicles Data Book. London: Greenhill Books, 2000.

Last updated 8 Feb 2023.
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