The M551 Sheridan was named after Civil War General Philip Sheridan and was an Armored Reconnaissance/Airborne Assault Vehicle that was developed by the U.S. The Sheridan had both airdrop and swimming capabilities and was armed with the M81/M81E1 152 mm gun/launcher and the MGM-51 Shillelagh guided anti-tank missile.
THE DESIGN OF THE M551
The Sheridan used a large 300-hp Detroit Diesel 6V53T engine and as a result had a good power-to-weight ratio and mobility, running at speeds up to 45 mph which was more than any other tracked vehicle at the time; however the downside to this was that the vehicle was very noisy and unreliable in combat conditions.
The Sheridan had cloth sides that could either be rolled up or tucked into rubber tubes on both sides of the tank’s hull edges, a combination wood/aluminum front shield and an acrylic class window for the driver to look through; the Sheridan could swim a river that was about 50 yards wide and afterwards the water would leak out of it or the crew could open the battle hatch located within the rear ramp door and let out the remaining water.
The swimming capability of the Sheridan was provided by a flotation screen; the front armor was overlaid with three folded layers, hinged together that made a hull that came all the way up to the turret. Unfortunately the tradeoff was that the Sheridan’s armor was thin and could be penetrated even by heavy machinegun rounds and it was also highly vulnerable to mine.
The Sheridan could be rigged for low-velocity airdrop from C-130 and C-141 aircrafts, meaning that in areas where landing was not possible, one of these aircrafts could drop the Sheridan onto a field while still in flight. The tank would have to be strapped down to a special pallet which would absorb most of the landing impact; the crew would parachute in from another plane as it was not safe for them to be dropped off in the tank. Once the crew landed, they would release the lines holding the tank to the pallet and drive away.
VARIATIONS OF THE M551
Variations of the M551 include the XM551/M551, the “Two Box” M551, the M551A1, M551A1 TTS and the M551 NTC. The XM551/M551 was the basic model that was produced in 1965; while in the Two Box model the Shillelagh missile systems were removed and replaced with two separate boxes containing a 7.62x51mm coaxial machine gun and a 12.7x99mm BMG .50 caliber ammunition, while the storage area was also redesigned to hold conventional rounds. The M551A1 brought an upgraded AN/VVG-1 laser rangefinder while the M551A11 TTS was fitted with a Tank Thermal Sight unit.
Finally the M551 National Training Center (NTC) had their variation of the M551 Sheridan, using M551 hulls to create mock vehicles for training purposes; these training vehicles were also known as ‘vismods’ which was short for visually modified. They have since been replaced with modified M113s and M1 Abrams; however the war versions of the Sheridan have not been replaced since it was retired in 1996.