XM551

XM551 Pilot #1

In 1959, numerous proposals were put forth for a tank that would replace the M41 Bulldog and M56 Scorpion. Inspired by the Soviet PT-76, the tank was required to be lightweight, amphibious, and it was intended to mount a 152 mm Gun-Launcher that could fire either conventional HEAT rounds or the experimental XM13 guided missile. Cadillac’s design ultimately won, it was now designated as the AR/AAV XM551 (armored reconnaissance/airborne assault vehicle) and a contract was signed in 1960 for the further development of their concept. The tank used a light aluminum hull surrounded by a thin structure filled with polystyrene foam for added buoyancy. In total, twelve pilots would be constructed before the XM551 was standardized as the M551 Sheridan in 1966.

76 mm, 90 mm, and 105 mm guns were all proposed for use on the XM551. While they wouldn’t actually be mounted on the tank (for testing) until after the Sheridan entered production, they were considered as far back as when the tank was still being conceptualized. The only differences in the turrets were a new ventilator cover and the elimination of an external .50 caliber machine gun.

Towards the end of 1963, XM551 Pilot #7 was constructed, representing the 3rd generation of the XM551 program. The front hull was significantly modified, switching out the conventional flat hatch for the driver with the rotating assembly found on the production vehicles. A new surfboard was also installed and bulbous covers were added to the road wheels to improve buoyancy. This also allowed for the waterjet propulsion system to be removed, as the tank could now propel itself in water with its tracks. An XM122 spotting rifle was also installed. A laser rage finder and simple optical sight were also evaluated for use.

Pilot #1, a 1st generation XM551.

Pilot #7, a 3rd generation XM551.

This tank is the current Tier X Sheridan moved down a tier to replace the old T49. However, the Sheridan as we have it is missing several options and is more indicative of the first generation pilots for the XM551, not the production M551 as the name implies. Dropping the tank a tier can give it more flavor and authenticity.

Compared to the old T49, this tank is notably slower, but it has more flavor and has some trollish foam spaced armor that obscures the tank’s actual hull. One would have to aim for the center of the tank in order to guarantee a hit. The second turret could give some improvements to reload and dispersion, on top of a slight view range buff, but it would also require mounting the second suspension/hull. This hull, however, has less of the foam.

M551 Sheridan mounting the 105 mm howitzer

M551 Sheridan mounting the 76 mm gun

Fun fact: this Sheridan (Tier X) model shouldn't have this clutter on the front hull.
It prevents the surfboard from deploying and obscures the driver's window.

Guns: 76 mm Gun M32; 90 mm Gun T132E3; 90 mm Gun M41; 105 mm Howitzer XM103E7; 152 mm Gun-Launcher XM81 (conventional)

Engines: GM 6V53T Early (275 hp); GM 6V53T Late (300 hp)

Turrets (Turret Armor (mm)): XM551 Pilot #1 (38/15/15); XM551 Pilot #7 (38/15/15)

Suspensions/Alternate Hulls, Total Weight (metric tons): XM551 Generation 1 (15.08); 
XM551 Generation 3 (15.08)

Hull Armor (mm): 15/32/13

Top Speed: 56 km/h

Crew: 4 (Driver; Gunner; Loader; Commander/Radio)

Parent: T49
Children: Sheridan
See Also: XM551 TB

Sources:

Hunnicutt, R. P. Sheridan, 2015.