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In the early 1960s, the US Army issued a requirement for a replacement to the ageing M41 Walker Bulldog light tank. General Motors’ Cadillac Gage division was awarded a contract for prototypes of what was officially an ‘Armoured Reconnaissance/Airborne Assault Vehicle’. The new tank, the M551 Sheridan, was superficially impressive, but the design was too ambitious for the technology of the period.
The tank’s futuristic main armament, a 152mm gun/launcher system, was an endless source of trouble. Although the concept was brilliant, the realities of trying to ‘shoehorn’ such a heavy weapon into a light tank caused problems. When firing over the forward arc, for instance, the recoil would lift the front road wheels off the ground and crew members had to brace themselves to avoid being thrown around the turret. The original idea was that the weapon could fire both conventional shells and the new Shillelagh anti-tank missile, but both suffered teething problems. As such, although the first Sheridans were completed in 1966, the new missiles were not finished in time to see use in Vietnam, and the tank used only conventional shells.
A total of more than 200 Sheridans saw service in Vietnam between 1969 and 1972, largely with armoured cavalry units. The lack of any significant threat from enemy armoured fighting vehicles (AVFs) led to the decision not to deploy the still troublesome Shillelagh missiles, but the heat and humidity accentuated the unreliability of the tank’s already temperamental electrical systems.
Fragile and sensitive
Within six months, there were a host of other problems, including engine-cooling issues and dangerous malfunctions with the combustible-case main gun ammunition. One summary listed ‘16 major equipment failures, 123 circuit failures, 41 weapons misfires, 140 ammunition ruptures, 25 engine replacements, and persistent failures related to the 152mm cannon.’ The airborne role and a requirement for amphibious capability meant that weight had to be kept to a minimum, so the armour was just sufficient to give protection from shell splinters and small-arms fire. This meant that, while the Sheridan was highly mobile, it was horribly vulnerable to mines and even the lightest anti-tank weapons. The first combat loss occurred as early as February 1969, when a Sheridan was ripped apart after a mine it hit detonated its own 152mm ammunition.
When everything worked properly, the Sheridan had impressive firepower – the M625 Canister round acted like a gigantic shotgun, spraying out 10,000 metal flechettes to an effective range of about 400 metres. The 23.4kg HE-T (High Explosive, Tracer) shell was highly destructive, but its ‘super-quick’ impact fuse was so sensitive that the gunnery manual warned: ‘Never fire the HE-T cartridge over friendly troops or through brush or other obstructions close to vehicle. The fuse on this round is very sensitive and could cause premature detonation.’
Weaknesses: over-ambitious design, dangerous recoil, electrical issues
The fragile combustible-case 152mm ammunition had to be handled carefully, too. Early production vehicles were fitted with a bore evacuator and a breech scavenging system, which blew carbon dioxide into the open breech after firing to get rid of any burning remnants of propellant. But this also blew any burning material around the turret, posing a real risk of detonating the stowed ammunition. A replacement closed- breech scavenging system was hastily developed to blow any debris out of the muzzle.
These continual tweaks did not suffice. To the relief of many crews, the Sheridan began to be phased out of service from 1978, although the last few were not retired until 1997.
Images: Wikimedia Commons
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