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Arsenal: M-551 Sheridan

By Christopher Miskimon
12/5/2018 • Vietnam Magazine

The light tank was ill-suited to Vietnam’s climate and environment, but with 152mm guns and modifications it still served well.

East of Tay Ninh City a soldier sat in his listening post, darkness all around. His unit, Troop A, 3rd Squadron, 4th Cavalry Regiment (3-4), was bivouacked for the night at a road junction. Suddenly, hearing movement, he reported it to his superiors, and word passed down the line to the tank crews in their M-551 Sheridan light tanks. Peering through infrared night vision scopes, the tankers searched the darkness for enemy soldiers and found them stealthily approaching. Main gun breeches were opened, and loaders pushed in canister rounds, the deadly “beehive” shells that carried some 9,000 flechettes. The North Vietnamese crept ever closer, unaware that the tanks’ 152mm cannons were trained on them. Suddenly the roar of cannon fire shattered the quiet and muzzle flashes lit the night. In an instant, tens of thousands of the tiny needle-sharp flechettes spread through the air, shredding leaves, grass and human flesh. Within minutes, the enemy’s will to continue was gone, and those left standing fled back into the night. After daylight the Americans walked the field and found the bodies of 40 enemy soldiers, including a battalion commander.

It was March 10, 1969, and the action helped build at least some faith in the new tank. The 3-4 Cav had only received its Sheridans in January, and initial opinions of the tank’s armor and reliability had been low. Weeks earlier one of the unit’s Sheridans had been destroyed when it struck a mine and its onboard ammunition detonated. This worried the crews since their previous tank, the M-48A3 Patton, would have survived with nothing worse than a few road wheels blasted off.

This was to be the legacy of the M-551 Sheridan tank in Vietnam: appreciated by some for its firepower but despised by others for its thin armor and poor reliability. Rushed into production and then sent to Vietnam, the Sheridan served the U.S. Army from the late 1960s to the mid-1990s.

The Sheridan’s development arose from the Army’s need for an improved light tank. In the post–World War II environment, such vehicles were commonly designed for reconnaissance, screening and defensive operations. One of their principal drawbacks was their main armament, usually a cannon too weak to penetrate the armor of any medium or heavy tanks they might encounter. In theory, light tanks were supposed to run from anything too powerful for them. In practice, they often either couldn’t run fast enough or were committed to missions that precluded easy retreat. America’s then-current light tank, the M-41 Walker Bulldog, had a 76mm main gun, too light to face contemporary Soviet medium tanks such as the T-54— although ARVN tankers would later use the M-41 against T-54s with some success. What the U.S. Army wanted was a light tank that was armed with a gun powerful enough to face larger opponents and was air-droppable.

What the Army got was an entirely new concept. Instead of using a cannon with conventional ammunition, the new tank would fire a guided missile capable of destroying any tank in service. For action against softer targets, there would be a normal High Explosive (HE) round but with a new twist: the ammunition would be caseless. Normal rounds had a brass case just like a huge rifle cartridge, which had to be ejected from the breech and disposed of. The Sheridan’s HE round was designed to be completely consumed in the process of firing, except for a small base cap still made of brass. The new gun/missile system, dubbed the Shillelagh, promised to herald a new age of tank gunnery. To reduce weight, the vehicle itself was to be made largely from aluminum. Rather than call the vehicle a tank, the Army dubbed it an Armored Reconnaissance/Airborne Assault Vehicle or AR/AAV. The airborne component was satisfied by para-dropping the vehicle out of a lowflying C-130 by use of drag parachutes, which pulled the tank, secured to a special shock-absorbing landing pallet, out the rear ramp of the plane so that it landed intact. The crewmen would follow by their individual parachutes, unhook the tank from the pallet and ready it for combat. That capability, however, was never used in Vietnam.

General Motors was selected to build the M-551 in 1960, with initial test firings of the new Shillelagh system beginning the following year. Testing revealed a number of problems that would plague the Sheridan throughout its development, its combat service in Vietnam and beyond. First, humidity warped the caseless ammunition so badly that the round often couldn’t be chambered in the gun. Once chambered, a round might ignite prematurely if it touched any burning embers still in the tube from the previously fired round. Attempts to cover the rounds with moisture-repellant coating didn’t work, so in the end they were wrapped in a plastic bag until ready to fire. An attempt was made to eliminate the burning propellant problem by fitting an open-breech scavenger system that sent air through the cannon chamber to clear the embers. But since the gun’s breech was open while the scavenger was operating, it blew both carbon monoxide and any burning propellant back into the turret, risking detonation of the stored ammunition. Eventually a closed breech system was installed. The Sheridan also suffered from a number of automotive problems.

Weapons as complex as armored vehicles usually encounter problems in their development and, despite the novel concepts used in the M-551, these problems would likely have been solved given time. Time, however, was not on the Sheridan’s side, and it was pushed into service in 1968. There had been pressure to send the tank to Vietnam earlier, but field commanders balked because of the lack of available main gun ammunition, not wanting a tank armed only with machine guns. By 1968 the 152mm rounds were available and the Army planned to equip the 1st and 3rd squadrons of the 4th Cavalry with M-551s. Both units’leaders were hesitant to take the Sheridans, fearing they were not as tough or versatile as their M-48A3 Pattons. In the end, only the 3rd Squadron got Sheridans, with the other tanks going to equip the 1st Squadron, 11th Armored Cavalry Regiment (11th ACR).

The new light tanks were very different from the big Pattons. The Sheridan was 20.7 feet long, 9.2 feet wide and 9.7 feet high. It weighed 17.4 tons fully loaded, powered by a 300 horsepower 6-cylinder Detroit Diesel engine. Top speed on roads was 43 mph with a range of 372 miles. The turret could hold nine Shillelagh missiles and 20 HE rounds, though the lack of an armored threat meant the missiles were never shipped. The beehive round was developed especially for use in Vietnam. Secondary armament was a coaxial 7.62mm machine gun and a .50-caliber machine gun mounted at the commander’s hatch. Eight smoke grenade launchers were spaced four on each side of the turret.

Both units began training with their M-551s in January 1969. The 3-4 Cav traded Pattons for Sheridans on a one-for-one basis, rotating soldiers through the training, while the 11th ACR swapped two M-113 Armored Cavalry Assault Vehicles (ACAVs) for three Sheridans, withdrawing an entire troop at a time from operations to undergo conversion training. Comparatively, the Sheridan was lighter and less well armored than an M-48A3, but was heavier, better protected and had more firepower than the ACAV.

With entry of the M-551 into actual combat, its strengths and weaknesses were quickly apparent. Mine blasts and RPG hits that a Patton would have survived destroyed or disabled a Sheridan. The caseless ammunition took much of the blame because all too often it would detonate in secondary explosions after a hit. That made Sheridan crews quick to evacuate in the event of a hit. To minimize risks, some crews stored only a few rounds of cannon ammunition or rode outside the vehicle as much as possible, adversely affecting combat performance. Wet weather caused problems with the fire control system and other electrical problems. In one tragic incident, an electrical malfunction caused a Sheridan traveling in a convoy to fire the canister round loaded in its gun into the vehicle ahead of it, a personnel carrier whose troops were riding on top.

In February 1969, the 1st Squadron, 11th ACR moved to Bien Hoa to serve as a reaction force against an anticipated enemy attack. On February 23, the VC fired a heavy barrage of mortars and rockets against the American position. Perceiving this as preparation for a ground attack, Troops A and B quickly assembled to make a sweep for VC troops. Led by Major William Privette, executive officer of the squadron, the force immediately ran into a sizable enemy element. Major Privette placed both of his troops on line and advanced. The heavy recoil of the 152mm guns caused the front ends of the tanks to lift a foot and a half with each shot. Overwhelmed by the daunting display of firepower, the enemy broke and retreated quickly, leaving behind the bodies of more than 80 of their comrades. This devastating victory was possible primarily because of the tank’s 152mm gun. Had they engaged this enemy a few months earlier in their ACAVs, the 1st Squadron’s troopers would have had only their .50-caliber and 7.62mm machine guns to bring to bear. The crews were suitably impressed.

Whatever the opinion of the troops, the Sheridan continued to be deployed to Vietnam. By the end of 1969 there were 200 M-551s in theater, and before long most of the cavalry units were equipped with them. The crews made the Sheridans their own, modifying them as soldiers always do. Drivers often installed a screen of chicken wire or mesh in front of their hatch to deflect branches. Armor shields fitted around the commander’s .50-caliber machine gun gave him added protection while firing. The normal plethora of extra equipment and comfort items soldiers carry with them adorned the Sheridans’ turrets and hulls as well. Extra ammunition, smoke grenades, food and personal gear hung off the vehicles, giving them the gypsy caravan appearance so typical of an army in the field. To protect against RPGs, rolls of chain link fence were often carried on the Sheridans, to be unrolled and set up around the tanks during stops to predetonate any incoming rockets.

After Vietnam the Sheridans equipped airborne tank battalions and both divisional and separate armored cavalry units. The tank fared much better in the European environment for which it was really designed, but its ammunition and recoil troubles forced its quick retirement from the cavalry units in the late 1970s. Because there was not a suitable replacement, the M-551 Sheridan continued as the airborne’s only tank until it was finally withdrawn in the 1990s—but not before serving in the invasion of Panama and Operation Desert Storm. Today, many M-551s are still in service at the Army’s National Training Center at Ft. Irwin in California, configured to resemble Soviet-era T-72 tanks and BMP infantry fighting vehicles.

While the Sheridan’s Vietnam story is one of a tank not suited to the climate or conditions of Southeast Asia, the crews operating these tanks knew full well their shortcomings but made use of their strengths to achieve some hard-won successes.

 

Originally published in the August 2007 issue of Vietnam Magazine. To subscribe, click here.  

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