Written by Matthew Copes
When developing tanks and other armored vehicles, designers have always struggled to find the “sweet spot” between protection, firepower, mobility and cost.
Since before the First World War, militaries around the globe have spent gobs of money developing tanks of all shapes and sizes.
That said, most missed the aforementioned sweet spot by a wide margin.
Bigger guns generally have longer ranges and more energy to penetrate thicker armor, but they’re also heavier, more expensive, and have lower rates of fire, and since their shells are larger, fewer rounds can be carried into battle.
Likewise, more armor means better protection for machine and crew, but it too makes vehicles heavier, slower, more expensive, and more difficult to transport.
Some, like Germany’s gargantuan 68-ton King Tigers of World War II featured nearly impenetrable armor and immense guns, but on the downside they were slow, unreliable and a nightmare to transport.
On the other hand, the American M18 Hellcat tank destroyer was the fastest armored vehicle of the Second World War.
Hellcats had potent guns, but their armor was criminally thin, and as a result crew survival rates were abysmal.
Fast forward nearly two decades to the early ‘60s.
After a relatively short development period, GM’s Cadillac division unveiled a revolutionary light tank – or more accurately, Armored Reconnaissance/Airborne Assault Vehicle – prototype for the US Army.
Later designated the M-551 Sheridan, the fast, tracked vehicles featured futuristic designs and powerful 152 mm cannons.
They’d go on to serve for more than three decades, but the question has always remained, were Sheridans the well-rounded light tanks they were made out to be, or overpriced, overly complex death traps that rarely made an impact on the battlefield?
Let’s find out.
Like most American tanks and armored vehicles, Sheridans were named after a general.
In this case, Union General Philip Sheridan, most known for his no-holds-barred devastation of the Shenandoah Valley in 1864, which locals took to calling the Great Burning.
But ironically, light and nimble Sheridans were introduced at a time when prevailing American military doctrine favored ultra-heavy main battle tanks over more mobile but less lethal light and medium tanks.
The idea was that standardization would speed production and reduce costs, and that in the event of war, large concentrations of 60 and 70-ton behemoths would win the day.
Of course, transporting these monsters would be tedious, time consuming and expensive, and thanks to their immense weight they couldn’t be airlifted to out-of-the-way hotspots at the drop of a hat.
What was needed was a fast, lightly armored vehicle with a big cannon that could be dropped by parachute directly onto the battlefield.
Try that with an M-1 Abrams.
Though slightly lighter than most contemporary tanks, American M48s and M60 tanks of the ‘50s, ‘60s and ‘70s were more than three times as heavy as Sheridans.
Especially during the Second World War, lightly armed airborne troops were particularly vulnerable immediately after deployment, because they were generally only equipped with small arms and hand-held anti-tank weapons like bazookas.
The Army did have 7.5-ton (16,500 lb / 7,500 kg) M22 Locust tanks that could be transported into battle via glider, but they only had piddly 37 mm cannons and very light armor, and most were used by British paratroopers.
After the war the Army pressed M41 Walker Bulldogs into service as light tanks, but at 25 tons they were far too heavy.
The truth was that light tank development just wasn’t a priority during the Cold War, at least until the amphibious Soviet PT-76 tanks burst onto the scene in the early ‘50s, after which the Army realized that it had some serious catching up to do.
General Motors Cadillac division – of later Cimarron fame – was tasked with developing a vehicle that was significantly lighter but packed a bigger punch than the Walker Bulldog.
Cadillac produced two designs designated T71 and T92, the latter of which won out despite being heavier.
But though the T92 was both agile and powerful, it wasn’t amphibious like the Soviet’s PT-76, and giving it the ability to swim across lakes and rivers would have meant a costly and time consuming redesign.
Hence, the T92 was scrapped in favor of an all new machine designated the XM551.
By then the Army had already begun deactivating its M103 heavy tank battalions and replacing them with M60s, while at the same time America and West Germany were co-developing the MBT-70 main battle tank.
But despite promises of an unstoppable “super tank” from both sides, the MBT project would never come to fruition, though by then it was drastically behind schedule and over budget.
So much so in fact, that the Army assumed Congress would never agree to fund two developmental tanks simultaneously, especially when the first had turned into such a debacle.
To get around this, ehe Army relied on a time-tested method for duping unsuspecting civil servants – smoke and mirrors.
Instead of calling the Sheridan a light tank which it really was, they classified it as an armored reconnaissance vehicle.
And abracadabra, their little trick worked, because in the spring of 1965 the project got funded and Cadillac was awarded a 115 million USD (1.25 billion USD today) contract to get the experimental prototype ready for production.
To keep weight low, the luxury car manufacturer used an aluminum hull and a lightly armored steel turret.
Though thin by heavy tank standards, Sheridan armor could stop fire from heavy machine guns up to about .50 caliber (12.7 mm), but when it came to mid-size cannons, tank rounds, mines and rocket-propelled-grenades (RPGs), it was usually lights out for the crews crammed into the tight fighting compartments.
Nonetheless, production began in late July of 1966 at Cadillac’s Cleveland, Ohio plant.
Approximately 20 feet (6 m) long, 9 feet (2.8 m) wide and 7.5 feet (2.3 m) high, Sheridans tipped the scales at just over 34,000 pounds (15,400 kg) and were crewed by a commander, driver, loader and gunner.
By comparison, they were only about two tons heavier than the M113 armored personnel carriers that were entering service at the same time, though the latter had no turrets or cannons.
Performance and Mobility
Powered by turbocharged, 2-stroke Detroit Diesel 6V53T engines producing about 300 horsepower, Sheridans had high power-to-weight ratios.
As a result on and off-road performance were exceptional, thanks largely to their rugged yet simple torsion bar suspensions.
With operational ranges approaching 350 miles (560 km) and maximum road speeds of approximately 45 miles per hour (72 km/h), they could get where they were needed in a hurry, but more importantly they could be airlifted by transport aircraft incapable of hauling heavier tanks.
When time was of the essence during rapid deployments and combat situations, Sheridans could be delivered by C-130, C-141 and C-5 transports using the Low Altitude Parachute Extraction System (LAPES).
These low-velocity airdrops occurred just meters over the ground, but instead of slowing the palletized vehicle’s descent, parachutes were deployed horizontally to yank them out of the cargo hold.
So you’re probably wondering if crews actually rode in the Sheridans when they were airdropped, right?
The answer is no, crews parachuted from other aircraft and rendezvoused with their machines on the ground.
In addition to their superior mobility on terra firma, Sheridans were capable of swimming across relatively narrow lakes and rivers at about 3.5 miles per hour (6 km/h).
Buoyancy was provided by flotation screens similar to those used on tanks and armored vehicles during the landings at Normandy and elsewhere.
But though effective, the wood, fabric and aluminum strut system was relatively tedious to operate and prone to damage even during routine non-combat operations.
Drivers were equipped with cheesy plastic splash guards that were generally considered useless, and steering orders were usually shouted from the commander in the turret down to the driver in the hull over the drone of churning tracks and thumping diesels.
Yet despite being mobile and reliable, packing big guns and rarely getting stuck in mud like their heftier main battle tank counterparts, Crews weren’t crazy about Sheridans because they just didn’t provide adequate armor protection.
On the flipside, infantrymen appreciated their ability to get on-scene quickly and deliver much heavier firepower than the .50-caliber machine guns on M113s, alongside which Sheridans commonly fought.
Previously, armored units had consisted solely of heavy tanks, while mechanized infantry units were supported by M113s.
Sheridans were designed to fill this gaping void, but in the end they saw relatively little combat.
Though the M81 gun and missile system was one of the Sheridan’s most noteworthy features, it also proved to be one of its biggest Achilles’ heel.
Designed to fire standard cannon rounds and MGM-51 Shillelagh guided anti-tank missiles, at least in theory it seemed like a revolutionary solution to the age-old problem of stuffing a potent weapons system into a light vehicle.
About 45 inches (110 cm) long, 6 inches (15 cm) in diameter and weighing approximately 60 pounds (27 kg), Shillelagh missiles might have been lethal tank and bunker busters, but despite ranges exceeding 2,000 yards (1,800 m), the Sheridan’s tracking and guidance systems didn’t work under 800 yards (730 m).
In addition to manufacturing, logistical and reliability issues, it’s no wonder they were never used in Vietnam.
The cannons were also capable of firing enormous 152 mm caseless shells that were devastating to structures like buildings and bunkers, and they were particularly effective against troop formations when utilizing M625 canister rounds which unleashed hundreds of knife-like flechettes that cut down nearly anything in their path.
However, the cannons’ short barrels and low muzzle velocities meant that they were far less capable of knocking out enemy armor than the smaller but higher velocity 100 and 105 mm guns on M48s and M60s.
But as was the case with the missiles, when Sheridans entered service the ammunition for the cannons wasn’t ready, and it wouldn’t make its first appearance until the end of 1968.
The following January General Creighton Abrams dispatched the first Sheridans to Vietnam and they began replacing M48 Pattons with select armored cavalry regiments, but the transition wasn’t a smooth one.
Due to slow and complex exhaust venting systems and complicated semi-automatic loaders, experienced Sheridan crews were lucky to squeeze off three rounds per minute from the cutting edge guns.
By comparison, M48 and M60 crews could put more than a dozen rounds down range in the same amount of time.
When the guns did fire the recoil was so great that it often resulted in damaged components, and the rear road wheels were known to leap inches into the air, frequently causing head and neck injuries to crewmen who hadn’t braced themself for the shock.
Some Sheridans were modified to carry smaller 76 mm cannons, but these never saw service.
Sheridans were equipped with one .30 caliber machine gun mounted in the hull and a .50 caliber heavy machine gun mounted on top of the turret, but due to limited space much of the ammunition had to be stored in compartments outside the vehicle.
In addition, Sheridans could only carry 20 main gun rounds compared to more than 50 for M60s, and the heavy, fixed, caseless shells were difficult to handle and prone to damage when being moved around tight turrets.
When projectiles and propellants separated due to rough handling, crews were instructed not to use them.
This became such an issue that it was common for the bottom of turret baskets to be littered with highly explosive and utterly unusable shells that were prone to detonating when the tank’s thin armor was penetrated by an RPG or mine blast.
In fact, once a round of nearly any size had penetrated the armor, it was standard operating procedure for the crew to abandon the tank immediately.
Sheridans in Vietnam
Sheridans officially entered service in the summer of 1967, but since no ammunition was ready the new machines were little more than high-dollar machine gun platforms, and even before they saw combat crews expressed concerns about their thin armor and aluminum hulls.
Most knew from previous experience that they’d be highly vulnerable to everything from mid-caliber anti tank guns and RPGS, to mines and unguided rockets.
Nonetheless, once in-country Sheridans were pressed into service, albeit tentatively, until their strengths and weaknesses could be determined.
But despite limited numbers, they saw their fair share of action in Vietnam, and were ultimately assigned to most armored cavalry units involved in the conflict.
Sheridan crews suffered much higher casualty rates than their counterparts in M48s and M60s, and as a result by 1969 they were largely relegated to reconnaissance and road clearing duties.
In mid-February of 1969, an M551 ran over a landmine, the charge from which penetrated the hull, ignited the 152mm shells stored inside and caused the vehicle to explode in dramatic fashion before the crew had time to escape.
A few months later, nine Sheridans were crossing a river when three hit mines and were destroyed, but the heaviest single-day loss took place in mid-March of 1971, when five Sheridans were lost to RPG fire in one engagement.
By then Sheridans hulls were commonly seen smoldering along roadsides, often with their turrets laying beside them after having been blown off when the rounds inside exploded.
To increase survivability, crews often made makeshift modifications to their Sheridans, including mounting steel plates around the exposed .50-caliber machine guns and welding additional plates under the hull to protect against mine blasts.
By late 1970 more than 200 Sheridans were in Vietnam, and those that survived stayed until the spring of 1972 when they were shipped back to the United States.
Post Vietnam Service
Though no replacement had been developed, the great Sheridan phase-out began in 1978.
In the early ‘80s many remaining M551s were modified to resemble Soviet tanks for use in mock combat at the National Training Center in Fort Irwin, California.
During Operations Desert Shield and Desert Storm, more than four dozen Sheridans were used by the 82nd Airborne Division, and they were among the first armored vehicles deployed.
But though they were commonly photographed in impressive formations, ostensibly ready to engage enemy armor, both in terms of firepower and protection they would have stood little chance against the Soviet main battle tanks used by Iraq’s once vaunted Republican Guard.
Like in Vietnam, in these conflicts Sheridans were largely used in reconnaissance and infantry support roles.
They tended to make themselves scarce during heavy tank engagements, and though nearly 90,000 Shillelagh missiles were produced, less than a dozen were ever used in Iraq and Afghanistan.
The Sheridan’s only combat airdrop occurred in 1989 during Operation Just Cause, the invasion of Panama.
Of the 15 Sheridans deployed, ten were dropped from C-5 Galaxies, though two were destroyed when they hit the ground.
The ones that survived aided ground troops tasked with securing high-value targets around Panama City, but again, their performance received lackluster reviews.
Since then multiple attempts have been made to improve Sheridans, especially in regard to armor protection and a simpler and more effective gun system.
One experimental version mounted a 105 mm cannon, but the recoil was too powerful for the Sheridan’s light frame and the concept was ultimately scrapped.
The 82d Airborne Division kept their Sheridans until the late ‘90s, not because they were particularly capable, but because they were the only light tanks in the inventory that could be deployed rapidly.
Late variants also included thicker armor and thermal sighting for night engagements, but by 2003 most were being used for target practice, and some were even sunk to create artificial reefs.
All told more than 1,600 units were built, and in the end their one claim to fame was that they could be transported to distant hotspots more quickly and efficiently than larger, heavier and more capable tanks.
These days Abrams tanks are just as fast, pack much bigger punches and can also be transported by C-5 Galaxies, despite the fact that at more than 70 tons, each weighs more than four Sheridans.
The Army planned to replace Sheridans with the M8 Armored Gun System, but this project was canceled late in development in 1996.
In modern combat, it’s generally agreed that airborne forces are better off with infantry anti tank missiles like Javelins.
Now, even the most underfunded and poorly trained enemy forces are equipped with weapons like RPGs, rockets and powerful IEDs, all of which are capable of penetrating the Sheridan’s thin armor.
After the Vietnam War, a congressional investigation determined that many crewmen died in Sheridans because when they were pressed into service they were “wholly unready for combat.”
In addition, though the Army tried to cover it up, a Government Accountability Office study determined that the cost to develop the Sheridan exceeded 1.3 billion USD, or about 12 billion USD today.
New York Congressman Samuel S. Stratton may have summed the situation up best, when he criticized the Army for what he called their “bumbling ineptness.”