Armored Reconnaissance Airborne Assault Vehicle
The M551 Sheridan was designed as a true light tank, droppable by parachute from aircraft, and featured a little-used amphibious capability.
Authored By: Staff Writer | Last Edited:
The M551 General Sheridan (or simply "Sheridan") was a light-armored and heavily-armed portable tank system utilized by the United States Army. The system evolved from the need to counter a similar Soviet design of the time and was thrown into combat deployment during the Vietnam War. While results there were relatively mixed to some extent (an argument can be made that the Sheridan was never meant for this type of intimate jungle-based combat), the Sheridan went on to prove highly-successful in ensuing deployments involving direct enemy action in more urban environments. The qualities of this "little" system shown clear and her firepower and air-droppable qualities proved highly-effective when used in within her limitations and designed role. While her Shillelagh missiles never really quite lived up to their billing (both figuratively and financially), her contributions to the American Army in the remaining decades of the Cold War are still of note.
The origins of the M551 Sheridan actually lay in the years following the close of World War 2. During that conflict, tanks were categorized by weight in classes aptly titled Light, Medium and Heavy characterized respectively by the M5 Stuart, M4 Sherman and M26 Pershing as examples (the M26 was later added into the Medium tank class). In late 1945, the American War Department Equipment Board sought to bring about changes that, in turn, would bring about something of a new face for the American Army. This included re-envisioning the "light tank" class that, during the war, had been primarily made up of the M24 Chaffee. The Board now categorized this class of light tank further beyond their reconnaissance and light action roles and devised that any new and similar developments be ready for the challenges of the modern battlefield beyond World War 2 Europe. This new light tank should have adequate armor protection - especially where the front hull is concerned - while maintaining an operating weight no higher than 25 tons. The tank system should be well-armed with a main gun potent enough to coincide with the type of "light" action it could face.
The Walker Bulldog - Serviceable, But Not Quite There...
While the M24 Chaffee was a success in itself, it never truly sported a powerful enough armament to best well-armored targets - especially those that the Germans were unleashing towards the end of the war. From the 1945 revision came forth the M41 Walker Bulldog in 1949, a light air-droppable tank accepted into service as a direct replacement for the M24 Chaffee. The M41 sported a more powerful main gun - this in the form of the long barrel 76mm M32 - and featured armor 1.5 inches at its thickest while taking into account lessons learned from World War 2 armored warfare. By 1953, the Walker Bulldog had all but eclipsed the M24's existence in the American Army inventory. While the M41 Walker Bulldog proved successful in most respects, the system also proved overweight for its air-drop role and its loud Continental gasoline engine was far from economical. Bulldogs were also known to have short lifespans in service and were only modestly better than the M24 Chaffees they replaced - perhaps an interim solution at best. In short, the Walker Bulldog was a better alternative to the M24 Chaffee but it was not the light tank system that the American military was looking for. On November 9th, 1950, the US Army did away with categorizing their tanks based on weight and instead focused their organization on the main gun caliber. The light, medium and heavy tank classes of World War 2 were no more.
By 1952, a new design was being approached to replace the M41 Walker Bulldog. The M24 Chaffee weighed in at 18 tons while the M41 Walker Bulldog bested 23.5 tons. This new design would again settle on an 18-ton weight limit and requests for proposals went out, ultimately leading to the selection of Aircraft Armaments, Incorporated's (AAI) smallish, low-profile "T92" with its 76mm gun. The tank's size was truly what the doctor had ordered but a change half-way across the world would become the T92's undoing.
The Soviet PT-76 Changes Everything
As it stood, the Soviets had taken development of a new light tank project of their own. The major difference here is that the Soviet requirement called for a similar system with full amphibious capabilities. While the AAI design was portable enough and armed/armored appropriately for the task, the tank was simply too small to make effectively buoyant (and therefore amphibious) without rewriting much of the existing design. By June of 1958, the T92 project was cancelled altogether. The aforementioned Soviet design - as fate would have it - went on to become the widely-exported and successful PT-76. This also proved that the Cold War could be won by more means that just military might and political pressures - you could best your enemy's intentions by staying a full step ahead in terms of development.
The XM551 is Given Life
With the AAI product dead, a new requirement for an Armored Reconnaissance Airborne Assault Vehicle (since the "light tank" class was all but extinct in the American Army, a new classification other than Main Battle Tank was needed) sprung up in July of 1959 to fulfill the Army role of an air-droppable system with amphibious qualities. No fewer than twelve companies all vied for the potential contract with Cadillac officially winning out in 1960. The new tank system based on Cadillac's design was afforded the designation of "XM551" to denote its developmental stage and future official designation (M551). In many respects, the design of the XM551 was being developed alongside the details being acquired about the Soviet PT-76. The PT-76 was amphibious, offered full cross-country support, supplied potent firepower for its crew and could ferry infantrymen to the frontlines as needed. By comparison, the US Army inventory had no one such vehicle that could match this foreign product's resume.
The name "Sheridan" was officially accepted into the tank's designation scheme a year later. Since the early days of (indirect) American involvement in World War 2 via Lend-Lease, the British began a habit of naming their American-produced tanks after famous Civil War generals (Stuart, Lee, Grant, Sherman, etc...). As such, the US Army maintained (and even does so to this day with the Patton, Abrams and Bradley) the same system of naming its tanks. The XM551 "General Sheridan" was therefore named after Civil War General Phillip Sherman.
Twelve XM551 pilot vehicles (prototypes) were ordered. Early form Sheridans had water jets for wading and a Detroit Diesel-brand 6V53 series engine. The initial pilot model was made available in June of 1962 and bore a large (though not exact) resemblance to the production models to come. By 1965, the production contract had been handed over, this occurring before the vehicle had even been given the complete green light for service. By the end of 1966, the Sheridan was finally cleared for operational use.
Design of the Sheridan was distinct from any previous American tank offering. The system fielded five road wheels to a side and a low-profile, angular turret which was situated about the center of the upper hull. The forward hull featured a lo-angled sloping glacis plate while the sides of the hull were straight-faced - though still maintaining a low surface area. The hull itself was constructed of welded aluminum to keep weight in check though the turret was of all steel for protection purposes. The turret system could be hoisted out of the hull via an engineer vehicle's crane and set aside for relatively easy access when maintaining (turret connections had to be severed first). This construction and design combination meant that the Sheridan weighed in at just under 17 tons and stood no higher than 8 feet tall. The cramped internal workings provided home to four personnel made up of the driver, commander, gunner and loader - quite conventional even by modern standards though made different from her World War heritage by the lack of a bow gunner.
The driver was situated in the front center hull just in front of the 360-degree traversing power turret with entry/exit made possible by his own hatch. The other three crewmembers all occupied their respective positions within the turret. As with most tanks (the immediate exception being the modern Israeli Merkava series) the engine and transmission were set in the rear of the vehicle hull. Suspension was of the torsion-bar variety and featured rubber-tired road wheels (five to a side). The track idler was set to the front while the drive sprocket was at the rear. Interestingly enough, no return rollers were provided for the tracks, hence its tight appearance along the sides.
Walking on Water (or How a Tank Swims)
Perhaps the most notable of all the Sheridans design additions came in the form of the collapsible flotation screen buried within the extreme edges of the hull, giving the hull edges their distinct rounded appearance. The crew could erect this screen around the upper perimeter of their Sheridan and utilize the tanks track system to gently (and slowly) wade through water. Vision was only allowed through the forward part of this screen via two (and upper and lower) clear rectangular areas in the canvas. A water jet-type approach was originally conceived (and included on early pilot vehicles) to accomplish water traversing but this was ultimately dropped in favor of using the tanks own tracks.
Power was derived from a General Motors 6V53T series 6-cylinder supercharged diesel engine (Cadillac was itself a division of General Motors). Engine output was 300 horsepower and provided top speeds of 43 miles-per-hour on roads and up to 3.6 miles-per-hour when wading through calm waters. The M551 maintained an excellent power-to-weight ratio though it suffered from being thinly armed - a sacrifice needed to make the 18-ton-or-less requirement and still be air-droppable for airborne elements. Its thin armor was most often times addressed in the field by crews as was the case in World War 2.
As an air-portable/air-droppable light tank, the Sheridan was designed to be brought to the battlefield by air. This action could be accomplished by unloading the vehicle through the rear ramp of a C-130 (or even the larger C-141) transports. The tank system would be rigged atop a specially-modified pallets and lowered by parachute to the ground. Mind you this required the aircraft to fly at low altitude and the Sheridan crew to be waiting on the ground (not sitting inside their Sheridans during the drop). Once landed, the Sheridan crew would remove all connections to the parachute and pallets and drive their Sheridan away to action.
The proceeding insight concerning Sheridan LVAD and LAPES is courtesy of Colonel Dan Miller, member of the 3-73 Armor, 82nd Airborne Division from 1985 through 1987:
Sheridans could be air-dropped by one of two different ways. The first was the Low-Velocity Air Drop (LVAD - also known as "Heavy Drop") and consisted of a Sheridan being set onto a pallets and dropped out of any conventional USAF cargo aircraft at about 1,500 feet. From there, the Sheridan would descend via no fewer than seven G-11 type cargo parachutes until it reached the ground. This method of insertion required that all G-11 parachutes function properly or there was a risk of losing the entire Sheridan / pallets system in the process. As USAF personnel needed no specialized certification for this insertion process, it became the preferred method of entry for airdropped Sheridans in the long run.
The second method became the Low-Altitude Parachute System (or LAPES) used in conjunction with a Lockheed C-130 Hercules transport. This method again required the Sheridan to mount a pallets but was only fitted with a single extraction chute. The C-130 would fly extremely low to the ground - no more than 15 feet in fact - and preferably over flat terrain and at a pre-required speed. The distance required for such an insertion equaled several hundred meters. The Sheridans were then dropped out with chute in tow. As one might expect, this particular method of entry proved the more dangerous of the two and the C-130 was the only such USAF aircraft cleared for LAPES operations - injuries and even deaths were attributed to these operations. Crews part of such an airdrop would require specialized training as well, in both day and night LAPES actions, and needed regular certification in the process.
In whole, either airborne insertion method for Sheridans involved some level of a risky action and the scene itself was quite impressive considering the sight of low-flying transports dumping out a 17-ton tank - a far cry from the M24 Chaffees in World War 2.
The Goods: Armament
Armament of the Sheridan centered around its potent M81E1 rifled 152mm main gun that could double as a launcher for the Ford-built MGM-51 Shillelagh anti-tank/anti-fortification missile. In fact, the main gun was originally designed with the primary role of it being a launcher first, with the cannon aspect of the system as a secondary use. In reality, the cannon aspect proved to be a primary function when the Sheridan was finally put into action. It is worth noting the main gun's rather short length and lack of a muzzle brake as well. The main gun was afforded 20 projectile rounds but these rounds had a temperamental attribute as "combustible" cartridge cases. They proved extremely susceptible to humidity in that any such moisture could compromise the round, making them more a danger to the Sheridan crew than to any enemy in the crosshairs. This was eventually rectified by the use of special ammunition covers that were removed immediately before loading a round. Ammunition types available to the crew were M409 High-Explosive, Anti-Tank (HEAT), M411 TP (Training) and M625 Canister round, the latter comprised of 10,000 internal flechettes. Early Sheridans featured an open breech scavenger system tube (in which the barrel appears to "grow" in diameter from barrel end to its base) while later Sheridans sported safer close-breech scavenger system tube (a cleaner, smooth-sided barrel of similar length).
The Ford Shillelagh
8 x MGM-51 Shillelagh ready-to-fire missiles would have been standard but their cost and lack of targets in Vietnam meant that they weren't even shipped to the theater. The Shillelagh was development of Ford and proved expensive and somewhat of a disappointment in the end. Despite it being made available by the time of the Vietnam War, the missile was not even used in anger until the Gulf War in 1990, to which the missile was unleashed successfully on Iraqi fortified bunkers (on at least two reported occasions). Despite the anti-tank prowess of the missile's design, the Sheridan was still a "light" tank and no match for squaring off head-to-head against modern Soviet T-72s in the conflict. As such, their role still remained that of reconnaissance. Though some 88,000 Shillelagh missiles were produced, barely a fraction of these were ever fired. The BGM-71 TOW's arrival made sure that the Shillelagh was all but extinct.
Additionally, crew protection was served through 1 x 12.7mm M2HB heavy commander's heavy machine gun and a 1 x 7.62mm co-axial machine gun (early models were fitted with the M73/M219 co-axial system whilst later ones had the M240C system in its place). The 12.7mm machine gun was affixed to the commander's cupola while the 7.62mm armament was fitted as a co-axial unit alongside the main gun and operated by the gunner. While early Sheridans sported protection-less machine gun cupolas, many Sheridans in Vietnam added M113 APC-type ACAV machine gun "shields" to their .50 caliber stations. Others also made use of an additional external M60 7.62mm machine gun for the loader (some complete with applicable ACAV shielding), an additional M2HB .50 caliber heavy machine gun as well as searchlights later added. 8 x smoke grenade dischargers were set to the front sides of the turret, four in a row to a turret side.
Sheridans in Vietnam
As early as 1966 there was talk of sending these new tanks to the combat warzones of Vietnam, Army personnel eager to see how the new system would fare. However, delays were almost inevitable with the type's specialized ammunition and armament. Additionally, while the Germans in World War 2 and the Soviets/North Koreans in the Cold War maintained a substantial collection of armored vehicles for which the high-priced Shillelagh missile was designed to defeat, it was a different matter in the jungles of Vietnam where tank-on-tank battles seemed less likely if at all. While a solution was eventually found to make the combustion rounds usable in the thick of combat (by use of special ammunition covers removed before firing), the Sheridan was finally cleared for service in the bush. 3rd Squadron, 4th Cavalry and 1st Squadron, 11th Armored Cavalry Regiment were all equipped with Sheridans, replacing their collections of M113 armored personnel carriers (the 11th) and M48 Pattons (the 4th) in the process, and made ready for service in Vietnam in 1968. In January of 1969, no fewer than 54 Sheridans arrived in Vietnam, sans their complicated missile systems of course.
As the war progressed, Sheridans proved to have some misgivings during her stay in the theater. Her thin belly armor proved highly susceptible to enemy landmines, generally the method of warfare as selected by the North Vietnamese. As a consequence, Sheridan crews sometimes opted to sit atop their rides, completely exposed to small arms fire, than to be trapped inside of the Sheridans walls (this was also a matter of comfort, considering the humidity and heat of the region). It was not uncommon for crews to add additional bolt-on steel armoring along the underside of the hull, at the risk of covering up the escape hatch for the driver. The thick brush of Vietnam soon began taking their toll against the fragile Sheridans, leading up to a rise in maintenance and mechanical headaches. Many engines and drive trains were simply overworked when tackling what could be miles and miles of heavy thick brush. The dust of Vietnam was also quick to collect on the outer hull, eventually making its home within the various subsystems while mud proved a constant problem to suspension systems and tracks. M551's operated in conjunction with M113 armored personnel carriers in reconnaissance teams.
Along with land mine warfare, the Vietcong were also lethal with attacks using their Soviet RPGs (Rocket Propelled Grenades). Though primitive in concept, these man-portable systems launched an explosive rocket grenade at distance to defeat enemy armor. As the Sheridan was designed from the outset to be a light system, it was inherently thinly-armored by nature. This meant that the RPG's tank-killing properties could be used to good effect when needed. To make matters worse, the combustible projectiles stored within the Sheridan tank would more often than not lead to a complete Sheridan loss - unfortunately including crew. As such, Sheridan crews were quick to adapt rather simplistic fences made of chain link to help defeat such dangers - or at least deflate their lethal effects to an extent. This fencing was most often times added to the front hull via "homemade" supports, an area most likely to encounter such enemy attack. Storage space within the Sheridan also proved a premium and often led to crews stowing much of their gear, ammunition or other supplies (including fuel canisters, track links and cots) externally on either the turret bustle rack or on the flat hull rear atop the engine.
Seeing it that tank-versus-tank battles were not to be in Vietnam, the main gun fire power of the 152mm M81 system proved rather effective against most anything on the battlefield including fortifications - though the Sheridan's original sighting system offered little assistance to her gunners. The M625 Canister was particularly effective (it contained 10,000 flechettes!) for "cutting out" areas of dense thicket. Make no mistake about it - the M81 was a powerful gun when the situation called for it. Its massive recoil could easily jolt the system rearwards, lifting the front road wheels clear off the ground. In essence, the Sheridan was a Light Tank with the firepower of a Main Battle Tank. Similarly, the heavy caliber 12.7mm machine gun had the power to cut a man in half and take on light-armored vehicles as well. The tanks relatively small size made it ideal for concealment (particularly in areas of tall grass) when traveling or finding cover.
The Improved Sheridan
On April 22nd, 1971, 505 Hughes AN/VVG-1 laser range finders were purchased by the US Army to help improve the Sheridan sighting system when firing of the main gun. This effectively created a new Sheridan designation in the M551A1 to help distinguish this new form from her base one. Externally, these systems were further distinguished from previous production Sheridans by the laser designation system mounted just underneath the commander's 12.7mm machine gun. A new M127A1 telescope was also added as part of the sighting upgrade.
In all, some 200 M551 Sheridans made their way to Vietnam. Ironically, the amphibious capabilities were rarely used in the thick jungle setting. Nevertheless, its contributions to the conflict are what most people remember of the Sheridan legacy.
The Sheridan, Post-War
With the Vietnam War over for some years by this time, the Sheridan was due for some modernization. In 1977, she had her reliability improved through the implementation of a revised drive sprocket and engine block. Her smoke grenade launchers were now fitted in quadruple mountings, four to a turret side, as opposed to the original's single-file arrangement. Her throttle was reworked and her original co-axial 7.62mm machine gun was replaced by the standard M240 general purpose machine gun system. In all, these changes addressed some issues of the vehicle as encountered in Vietnam and made for a marked improvement in performance (albeit slightly heavier than the original Sheridan), no doubt extending her usefulness.
Operation Just Cause
In America's bid to depose Panamanian dictator Manuel Noriega in Operation Just Cause in December of 1989, the Sheridan was called into action once again - her first since the days of the Vietnam War. In the ensuing action, four Sheridans were covertly airlifted into the country and these were later joined by eight more (the eight were actually from a group of 10 air-dropped Sheridans - two being lost in the drops, the first was salvageable while the second purposely blown up with irreparable damage). Incidentally, these air-dropped Sheridans garnered Charlie Company, 3/73rd Armor a spot in armored warfare history when they became the first tanks to be "dropped" into a combat zone on December 20th, 1989. Their landing took place at Torrijos-Tocumen International Airport. Once again, the main guns proved highly effective in silencing the defenses and blockades of the Panamanian Army. These Sheridans also showcased their new smoke grenade discharger arrangement and were fitted with the M60A3 Tank Thermal Sight (TTS), assisting the gunners aim substantially over original Sheridans. Sheridans with the TTS system were further designated as M551A1(TTS) to differentiate the improvement.
The M551 was revered in her actions during Operation Just Cause, in particular by her lethal 152mm main gun. The Sheridan proved a versatile mount and its relatively compact size ensured that it could go where larger tank systems were restricted. In many ways, the limitations as seen in the image of a Sheridan trudging through foreign territory in Vietnam were gradually undone by the Sheridan in Panama. The M551 proved a vital and flexible implement in the opening rounds of the action and impressed warplanners and commanders alike thanks to her versatility and firepower.
The 82nd Airborne Embraces the Sheridan
While most other frontline units stopped operational use of Sheridans as the decades rolled on, the 82nd Airborne Division stayed true to their mount. Most importantly, the tank was still an air droppable resource that worked well within the confines of an airborne unit - the only such system in the US Army inventory (even to this day, though the Sheridan is no longer in active service). It could (relatively) easily fit inside of the cargo bay of a Lockheed C-130 Hercules and be air-dropped to waiting airborne personnel on the ground, adding immediate fire support to advancing infantry squads.
Operation Desert Shield
M551 Sheridans were deployed to ally Saudi Arabia in 1990 for Operation Desert Storm, a massive coalition build-up of force enacted to remove Saddam Hussein's army from their illegal take-over of Kuwait. In fact, Sheridans actually represented some of the earliest units to arrive in the theater, this accomplished through use of the massive Lockheed C-5 Galaxy transports (C-141 transports were also cleared for use by Sheridans by the way). While early arriving Sheridans still sported their European camouflage colors, they were eventually converted to a three-color "desert-friendly" pattern that was eons away from their olive shade used during the Vietnam days. When they needed to move intra-theater, the C-130 was ready to take the little machines airborne. By the end of the year, Sheridans in the Gulf were sporting the Shillelagh anti-tank missiles not brought to Vietnam in preparation for facing off against Hussein's impressive collection of armor. Driver stations were also improved by the inclusion of the same night vision system as found on the M2 Bradley carriers.
The Sheridans in the gulf - for the most part - retained their visual similarities to their Vietnam counterparts. Thanks to its actions in that previous conflict, the cupola shielding was now standard fare for crew protection. Improvements to the sighting system by this time were also a large step forward in the Sheridan's evolution. External stowage of essential and non-essential gear was still the norm. The fact that the system was also nearly a quarter of a century old was something special to note as well - considering the digital nature of the Gulf War to come, complete with stealth fighters, laser-guided bombs and heavy use of digital implements.
Apart from direct war participation, the Sheridan was utilized in Cold War exercises by American personnel in training. The National Training Center (NTC) at Fort Irwin, California made use of 300 such vehicles (designated as M551NTC). The fiberglass and plywood VisMod (Visually Modified) kits were an effort to alter the external appearance of Sheridans to mimic that of Soviet armor systems such as the BMP armored personnel series and the T-72 Main battle tank for use in specially-crafted war games. perhaps the most-closely associated use of the Sheridan in this fashion was in copying the look of the lethal four-barreled ZSU-23-4 anti-aircraft systems. While retaining their lower hulls, these Sheridans had a superstructure superimposed on their existing hulls and closely resembled their Soviet counterparts. After the end of the Cold War, actual Soviet equipment became readily available for use in such war games and these types of modifications were no longer needed.
Production of the M551 General Sheridan began in 1966 and ended in 1970 with totals topping a reported 1,562 systems. The Sheridan served actively from 1969 up until 1996 - quite a feat for any tank especially those with an origin dating back to the 1960s.
The Sheridan was evaluated by a number of American-friendly potential nations in its life but none of these ever secured any production orders. The Sherman and her legacy, therefore, were "all-American" from beginning to end. The Sheridan was stationed in South Korea with deterrent forces on watch and, similarly, in Europe. Sheridan crews trained extensively in Germany for the seemingly inevitable World War 3 scenario. How the Sheridan would have fared in such an all-out war against the cream of the Soviet crop is left to the imagination, though the system itself was developed indeed for such a war.
As it stands and despite her not in active service with the US Army, the M551 Sheridan has yet to be directly replaced in the Army inventory by a capable like-system.