Staff Writer (Updated: 9/23/2015):
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).