M551 Sheridan

M551 The Sheridan Story

The M551 General Sheridan (or simply "Sheridan") is a lightly armored and heavily armed man-portable tank system used by the United States Army. The system evolved out of a need to counter similar Soviet designs of the time and was put into combat use during the Vietnam War.

While the results were somewhat relatively mixed (it's fair to say that Sheridan was never intended for such an intimate jungle fight), Sheridan continued to demonstrate in subsequent engagements involving more direct enemy action, including urban environments was very successful. The qualities of this "small" system are clearly demonstrated, its firepower and airdrop qualities proving very effective when used within its limits and intended role. While their Shillelagh missiles never quite lived up to their claims (both figuratively and financially), their contribution to the U.S.

Army in the remaining decades of the Cold War remains noteworthy.

Background

The origins of the M551 Sheridan actually date back a few years after the end of World War II. In this conflict, tanks were divided into light, medium and heavy by weight, exemplified by the M5 Stuart, M4 Sherman and M26 Pershing (the M26 was later incorporated into the medium tank category). In late 1945, the U.S.

War Department Equipment Committee attempted to introduce some changes to bring some changes to the U.S. Army. This included an overhaul of the "Light Tank" class, which consisted mainly of M24 Chaffees during the war. The Executive Committee now categorizes this class of light tanks further beyond their reconnaissance and light combat roles, designing all new and similar developments to be ready for the challenges of modern battlefields outside of WWII Europe. This new light tank should have decent armor protection - especially as far as the front hull is concerned - while maintaining a working weight of no more than 25 tons.

The armored system should be equipped with a main gun powerful enough to match the type of "light" operations it may face.

Walker Bulldog - Available but not quite there. .

While the M24 Chaffee was a success in its own right, it was never really a weapon powerful enough against well-armored targets - especially those unleashed by the Germans at the end of the war. The 1945 redesign gave birth to the M41 Walker Bulldog in 1949, a light, air-droppable tank that entered service as a drop-in replacement for the M24 Chaffee. The M41 was fitted with a more powerful main gun - a long-barreled 76mm M32 gun - with 1.5 inches of armor at its thickest, drawing lessons from World War II tank warfare. By 1953, the Walker Bulldog nearly overshadowed the M24's presence in the U.S. Army inventory.

While the M41 Walker Bulldog proved successful in most respects, the system also proved to be overweight due to its airdrop effect, and its noisy Continental petrol engine was far from economical. Bulldogs are notoriously short lived, only marginally better than the M24 Chaffees they replace - perhaps a temporary fix at best. In short, the Walker Bulldog is a better replacement for the M24 Chaffee, but it's not the light armor system the US military is looking for. On November 9, 1950, the U.S. Army removed the weight classification of tanks and instead focused its organization on main gun calibers.

Gone are the light, medium and heavy tanks of WWII.

By 1952 a new design was settled to replace the M41 Walker Bulldog. The M24 Chaffee weighs 18 tonnes, while the M41 Walker Bulldog is over 23.5 tonnes. This new design would again establish a weight limit of 18 tons and a reduced request for proposal, culminating in the selection of a small flat "T92" with a 76mm gun from Aircraft Arms (AAI).

The tank is sized to the doctor's orders, but moving halfway through the world will spell doom for the T92.

The Soviet PT-76 changed everything

On the surface, the Soviet Union has taken over the development of the new light tank project. The main difference is that Soviet requirements required a similar system with full amphibious capability. While the AAI was designed to be portable enough and adequately armed/armored for the mission, the tank was simply too small to be effectively buoyant (hence amphibious) without rewriting most of the existing design. By June 1958, the T92 project was completely cancelled. The aforementioned Soviet design - as fate would have it - became the widely exported and successful PT-76.

It also proves that the Cold War can be done through more means than just military force and political pressure - you can outsmart the enemy's intentions by being one step ahead in development.

XM551 comes to life

With the demise of the AAI product, new requirements were placed on the armored reconnaissance airborne vehicle (as the "light tank" class has all but disappeared in the US Army, a different classification from the main battle tank is required). The Army role of the amphibious airdrop system was completed in July 1959. No fewer than a dozen companies vie for a potential contract with Cadillac, which they officially won in 1960. The new fuel system designed by Cadillac was named "XM551" to denote its stage of development and its future official name (M551).

In many respects, the design of the XM551 evolved in parallel with details obtained from the Soviet PT-76. The PT-76 is amphibious, provides all-round terrain support, provides its crew with significant firepower, and can bring infantry to the front lines when needed. By contrast, there is no such vehicle in the U.S.

Army's inventory that rivals the CV of this foreign product.

A year later, the name "Sheridan" was officially added to the tank's naming scheme. From the early days of America's (indirect) involvement in WWII through Lend-Lease, the British began naming their American-made tanks after famous Civil War generals (Stuart, Lee, Grant, Sherman, etc.).

As a result, the US Army retains (to this day it does with Patton, Abrams, and Bradley) the same system of tank nomenclature. The XM551 "General Sheridan" was thus named after Civil War General Philip Sherman.

Ordered 12 XM551 pilot vehicles (prototypes). Early forms of Sheridans had water jets for wading and Detroit Diesel brand 6V53 series engines. The first pilot model came out in June 1962 and was very similar (though not exactly) to the future production model.

By 1965, the production contract had been handed over, and that was before the vehicle was even licensed for repair. In late 1966, Sheridan was finally approved for use.

Sheridan Tour

The Sheridan's design is different from all previous American tank products. The system consisted of five wheels on one side and a flat square turret located roughly in the middle of the upper part of the fuselage. The front fuselage has a gently sloping front panel, while the sides of the fuselage are straight - although it still retains a small surface area.

The hull itself is made of welded aluminum to control weight, although the turret is all steel for protection. The turret system can be lifted from the hull using a construction vehicle crane and set aside for relatively easy access during maintenance (the turret must be disconnected first). This combination of engineering and design means the Sheridan weighs less than 17 tons and stands no more than 8 feet tall.

The cramped interior accommodates four people, including the driver, commander, gunner and loader - fairly traditional even by modern standards, although they are a departure from the WWII legacy without the archer.

The pilot is located in the center of the front of the fuselage, just in front of the 360-degree rotating propulsion tower, allowing entry and exit through its own hatch. The other three crew members all took their place in the tower. As with most tanks (except the modern Israeli Merkava series), the engine and gearbox are located in the rear of the hull. The suspension is of the torsion bar type and is equipped with rubber tire wheels (five on each side).

The chain idler is at the front and the drive sprocket is at the rear. Interestingly, no pulleys are provided for the rails, so the sides look tight.

Walk on water (or swim like a tank)

Perhaps the most striking of all the design additions to the

Sheridans is the collapsible flotation screen, which is buried in the very edge of the hull, giving the hull edge a distinctly rounded appearance. Crews can deploy this screen around the top perimeter of their Sheridan and use the tank's rail system to gently (and slowly) wade through the water. Only two clear rectangular areas of the screen (and above and below) are allowed to be viewed through the front of this screen.

A water spray method was originally envisioned (and included in early pilot vehicles) for water traversal, but was eventually abandoned in favor of the tank's own tracks.

Power comes from a GM 6V53T series supercharged 6-cylinder diesel engine (Cadillac itself is a division of GM). The engine makes 300 horsepower and has a top speed of 43 mph on the road and 3.6 mph when wading in calm waters.

The M551 maintains an excellent power-to-weight ratio despite its low armament - a sacrifice to meet the requirements of 18 tons or less, and still has airdrop capability. Its thin armor was mostly tackled on the battlefield by the crew, as was the case in WWII.

Skydive Sheridan

A portable/droppable light tank, the Sheridan is designed to be airdropped to the battlefield. This can be done by unloading the vehicle via the rear ramp of the C-130 (or even larger C-141) transporter.

Tank systems will be mounted on specially modified pallets and lowered to the ground using parachutes. Note that the plane must be flying low and the Sheridan crew must wait on the ground (do not sit on their Sheridan aircraft during the landing).

After landing, the Sheridan crew will remove all connections to parachutes and trays and deploy their Sheridan.

The following insights on Sheridan LVAD and LAPES were provided by Colonel Dan Miller, 3-73rd Armor, 82nd Airborne Division, 1985-1987:

Sheridans can be airdropped in two different ways. The first, the Low Velocity Airdrop (LVAD - also known as the "Heavy Airdrop"), consisted of Sheridan aircraft placed on pallets and dropped from approximately 1,500 feet from any U.S. Air Force conventional cargo aircraft. From there, Sheridan will descend via no fewer than seven G-11 cargo parachutes until it reaches the ground.

This method of deployment requires all G-11 parachutes to function properly, otherwise the entire Sheridan/tray system may be lost in the process. Since USAF personnel did not require special certification for this insertion process, it became the preferred method of airdropping Sheridan in the long run.

The second method is the Low Altitude Parachute System (or LAPES) used in conjunction with the Lockheed C-130 Hercules transport aircraft. This method also requires Sheridan to assemble a tray, but only comes with an extraction chute. The C-130 flies very low - no more than 15 feet in fact - preferably on flat terrain and at the desired speed.

The distance required for such insertion is several hundred meters. The Sheridan was then released with a parachute.

As expected, this particular method of entry proved to be the more dangerous of the two, with the C-130 being the only aircraft of its kind approved by the USAF for LAPES operations - where injuries and even fatalities were attributed. Aircrews involved in such airdrops also require specialized training in day and night LAPES operations, requiring periodic certification along the way.