M551 Sheridan

U.S.A. (1965)
Light Tank – 1662 built

The Missile-Firing tank

The Sheridan was one of these developments tailored to explore a tactical theory and put it to the test. At the time the perpetual chase between armour and weaponry seen lost in favour or the new ammunition, and the tactical possibilities introduced by lighter missiles met US Army Ordnance specifications for a new light tank. The previous M41 was too heavy, had a short range and inadequate weaponry already, so lighter models like the T71 and T92 were tested. None were amphibious however, and after reports about the new Soviet PT-76, the XM551 was built and tested. To meet all conflicting requirements the new design showed a combination of an aluminium hull with a steel turret, to keep the weight down while providing buoyancy and the required level of protection. The vehicle had yet another remarkable feature with a 152 mm gun capable of firing the MGM-51 Shillelagh AT missile.

Development History

The task of the designers of the XM551 was daunting. Providing an amphibious tank (none were built since the specialized World War 2 LVT families and DUKW and SEEP) light enough to be airlifted seemed only possible with a small model build from light alloys, but at the same time, mobility and range were to be greatly enhanced and moreover the armament needed to be enough to knock-out a 1960s main battle tank. This squaring of the circle resulted in the Sheridan. Since a “normal” gun powerful enough for the task would have needed a long barrel, large turret, and hull, the solution was to use missiles instead, which provided their own velocity and could be launched from a simple tube. At the end the solution was so unusual it was called an “Armoured Reconnaissance/Airborne Assault Vehicle” but not a light tank.

XM551 Pilot

Design

Obviously the protection seemed to be sacrificed on this model. The aluminium hull and the steel turret could only hold against 0.5 inches fire (heavy MG) but remains vulnerable to grenade, ATGMs and mines. The hull was compartmented into lateral flotations screens, with a front “surfboard” made of wooden hinged folded layers. It could be deployed into a frontal sloping vertical surface, with canvas to cover the flanks, maintained by poles at the rear, sides, and high as the turret top. The system was directly taken from World War 2 DD tanks. The front “bow” comprised a plastic window for the driver which provided a mediocre visibility was later eliminated. The driver steered the tank blind, guided by the Commander towering above his cupola. In all, the tank was only 15.2 tons which was light enough to be airlifted by “heavy duty” helicopters like the Boeing Ch-47 Chinook and the Ch-58 Sea Stallion and most air transporters. There was sufficient buoyancy to allow the tank to swim at at 5.8 km/h (3.6 mph), rather than just sink like most MBTs to cross deep rivers. Therefore it was amphibious and NBC-proof, like the Soviet PT-76. This swimming characteristics, lightweight hull allowing air transportation combined with a top speed in excess of 70 kph gave an excellent strategic and tactical mobility, at least on the paper.

CH-54B carrying an M551 Sheridan tank.

The aluminium hull was entirely welded, with some cast elements at the front and rear. Armour thickness ranged from approx. 8 mm to 13 mm at the front. The Detroit Diesel (General Motors) 6V53T, 6 cylinder, turbocharged diesel gave indeed an output of 300 hp (220 kW), and a favourable power/weight ratio of 19.7 hp/tonne. The driver train reused standard roadwheels and tracks (five doubled rubber-clad roadwheels per side, suspended by torsion bars) but there were no return rollers and the drive sprocket and idler were both specific to this model. The crew of 4 comprised the driver, located in the center, with his own hatch and three vision blocks, and two headlights/blackout lamps protected by armoured covers. The commander, gunner and loader were all located in the center fighting compartment, in a rather cramped turret. The latter was relatively flat and was given extremely sloped sides to maximize the effective thickness of the armour. The commander cupola was located on the right hand side, and was given a cal.50 HMG ring mount. The loader’s hatch was located at his left. There were also two banks of height electrically fired smoke dischargers on each side.

XM-551 Pilot #12 at Fort Knox, Kentucky.  Mid 1960’s

The armament was remarkable as a lightweight solution which could deliver a potentially killer blow. The M81E1 Rifled cannon was a short barrel, able to fire either missiles or tailored ammunitions for fire support. Due to the unfavourable velocity ratio, only traditional HE rounds were fired, for infantry support, while the AP capability was entirely provided by the new Shillelagh missile system. The latter was developed in 1958 by Sperry and Ford Aeronautics (Later Martin Marietta) as the XM13, produced in 1964 as the MGM-51. It was 152 mm in diameter with the winglets folded, and 290 mm wingspan unfolded, with a solid-fuel rocket and a 15 pdr shaped-charge (6.8 kgs) warhead. Guided by infra-red signals it could fly at 1,060 feet (320 m) per second. This was sufficient to defeat 150 mm of RHA plate at a 60º angle at the range varying from 2000 to 3000 m depending of the models A, B or C. 88 194 of these were produced until 1971. It was also deployed by the M60A2 “Starship” and the first prototype of the MBT-70. This weapon was completed by a roof-mounted cal.50 M2 (1000 rounds) and a coaxial M73 LMG with 3000 rounds.

 

Production & Variants

Production started on 29 July 1966, and the Sheridan entered service in June 1967. In all 1,662 M551s were built between 1966 and November 2, 1970 for a total cost of $1.3 billion for the entire program. However problems with the M81 gun quickly showed, as cracks developed near the breech after intensive firing, later linked to the Shillelagh “key” running in a slot cut into the barrel. The modified M81E1 introduced a shallower slot and matching modification to the missile. Still, the gun was criticized for having too much recoil for the vehicle’s weight and frail construction. The blast was enough to lift-off the second and even third road wheels. Experimental 76 mm guns were also tried but never adopted.

The M551 in Action

Vietnam War

In June 1967, the first batch of Sheridans entered service with 1st Battalion, 63rd Armor Regiment at Fort Riley. At that time, the concept was ready to be battle tested in Vietnam although there was no immediate use for the cavalry unit. This came in late 1968 when General Creighton Abrams met with Colonel George S. Patton IV and his 11th Armored Cavalry Regiment (11th ACR Blackhorse) as despatched in Vietnam (and remained the only cavalry unit in service here). These tanks were evaluated by the 3rd Squadron, 4th Cavalry, and the 1st Squadron, 11th ACR from January 1969 to the fall of 1970. These suffered badly from mines and RPGs, always fatal contrary to the M48 “Patton”. Sheridans there assumed reconnaissance, night patrol and road clearing duties and totalled 39,455 road miles and 520 combat missions with a combat readiness of 81.3%. Reports were enough to decide to equip all other cavalry squadrons with this tank. However it was also detected that the caseless 152 mm main gun rounds used were easy to ignite when the tank was hit by a mine.

Sheridan M-551 and crew members of the 3rd Squadron, 4th Cavalry, Vietnam. Circa 1969

This was demonstrated in late 1969 when three out of the nine Sheridans of the 4th Squadron, 12th Cavalry detonated on mines when crossing a river near the DMZ, being total losses. On march 1971, five (11th ACR) were lost in a row by a Vietcong ambush operating RPGs. In all these cases, the Sheridan just “melted” due to the heat intensity and aluminium nature, gaining a sinister reputation. However their mobility was excellent in mud and in general all terrains. They were found very effective in infantry support, bursting M657 HE shell or the M625 canister round which launched a devastating bunch of flechette rounds, despite their slow reloading time. Their low ammunition stock was compensated by their combination with combination with ACAVs (M113s) that carried extra rounds.

MGM-51 Shillelagh Anti-tank miisile fired from M551 Sheridan light tank.

By May 1970, the 11th Armored Cavalry Regiment entered Cambodia. Most vehicles has been modified with a large steel shield (“ACAV set”) to protect the commander when firing the 12.7 mm HMG buttoned-up in his open cupola. The driver was given a modified rotating hatch and an extra layer of bolted steel was applied to the belly to protect against mines, but mostly protecting the front part of the belly due to weight issues. The 11th Cav. also participated in Operation Dewey Canyon II in support to ARVN Lam San 719 Operation, taking heavy losses in the process.

A C-130 delivering an M551 Sheridan tank using LAPES (Low Altitude Parachute Extraction System).

The 1980s

The Army began to retire the Sheridan in 1978. But due to the lack of suitable replacement some units like the 82d Airborne Division retained them until the 1990s (1996 for the 82 AD). Another task was found to keep them active: Doing the bad guys in training, modified to resemble mock T-72 and T-80s. These vehicles were active at the National Training Center at Fort Irwin, California in the 1980s. They were retired at the end of 2003, and either scrapped, ended as targets, given to collections or dumped into the sea. During the invasion of Panama (Operation Just Cause) in 1989, fourteen M551s were deployed, among which four were transported by C-5 Galaxys and ten air-dropped by C-130s (two Sheridans destroyed upon landing). They were attached to TF Bayonet (193rd Infantry Brigade) part of TF Gator, taking part on the attack on the Commandancia and later provided support to JSOC elements inside Panama City. Height fought at Torrijos-Tocumen Airport. Their performance received mixed reviews.

First Gulf War (1990s)

Fifty-one Sheridans were deployed by the 82nd Airborne Division during Operations Desert Shield and Desert Storm. Their role was limited to reconnaissance and possibly six or fewer Shillelagh missiles were fired on anti-tank guns or T-55s in operation, the only operational use of these ordnance among 88,000 missiles produced. These were deactivated after the war. Attempts to provide them a standard NATO 105 mm failed. In fact replacement came with the M1128 Mobile Gun System variant of the Stryker. So the Sheridan was perhaps the last of American light tanks. 

M551 Sheridan Links & Resources

The M551 Sheridan on Wikipedia

M551 Sheridan Specifications

Dimensions (L-W-H) 20’6″ x 9’2″ x 9’6″ ft.in
(6.29m x 2.81m x 2.94m)
Total weight, battle ready 15.2 tons (34 000 lbs)
Crew 4 (Commander, Driver, Loader, Gunner)
Propulsion Detroit 6V53T 6 cyl. supercharged diesel 300 hp (220 kW).
Top speed 43 mph (76 km/h) road, 3.6 mph water
Suspensions Torsion bars
Range 348 miles (560 km)
Armament Main: 152 mm (5.98 in) gun M81E1, 9 missiles, 20 roundsSec: 1 cal.50 M2 (12.7 mm)+ 1 cal.30 (7.62 mm) M73
Armor Aluminium & steel – Max 0.3 in (8 mm)
Production (all combined) 1662

M113 APC (Armored Personnel Carrier)

A combined arms operation in Vietnam. M113s clear the way through heavy bush while infantry follows. Circa 1967
A combined arms operation in Vietnam. M113s clear the way through heavy bush while infantry follows. Circa 1967

The M113 APC

The M113 is a fully tracked APC (Armored Personnel Carrier) that was developed by Ford Machinery Company (FMC). The vehicle was first fielded by the United States Army’s mechanized infantry units in Vietnam in April 1962. The M113 was the most widely used armored vehicle of the U.S. Army in the Vietnam War, earning the nickname ‘Green Dragon’ by the Viet Cong as it was used to break through heavy thickets in the midst of the jungle to attack and overrun enemy positions. It was largely known as an “APC” or an “ACAV” (armored cavalry assault vehicle) by the allied forces. The M113 introduced new aluminum armor that made the vehicle much lighter than earlier vehicles; it was thick enough to protect the crew and passengers against small arms fire but light enough that the vehicle was air transportable and moderately amphibious. In the U.S. Army, the M113 series have long been replaced as front-line combat vehicles by the M2 and M3 Bradley’s , but large numbers are still used in support roles such as armored ambulance, mortar carrier, engineer vehicle, and command vehicle. The army’s heavy brigade combat teams are equipped with around 6,000 M113s and 4,000 Bradley’s.

Development

The M113 was developed by Ford Machinery Company (FMC), which had produced the earlier M59 and M75 Armored Personnel Carriers. The M113 bears a very strong resemblance to both of these earlier vehicles. The M75 was too heavy and expensive to be useful; its weight prevented amphibious capability, and being transported by air. The lightened M59 addressed both of these problems, but ended up with too little armor, and was unreliable as a result of efforts to reduce its cost.

The army was looking for a vehicle that combined the best features of both designs, the “airborne armored multi-purpose vehicle family” (AAM-PVF). of all-purpose, all-terrain armored fighting vehicles FMC had been working with Kaiser Aluminum and Chemical Company in the late 1950s to develop suitable aluminum armor. It was known that use of this armor could produce a vehicle that provided the protection of the M75, and the light weight and mobility of the M59.

Food Machinery Corp. responded with two proposals; two versions of the aluminum T113—a thicker and a thinner armored one—along with the similar but mostly steel T117. The thicker-armored version of the T113, effectively the prototype of the M113, was chosen because it weighed less than its steel competitor, while offering the same level of protection. An improved T113 design, the T113E1, was adopted by the U.S. Army in 1960 as the “M113”. A diesel prototype, T113E2, was put into production in 1964 as the “M113A1”, and quickly supplanted the gasoline-engined M113.

The M113 was developed to provide a survivable and reliable light tracked vehicle able to be air-lifted and air-dropped, by C-130 and C-141 transport planes. The original concept was that the vehicle would be used solely for transportation, bringing the troops forward under armor and then having them dismount for combat; the M113 would then retreat to the rear. Entering service with the U.S. Army in 1960, the M113 required only two crewmen, a driver and a commander, and carried 11 passengers inside the vehicle. Its main armament was a single .50-caliber (12.7 mm) M2 Browning MG (machine gun) operated by the commander.

On 30 March 1962, the first batch of 32 M113s arrived in Vietnam, and were sent to two Army of the Republic of Vietnam (ARVN) mechanized rifle companies, each equipped with 15 of the APCs. On 11 June 1962, the two mechanized units were fielded for the first time. During the Battle of Ap Bac in January 1963, at least fourteen of the exposed .50 caliber gunners aboard the M113s were killed in action, necessitating modifications to improve crew survivability. Soon, makeshift shields formed from metal salvaged from the hulls of sunken ships were fitted to the carriers, which afforded better protection. But, finding that this material could be penetrated by small arms fire, subsequent shields were constructed from scrapped armored vehicles.

The ARVN 80th Ordnance Unit in South Vietnam developed the shield idea further and commenced engineering general issue gun shields for the M113. These shields became the predecessor to the standardized armored cavalry assault vehicle (or ACAV) variant and were issued to all ARVN mechanized units during the early 1960s. The ARVNs had modified the M113s to function as “amphibious light tanks” and not as battle taxis as U.S. designers had intended. Instead of an armored personnel carrier, the ARVN used the carried infantry as extra “dismountable soldiers” in “an over-sized tank crew”. These “ACAV” sets were eventually adapted to U.S. Army M113s with the arrival of the army’s conventional forces in 1965. The vehicles continued to operate in the role of a light tank and reconnaissance vehicle, and not as designed in theater. Still, the M113 could carry 11 infantrymen inside, with two crewmen operating it. The U.S. Army, after berating the Vietnamese for flouting battle doctrine, came out with their own ACAV version. This more or less standardized ACAV kit included shields and a circular turret for the .50-caliber M2 machine gun in the track commander (TC) position, two M60 machine guns with shields for the left and right rear positions, and “belly armor”—steel armor bolted from the front bottom extending 1/2 to 2/3 of the way towards the bottom rear of the M113. The two rear machine gunners could fire their weapons while standing inside the rectangular open cargo hatch. This transformed the M113 into a fighting vehicle, but the vehicle still suffered from its lightly armored configuration, having never been designed for such a role.

 

 

Design

Armament

The basic M113 armored personnel carrier can be fitted with a number of weapon systems. The most common weapon fit is a single .50 caliber M2 machine gun. However, the mount can also be fitted with a 40 mm Mk. 19 automatic grenade launcher. A number of anti-tank weapons could be fitted to the standard variant: the U.S. Army developed kits that allowed the M47 Dragon and BGM-71 TOW anti-tank missile systems to be mounted. In the case of the M47, the system mated to the existing machine gun mount, without having to remove the machine gun. This allowed the commander to use both weapons. A large array of turrets and fixed mounts are available to mount high explosive cannon ranging from 20 mm to 105 mm on to the M113 series, making them function as assault guns and fire support; while in many cases still having room inside to carry dismounted infantry or cavalry scouts.

Armor

The M113 is built of 5083 aircraft-quality aluminum alloy, which gives it some of the same strength as steel at a slightly reduced weight, as the greater thickness allows structural stiffness. The M113A3 was designed to provide protection against 7.62mm threat, and this proved not to be enough when tested in combat.

In comparison, modern APCs like the Stryker have an all-around 7.62mm armor-piercing protection, plus 14.5 mm protection on the front, sides, and rear. Also protection against antipersonnel mines through the vehicle floor is installed.

Mobility

Its weight allows the use of a relatively small engine to power the vehicle, a Detroit 6V53 V6 two-stroke diesel engine of 318 cubic inches (5,210 cc) with an Allison TX-100-1 three-speed automatic transmission. This allows the vehicle to carry a large payload cross-country and to be transported by fixed- and rotary-wing aircraft. Original production M113s can swim without deploying flotation curtains, using only a front-mounted trim vane; they are propelled in the water by their tracks.

 

An M113 armored personnel carrier, foreground, M1A1 Abrams main battle tanks and other equipment of the 3rd Armored Division move out on a mission during Operation Desert Storm. February 15, 1991

Specifications:

Type: APC (Armored Personnel Carrier)

Weight: 12.3 tonnes

Length: 16 ft tall (4.86 m)

Width: 8 ft 9 in (2.68 m)

Height: 8 ft 2 in (2.5 m)

Crew: 2

Passengers: 11

Armor: Aluminum, 12-38 mm (0.47-1.50 in)

Main Armament: M2 Browning MG

Engine: Detroit Diesel 6V53T, 6-cylinder diesel engine

Operational Range: 300 mi (480 km)

Speed: 42 mph (67.6 km/h), 3.6 mph (5.8 km/h) swimming