Armament fired from XM551 Sheridan pilot #12 in 1967 ;Primary armament The 152 mm M81
gun-launcher that came to be used in the Sheridan was developed by
Ford-
Aeronutronic under a contract awarded by the Army in 1959. The gun's primary purpose was to fire the Shillelagh missile, but it could also fire more conventional combustible case ammunition. Two factors contributed to the selection of such a large-caliber gun. The Army desired a gun that would be capable of penetrating 150 mm of armor at 60 degrees, more than the 76 mm guns of the M41 or T92 were capable of. Secondly, the Shillelagh missile housed complicated electronics in addition to its warhead. As a consequence of the pairing of a large caliber gun and relatively lightweight vehicle, only low-velocity rounds would be compatible with the gun. A number of existing vehicles already mounted only ATGMs, or alternately
recoilless rifles like the
M50 Ontos, but these typically had limited utility in the infantry support role, or in the case of Ontos could not be reloaded from within the vehicle. The XM551 appeared to offer a superior balance between anti-tank and infantry support. The gun was ideal for infantry support. The large, low velocity gun could fire a large high explosive projectile or canister shot. In comparison, high-velocity anti-tank guns over-penetrated soft targets, while smaller caliber weapons could not carry as great a payload. The Shillelagh missile system experienced problems with its missile guidance system early in its development. Two problems were identified. First, smoke from the rocket motor disrupted the infrared signal, necessary for guidance, between the tank and missile. A similar issue was identified during conditions where the sun was behind the turret. The first issue was resolved by changing the propellant to a composition that produced less smoke. This and other fixes were successful in resolving the problems with the missile guidance. In a year-long test that concluded in 1964, 58 out of 63 launches were successful. The M81 fired combustible case conventional ammunition in addition to the Shillelagh missile. This solution was chosen rather than a conventional metal case because the Army desired that the Sheridan use the same ammunition as the
MBT-70. In theory the combustible
nitrocellulose case would be completely consumed during the firing of the ammunition. In practice, as often as 39 percent of the time, the case could absorb moisture from the air and only be partially consumed. This was dangerous to the crew as the smoldering case could prematurely detonate the next loaded round, as happened three times during five months of testing from November 1966 to March 1967. The cases could also break open, or swell so much with moisture that they could not be chambered. The Army started packaging the rounds in
Neoprene bags to reduce the impact of moisture on the combustible case rounds. These were removed by the loader before the round was used. The combustible case was made more durable, and less likely to break, in subsequent generations of ammunition. An open-
breech scavenging system using compressed air to blow any debris out of the gun was installed to resolve the issue of smoldering debris in the breech. This created problems of its own as the system could blow smoldering debris into the crew compartment. Recognizing the danger, the Army canceled production of this system. A closed breech system that solved the problem was put into production in 1968. Vehicles without this system, the first 700 off the production line were waylayed in storage pending the installation of this system. The gun with this new system installed was called the M81E1. Sheridans with this system installed carried 29 rounds of ammunition instead of 30 due to space constraints. The second and even third road wheels could clear off the ground when the main gun fired
high-explosive anti-tank rounds. The recoil was so violent that components such as the commander's birdcage armor and the searchlight were liable to come off. In the
Vietnam War, firing the gun often adversely affected the delicate electronics, which were at the early stages of the transition to solid state devices, so the missile and its guidance system was omitted from vehicles deployed to South Vietnam. The expensive missile was
fired in anger only in the Persian Gulf War's
Operation Desert Storm, despite a production run of 88,000 units. Ammunition: • MGM-51 Shillelagh missile. • MTM-51 Shillelagh missile training round. • M409,
high-explosive anti-tank multipurpose (HEAT-MP). • HE-T XM657E2 Shell, high explosive. Never standardized. • Canister M625, beehive anti-personnel round launching 10,000 13-grain
flechettes from the muzzle. Could be used to clear foliage or against personnel. • XM617, anti-personnel round firing 8,200 fléchettes. • XM410-WP,
white phosphorus smoke round. Never standardized. • M411A1 Shell (TP-T), training round. ;Secondary armament The commander's cupola was armed with a
.50 caliber M2HB in a flexible
anti-aircraft mount on the turret. Stowage capacity was for 1000 rounds Early production Sheridans were armed with the 7.62 mm
M73 coaxial machine gun. These were soon replaced with the M73E1 model and then the 7.62 mm M219 variant. The M73 and M219 were problematic designs and were replaced by the 7.62 mm
M240 machine gun. Stowage capacity was for 3,000 rounds. Early XM551 pilots had a .50 caliber
spotting rifle, and pilot #7 had a 15mm XM122 spotting rifle. Development of spotting rifles was ended when it was decided to eventually install
laser rangefinders on the production vehicle.
Armor The vehicle had a steel turret and aluminum hull. Although the hull could deflect heavy machine gun fire of up to 12.7 mm AP, it was easily penetrated by
rocket-propelled grenades (RPGs), which could destroy the vehicle if the spalling contacted the
caseless main gun rounds. Like the
M113 armored personnel carrier, it was vulnerable to mines.
Mobility Tactical mobility , Alabama The Sheridan was powered by a large 300-hp (224 kW) Detroit Diesel 6V53T diesel engine, and a
flat track (no support rollers). The XM551 thus had an excellent
power-to-weight ratio and mobility, able to run at speeds up to 72 km/h (45 mph). However, the vehicle proved to be very noisy and early Sheridans were unreliable under combat conditions. Swimming capability was provided by a flotation screen. The front armor was overlain by folding "surfboard". This could be opened up into a sloping vertical surface in front of the driver, providing a bow of a boat hull, about level with the top of the turret. Fabric formed the rest of the water barrier, folding up from compartments lining the upper corner where the side met the top of the hull, and held up at the back with poles. The front of the "hull" was provided with a plastic window, but in practice it was found that water splashing onto it made it almost useless, and the driver instead usually had to take steering directions from the vehicle commander. The
M2 Bradley adopted a similar solution, but dropped it with upgraded armor. The Sheridan could swim across a river that was about 46 m (50 yards) wide. Tanks in the Patton series (
M46,
M47,
M48), as well as the
M60 tank could not perform these operations; they would have to crawl along the river bottoms using
snorkels. Not by design, it was found that the swimming hardware acted to reduce the effectiveness of
RPG hits. Although it was rarely used in Vietnam, fording was frequently used during
Reforger exercises in Europe. The Sheridan was capable of being lifted by the
Sikorsky CH-54 Tarhe heavy-lift helicopter.
Strategic mobility delivering an M551 Sheridan using LAPES (
Low Altitude Parachute Extraction System). The Sheridan could be rigged for low-velocity airdrop from
C-130 (19,000 kg, 42,000 lb max load),
C-141 aircraft (17,460 kg, 38,500 lb max load), and the
C-5. Many films exist showing the Sheridan being pulled out of a
C-130 Hercules transport by brake chutes and skidding to a stop. The
Low Altitude Parachute Extraction System (LAPES) is a somewhat risky maneuver that allowed accurate delivery onto a field when landing is not possible, and the practice was stopped in the late 1990s. The tank was strapped down to a special pallet which absorbed most of the landing impact. The crew did not ride in the tank during extraction, but parachuted from another plane. On landing, they would go to their tank, release the lines, and drive it away. ==Variants==