HISTORY
The M551 VN is a Vietnam War variant of the M551. When the M551 was first introduced, it was deployed to Vietnam in 1969 and was withdrawn from service officially in 1973 when the US withdrew from vietnam. The M551 was not withdrawn from service at this time; rather, it was withdrawn from the conflict along with the rest of the American forces. The M551 was officially withdrawn from service in 1996.
The development of the M551 Sheridan Armored Reconnaissance/Airborne Assault Vehicle (AR/AAV) originated in 1959 as the U.S. Army sought a lightweight successor to the 26-ton M41 Walker Bulldog. The objective was a platform capable of being air-dropped by parachute and swimming across inland waterways while providing the fire support of a heavy tank. Cadillac Motor Car Division was awarded the contract to produce a vehicle that adhered to a strict 15-ton (approx. 30,000 lbs) weight limit—a constraint that forced engineers to abandon traditional steel armor in favor of radical, lightweight materials.
To achieve this weight threshold, the hull was constructed from 7039 aluminum alloy, which kept the final combat weight at roughly 33,500 lbs (16.75 tons). This lightweight construction, powered by a 300-horsepower Detroit Diesel 6V53T engine, gave the Sheridan a high power-to-weight ratio of 18.9 horsepower per ton and a top road speed of 43 mph. However, the use of aluminum introduced a significant technical drawback: the hull provided only 0.5 to 1.25 inches of protection, sufficient against .50-caliber rounds but notoriously vulnerable to anti-tank mines and rocket-propelled grenades (RPGs).
The centerpiece of the development was the M81 152mm Gun/Launcher, designed to fire both conventional high-explosive rounds and the MGM-51 Shillelagh guided missile. The Shillelagh, which carried a 15 lb shaped-charge warhead, was intended to engage Soviet armor at ranges exceeding 2,200 yards. Because the 152mm (6-inch) bore was exceptionally large for such a light vehicle, the recoil was massive; early tests showed that firing a conventional round could cause the 19-inch ground clearance to fluctuate violently as the front road wheels lifted off the ground, often damaging the sensitive infrared electronics used to guide the missile.
Logistical requirements further complicated the design by introducing caseless ammunition. To maximize the internal stowage of 29 rounds within the cramped turret, engineers utilized a fully combustible nitrocellulose propellant. In practice, these charges were extremely fragile and sensitive to the high humidity of tropical environments. More critically, unburnt residue often remained in the breech, leading to “flarebacks” that could ignite the next round being loaded. This necessitated the late-stage addition of the Closed Breech Scavenging System (CBSS), which used a high-pressure air blast from a 3,000 psi tank to clear the tube before the breech was cycled.
The testing phase at Aberdeen Proving Ground throughout the mid-1960s revealed staggering unreliability in the Shillelagh system, with hit probabilities falling far below Army requirements. Despite these technical “red flags” and a total program cost that ballooned to over $1.3 billion, the Sheridan was pushed into full-rate production in 1966. The Army faced significant pressure from the Department of Defense to field a “strategically mobile” asset, even as the 152mm gun system struggled with persistent electronic failures and environmental degradation of the ammunition.
By the time the M551 was deployed to Vietnam in 1969, “development” had transitioned to a series of field modifications, both authorized and unauthorized. This included the equipment needed to remove the missile, aside from critical components, meaning it could not fire the missile. This was done due to the recoil and environment, causing components of that system to fail frequently, and the lack of heavily armored targets.
This specific variant would be based on the many field modifications; most crews in vietnam did everything they could to increase the protection that their vehicles could offer. This resulted in crews often “acquiring” the gun shields from the M113 ACAV and mounting them on their 50cal to protect the commander from small arms fire. This offered almost all-around protection from rounds up to 7.62mm. In Vietnam , mines were a constant threat to the point that crews would often ride outside of the tank due to the threat they posed, however, the driver was unfortunate enough not to have that luxury, hence the army introduced an additional steel armor plate that was located underneath the driver to offer just a little bit of extra protection.
One other field mod was a form of “slat armor” comprised of chicken wire and wood that offered protection against weapons such as small ATGMS and RPG-7s. However, this armor was only located in a position that protected the driver from attacks of this kind from the front and would likely only offer protection from one or maybe two such attacks. Additionally, crews often created their own stowage racks on the rear of the turrets to pack as many supplies as they could.
VEHICAL STATS
BASIC
Crew: 4
Length: 22ft 4in
Width: 13ft 6in
Height: 12ft 6in
Weight: 15.2 tons (basic) aprox 16 tons (with additional armor and equipment)
WEAPONRY (spaded)
1x 152mm M81E1 Gun (ammo: 30)
2x 12.7mm M2HB (ammo: 1500) (Only one .50 is availible stock)
1x 7.62 coax (Ammo: 3000)
8x Smoke grenades
1x coax AN/VSS-3 IR/Visible spotlight
AMMO
HEAT: M409A1
HE: M657A2
(Note: During Vietnam, most M551s had a majority of their equipment for firing the shiliage MSL removed and did not use them in combat)
Armor
Turret: 25/25/25 (Steel)
Hull: 31/25/19 (rolled Aluminum alloy 7039)
Hull (mine protection): 12.7mm steel
Gun shield: 12.7mm steel
MOBILITY
Engine: Detroit 6V53T 6 cylinder supercharged diesel 300 HP
Allison XTG-250-1A, 4 ranges forward, 2 reverse
Max speed: FWD: 43mph RVS: 10mph
SOUCES
M551 Sheridan Light Tank - The Army Historical Foundation
M551 Sheridan in combat 1969-91 – Mike's Research
152mm Gun-launcher AR/AAV M551 Sheridan
M551 Sheridan Operator Manual | PDF | Transmission (Mechanics) | Gun Turret
TM 9-2350-230-10
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