US Armor in Vietnam - the M551 Sheridan

Page Title - United States Armour
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M-551 Sheridan


Developed in 1959 as a replacement for the M41 light tank and the airborne M56 Scorpion self propelled antitank gun, the Sheridan was intended as an airborne reconnaissance and assault vehicle. 

Standard Vehicle Data

Type: Light Tank
Crew: 4 (Commander, Driver, Gunner and Loader)
Range: 600 km
Max Speed: 70 km/h (5.8 km/h in water)
Armour:
Fording: Amphibious
Obstacle: Capable of traversing obstacles up to 0.8 meters high
Crossing: Capable of crossing gaps of up to 2.5 meters

Armament:

  • 1 x 152mm Gun/Missile Launcher
  • 1 x 7.62mm co-axial MG
  • 1 x .50 cal on commander's cupola.
  • 8 Smoke Dischargers

In 1968 plans were approved to equip two divisional cavalry squadrons, the 1st and 3rd Squadrons of the 4th Cavalry with the new tank  Neither unit actually wanted the Sheridan because it was suspected of being too vulnerable to mines and RPG's.

In a last minute change of plan, the new M551's were sent to the 1st Squadron, 11th Armored Cavalry Regiment and the 3rd Squadron, 4th Cavalry with 'B' Troop 3/4 Cavalry receiving their first Sheridan's in late January 1969.

It was subsequently decided to replace the M48A3 Patton's in cavalry platoons of divisional cavalry squadrons with the M551. Some cavalry platoons of regimental cavalry squadrons were actually using M113 ACAV's as substitutes for the M48A3's in their cavalry platoons and it was intended that the M551 should replace these also. By 1970 almost every cavalry unit in RVN was equipped with the Sheridan.

The M551 suffered from many defects (see below). One particular problem concerned the vehicle's all electric fire-control system which malfunctioned continuously in the rainy season, and despite pre-deployment tests which highlighted these problems - in particular the combustible 152-mm ammunition - and the general unsuitability of the vehicle for the hot and humid Vietnamese environment, the Army sent the M551 anyway.  According to Jerry Headley the picture above of 'Hard Core 7',

"... the track belongs to B Troop, 2d Platoon. It was taken in the Ho Bo Woods in Feb '69. I was the Troop Commander at the time. The photo was taken by Army photographers. They also took films. The purpose was to send them to Congress. They were investigating complaints about the Sheridan, i.e., too noisy, gave off a plume of white smoke when moving, etc. Notice the "RPG Screen" in front of the driver.  The crew made this themselves as additional protection from RPGs."

The Sheridan was armed with a 152-mm main gun (vehicles dispatched to Vietnam had the guidance system for their ATGW missiles removed) which fired a selection of combustible-case antitank rounds and also the 'beehive' round. Other armament consisted of a co-axial 7.62-mm MG and cupola mounted .50-cal M2 HB for use by the tank commander. When the M551 first appeared in RVN they did not have any armor protection for the tank commander's .50-cal MG. Many crews utilised the armored gun shields from M113's to provide some protection to the tank commander. Eventually, a production armor kit was developed, known as the 'bird cage', to provide all-around protection for the commander.

Whilst the Sheridan had a steel turret it only had a thin aluminum hull which was vulnerable to RPG's. In particular the M551 had a thin underbelly which, unlike the heavy steel belly armor of the M48A3, was very susceptible to damage from mines. As a result, many crews refused to ride inside the vehicle (just as they did with the M113).  The Army attempted to remedy this by retro-fitting steel belly armor. In contrast to the M48A3 which could absorb a lot of hits and still continue to fight, in combat the Sheridan was prone to catastrophic explosions (due in part to the highly combustible 152-mm ammunition carried). 

DESIGN DEFECTS

When asked to highlight some of the problems experienced with the M551, Stanley Homiski, Commo Sergeant with B Troop, 3/4 Cav replied,

"... I can tell you one thing about the Sheridan is that at first we had a lot of radio problems with them. That was because when that 152 MM main gun fired it would lift the tank up off the ground about two feet the second road wheel back and the action of the tank coming back into firing position would severely damage the radio mount. It wasn't shock mounted properly. This was in addition to all the other problems we had with it."

"... electrical problems with the Sheridan happened more during the wet season... we found that due to temperature changes between the daytime (100+ degrees F) and nighttime temperatures (around 70 degrees F) that severe condensation problems occurred. There would be small droplets of water dripping in the turret usually around the area that the radio was installed. At one point we resorted to covering the radio with a poncho to keep out the moisture but this in turn caused problems with the radio starting to overheat due to lack of ventilation."

"... I don't know if you are aware that one of the major problems with it in Vietnam was one of engine failure, the vents would become plugged with jungle vegetation and the damn engine would fail, one of the other things was the main gun ammo - it didn't have a brass shell casing, the whole shell was consumable and you had to keep asbestos covers over the ammo up until you loaded it into the breech."

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Sources:

Jerry Headley, Commanding Officer 'B' Troop 3/4 Cavalry, RVN 1968 - 1969.

Stanley 'Ski' Homiski, Commo Sergeant 'B' Troop 3/4 Cavalry, RVN 1968-1969

I owe a special thank you to these two men who have been extremely patient with my constant questions and also more than generous with their interesting and illuminating replies. Thank you.

Armor of the Vietnam War; (1) Allied Forces, Michael Green & Peter Sarson, Concord Publications Ltd

Armor in Vietnam; A Pictorial History, Jim Mesko, Squadron/Signal Publications Inc.


 

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