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M551
M551A1(TTS)


152mm Gun-launcher Armored Reconnaissance/Airborne Assault Vehicle M551 Sheridan1-6
M551: General
Date of first acceptance July 1966 Total acceptances 1662
Manufacturer Allison Division of General Motors Corp. Crew
4 men:
  • Commander in turret right rear
  • Gunner in turret right front
  • Loader in turret left rear
  • Driver in hull front center
M551: Dimensions
Combat weight 33,460lbs
15,120kg
Height over AAMG 116"
295cm
Length without gun-launcher 248.3"
630.7cm
Gun-launcher overhang forward 0"
Width over tracks 110"
280cm
Tread 92.5"
235cm
Ground clearance 19"
48cm
Fire height ~76"
~190cm
Turret ring diameter 76"
190cm
Ground pressure, zero penetration 6.8psi
.48kg/cm²
M551: Armament
Type Mount Ammunition Traverse Max traverse rate Elevation Max elevation rate
152mm Gun-launcher M81, M81 Modified, or M81E1 Turret 20 rounds;
10 MGM-51A, -51B, or -51C Shillelagh missiles
360°
(manual and electric)
36°/sec +19.5° to -8°
(manual and electric)
4°/sec
.50cal M2HB MG Flexible in turret AA mount 1000 rounds 360°
(manual and electric)
54°/sec Manual --
7.62mm M73, M73E1, M219, or M240 MG Coaxial to 152mm gun-launcher 3000 rounds 360°
(manual and electric)
36°/sec +19.5° to -8°
(manual and electric)
4°/sec
Stabilizer
Aziumuth and elevation
Night vision
Infrared periscope M48 for driver, passive periscope M44 for gunner
M551: Armor
Assembly
Welding
Hull
Rolled 7039 aluminum alloy
Turret
Rolled and cast homogeneous steel
M551: Automotive
Engine General Motors 6V53T; 6 cylinder, 2 cycle, vee, supercharged diesel
Horsepower Net: 255@2800rpm
Gross: 300@2800rpm
Torque Net: 520 ft-lb@2100rpm
Gross: 615 ft-lb@2100rpm
Fuel capacity 158gal
598L
Transmission Allison XTG-250-1A, 4 ranges forward, 2 reverse
Steering Mechanical, T-bar
Brakes Multiple disc
M551: Suspension
Type Road wheels Track return rollers
Torsion bar 5 individually sprung dual/track Flat track
Drive sprockets Idlers Shock absorbers
11-tooth rear drive Dual adjustable at front of track On first and last road wheels/track
M551: Track
T138
Double center guide, single pin, cast steel with rubber pads
Width 17.5"
44.5cm
Pitch 4.7"
12cm
Shoes/track 102 Ground contact length 140"
360cm
M551: Performance
Max level road speed 43mph
69kph
Max water speed 3.6mph
5.8kph
Max trench 96"
240cm
Max grade 60%
Max vertical obstacle 33"
84cm
Min turning diameter Pivot
Max fording depth Floats Cruising range ~350mi, roads
~560km, roads


As can be surmised from the designation, the M551 Sheridan was intended to fill a multitude of roles. It was a light airdroppable tank, a reconnaissance vehicle, and was able to take on enemy armor thanks to its 152mm combination gun/missile launcher. The driver's hatch was unusual in that it rotated around the driver on a vertical axis, while the commander's cupola hatch and loader's hatch were more conventional. M551's hull was ringed by a lightweight structure filled with closed cell polystyrene foam for buoyancy, and a flotation screen could be raised to allow the vehicle to float across water obstacles. There were four smoke grenade launchers on either side of the turret which were supported by a brace on early vehicles. Early M551s had a turret ventilator dome on the left rear side wall; this was moved to the turret left front on later Sheridans.

The original M81 152mm gun-launcher on the M551 was equipped with a bore evacuator and an open breech carbon dioxide scavenging system, which was composed of two jets that blew CO2 into the open breech after firing to get rid of any remnants of the combustible case ammunition. However, this system also blew any burning pieces of left over combustible cases around the turret, which was unpleasant for the crew and deadly with the rest of the ammunition around. A closed breech scavenging system (CBSS) was developed by Allison to remedy this problem. CBSS blew any remaining pieces of the ammunition out of the weapon while the breech was still closed. CBSS was first installed on M551 number 700, and retrofit kits were developed for the previous vehicles. In vehicles with CBSS, ammunition stowage was reduced to a total of 29 rounds and missiles since one of the air cylinders replaced an ammunition rack.

The 152mm gun-launchers differ in the following ways: M81 was fitted with a bore evacuator and the open breech scavenging system; M81 Modified was an M81 retrofitted with the CBSS; M81E1 had a shallower missile keyway to reduce firing stresses on the barrel, and the barrel was thickened near the muzzle. When CBSS was introduced, the bore evacuator was omitted from new-production gun-launchers.

The MGM-51 Shillelagh missile was a SACLOS infrared-guided missile fitted with a shaped-charge warhead. MGM-51B extended the maximum range to 3km from the MGM-51A's 2km, and the MGM-51C was fitted with a shorter key compatible with the M81E1 gun-launcher. Sheridans 140 through 223 and 740 through 885 were produced without the missile guidance hardware.

In early 1972, laser rangefinders AN/VVG-1 were fitted to the commander's cupola, and the transceiver replaced the forward vision block. The rest of the laser rangefinder was mounted behind the commander, and a cable cover ran around the right side of the cupola from the transceiver to the rear of the TC's position. Sheridans fitted with the laser rangefinder were designated M551A1. M551A1s also replaced the gunner's M127 sighting telescope with the M127A1, which provided protection against laser light, and the cupola could be aligned with the main gun-launcher automatically.


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152mm Gun-launcher Armored Reconnaissance/Airborne Assault Vehicle M551A1 (Tank Thermal Sight) Sheridan1,4
M551A1(TTS): General
Date of first acceptance 1989
Manufacturer Anniston Army Depot Crew
4 men:
  • Commander in turret right rear
  • Gunner in turret right front
  • Loader in turret left rear
  • Driver in hull front center
M551A1(TTS): Dimensions
Combat weight 33,600lbs
15,200kg
Height over AAMG 116"
295cm
Length without gun-launcher 248.3"
630.7cm
Gun-launcher overhang forward 0"
Width over tracks 110"
280cm
Tread 92.5"
235cm
Ground clearance 19"
48cm
Fire height ~76"
~190cm
Turret ring diameter 76"
190cm
Ground pressure, zero penetration 6.9psi
.48kg/cm²
M551A1(TTS): Armament
Type Mount Ammunition Traverse Max traverse rate Elevation Max elevation rate
152mm Gun-launcher M81E1 Turret 20 rounds;
9 MGM-51C Shillelagh missiles
360°
(manual and electric)
36°/sec +19.5° to -8°
(manual and electric)
4°/sec
.50cal M2HB MG Flexible in turret AA mount 1000 rounds 360°
(manual and electric)
54°/sec Manual --
7.62mm M240 MG Coaxial to 152mm gun-launcher 3000 rounds 360°
(manual and electric)
36°/sec +19.5° to -8°
(manual and electric)
4°/sec
Stabilizer
Aziumuth and elevation
Night vision
Infrared periscope M48 for driver, AN/VSG-2 tank thermal sight for gunner
M551: Armor
Assembly
Welding
Hull
Rolled 7039 aluminum alloy
Turret
Rolled and cast homogeneous steel
M551A1(TTS): Automotive
Engine General Motors 6V53T; 6 cylinder, 2 cycle, vee, supercharged diesel
Horsepower Net: 255@2800rpm
Gross: 300@2800rpm
Torque Net: 520 ft-lb@2100rpm
Gross: 615 ft-lb@2100rpm
Fuel capacity 158gal
598L
Transmission Allison XTG-250-1A, 4 ranges forward, 2 reverse
Steering Mechanical, T-bar
Brakes Multiple disc
M551A1(TTS): Suspension
Type Road wheels Track return rollers
Torsion bar 5 individually sprung dual/track Flat track
Drive sprockets Idlers Shock absorbers
11-tooth rear drive Dual adjustable at front of track On first and last road wheels/track
M551A1(TTS): Track
T138
Double center guide, single pin, cast steel with rubber pads
Width 17.5"
44.5cm
Pitch 4.7"
12cm
Shoes/track 102 Ground contact length 140"
360cm
M551A1(TTS): Performance
Max level road speed 43mph
69kph
Max water speed 3.6mph
5.8kph
Max trench 96"
240cm
Max grade 60%
Max vertical obstacle 33"
84cm
Min turning diameter Pivot
Max fording depth Floats Cruising range ~350mi, roads
~560km, roads


In early 1989, the tank thermal sight from the 105mm gun tank M60A3(TTS) was also fitted to the M551A1. These vehicles were known as M551A1(TTS).

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References

  1. Hunnicutt, R.P. Sheridan: A History of the American Light Tank, volume 2. Navato, CA: Presidio Press, 1995.
    Reprinted with permission from Sheridan, R.P. Hunnicutt ©1995, available from Presidio Press, 505B San Martin Drive, Suite 160, Navato, CA 94945.

  2. Crismon, Fred W. U.S. Military Tracked Vehicles. Osceola, WI: Motorbooks International, 1992.

  3. Miller, David. The Illustrated Directory of Tanks of the World. Osceola, WI: MBI Publishing Co., 2000.

  4. Federation of American Scientists. "M551A1 Armored Reconnaissance Airborne Assault Vehicle (Sheridan)." DOD 101. 12 Sep 1998. 25 Apr 2001 <http://www.fas.org/man/dod-101/sys/land/m551a1.htm>. DOD 101

  5. Holloway, Mark. "M551." E-mail to the author. 2 Dec 2001.

  6. Holloway, Mark. "RE M551." E-mail to the author. 2 Dec 2001.



Last updated 25 Apr 2012.

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© Copyright 2001-12 Chris Conners