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ANNEX G-4 – A Brief History of the Three Marines’ Armored Fighting Vehicles

ANNEX G-4

A Brief History of the Three Marines’ Armored Fighting Vehicles

M48A3 Patton Tank Gun Tank

On 27 February 1951, the Ordnance Technical Committee Minutes (OTCM) initiated the design of the new tank, designated the 90mm Gun Tank. A deeper modernization than the M46 and the M47, the M48 featured a new hemispherical turret, new redesigned hull, and an improved suspension. The hull machine gunner position was removed, reducing the crew to 4. It was essentially a new tank. On 2 April 1953, the Ordnance (OTCM) standardized the last of the Patton series tanks as the 90mm Gun Tank M48 Patton.

Nearly 12,000 M48s were built from 1952 to 1959. The early designs, up to the M48A2C's, were powered by a gasoline 12-cylinder engine which was coupled with an auxiliary 8-cylinder engine (called the "Little Joe"). The gasoline engine gave the tank a short operating range and were prone to catching fire when hit. This version was considered unreliable Beginning in 1959, most American M48s were upgraded to the M48A3 model which featured a diesel power plant. By 1964, the US Marine Corps had received 419 Patton tanks. Because all M48A3 tanks were conversions from earlier models, many characteristics varied among individual examples of this type. M48A3 tanks could have either 3 or 5 support rollers on each side and might have either the early or later type headlight assemblies.

There were more than 600 Pattons deployed with US Forces during the Vietnam War most extensively by the Marines Corps 1st, 3d, and 5th Tank Battalions. The initial M48A3's landed with the Marine's 3d Tank Battalion in Da Nang, Republic of (South) Vietnam in March of 1965; by the forward elements of the 1st Tank Battalion in July of 1965; later by the 5th Tank Battalion.

The M48A3's performed admirably in Vietnam in their primary mission of infantry support using all of its many capabilities. While Marines were denied the opportunity to go one-on-one against the NVA's Russian and Chinese supplied T-54/55 and PT-67 in I Corp, the South Vietnamese Tankers’ Pattons scored well and made an excellent accounting of themselves.

M48 Specifications

Weights:


Armament:


Hull

20 tons

Main gun

90 mm rifled cannon 64 rds

Turret

18 tons

Tank Commander

.50 cal M2 HB machine gun 3000 rds

Engine/Transmission

6 tons

Coax

7.62mm M73 machine gun 10,000 rds

Engine cover

2 tons

Ammunition:

HEAT (High Explosive Anti-Tank)

Track

2 tons each


HEP (High Explosive Plastic)

Basic load

2 tons


Canister

Total

52 tons


WP (White Phosphorus)

Performance:



Beehive

Range

258 miles / 463 km

Armor:

Homogeneous cast steel

Top speed

40 mph / 48 km/h

Hull Front

120 mm

Fuel consumption:

1 g/mi road, 2 g/mi offroad

Hull Side, front

76 mm

Fording:

1.2 m

Hull Side, rear

51 mm

Vertical Obstacle:

0 .9 m

Hull rear

44 mm

Gap Crossing:

2.59 m

Hull floor

25 mm

Powerplant:


Turret Front

110 mm

Engine

Continental AVDS

Turret Side

76 mm


1790 cu in 690 horsepower

Turret Rear

50 mm

Transmission

Alison CD-850

Crew:

4 (driver, gunner, loader, TC)

M67A2 Flame Tank

Flame Thrower Tank M67: also known as "Zippo", nicknamed after a popular brand of cigarette lighter was a medium tank that was designed in the years 1952—1954 on the M48 tank chassis, at the initiative of the US Marine Corps. During the production, which lasted from 1955 to (according to various sources) 1956 or 1959. 109 M67 tanks were produced for the Marine Corps and also the US Army which relatively quickly took them out of service in favor of the lighter flame thrower tanks the M132. In the Marine Corps the M67’s were modernized in the early 1960’s, and were actively used during the Vietnam War. M67 tanks were finally removed from service in the years 1972—1974, after the withdrawal of the Marine Corps from Vietnam.

The Patton Tank

By Gerry Hodum

Chapter One

When I was asked if I would write about the Patton tank, my first question was, “Which One?” Since the request came from the “Sponson Box” it had to be the M48A3 of Viet Nam fame, right? Well, as it turned out it was the Patton series, all of them. After all, since the tank was the M48A3 that meant there were M48’s before. Didn’t veteran tankers talk of Patton’s in Korea? Sure they did, but it couldn’t have been the M48 right? Weren’t there “tankers tales about sitting for hours and hours running “geese””? Sure, but that could not have been the A3, could it? No, that’s true, it wasn’t. The Patton series tanks started with the M46 medium tank that appeared in the late 1940’s. The M46 had its origin’s in the M26 General Pershing heavy tank (soon re-designated as a medium tank) that was used in the last months of World War II in Germany where they were found to be able to stand and shoot it out with Panthers and Tigers and with the Army on Okinawa (these arrived late and saw no combat). The production M26 Pershing was 46 ton, had a 90mm gun, two .30 cal machine guns and a .50 caliber machine gun for the commander to use. A cast/welded slab sided hull and cast turret with power traverse and manual elevation, a five-man crew (commander, gunner, loader, assistant driver (bow gunner), and driver), a torsion bar suspension and was gasoline fueled. All in all you could say the Daddy of the Patton, but it had one characteristic that was not Patton related. It was underpowered, the Ford GAF engine was liquid cooled; and had the same 500-horse power as the engine (GAA) in the M4 Sherman tank the M26 Pershing was destined to replace. It was the same Ford V-8 but adapted to accept a 900-F2-torquematic transmission with three forward and one reverse range selection (no pivot). Marine tankers of 1st Tank Battalion used the M26 series in Korea in 1950-51. Because of the Pershing shortcomings known to the Ordnance Corps, one of which was the tendency to throw cooling fan belts if shifted improperly, testing of improved M26 vehicles continued in the late 1940’s. The M26 vehicle was modified with the Continental AV-1790-5A engine and the Allison CD-850-3. After considerable testing the tank was designated M46 and named the Patton

by US Army order. The 90MM gun M3 was retained after various calibers were proposed and tested. The M46 was, at the time, unusual in that the steering and drive range selection was controlled by a “wobble stick” at each position allowing the driver or the assistant driver to operate the vehicle. The engine was the air-cooled AV-1790-5A, gasoline fueled with a gross horsepower of 810 and a cross-drive CD-850-3 transmission with two forward and one reverse range. The transmission allowed the tank to pivot on its own axis while the lockup feature of the transmission moved the M46 at about 30 miles per hour. Fire control improvements included an improved telescope, M83, in a cant correction mount. The 1st Tank Battalion in Korea put the M46 series to good use.


There the 18 inch searchlight was tested and mounted on the gun tanks. It was found that the lights were hard to hit when lit at night and proved to be demoralizing to the enemy. The 90mm M3 gun was improved with the addition of a gun tube bore evacuator and a single baffle muzzle brake replacing the double baffle muzzle brake (making the gun an M3A1) with the bore evacuator correcting the problem of “flashback” on the M26 with the M3 gun. Also installed in later production was the improved AV-1790-5B engine and CD-850-4 transmission. The M46 could mount a bulldozer blade kit and soon replaced the M4 dozer tanks still in use by 1st Tank Battalion in Korea. The Patton M46 proved to be a reliable tank but the Ordnance Corp knew there were some features that were obsolete and had been testing improved vehicles even while the M46 program was ramping up. A family of vehicles was in the making. The method of classifying tanks changed from weight-to-gun size. The basic program looked for a new 76mm gun tank T41a 90MM gun tank T42 and a 120MM gun tank T43.

The T41 went on to be the M41 series Walker Bulldog tank with a 76MM cannon; an air-cooled 6 cylinder opposed gasoline 500-horse power gasoline engine (AOS-895-3) and an automatic transmission (CD-500-3).

TheT43 was developed into the M103 with a 120MM gun; an air-cooled V-12 810-horse-powered gasoline engine (AV-1790-7) and a cross drive automatic transmission (CD-850-4B). The Korean War brought the shortage of tanks to the Army’s attention. The development of the T41 and T43 continued, but the questionable performance of the T42 raised doubts as to its continued development. The tank used the same power package as the T41 and was considered by the Armored Force to be underpowered. The fact that the supply of M46 and the M26’s available for conversion was short of the worldwide requirements of the Army showed a new tank was needed. It was suggested, as an interim measure, that the advanced fire control and improved 90mm gun and mount turret of the T42 with the reliable but modified M46 hull be produced as the M47 tank.

Removing the vent blower between the driver and assistant driver and the turret race area enlarged from 69 inches to 73 inches to match the T42 turret race ring changed the hull of the M46. Improvements in wiring, fire extinguishers and the removal of the assistant driver’s controls were some of the items resulting from testing at Aberdeen MD.

The tank was standardized as the M47 and named the Patton II but became quickly known as “Patton”. The M12 stereoscopic range finder was at the gunner’s position and served as his primary sighting unit with a M20 periscope as his auxiliary sight unit. The M12 was a binocular unit. There was no telescope as an auxiliary sight. The gunner observed a reticule that had five vertical lighted lines in a “V” pattern which had “Depth” that were “flown” out by turning the ranging knob, until the “Geese” appeared to be at the same distance as the target. To be proficient at this a gunner would sit in the tank and “Fly the Geese” over and over at a measured 1200 yard target. One was required to log the exercise on log sheets (at least at 8th Tank Bn. FT, ClNC) and these were maintained for record. If you had bad eyesight, cold, hangover, or any other problems the damn “Geese” would not fly! Many a gunner used his M20A1 auxiliary sight rather than the range finder and “Guesstimated” the range. Another small problem was that, unless you had turret power on when the tank was moving, the gun would oscillate from stop to stop if not secured in the travel lock. The electric motor on the hydraulic power pack made a very high annoying screeching sound and made conversation in the turret very difficult even with intercom. If you have seen the movie “Battle of the Bulge” the German tanks are M47’s. All these developments, the M46, the T42, the M47 programs overlapped and were accelerated by the Korean War. From the late forties to the early fifties even as the M46 and the M47 were being produced and issued the Armor Board had been developing its replacement. In 1950 design work was begun by the Ordnance Corps and shortly given to Chrysler with a letter of intent to design a new 90mm gun tank was begun by the Ordnance Corps and shortly given to Chrysler with a letter of intent to design a new 90mm gun tank. This tank was designated T48.

Chapter Two

The T48 (APGT48) design was an elliptically shaped hull and turret based on concepts forwarded from Ordnance Tank Automotive Command (OTAC) to the Chrysler Ordnance Development Department, designated as Vehicle Design Agency. These closely resembled the hull and turret shape of the T43 120mm gun tank. This shape provided for maximum armor protection at minimum weight. The vehicle was to be equipped with the same basic power package as the M47 to make the transition to the user easier and to answer user complaints about the questionable power of the T42. The vehicle had six road wheels and a wider track (28 inch compared to 23 inch). The T48 had an 85-inch turret ring (M47-73.0 inch) the same as the T43, which allowed for heavier weapons systems in the future and for the turret walls to slope smoothly to the hull. The crew of four men consisted of a driver, who was positioned in the center of the hull eliminating the assistant driver/bow gunners’ position. This allowed main gun ammunition to be positioned to the drivers right and left. The turret contained the rest of the crew, tank commander, gunner, and loader. The main gun was a lightweight 90mm T139 with a quick-change gun tube. The tube could be removed with out removing the breech ring or gun mount from the turret due to interrupted threads attaching the tube to the breech ring. Coaxially mounted with the main gun on the left were a 0.50 caliber M2HB machine gun (removed in production vehicles) and a 0.30 caliber M1919A4 machine gun on the right (moved to the left side and replaced by a telescope in production vehicles). At the tank commanders position a 0.50 caliber M2HB machine gun was mounted to the commander’s cupola in a manually controlled mount. This was called the Chrysler mount by the users and could be elevated/depressed and traversed from inside with the hatch closed. The 0.50 cal M2HB could be reloaded by using a crank to unlock the machine gun mount and then cranking the mount to pivot over the open loaders hatch where another 105 round (as 0.50 caliber was packaged then) box of ammunition could be installed. The mount was then cranked back, relocked and was now ready to fire. The tank was to be equipped with a range finder but the original T48s that were used in 2d Tank Battalion in the training platoon, under Gunny Dick Wenner, had the Phase 1 fire control. This was an off set telescope for the tank commander connecting to a ballistic drive unit that connected to a range drive unit and gunners periscope This simple fire control was replaced by a Phase IV fire control as more components were produced by industry. This had a stereoscopic range finder ballistic computer ballistic drive and gunner’s periscope with a straight tube auxiliary telescope There were various marks of each instrument series until they were given model numbers. Hull design changed as a result of testing of the T48 at Fort Knox KY, Aberdeen MD and Chrysler proving grounds Chelsea MI. Some of the changes were the small drivers hatch and linkage operated driver’s periscopes that had to be lowered before the hatch could be opened left side, being replaced by a larger hatch and periscopes that remained in place during hatch opening and closing right side. It was found that with the two gasoline heaters operating the short exhaust vents mounted allowed heater exhaust to enter the crew compartment. The change there was to make the tubes longer and directed them to the sponson box area on the left side. Further testing revealed other shortcomings in the suspension and hull configuration. To reduce the chance of throwing track a track tension idler was placed between the last road wheel and the sprocket and hub assembly. At the engine exhaust area it was found the exhaust was directed straight at the gun travel lock and gun tube. This caused a heat hazard unless asbestos gloves were used and the heat also “froze” the lock with the gun in travel lock. Deflectors were designed to aim the exhaust away from the travel lock and the gun tube. The original travel lock was short and a longer one soon appeared to lift the gun tube out of the heated area. As need increased and production of the M48 started to ramp up and testing continued it was found that casting of the one piece hull was limited by available facilities. A design study developed a hull that could be made from seven pieces and welded together and still provide the ballistic integrity required. This permitted hull availability and the tank to be being manufactured by Ford Motor Company (Livonia, MI) General Motors Corporation (Grand Blanc Tank Arsenal) and Chrysler Corporation (Chrysler Newark Delaware Tank Plant). (A little Tank Trivia: Chrysler made ALL their M48’s at Newark Delaware. No M48’s were ever made at Detroit Tank plant!) To avoid confusion on the part of the user it was decided that early vehicles with the small hatch and the Chrysler cupola were designated M48 (this included a number of mild steel “Non Ballistic” hulls and turrets that were designated M48C For “Training Use Only.”) At Camp Lejeune you could find, in the mid to late’50’s, T48’s (APGT48), M48’s and M48C’s at 2d Tanks. You also could find at 8th Tanks, Force Troops, 54 brand new M48A1’s Chrysler manufactured (of which 50% were in open storage in the tank park). These were equipped with full up completed fire controls system with a stereoscopic rangefinder (T46 or M13), ballistic computer (T30 or M13), ballistic drive (T24 or M5) with elevation quadrant (M13), gunners sight (M20) and a straight tube telescope (T156 or M97) as an auxiliary sight. They also had the production version of Aircraft Armaments Industries Model 30 commanders cupola that was designated “Cupola M1”. This cupola gave the commander the ability to service his 0.50 cal under armor and with the hatch in the upright position protected his back somewhat. During testing, this cupola had a powered ammunition drum unit mounted on the turret basket that held 500-600 rounds feeding up thru a flexible chute to the M2HB 0.50 that was mounted horizontally in the cupola. Feed was from the right side of the receiver with link and brass being “flex link chuted” from the right side out of the cupola. It was found that the feed system interfered with the access of the gunner’s area and the commander’s use of the range finder. This system was replaced by an ammunition box, attaching to the cupola edge, with 105 rounds (as packaged then per box) available for use. The box lay along the left side of the head of the commander when he was at the sight. The ammo fed forward into a flexible link chute, went forward, did a 180 degree turn to the rear, a 90 degree turn to the viewers right and up in the receiver (now mounted at an angle towards the left side so the cover could be opened somewhat) where the rounds ("DOUBLE LINK INBOARD AND FORWARD”!!) could be laid in the receiver. The charging handle, a very small wire called a “Charging Cable” was used to arm the weapon. The trick we learned early on, if your weapon fired at all, was to remember to stop with two rounds hanging before the end of the chute so you could juggle a 105 round box with one hand while pushing a round through the double link of the fresh ammo and the single link tail hanging from the flexible link chute with the other hand. Simple, right? As you can see I’m a great fan of this cupola and it didn’t get any better. Many suggestions were submitted regarding this cupola (some not printable) and efforts were made to alleviate the situations but NONE proved satisfactory, the worst (in this writers mind) was the 50 round ammo supply box and the continued use of the small cable charger. When 8th Tanks was stood down the tanks (including all the storage ones) were taken up by 2d Tanks, which had become the Force Troops unit. It might be of interest to know that the Marine Corps pushed for flame tank development right along with the T48-M48. The T67 was a Chemical Corps Flame Unit M7-6 mounted in a T48 modified structure. This had a reduced crew of three (no loader) and mounted, beside the flame unit, a .30 caliber machine gun and a 0.50 caliber M2HB in a Chrysler cupola. In 1955 the vehicle was designated the M67 (on modified M48A1 structures). These units saw substantial service in the Marine Corps in the mid 1950’s and early 1960’s. (The flame tank developed from the below mentioned M48A2 was the designated M67A1 which saw no service in the Marine Corps). Continued testing of the M48 series vehicle showed that limited operational range had to be addressed. Improved engines and transmissions were tested at the proving grounds and the user. The Army accepted a fuel injected gasoline engine AVI-1790-8 and a CD-850-5 transmission along with improved fire control Instruments and Cadillac Gage turret power controls, after much testing as the M48A2. Improved suspension components included an upgraded front idler mount and arm along with added snubbers and reduction of the support rollers from five to three. M48A2’s were not used by the Marine Corps, as they did not give that large an improvement for the Corps investment. The testing was followed but the Corps was also following the developments of the Army’s tank upgrade program. The Corps finally looked at the M48A3.

Chapter 3

           Though the Corps passed on the M48A2 it did realize that the shortcomings of the M48 series had to be addressed. It had monitored the experimental vehicles and different up-grade programs ongoing and was looking for the    best bang for the buck as it has always done. It appears that the feelings were that nothing offered was that much better over what the Corps had in hand. The Army was considering an upgrade to the M48A2 of a diesel power package and a 105mm gun. One major shortcoming was the M1 cupola; it would not permit the installation of the upcoming night vision equipment in development, was cramped, had limited vision, and a small ammunition supply. A new tank program, XM60, was addressing that problem with a new cupola, the M19 and a new hull configuration. This program initially used an M48A2 turret with a 105mm gun installed, modified to accept the new M19 cupola using an adapter ring. The cupola used a newly developed short receiver .50 caliber machine gun (M85). The Corps however looked at the stock of 90mm gun tubes and the 90mm ammunition stocks, including the new T300 HEAT round (MV4000 fps), and felt the upgrade program was best at the time but not with the 105mm or the new M19 cupola and the newly developed and scarce M85 machine gun. Modification and upgrade of the M48A1’s (approximately 420-425 initially) for the Corps was to be done at two Army Depots, Red River, Texas and Anniston, Alabama. The tank went through a number of designations but was fielded as M48A3 Patton tank early on in 1963.

The Corps sent enlisted and officer maintenance personnel to the New Vehicle School at Detroit, Michigan, located at the site of the Chrysler Tank Plant which was producing M60’s and M60A1’s. There were three phases to this training, automotive, turret, and fire control. This schooling showed that the M48A3 was a vast improvement over the M48A1. The automotive improvements were the AVDS-1790-2A engine. One note here, the -2A engine was called the “smokeless” engine. Having operated an XM60 with a –2A engine installed, I can truthfully say that “smokeless” part was true, a –2A engine at high RPM’s emitted a LARGE BLACK CLOUD of exhaust smoke that disclosed positions and made crewmen sick from the fumes. An improved CD-850-6A transmission, improved drivers controls, large aluminum fuel tanks (385 gallons, vs. 200 A1) gave a longer operating range (300 miles vs. 70/90 miles M48A1). This capacity was made possible because the auxiliary generator could be deleted since the diesel engine at idle supplied sufficient power to maintain the six (6) batteries. The upgraded suspension included removal of any track tension idlers, improved compensating idlers, road wheel arms and housings. One particular change was the addition of an access port so the anchored end of a torsion bar could have a drift pin and sledge used to assist in removal when required. Major rework included new top deck armor and rear grill doors that improved power pack cooling and reduced the infrared signature from anti-tank missile seeker observation. Some of the M48A3’s were received with only three support rollers and two removed with armored plates bolted to the hull. This practice was stopped and parts were received to install the two support rollers. An M60 type diesel heater was installed along with a two shot CO2 fire extinguishing system. New and upgraded wiring and routing of linkages away from the hull floor and into the “V” of the hull increased protection from mine damage. Fender mounted, side loading, removable filter air cleaners were installed to supply the vast air requirements of the diesel engine and making it easy to service them. Engine air could be through the turret (for fording or dusty area’s) or from the engine compartment (for cold weather operations) depending on how the air filter access plates were installed. The hull and turret were machined in the race ring area for the installation of a fording seal. Mounted to the hull, this seal was inflated by a hand pump in the driver’s area. This made installation of the fording kit much easier (remember that “belly band”?).

In the turret area the mounting of the Cadillac Gage constant displacement hydraulic system was a vast improvement over the Oil-gear system with it’s LOUD electric motor used in the earlier series M48’s. A large hydraulic accumulator permitted some traverse and/or power elevation without the electric motor running. The super elevation actuator, in the system, kept the reticules (gunners sight and TC’S range finder) on the aiming point as the tank commander introduced range. The fire control improvements included a “split image” coincidence range finder (M17 series) (NO MORE FLYING GEESE!!), a new ballistic drive (M10B1 series) that include a “temperature compensating link” that maintained the correct parallelograms in the ballistic drive as the turret expanded and contracted from solar radiation. This insured that bore sight and zeroing inputs were retained and not compromised. The M31 eight power daylight periscope and M44 infinity sight were provided for the gunner in a new mount. An articulated telescope (M105) mounted to the gun mount had the eyepiece at head height for the gunner. This replaced the straight tube telescope of earlier tanks and was welcomed by all. The ballistic computer had updated cams and designations installed. All fire control items were presented in metrics and bore sight was at 1200 meters rather than 1500 yards as in the past. The Corps retained the .30 cal machine gun (M1919A4/M37) and rejected the new design 7.62mm M73. Someone had good sense in this! Searchlight mounting points were installed on the gun shield along with a new waterproof gun shield and storage points on the ventilator bulge. Electrical wiring changes included a permanent searchlight cable connector on the turret roof. At first the 18” searchlight was modified mount-wise and cable connector-wise to fit the mounts and connector. These units were old and worn but did the job while new equipment was being developed and procured. As new AN/VSS-1, 2.2 KW xenon/infrared searchlights became available the 18-inch lights were replaced. All M48A3 vehicles came with the installed NBC M13A1 gas particulate units in the hull and turret with hoses routed and secured at all crew stations.

Along with the procurement of the M48A3 vehicle was the development of the M67A2 flame tank at the same time. The Corps M48A1 fleet had M67 flame tanks based on the M48A1 structure, The M67A1 was a flame tank used by the US Army based on the M48A2 vehicle. Thus, when the Corps requested conversion along M48A3 lines the result was M67A2. However, the Corps again turned down the troubled M73 machine gun and retained the reliable Browning M1919A4/M37 0.30 caliber. The Corps took its M48A3’s and it’s M67A2’s into Viet Nam and Dominican Republic combat with many very happy Marine tankers glad not to be using gas fueled M48A1’s or M48A2’s. However, complaints and combat usage data became available from the tanks use in Viet Nam. Units sent complaints to headquarters both system wise and some backdoor efforts. It is to be noted that the M1cupola was high on the list and was never to satisfy any one. Units mounted the .50 caliber machine gun on top, on the side of the cupola, the turret roof and fired from exposed positions. Various attempts were made but NONE ever solved the space, poor vision, constricted mounting of the M2, and a proper ammunition supply. One attempt was to install the vision ring (“Doughnut Ring”) between the cupola and turret structure. This gave 360 degrees of vision and some space improvement, particularly if the new extended (bulged) commanders hatch was installed (not all were done) but it still didn’t solve the gun problem. Various “modified” chutes for the links were tried (longer, shorter, lined rigid and flexible) links still jammed. Front mounting pins were modified to permit easier mounting and dismounting of the gun. The available ammunition supply was never solved to satisfy anyone. The attempt to mount the M19 cupola was terminated, as it was determined that the loaders hatch was blocked when the cupola was rotated to the right preventing the hatch from being opened.

There were changes with the “Mod B” program. These included the vision ring with hatch cover, modification of the rear grill door area with protection of the grills to prevent them being bent in the bush and reducing the cooling of the power pack. The taillights were also protected by added armor. The tank infantry phone box was raised and moved away from the right rear fender area to prevent damage when fenders were raised or damaged by the jungle during trail busting. A gunners M32 daylight and infrared sights replaced the M31 day sights to be used with the new AN/VSS-1 day/infrared searchlights. Most Mod B’s were completed by Bowen-McLaughlin-York (BMY) at York, PA. Though still with , the M48A3 gave reliable service and powerful and accurate firepower and holds a special place in the Marine Tankers history with the Patton series of vehicles.

M51A1 Ontos Antitank

The ONTOS Anti-Tank Vehicle

by Peter Brush

By any measure, the ONTOS was one of the most interesting “things” to come down the road of United States military armored development. The idea for this vehicle was born in the aftermath of World War II when the U.S. Army perceived the need for a new reconnaissance vehicle. Then it evolved into a tank destroyer for use with the Army on the nuclear battlefields of Europe. Next it was deployed in Marine Corps anti-tank battalions. The ONTOS most significant contribution was in the Vietnam War, but in a role much different from the role for which it was designed. This is the story of the ONTOS, officially the “Rifle, Multiple, 106mm, Self-Propelled M50 of the anti-tank company, infantry regiment, Marine Division.”

The adaption of the internal combustion engine to warfare brought about the removal of the horse from the battlefield. The reconnaissance mission formerly performed by cavalry remained. By the end of World War II, the motorcycle, jeep, armored car, and light tank all tried to fill the gap, all without complete success. A classified 1953 U.S. Army report noted:

There is an urgent and immediate need in our army for a vehicle similar in performance to the jeep, but at the same time affording some armored protection and greater cross-country mobility, for use by reconnaissance personnel, commanders, messengers, and liaison officers who are frequently exposed to small arms fire.”

At that time jeeps and half-tracks were authorized in the command, scout, and support elements of the Army reconnaissance platoon. The ONTOS was considered as a replacement. After considerable study the Army concluded that although the vehicle had outstanding cross-country mobility and armor protection, it had deficiencies in the areas of storage space, lack of speed, lack of range, and excess weight. Ironically, given the vulnerability of the M50 to enemy mines in Vietnam, the Army concluded these test vehicles “offered protection against atomic bombing.” The Army decided to stick with its M38A1 half-ton trucks and M21 mortar carriers for reconnaissance platoon use.

During World War II, the Army embraced the tank destroyer concept, which called for the placement of large-caliber anti-tank guns on lightly armored carriages. These could quickly be moved to any area under enemy tank threat. This concept was never embraced by the Marine Corps to any extent. The tank remained the favored anti-tank weapon for the Marines in the immediate postwar period. In addition to duties as naval infantry, postwar planners envisioned a role for the Corps in any European conflict between the United States and the Soviet Union. Late 1940s war planning put the Marines into direct conflict with front-line units of the Red Army. In the Pacific War the Marines dealt with sporadic attacks by small Japanese tanks. In the future war Marine tankers would have to face a highly mechanized Soviet force equipped with large numbers of medium and heavy tanks.

Using tanks to destroy enemy tanks proved less than satisfactory in the Korean War: too often the weight of American medium tanks rendered them too road bound. Both Army and Marine planners, cognizant of the formidable threat posed by Communist armor, returned to the World War II tank destroyer concept.

In 1949 the USMC Armor Policy Board specifically noted “There is a requirement for a destroyer-type tank to destroy hostile heavily armored vehicles….”

As early as 1944, Army production and logistics considerations began to determine Marine Corps tank decisions. Although some of the USMC armor requirement was developed and produced by the Navy’s Bureau of Ships (e.g., amphibious tractor or amtracs), the Corps came to fully depend on the Army for its tank procurement. In 1957, Marine Corps planners and engineers from the Allis-Chalmers Corporation began development of this new anti-tank vehicle. It would be built at the company’s La Porte, IN, factory.

In 1953, Michigan Congressman Gerald R Ford held congressional hearing for Army appropriations. When discussion turned to anti-tank capabilities, the testimony of Army generals was taken off the record and not included in the printed transcript. The public became aware of ONTOS development only by mistake. According to a report in the New York Times dated June 26, 1953, the congressional testimony was classified “Secret”. The newspaper noted “An entirely new weapons-carrying vehicle, nicknamed ‘The Thing’ but carrying the official designation ONTOS, to be used variously, including as a mount for a new ‘highly-powered’ recoilless rifle and for a quadruple .50 calibre antiaircraft weapon against low flying planes.’ Army officials expressed amazement and appeared appalled when copies of the 1,667-page printed testimony released by the subcommittee reached the Pentagon.

The original M-50 ONTOS emphasized firepower over crew comfort. The hull was derived from the T55/T56 series of tracked armored personnel carriers. It was powered by a six cylinder in-line gasoline engine, the General Motors SL 12340, which developed 145 horsepower at 3,400 rpm (this was later upgraded to a Chrysler V-8). This power source was coupled to a XT-90-2 transmission, which drove the front sprockets, which turned the tracks. Maximum road speed was 30 mph on improved roads. The ONTOS had terrain navigation ability superior to tanks. Range was 190 miles on primary roads, 120 miles on secondary roads, and 50 miles cross country with a 47 gallon internal fuel tank. With fording kit installed the vehicle could cross streams as deep as 60 inches. This three-man vehicle weighed nine tons. It was not portable by an available helicopter although it could be air transported by R4Q aircraft. Two ONTOS could be landed over the beach in a LCM-6 (Landing Craft, Mechanized).

The main weapon consisted of six 106mm M40A1C recoilless rifles mounted on a central turret that overhung the hull on both sides. Built with simplicity in mind, this rifle was the same weapon used by infantry on a fixed mount. These guns could be fired individually, in pairs, or all at once. Fifty caliber spotting rifles were mounted on four of the recoilless rifles. Two of the recoilless rifles were equipped with a spotting rifle and sigh and could be removed from the vehicle for use on ground mounts (these spotting rifles could not be fired from inside). The ONTOS also had a .30 caliber machine gun. These weapons were externally and coaxially mounted and were fired electrically. Rate of fire was four aimed rounds per minute with all guns loaded. Average reload time was one minute.

The trajectories of the spotting rounds and the 106mm rounds were very similar to a distance of 1,100 yards. Beyond 1,100 yards the trajectories differed, causing the effectiveness of this spotting system to decrease as range increased. The spotting rifle could not be used beyond 1,500 yards, necessitating the use of burst-on- target and bracket techniques of fire adjustment at these greater ranges. High Explosive Anti-Tank (HEAT) ammunition for the 106mm rifle would penetrate the armor of any known tank (16” of armor at 0 degrees obliquity). The ONTOS’ armor was one-half inch plate except for the floor, which was only 3/16” inch thick. The upper sloped armor would withstand all small arms fire, but was vulnerable to .50 caliber armor piercing rounds. Artillery airbursts could cause severe damage to the ONTOS’ guns and external fire control equipment.

Frank Pace, Jr., Secretary of the Army during the Truman administration, initially supported the M50 for Army use. Pace noted, “If ONTOS is there, tanks had better get the hell off the battlefield.” [8] Not everyone agreed with Pace. Others felt it was too lightly armored, underpowered, and incapable of sustained combat. The Marine Corps accepted the ONTOS after the Army rejected it. The Marines did not have the specialized supply and maintenance capabilities of the Army, and the ONTOS was a simple vehicle. It had fewer parts than other armored vehicles. There was no heavy turret. The engine was a common truck engine found on various military and civilian vehicles. The fire control system was simple: according to LtCol. E.L. Bale, Jr., a Marine instructor at the Army Armor School, the average Marine could master the system “as easily as he has the pinball machine in the local drug store.” The Corps ordered 13 million dollars worth, about 300 vehicles. Production was to run for about one year beginning in mid-1956.

The ONTOS was quickly integrated into armored units of the Fleet Marine Force. Its first non-trailing deployment abroad came in July 1958. The Lebanon Crisis saw Marine Battalion Landing Teams (BLT) of the Navy’s Sixth Fleet come ashore to stabilize the weak Lebanese national government. This 2nd Provisional Marine Force included 15 M48 tanks, 10 ONTOS, and 31 LVTP5 amtracks (Landing Vehicle, Tracked). These vehicles provided general Force security and protection for armored patrols until a larger Army tank force could be sent from Germany. The Marines began reembarkation in mid-Aug. Marines got their chance to destroy tanks with ONTOS in 1965 in the Caribbean. In April the Dominican Republic was sliding into civil war as reformers did battle with right-wing military forces. The 6th Marine Expeditionary Unit, the Caribbean quick-reaction force, was ordered to ashore in order to evacuate civilians and reinforce security at the American embassy. A full company each of tanks, amtracs (LVTs), and ONTOS were part of the landing team. The U.S. force took the side of the Dominican military. The rebel army possessed less than formidable armor capabilities. Before leaving in June, M50 HEAT rounds scored hits on an old Swedish Landsverk L-60 light tank and a French AMX-13 light tank. But it was halfway around the world in Vietnam where the ONTOS mettle would be truly tested.

ONTOS were organized in the Marine Division into Anti-Tank Battalions. Each battalion was composed of one headquarters and service company plus three anti-tank companies. Each of these latter companies contained three platoons of five ONTOS for a total of 45 ONTOS vehicles per battalion. Planned distribution in the Marine Division was for 15 ONTOS (three platoons) for each of the division’s three infantry regiments. ONTOS companies, along with tanks and amtracs, landed with the 9th Marine Expeditionary Brigade at Da Nang, Vietnam, in the first half of 1965. Within one year, both the 1st and 3rd Anti-Tank Battalions were ashore in Vietnam. Unlike the enemy in the Korean War, the Vietnamese Communist military forces possessed significant anti-armor capabilities in the form of recoilless rifles and rock propelled grenades. The ONTOS’ thin floor armor (3/16”) made it especially vulnerable to mines. Consequently, and as opposed to its designed role, ONTOS spent a great portion of their time in static defense positions.

Initially ONTOS units were deployed in defense of the Da Nang airfield. In August, 1965, the Marines began Operation Starlite, the first big battle of the war. At 0730 on August 17, tanks and ONTOS rolled off amphibious landing craft (LCUs and LCMs) and made their way ashore south of Chu Lai in support of the assault companies. Later in the day a Marine armored column was halted when a M-48 tank was hit with recoilless rifle fire. The Viet Cong (VC) poured mortar and small arms fire into the Marine positions, quickly killing five and wounding 17. The ONTOS maneuvered to provide frontal fire and flank protection until enemy fire let up. The following month, in Operation Golden Fleece, a combined infantry-armor assault force including ONTOS attacked a VC main force unit trying to collect a rice tax in a Vietnamese village near Da Nang. The enemy was forced to break contact and flee the area.

After establishing themselves at Da Nang and Chu Lai, the Marines built their third base at Phu Bai, in Thua Thien Province 35 miles northwest of Da Nang. Initially, defense of Phu Bai was provided by the 2nd Battalion, 1st Marines (Reinforced) which had a platoon of M50s attached. It was not only the Marines who were expanding their forces in the northern part of South Vietnam: both the VC and North Vietnamese Army (NVA) also increased their forces, and both sides sustained heavy casualties. In late June, on Operation Jay, a large, heavily armed VC force ambushed a South Vietnamese Marine Corps convoy moving north on Route 1, the main north-south highway in Vietnam. At 0830 hours on June 29, the attacking force struck the convoy with mortar and recoilless rifle fire, quickly hitting ten trucks. U.S. Marines quickly sent reinforcements, including ONTOS, to assist the SVN Marines. The VC force lost interest and tried to break contact. While crossing open ground, the M50 platoon opened fire and “obliterated a VC squad on a ridgeline with a single 106mm salvo.” An M50 platoon commander even captured an enemy soldier. Over 185 enemy soldiers were killed in this action.

Marines and their armor were deployed in I Corps, the northernmost of four military districts in Vietnam. An exception to this was Special Landing Force (SLF) of the Navy’s Seventh Fleet, the strategic reserve for the Pacific Far East. The SLF was available for amphibious landings in South Vietnam. Gen. William Westmoreland, commander of U.S. forces in Vietnam, decided to use the SLF to clear Viet Cong forces from the Rung Sat Special Zone south of Saigon. VC gunners were firing on ships using the river channel that supplied the Vietnamese capital. The result was Operation Jackstay, March 26-April 6, 1966. The operation had limited success but not due to lack of ingenuity of the Marines, who experimented with riverine warfare techniques including mounting an ONTOS on a LCM to provide fire support. Only 63 enemy were killed; however, the shipping channel was at least temporarily clear.

The following incident illustrates the vulnerability of the M50 to enemy mines. It was spring, 1966. An armored column supporting Company “K”, 3/9 was returning to base camp near Da Nang. Three tanks and an ONTOS went over a stream at a place called Viem Dong Crossing without mishap. As the second M50 crossed, Platoon Commander (Lt) Allen Hoof heard a “pop”, turned rearward, and saw the upper half of the vehicle blown off the lower half, and lying upside down next to it. All three crewmen were wounded. Acting ONTOS Commander PFC Greg Weaver was quickly removed from the vehicle but died almost immediately. The mine explosion, perhaps either command detonated or activated by a counter, caused the detonation of a 106mm round stored directly under the commander. This secondary explosion blew the turret off the vehicle and killed Weaver.

Since enemy tanks were not a problem for Marines in Vietnam, ONTOS use reverted to its secondary mission: providing direct fire support for infantry. By late 1966 problems with ONTOS became evident. The supply of tracks was depleted, which caused breakdowns on operations. This caused a reluctance to utilize the M50. An even more important reason was several incidents of accidental firings of recoilless rifles which cost some Marine lives. This was an extremely serious problem for ONTOS on convoy duty. These accidents were caused by overly tight adjustment of the firing cable allowing the firing pin to release prematurely. This adjustment was a crew responsibility and required thorough understanding of the firing cable, sear, and trigger. These mishaps caused restrictions to be placed on ONTOS’ use.

By 1967 the Marines were fighting two wars in Vietnam. The 1st Marine Division engaged in counter guerrilla operations in the southern part of I Corps while the 3rd Marine Division conducted mostly conventional war against NVA along the Demilitarized Zone (DMZ) in the north. As the Marines moved northward to counter the NVA threat, ONTOS and tanks provided important support. In May 1967, the 2nd Battalion, 9th Marines (2/9) and 2nd Battalion, 26th Marines (2/26) began Operation Hickory north of Con Thien. Fighting against enemy forces in well prepared bunkers and trenches was heavy. M50s, using the proper ammunition, proved to be devastating antipersonnel weapons. After the conclusion of Hickory, 2/9, accompanied by tanks and ONTOS, was sent on a spoiling attack into the DMZ. On this operation the tracked vehicles proved more of a liability than a tactical asset as the terrain restricted them to the road. Instead of providing infantry support, the M50s and tanks required infantry protection against NVA rocket propelled grenade (RPG) attack. Using these vehicles as ambulances to evacuate the wounded further reduced their offensive capabilities.

1967 saw the introduction of CH-53 Sea Stallion heavy-lift helicopters for the Marines in Vietnam. The first models had a six-ton external lift capability. This meant an ONTOS could be transported by helicopter if it was broken down into components with the hull transported externally. It could then be reassembled and operated at destination, giving it a transportability beyond its design considerations. M50s could also go where tanks feared to tread (or should have): in a 1966 operation, tanks got stuck in flooded rice paddies. ONTOS, with less ground pressure, were able to drag timbers up to the tanks without bogging down. In Operation Jay, mentioned above, the ONTOS of B Company, 3rd Anti-Tanks were able to assist the SVN Marines because they could cross a pontoon bridge - - the only tracked vehicles light enough to drive to the operation. ONTOS could go more places than many people thought possible.

In December 1967, the 1st and 3rd Anti-Tank Battalions were de-commissioned in Vietnam and their vehicles were attached to the tank battalions. 1968 saw ONTOS assume an important role in some of the heaviest fighting of the entire war, the Battle of Hue during the Tet Offensive. In February, 14 NVA battalions seized control of most of the city. The Americans and South Vietnamese faced the formidable task of retaking this important cultural center of the nation. The result was urban fighting unlike anything seen in the war. The attacking Marines had to take each building and each block one at a time. This close-quarter combat and low flying clouds, coupled with the desire to minimize damage to the city itself, meant there could be little reliance on artillery and close air support.

Four tanks from the 3rd Tank Battalion along with a platoon of ONTOS from the Anti-Tank Company, 1st Tank Battalion, joined the advance against strong enemy resistance. LtCol Ernest Cheatham, commander of 2/5, had reservations about using tanks. One tank sustained over 120 hits and another went through five or six crews. Infantry commanders liked the ONTOS better. Cheatham described the M50 “as big a help as any item of gear we had that was not organic” to the battalion. Regimental commander Col. Stanley Hughes went even further when he claimed the ONTOS was the most effective of all the supporting arms the Marines had at their disposal. Its mobility made up for its lack of armor protection, noting that at ranges of 300 to 500 yards, its recoilless rifles routinely opened “4 square meter holes or completely knock[ed] out an exterior wall.” The armor plating of the M50 was sufficient protection against enemy small arms fire and grenades. However, B-40 ant-tank rockets were another story: an ONTOS (with) 1/1 was knocked out and the driver killed on February 7 while supporting the 1st Battalion, 1st Marines. The essential role of tanks and M50s in the fighting illustrated by the fact that Marines had to hold up their advance from time to time for lack of 90mm tank and 106mm ONTOS ammunition.

The Perfume River flows through Hue. After clearing the south bank on February 11, the 1st Battalion, 5th Marines pushed north to clear NVA forces firmly entrenched in the 4-square-mile Citadel, location of the former Imperial Palace. USMC M-48 tanks and ONTOS were placed under the command of the attached tank platoon commander. Tactically, the tank or ONTOS commander, working with the infantry commander, would reconnoiter a particular target area, usually a masonry structure blocking the Marine advance. Returning to their vehicle, the tank or ONTOS commander return to his vehicle, then move forward at full speed while the infantry laid down a heavy volume of fire. Upon reaching a position where fire could be directed on the target, the vehicle commander halted the vehicle, fired two or three rounds into the structure, then quickly reversed direction and returned to friendly front lines. Casualties among armor crews were high. On February 24, South Vietnamese troops finally dislodged NVA forces from the Citadel. By the time the battalion for Hue was over, 50 percent of the city was destroyed.

Before, during, and after the Battle of Hue, the 26th Marine Regiment was fighting the North Vietnamese at Khe Sanh. Here the enemy tank threat was real: 17 days into the battle at Khe Sanh, NVA tanks helped overrun the nearby Special Forces camp at Lang Vei. Ten M50s from the 3rd Tank Battalion were incorporated into Khe Sanh’s defenses. They were sometimes used for reconnaissance but more often in static perimeter defense roles. Author Robert Pisor notes the ONTOS at Khe Sanh had “enough flechette [anti-personnel] ammunition to pin the entire North Vietnamese Army to the face of Co Roc Mountain.”

The Marine Corps began to deploy its forces out of Vietnam in 1969. Tank and amtrac units rotated early as fighting had ebbed in the Corps’ area of responsibility. By this time the M50 parts supply was depleted and the 106mm rifle was about to be replaced by other weapons. M50 mechanics cannibalized disabled machines to keep others running, but after Hue the ONTOS were worn out. Ironically, excess ONTOS were given to Army forces (recall that the Army initially rejected the ONTOS as being unsuitable for its requirements). These Army ONTOS were formed into Company D, 16th Armor, for use with the 173rd Airborne Brigade. The Army used its ONTOS until they ran out of spare parts, then employed them in fixed bunkers. In the United States, the Marine 2nd Anti-Tank Battalion was disbanded along with the 5th Marine Division. The last ONTOS garrison was stationed at Guantanamo Bay, Cuba. It continued to operate until 1980 by which time it had one operation vehicle. Two others were used for parts.

Upon return to the United States, the tops of the vehicles were removed. Many of the chassis were sold for use as construction equipment or give to local governments for rescue work. One “platoon” of surplus M50s wound up in the service of the North Carolina Forestry Service for use as fire fighting vehicles. According to Vietnam veteran and former Marine Mike Scudder, ONTOS today are scarce. In fact, there are more surviving World War I tanks than ONTOS. Scudder should know: he bought the seven from North Carolina and is restoring two of them.

Was the ONTOS a successful addition to the Marine Corps arsenal? The answer is quite simply, yes and no. The primary mission of the M50 was a tank destroyer. In the actual tactical environments in which it was deployed, there was little use for this ability.

Its secondary mission was the provision of direct fire support for the infantry. In this role the ONTOS was underutilized. The reason, according to Major D.C. Satcher writing in the Marine Corps Gazette, is because, unlike artillery, air, and tanks, ONTOS were little emphasized in Marine officer training. ONTOS were never used in any tactics problems in The Basic School. ONTOS crew did not have their own MOS. An ONTOS officer normally served one tour with an M50 unit, then moved on. A weapons system that is under-emphasized will be underutilized.

Rejected in the beginning by the Army as a reconnaissance vehicle, it was used in this role by the Marines in Vietnam. The M50 could go places no other Marine armored vehicle could go. It had limitations. In addition to the problems previously noted (premature firing and vulnerability to mining), the recoilless rifles had to be loaded externally which meant the crew had to leave the protection of an armored hull in order to reload. The 106mm recoilless rifle is no stealth weapon: when fired, the tremendous back blast makes the ONTOS’ location visible to the enemy. ONTOS crew had to ensure no friendly troops were in the large back blast area when operating in confined areas.

There was no enemy armor for the ONTOS to destroy in Vietnam. Still, Marines are famous for their ability to improvise, and the enemy infantry were plentiful. The M50 was a formidable anti-personnel weapon. A couple of ONTOS on the perimeter could decimate Communist forces attack on Marine fixed positions, a static role quite the opposite of its designed high-mobility anti-armor role. My favorite example of Marines ability to adapt to local tactical conditions is the main streets of Hue City in February 1968. Not only good at destroying structures, ONTOS were able to provide a “smoke screen” for infantry attacks: when white phosphorus rounds were unavailable, “beehive” rounds (explosive shells filled with thousands of small darts) fired into masonry structures created a dust cloud that screened infantry movement. Marine infantry loved their ONTOS. In Phase Line Green: The Battle for Hue, 1968, author Nicholas Warr describes how the M50 platoon pounded the enemy positions, accompanied to the choruses of “Get some!” sung by infantry holed up in houses, waiting to move forward. Fact is, even with its limitations, the ONTOS was used and, to a considerable degree, used up in Vietnam, providing invaluable support for the Marines in I Corps.

Peter Brush

While the M50 was initially designed for the US Army as a tank destroyer and later obtained by the Marine Corps, in Vietnam the enemy deployed no tanks or even other moving targets against the Marines so it was more widely used for direct fire support for the infantry in combat, though that role that was seldom emphasized in training or doctrine. Its light armor was effective against small arms up to .30 cal but vulnerable to mines and rocket-propelled grenades. Ontos were deployed in static defense positions and convoy escorts.

The Ontos was particularly liked by its crews, and praised by the supported infantry unit commanders. Their relatively light weight and size meant that the M50’ss could also go where tanks got bogged down in the field and restricted in urban operations. The Ontos, with its lower ground pressure, could drag timbers up to the tanks to help get them unstuck. In other operations, the Ontos was the only tracked vehicle light enough to cross a pontoon bridge.

In the Battle of Hue City, the Regimental Commander, Colonel Stanley Hughes, felt the Ontos was the most effective of all (available) Marine supporting arms. Its mobility made it less vulnerable than tanks while at ranges of 300 to 500 yards (270 to 460 m), its recoilless rifles could knock holes in, or completely knock down, walls allowing assaulting Marine infantry to move from building to building without exposing themselves to the open street. The appearance of an Ontos was sometimes enough to make the enemy break and run. Anecdotal accounts given describe the enemy evacuating occupied buildings when an Ontos' .50 cal. spotting round entered a window. In Operation De Soto, the introduction of the large CH-53 Sea Stallion helicopter made possible moving a platoon 25 miles (40 km) south of Quan Ngai City carrying Ontos in slings underneath the aircraft.

The Ontos units were deactivated in May 1969, and some of the vehicles were handed over to an army light infantry brigade. They used them until they ran out of spare parts, and then removed the turrets and used them as fixed fortifications. Both these and the rest of the vehicles returned from Vietnam in 1970 and were cut up for scrap, with some of the chassis being sold off as construction vehicles. Allegedly the “bone yard” at the MCSD, Barstow, CA contains “many” Ontos.