As a first step towards developing a new medium tank, the US authorities decided to improve the existing M26 Pershing design- They substituted an air-cooled 800hp Continental AV-1790 V-12 engine and a beefed-up version of the Allison cross-drive transmission and steering system for the Pershing’s Ford GAF and Torquematic transmission. The improved tank also received a new model of the M3 gun with a fume extractor (bore evacuator) and muzzle brake. Although these improvements probably did not warrant a change of designation, the upgraded tanks were reissued as the M46 ‘General Patton’, in honour of George ‘Blood and Guts’ Patton, who had died of heart failure just before Christmas 1945.
The early postwar American monopoly on the atomic bomb led its senior political and military leadership to believe the threat of another large-scale ground war had greatly receded. In line with this, funding devoted to the development of the next generation of ground weapons for the United States armed forces, such as tanks, was slashed. With no money to replace the Second World War era M26 series tanks, the U. S. Army decided in January 1948 as an interim measure to modernize its inventory of roughly 2,200 M26 series tanks.
The biggest design shortcoming of the M26 series tanks had always been the relatively low power of their liquid-cooled Ford GAF gasoline engines that produced only 500 horsepower. Fortunately, the U. S. Army had initiated the development of the “ideal” tank engine in July 1943. What eventually sprung forth from this line of development was the Continental Motors Corporation AV-1790-1 gasoline engine, which boasted 740 gross horsepower. It was a 12cylinder, V-type, four-cycle, air-cooled engine.
In 1946, one of the first three examples of the new, more powerful Continental AV- 1790-1 engine was installed in a modified M26 series tank that was then designated as the M26E2 tank. Coupled to the new engine was a newly-designed Allison Corporation CD- 850-1 cross-drive transmission, which transmitted the power generated by the engine to the tank’s final drives and drive sprockets. The CD-850-1 was referred to as a cross-drive transmission because of its transverse mounting in the rear hull engine compartment of the M26E2 tank.
After testing at the Detroit Tank Arsenal, the single M26E2 tank was shipped to Aberdeen Proving Ground, Maryland, home of the U. S. Army’s Ordnance Branch. Positive results with the M26E2 tank led to the authorization of ten more to be designated as the T40 medium tanks. The T40 tanks had a new power pack arrangement consisting of an upgraded Continental AV-1790-1 engine designated as the AV-1790-3, which boasted 810 gross horsepower. It was coupled to an improved version of the original Allison Corporation CD850-1 cross-drive transmission, designated CD-850-5.
The new power pack arrangement required some design changes to the top and rear of the T40 tank hull. The most noticeable external changes on the T40 tank were the engine exhaust pipes that extended out sideways from the roof of the tank’s engine compartment to mufflers mounted on each of the vehicle’s rear fenders. There were also three square armored access hatches on the lower vertical rear face of the tank’s rear hull plate for servicing of the CD-850-5 cross-drive transmission.
Another external change to the T40 tank was the addition of a small track tension idler between the dual rear road wheels and the rear hull mounted drive Sprockets on either side of the vehicle’s suspension system. This was done to prevent the tank’s tracks being shed on sharp turns or when traveling over rough terrain. Like the front compensating idler (which appeared on the entire Patton tank series) it also helped to eliminate slack in the track under dynamic conditions such as hard braking. The track tension idler was originally referred to as the compensating idler wheel.
The U. S. Army considered mounting a new, more powerful 90mm gun on the T40 tank. In the end, a decision was made to use a modified M3 90mm gun, designated the M3A1, on the T40 tank. Unlike the M3 90mm main gun, the M3A1 90mm main gun sported a bore evacuator as well as a new, lighter and smaller single baffle muzzle brake. The optical sighting system on the T40 tank was improved with the addition of a new M83 sighting telescope.
Pleased with the results of testing the T40 tank, the U. S. Army decided on 30 July 1948 to standardize the vehicle as the medium tank M46. The vehicle also received the official name the “General Patton” in honor of the late General George S. Patton of Second World War fame. Most American tankers simply referred to it as the “forty-six.”
According to the U. S. Army’s TACOM (Life Cycle Management Command) historical office, between 1948 and 1951, a total of 1,170 M26 series tanks were converted by the Detroit Tank Plant to the M46 tank configuration.
The last 360 production units of the M46 tank featured a number of modifications based on continued testing and user input from the field and were assigned the designation medium tank M46A1. The original plan calling for converting almost all 2,200 M26 tanks in the U. S. Army’s inventory into the M46 tank configuration proved impossible to complete as some of the M26 tanks had been diverted to take part in the Korean War.
The new M46 series tank differed externally from the T40s tanks that preceded them by having three circular armored access hatches on the lower vertical rear face of the vehicle’s rear hull plate, rather than the three square armored access hatches seen on its predecessor. Instead of the Continental AV1790-3 engine as fitted to the T40 tank, the M46 series tanks would receive power from progressively improved models of the same engine, the last one bearing the designation AV-1790-5B. The M46 series tanks weighed in at about 97,000lbs (44mt) combat loaded.
Like the M26 series tanks, the M46 series tanks were rushed to South Korea to help stem the North Korean onslaught, the first ones arriving in August 1950 as part of the U. S. Army’s 6th Tank Battalion. Eventually 200 M46 series tanks would be deployed to Korea. They would serve alongside roughly 300 M26 series tanks and 680 later production versions of the M4 series tanks, the majority armed with a 76mm main gun.
Of the three types of American medium tanks to see service during the Korean War, the most favored for tank-versus-tank combat were the 90mm main gun equipped M26 series tanks and the M46 series tank. Tank-versus-tank action in Korea, however, became extremely rare after November 1950.
For American tanks, the greatest cause of losses in the Korean conflict was mechanical failure, with the engines in the M26 and M46 series tanks being the biggest culprit. The next most common mechanical failure with the same tanks where their transmissions, including the clutches and gearing; with the M46 series tank transmission being the most trouble prone. Neither tank was up to the demands imposed by the mountainous and heavily-wooded Korean terrain. The biggest enemy-caused casualty producer for American tanks during the Korean conflict was antitank mines.
Like the other American medium tanks to see action during the Korean War, the main role of the M46 series tank generally became infantry support, with occasional stints as artillery pieces when poor terrain conditions or weather ruled out the use of the vehicle in its normal roles. Other roles for the M46 series tanks included bunker busting, which the U. S. Army eventually considered as being extremely uneconomical and wasteful in main gun ammunition expenditure for the minor results achieved. The British experience of bunker-busting with tanks, however, was exactly the opposite, with the British Centurion earning a great reputation for this work. There were also armored raids and reconnaissance in force operations conducted during the Korean War which achieved excellent results against their Communist opponents.
A U. S. Army research report titled “Employment of Armor in Korea: The Second Year,” issued in April 1953, concluded that the later production versions of the Sherman tanks that served during the conflict were more mechanically reliable than the M26 and M46 series tanks and easier to maintain. Some of the blame for the poor mechanically reliability for these tanks can be attributed to the general failure of the U. S. Army logistical system to push the needed spare parts to the tanks in the frontlines. Added to this problem were the lack of training and experience among the tank mechanics tasked with keeping the M46 series tanks running, as well as the poor level of training among the men assigned to the vehicle. To make matters even worse, the weight of the M46 series tanks overtaxed the M4 series tank-based armored recovery vehicles employed in Korea.
Shown is a U.S. Army M46 series tank fitted with a large infrared searchlight mounted on the right side of the gun shield. The infrared viewer is mounted on the front of the tank commander’s cupola. The codename for the installation shown was Leaflet II.
ALLISON’S DRIVE SYSTEM
The Allison CD-500/CD-850 cross-drive system, as fitted to the M41 and M46 respectively, actually contained transmission, steering and brakes all in one and was a considerable improvement on the simpler systems that had gone before. It utilised two forward speed ranges and one reverse, each hydraulically selected, and in both senses, part of the engine’s power output was transmitted via the hydraulic torque converter or fluid flywheel and the remainder through a conventional mechanical path. The power from both transmission trains was available to both output paths when the tank was proceeding in a straight line, but when the ‘wobble stick’, or joystick, was actuated, all the mechanically transmitted power was applied to just one track. The wobble stick acted on a hydraulically controlled differential – moving it to one side while the tank was moving caused more power to be applied to one track; the same action while the driving gearbox was in neutral caused the two tracks to move in opposite directions and spin the tank on its axis
(The Merritt-Brown system of World War II achieved the same end, but the Allison system was simpler to use.) Disc brakes, operated by a foot pedal, were installed in each drive shaft. Later models of the CD-850 had a split hydraulic power train instead of the hydraulic/mechanical path found in the original, but the principle remained the same.
Marines are shown installing heavy wire fencing material around the turret of a M46 tank, sometime during the Korean War. The wire screen was intended to protect the tank from the high explosive antitank (HEAT) warheads of enemy rocket launchers. Notice that the M46 tank has three circular cross-drive transmission access panels instead of the three square ones on the T40 tank.
Rolling past the photographer is a U.S. Marine Corps M46 tank fitted with an unarmored 18-inch diameter Crouse-Hinds searchlight. Originally, the American military thought about fielding highly specialized searchlight tanks, however, it was more cost-effective to buy individual searchlights for each tank and risk losing them in battle then to fund the development and production of specialized searchlight tanks.
Marines in Korea
The old M26 tanks were worn beyond repair, and between July and November of 1951 they were gradually replaced by the improved M46. The Anti-Tank Platoon did not receive new vehicles until later, around late December. In many respects the new tank was an improvement over the M26, with a more powerful V-12 engine and Hydramatic automatic transmission, a bore evacuator to reduce the backflow of gasses through the main gun breech, and other minor changes. However, there were problems with the new model.
According to the official Marine Corps history, the new tanks suffered engine problems from faulty oil cooler fans, but for the crews the new vehicles had other, terrifying, quirks. Pete Flournoy expounded on one of its more serious defects: “The forty-six had a bad flaw, particularly in that kind of terrain…. As long as you kept it revved up you had good steering. You started backing off the gas pedal going downhill, and your engine slows down, you lose your steering. You had to keep it revved. We banged up a lot of tanks over there, doing that. You couldn’t steer ‘em. You run into embankments, damn near off the road down in the valleys. Hit some truck and knock him off the road trying to stop the damn thing.”
August 9, 1952
Siberia was a squad outpost about a quarter of a mile forward of the American MLR. At 0100 hours on August 9, the Chinese precipitated a nineteen-day bloodbath when they drove the Marines off the hill. The hill changed hands twice after the initial Chinese victory, deadly fighting that took place under torrents of artillery and mortar fire. By 1130 hours the Marines were again in nominal control of Siberia, but enemy fire directed against the northern slope and crest was so heavy that the Americans were able to reoccupy only the southern slope. The seesaw action continued that afternoon, when a powerful Chinese attack inflicted heavy casualties on the defenders and once again drove E/2/1 from the hill. That night a fresh rifle company, C/1/1, recaptured the terrain, only to be driven off yet again by enemy artillery at dawn on August 10.
Following this costly reversal, the division decided to take higher, more rugged, but more desirable Bunker Hill to the southwest. Chinese artillery and mortar observation from Bunker Hill had been a deciding factor in the Siberia struggle. American control over it would neutralize this advantage and allow observation of the enemy’s routes to and from Siberia. It would also extend artillery observation as far north as the next range of hills about 3,000 yards to the northwest, and allow American artillery to dominate a broad lowland to the north and northeast.
The struggle for Siberia became a secondary part of the battle. The next assault against it was designed to serve as a diversion, and the highly visible tanks provided the major diversionary element. The Chinese force on Siberia, a reinforced platoon, was inundated with fire from artillery and tank guns, while enemy positions on Bunker Hill and other positions to the north and east were similarly bombarded.
At dusk on August 11, four M46s from Captain Gene McCain’s C Company and four M4A3 flame tanks moved toward Siberia to support the night assault. The M46s opened fire on the hill at 2100 hours. Two of the flame tanks threaded their way along a rocky stream bed to the base of Siberia. They moved cautiously up the southern slope of Siberia, and then partially down the northern side of the hill, burning off dense vegetation as they drove forward. When the two tanks exhausted their flame fuel, they returned to the MLR and a second section followed, completing the work of the first pair of flame tanks and working their way down the reverse slope of the hill.
Infantrymen followed the tanks, rooting out the last of the defenders and consolidating a hasty defense. After exhausting their limited supply of flame fuel, the flame tanks withdrew. The M46s remained to support the infantry, blasting away at both Siberia and Hill 110 to the northeast, covering the movement of an assault team from D/2/1. The low ground between the hills was exposed to fire and swarming with enemy infantry, and M39s were used to resupply the beleaguered outposts.
Much of the fighting took place at night, but in those days before the appearance of thermal imaging systems, the tanks were virtually blind in the darkness. The only way to effectively control the actions of the tank was for the tank commander and driver to expose their heads to enemy fire, and this practice resulted in heavy losses among experienced men. The M46s were equipped with the new fighting light, a powerful incandescent spotlight mounted above the main gun so that it traversed and elevated with the tube. The light was mounted in a thinly armored box. Steel shutters that provided some protection for the lens and screened the glow of the lamp could be controlled from inside the tank. This device provided enough light to reveal and target enemy positions, while temporarily blinding an enemy who looked directly at the tank.
On Siberia, the M46s took full advantage of the new fighting lights, flicking the shutters open and shut. All night the tank-infantry force blasted away at desperate Chinese counterattacks until ammunition was exhausted. The D Company riflemen fought off one last fierce Chinese counterattack and then withdrew, the diversion completed.
While the Chinese were preoccupied with these provocative actions on Siberia, B/1/1 (under the operational control of 2/1) assaulted nearby Bunker Hill, which was the primary objective of the entire action. By 0230 hours, American forces were in possession of the crest. American and South Korean Marines hauled tools and building materials through a storm of enemy fire to construct defensive positions on the precious ground.
All day on August 12 the Marines burrowed into the crest and south slope of Bunker Hill, while the M39s evacuated the wounded. The Chinese still occupied the low ground below the northern side of the hill. At 1600 hours, the Chinese launched their strongest effort to recapture Bunker Hill. Wave after wave of CCF infantry surged up the northern slope. By 1740, the enemy controlled the northern slope and were trying to push the Marines off the southern slope. Neither side could hold the open ground on the rounded crest, swept clean by deadly fire from both sides.
The tanks of C Company replenished their ammunition and continued to flail away at the Chinese on the nearby slopes. More tanks—the rest of C Company, five M46s of the Anti-Tank Platoon of 5th Marines, and five from the division reserve—were brought forward to subdue the enemy positions on nearby hills.
The war in Korea was evolving into one of increasingly lavish use of firepower. At Bunker Hill alone the tank force expended 817 rounds of 90mm. ammunition and 32,000 rounds of machine gun ammo during the two days of heaviest action.
The contest for Bunker Hill inevitably spread to nearby hills as each side strove for any advantage to be gained. The direct fire capability of the tanks again proved useful, eliminating enemy heavy machine guns that opened fire from any adjacent high ground. The fighting was particularly heavy on a small outpost, appropriately code-named Stromboli, about 2,500 yards east of Bunker Hill. It was originally believed that the squad occupying Stromboli had been overrun, but the Marines trapped there eventually managed to communicate with their parent unit. The tanks took the slopes of the hill under fire from the MLR, and helped the isolated squad hold out against repeated enemy assaults.
Night combat still presented problems for the tank gunners. The gunner could illuminate the target with the fighting light and fire, but light reflected back from dust raised by the muzzle blast obscured his vision. Waiting for the dust to settle before reacquiring the target meant that considerable time was required to adjust fire for following shots. An aggressive enemy wisely used this interval to rain down a deluge of artillery and mortar fire upon the tank, which was easy to spot because of its glaring spotlight.
One solution was to work the tanks in pairs. When a machine gun position was spotted, one of the tanks would illuminate it while a second tank, hidden in the darkness, took it under fire. This allowed the second tank gunner to fire several rounds in rapid succession, thus minimizing the exposure of the illumination tank.