M18 Hellcat

When the Tank Destroyer Force was organized in 1941, their commander, Lieutenant Colonel (later General) Andrew Davis Bruce envisioned the units being equipped with something faster than a tank, with a better gun but less armor to allow for speed; a cruiser rather than a battleship.[7] He objected to the 3 inch M10 Gun Motor Carriage because it was too heavy and slow for his needs,[8] and later on to the 90 mm M36 Gun Motor Carriage because it was essentially an M10 with a bigger gun.[9] The United States Ordnance Corps made several failed attempts to provide said vehicle using the weapons (the 37 mm, 57 mm, 3 inch, 75 mm and finally the lightweight 76 mm of 1942–1943) and technology available, including mounting the 3-inch gun on the fast M3 Light Tank chassis.[10] The M18 was the end product of a long line of research vehicles aimed at providing the desired machine.

In December 1941, the Ordnance Department issued a requirement for the design of a fast tank destroyer using a Christie suspension, a Wright-Continental R-975 radial aircraft engine, and a 37 mm gun. Two pilot vehicles were to be built.[11] What became the M18 originated in Harley Earl's design studio, part of the Buick Motor Division of General Motors. Previously, basic designs for other kinds of vehicles had mostly originated from within the Ordnance Department. Buick's engineers used a torsion bar suspension that provided a steady ride.[lower-alpha 1] Though it weighed about 20 tons, the Hellcat was capable of traveling upwards of 45 mph. Its power came from Wright R-975, a nine-cylinder, 350-to-400-horsepower radial aircraft engine, paired to a three-speed 900T Torqmatic automatic transmission.

Changes to the specification mean that the first pilot – the 57 mm Gun Motor Carriage T49 – was built with the British (57 mm) QF 6-pounder gun instead of the 37 mm and a torsion bar suspension instead of the Christie suspension. It was tested in 1942 but the army wanted a heavier gun – the same 75 mm gun M3 as used on the M4 Sherman medium tank. The T49 project was cancelled and the second pilot was built with the 75 mm gun as the 75 mm Gun Motor Carriage T67. This met approval, but in early 1943 the army requested a more powerful gun – the 76 mm gun M1 under development for the Sherman. Six pilot models – as the 76 mm Gun Motor Carriage T70 – were built with this gun. The trials of these models led to a new turret and changes to the hull front, but the design was otherwise accepted for production, which began in July 1943.[11]

Once developed, the Hellcat was tested in the same manner as passenger cars before and after it, at the General Motors Milford Proving Ground. Top speed testing was done on a paved, banked oval and ride quality tests were done over specially developed stretches of bumps. The M18 also required tests of its ability to ford six feet of water, climb small walls, and ram through structures.

The first models of the tank destroyer were tested by the US Army's 704th Tank Destroyer Battalion. The unit had originally been trained on the M3 Gun Motor Carriage (a 75 mm gun installed in the bed of an M3 half-track). Despite its T70 prototypes requiring several improvements, the 704th had a "superlative" testing record, and the unit was later issued production Hellcats after many of their suggestions were integrated into the vehicle. The testing phase of the Hellcat proved that teamwork was an essential element of the new light tank destroyer units, and replaced the fixed, rigid structure of other units with a much more flexible command structure that allowed adapting to more complicated tasks.[12]

The M18's new design incorporated several labor saving and innovative maintenance features. It used the same Wright R975 engine as the Sherman tank. The fully unitized drivetrain was much easier to maintain, as it was mounted on rails equipped with steel rollers that allowed maintenance crews to disconnect it easily from the transmission, roll it out onto the lowered engine rear cover using rails, service it, and then reconnect it to the transmission. The transmission could also be removed easily and rolled out onto a front deck plate to facilitate quick inspection and repairs.

In contrast to the M10 and M36, tank destroyers, which used the heavy chassis of the M4 Sherman, the M18 Hellcat was designed from the start to be a fast tank destroyer. As a result, it was smaller, lighter, more comfortable, and significantly faster, but carried the same gun as the Sherman 76 mm models. The M18 carried a five-man crew, consisting of a commander, gunner, loader, driver, and assistant driver. 45 rounds of main gun ammunition were carried, 9 in the turret and 18 in each sponson. An M2 Browning machine gun with 800 rounds of ammunition was provided on a flexible ring mount for use against enemy aircraft and infantry.

Original plans called for a total of 8,986 M18s to be supplied – 1,600 for Lend-Lease to other countries and 7,386 for the U.S. Army. The production plans of the M18 were curtailed to 2,507 vehicles, including the six pilot models. 10 were later converted into T41/T41E1 command vehicle and prime mover prototypes, and 640 were converted into M39 Armored Utility Vehicles. The reasons behind the reduction (in no particular order) were:

• The 76mm gun was already inadequate for later German tanks and The Army Ground Forces preferred to get the 90 mm Gun Motor Carriage M36 into service, despite Tank Destroyer Force commander Andrew Bruce's objections to adopting it
• The number of self-propelled tank destroyer battalions had been approximately halved due to a policy change forced by the AGF, who wanted towed guns to be used and hence far fewer self-propelled units were needed for the Tank Destroyers
• There was little potential Lend-Lease activity: Britain and the Soviet Union "had little interest".[17] Two, listed as "T70", were transferred to the United Kingdom, and five to the Soviet Union.[18]

Production of M18 Hellcats ran from July 1943 until October 1944, with 2,507 built. Several changes were made during production. The first 684 M18s experienced problems with their transmission gear ratios which meant they could not climb steep slopes, and were returned to the factory for modification. Most of these early vehicles remained at the Buick factory after modification. Ten were later converted to T41/T41E1 command vehicle and prime mover prototypes, and 640 were converted to M39 Armored Utility Vehicles. The rest of the M18s built featured an improved transmission. The 76 mm gun M1 fitted to most Hellcats kicked up large amounts of dust when fired. This was enough to impede the vision of the crew, who had to wait until the muzzle blast cleared to fire accurately again. To solve this problem, the 76 mm gun M1 was replaced with the muzzle brake-equipped 76 mm gun M1A2 as soon as it became available; in the interim, the M1A1C gun, threaded for a muzzle brake but not so equipped, was used. Beginning in June 1944, roughly the last 700 Hellcats received the M1A2 gun. Hellcats with serial numbers 1350 and below had the naturally-aspirated R975-C1 engine, which produced 350 horsepower. The ventilation grate for the transmission and oil coolers behind the driver's hatch on these early Hellcats had a bulged shape, protruding above the line of the upper hull. M18s with serial numbers 1351 and above had the internally-modified supercharged R975-C4 engine, which produced 400 horsepower. At roughly the same time as the change in engine type, the shape of the ventilation grate was changed to be flush with the upper hull.[19]

There were three production contracts for the Hellcat: RAD-563 covered the six pilot models. T-6641 was for the first 1,000 vehicles, and T-9167 was for the final 1,507 vehicles.[20][21]

U.S. combined arms doctrine on the eve of World War II held that tanks should be designed to fulfill the role of forcing a breakthrough into enemy rear areas. Separate GHQ tank battalions supported infantry in destroying fixed enemy defenses, and armored divisions then exploited the breakthrough to rush into the enemy's vulnerable rear areas. U.S. tanks were expected to fight any hostile tanks they encountered in their attack, but the mission of destroying massed enemy armored thrusts was assigned to a new branch, the Tank Destroyer Force. Tank destroyer units were meant to counter German blitzkrieg tactics. Tank destroyer units were to be held as a reserve at the corps or army level, and were to move quickly to the site of any massed enemy tank breakthrough, maneuvering aggressively and using ambush tactics (charging or chasing enemy tanks was explicitly prohibited) to destroy enemy tanks. This led to a requirement for very fast, well-armed vehicles. Though equipped with turrets (unlike most self-propelled anti-tank guns of the day), the typical American design was more heavily gunned, but more lightly armored, and thus more maneuverable, than a contemporary tank. The idea was to use speed and agility as a defense, rather than thick armor, to bring a powerful self-propelled gun into action against enemy tanks.

This doctrine failed on the European battlefield where attacking American tanks ran into defending German tanks and self-propelled guns far more often than intended, and the tank destroyers were called in to assist. Tank destroyers were often deployed to attack enemy armor at long range from an ambush position. During the battle for Italy, tank destroyers compensated for a shortage of 155 mm artillery ammunition by using their 3-inch or 76 mm guns in indirect fire role. Near the end of the war, there were so few German tanks that tank destroyers were increasingly used as self-propelled artillery in support of infantry for lack of any other targets. In effect, they were used exactly like tanks.

In practice, tank destroyer battalions were not held in reserve but were attached nearly permanently to divisions. Tank destroyer battalions assigned to divisions were often split up, with tank destroyer companies attached to infantry regiments, or platoons attached to infantry battalions. When so attached, defending tank destroyer units supplemented organic antitank weapons (bazookas and 57mm towed guns).[31]

While the M18 was capable of high road speeds, this attribute was difficult to use successfully in combat.[29]

Although sustained travel at road speeds was hardly ever used outside of the Allied response during the Battle of the Bulge,[29] most Hellcat crews found the higher speeds especially useful in a sprint to flank German tanks, which had relatively slow turret traverse speeds, and such maneuvering allowed the tank destroyer crew to direct a shot into the enemy's thinner side or rear armor. The open-top proved an advantage in terms of allowing the TD crews significantly better visibility than their opponents, many of which were German tank destroyers that did not have revolving turrets and were handicapped by poor lateral visibility.

In general, Hellcat crews were complimentary of their vehicle's performance and capabilities but did complain that the open-top created a cold interior in the Northern European winter of 1944–45.[29] This problem was exacerbated because the air-cooled engine pulled a percentage of its cooling air through the crew compartment, creating, in effect, a large armor-plated refrigerator. It was not designed to do so, but it proved impossible to seal off the crew compartment entirely from engine induced drafts.

After World War II, many M18s were sold to other countries. These were rebuilt and refurbished by Brown & Root in northern Italy in the late 1940s and early 1950s, and bear data plates that indicate those rebuilds. One of the users was Yugoslavia, which received 260 Hellcats during the Informbiro period[32] and kept some of them in reserve until the early 1990s. A number of these vehicles were later used by the Military of Serbian Krajina and Army of Republika Srpska during the Yugoslav wars. One example was used on an armored train named the "Krajina express" (Krajina Ekspres).[33]

The Military of the Republic of China also operated several M18s until their chassis and hulls were worn out, at which point the turrets were salvaged and installed onto surplus hulls of M42 Duster anti-aircraft vehicles to produce Type 64 light tanks.

The Greek Army received 127 M18s from 1952 till 1954. Initially, these were organized in three Tank Destroyer Regiments numbered 397, 398 and 399. In 1959 the Tank Destroyer Regiments were reorganized in three Tank Destroyer Battalions with the same numbers. Most of the M18s were retired in the end of 60s but a few of them remained in service till the middle 70s for training.[34] The hulls of the M18s were dismantled and the turrets were used as gun emplacements in the northern borders of Greece and the Aegean islands.[35] One M18 is preserved in the Greek Army Tank Museum.[36]

The Venezuelan military still operates M18s, with 75 still in reserve service. An unknown number of these vehicles were modernized by a Yugoslavian firm in 1991.

M18 Hellcat "Amaz N Grace" is on loan to the Museum of American Armor in Old Bethpage, New York, from the US Military Museum. An M18 is on display at the Army Heritage and Education Center in Carlisle, Pennsylvania.

M39 Armored Utility Vehicle

Armored Utility Vehicle M39 in Korea, 1952

The only M18 variant produced in significant numbers was the M39 Armored Utility Vehicle, a turretless variation used to transport personnel or cargo or as a gun tractor. This version was armed with a single M2 machine gun on a flexible mount. Some 650 early production M18s were converted into M39s by removing the turret and fitting seats for up to eight men in the open fighting space.[37] M39s saw combat during the Korean War, primarily as armored personnel and munitions carriers and were finally declared obsolete on February 14, 1957.[38] About 100 M39s were transferred to the West German Bundeswehr in 1956, where they were used to train the reestablished Panzergrenadier armored infantry units.[39]

• T41 Prime Mover One Hellcat with its turret removed and internal changes made, converted into a prime mover for the 3-inch Gun M5.
• T41E1 Command and Reconnaissance Vehicle: One Hellcat with its turret removed and internal changes made, meant as a replacement for the M20 Armored Utility Car in tank destroyer battalions. The command and reconnaissance vehicle configuration was found to be superior, and it was decided to use it for both the T41 and T41E1. Ten prototypes, in a program consolidated as T41, were built
• M39 Armored Utility Vehicle A standardized T41; 640 converted from October 1944 to March 1945
• 105 mm Howitzer Motor Carriage T88: M18 with the 76 mm gun replaced with a 105 mm T12 howitzer. A pilot was built in 1944 but project cancelled after the end of the war.
• 90 mm Gun Motor Carriage M18: M18 with the 76 mm gun replaced with a 90 mm gun on a M36 turret; cancelled after the end of the war [40]
• 76 mm Gun Motor Carriage T86 (Amphibious)

M18 with a much larger and lightweight flotation hull, using its tracks for water propulsion. The T86E1 variant test vehicle used propellers for propulsion.

• 105 mm Howitzer Motor Carriage T87 (Amphibious)

The T86 vehicles had shown that track propulsion was better and this was used for the T87 which had the same 105 mm T12 howitzer of the T88.

All work on the three amphibious models was canceled after the end of the war.

• G-numbers
• M36 tank destroyer, the heaviest-gunned (90 mm) US tank destroyer of World War II.

• Panther Soup, John Gimlette, Hutchinson 2008, ISBN 9780091921385 / 9780091796730

Wikimedia Commons has media related to M18 Hellcat.

• 76mm Gun Motor Carriage M18 (with photos) – AFV Database
• Armored Utility Vehicle M39 – AFV Database
• M18 Tank Destroyer, Hellcat – World War II Vehicles (photos and video)
• "WWII tank to join in parade" – Daily Herald (Utah), 3 July 2008
• M18Hellcat.com, a website dedicated to the history and preservation of the M18 Hellcat
• Tankdestroyer.net (Web based United States tank destroyer forces information resource)