The M18 Hellcat (officially designated the 76 mm Gun Motor Carriage M18 or M18 GMC) was a tank destroyer used by the United States Army in World War II and the Korean War. Despite being equipped with the same main gun as some variants of the much larger Sherman tank, the M18 attained a much higher top speed of up to by keeping armor to a minimum, and using the innovative Torqmatic automatic transmission.
The M18 Hellcat was the culmination of the development of various prototypes of fast tank destroyers dating back to 1941. Entering production in summer 1943, the M18 first saw combat service in spring 1944. The M18 served primarily in Western Europe, but was also present in smaller numbers in Italy and the Pacific. Production continued until October 1944, with 2,507 built.
The M18 was the most effective U.S. tank destroyer of World War II. It had a higher kill-to-loss ratio than any other tank or tank destroyer fielded by U.S. forces in World War II. Kills claimed were 526 in total: 498 in Europe, 17 in Italy, and 11 in the Pacific. The kills-to-losses ratio for Europe was 2.3 to 1, and the overall kill to loss ratio was 2.4 to 1.Zaloga, 2004 p36 M18s were "...not primarily used for tank fighting, but were committed more often to improvised roles, usually direct fire support for infantry."Zaloga, 2004 p37 Although the M18 was retired from U.S. service immediately after the end of World War II, a variant, the M39 armored utility vehicle, served in the Korean War, and M18s continued in service with some countries until 1995.
The M18 Hellcat was an example of the balancing act among firepower, armor, and mobility in armored fighting vehicle design. Despite its excellent mobility and reasonably powerful main gun, the M18 Hellcat also had drawbacks, including thin armor and a poor high explosive shell for its main gun. Historian Steven J. Zaloga characterized the overall design of the M18 as "poorly balanced" and stated that "the Hellcat's combat record is attributable to the training and dedication of its crews, not to its ill-conceived design."
Development 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.Zaloga, 2004 p5 He objected to the 3-inch M10 gun motor carriage because it was too heavy and slow for his needs,Zaloga 2002 p13 and later on to the 90 mm M36 gun motor carriage because it was essentially an M10 with a bigger gun.Zaloga 2002 p19 The United States Ordnance Department made several failed attempts to provide such a 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.Hunnicutt, 1992 p298 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 on the chassis of the Light Airborne Tank T9 as the 37 mm Gun Motor Carriage T42.Hunnicutt 1992 p304 In April 1942, the Army requested that the 37 mm gun be replaced with a 57 mm and a coil spring suspension be substituted for the Christie suspension as the 57 mm Gun Motor Carriage T49, and authorized the construction of two pilot models. The Tank Destroyer Force, meanwhile, concluded that the same 75 mm gun M3 as used on the M4 Sherman medium tank would be better for a tank destroyer. The second pilot T49 was built with the 75 mm gun in an open-topped turret as the 75 mm Gun Motor Carriage T67 and was delivered in November 1942. After testing, it was revealed that a more powerful engine was necessary, and the 76 mm gun M1A1 being developed for the M4 Sherman had also impressed the Tank Destroyer Force. The T67 project was closed in January 1943, and the Ordnance Department directed that six pilot models of the 76 mm Gun Motor Carriage T70 be built with torsion bar suspensions, more powerful engines, new turrets and minor changes to the hull front.Hunnicutt 1992 pp305, 307 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.
The first pilot was delivered in April 1943, and all six were completed by July. Once developed, the Hellcat was tested in the same manner as passenger cars before and after it, at the General Motors Milford Proving Ground. Although the Hellcat was "relatively mature" when it came to automotive performance, problems had to be rectified that included issues with the engine and new automatic transmission, weak front shock absorbers, and the position of the gun to provide more working space for the gunner.Hunnicutt 1992 p307
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. Though it weighed about 20 tons, the Hellcat was capable of traveling at . Its power came from Wright R-975, a nine-cylinder, radial aircraft engine, the same as that used on the M4 Sherman tank, paired to a 900T Torqmatic automatic transmission.
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.
Design
The M18's new design incorporated several innovative labor-saving maintenance features. Drivetrain maintenance was performed by removing a large access plate on the front of the hull, disconnecting the drivetrain from the driveshaft, attaching extension rails to the drivetrain mounting, and sliding the entire assembly out of the hull on the rails.TM 9-755, 1945, pp. 281–284 For access to the bottom of the engine without having to remove it completely from the hull, it was possible to slide the engine out onto a rail system integrated with the rear hull access door.TM 9-755, 1945, pp. 196, 435–437
The 900T Torqmatic transmission had three forward gears, with ranges of up to 16 mph, 12 to 34 mph, and 30 to 60 mph (corresponding to cruising speeds of 12 mph, 25 mph, and 45 mph, respectively), and one reverse gear with a cruising speed of . The engine speed was governed such that the vehicle normally could not exceed in third gear.TM 9-755, 1945, p. 46 The M18 carried a five-man crew, consisting of a commander, gunner, loader, driver, and assistant driver. The driver and assistant driver were seated in the front left and right of the hull, respectively. Uniquely, the driver and assistant driver were provided with identical sets of controls and either could be used to operate the vehicle, but only the driver's set of steering levers could be used as parking brakes. The commander, gunner, and loader were positioned in the turret; the commander at the left rear, the gunner at the left front, and the loader on the right. The turret crew were all provided with seats, and the commander could stand on his seat to operate the .50 caliber M2 Browning machine gun on a flexible ring mount on the left rear of the turret. An SCR-610 radio was placed in the rear of the turret. An escape hatch was provided in the hull floor directly underneath the turret.
Armament
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, while carrying the same gun as the Sherman 76 mm models. The turret could rotate through 360 degrees in 24 seconds using an electro-hydraulic traverse mechanism, and the 76 mm gun could depress 10 degrees, and elevate 20 degrees. The maximum firing rate was 20 rounds per minute. The breech block of the gun was tilted 45 degrees to the right to aid the loader in manipulating and inserting ammunition in the confined space of the turret.Hunnicutt 1992 p313 45 rounds of main gun ammunition were carried, 9 in the turret to the right of the gun, and 18 in each sponson. A .50 caliber M2 Browning machine gun with 800 rounds of ammunition was mounted on the left rear of the turret in a flexible ring mount. Each crew member was provided with an M1 carbine with 90 rounds for self-defense, and six Mk 2 grenades, six M50 white phosphorus smoke grenades, and six smoke pots were also carried in the vehicle.
One disadvantage of the M18 was the inconsistent performance of its 76 mm gun against the thick and steeply sloped frontal armor of later German designs such as the Tiger and Panther. The problem of the main gun performance was somewhat remedied with High Velocity Armor Piercing (HVAP) ammunition beginning in fall 1944, which allowed the 76 mm gun to achieve greater armor penetration, but this was never available in quantity.Zaloga 2008 p194 The 76 mm gun with standard ammunition could penetrate the frontal turret armor of Panther tanks only at very close ranges, whereas the HVAP ammunition gave it a possibility of effectively engaging some of the heavier German tanks and theoretically penetrating the front of the Panther turret at ranges of about .
Armor
The armor of the M18 Hellcat was quite light to facilitate its high speed, and only provided protection from small arms fire. The lower hull armor was thick all around, vertical on the sides, but sloped at 35 degrees from the vertical at the lower rear and angled twice at the lower front to form a nearly rounded shape; 53 degrees from the vertical and then 24 degrees from the vertical. The hull floor was only thick.
The upper front hull also had an angled construction to form the Hellcat's sloping glacis; two plates were angled at 38 and 64 degrees from the vertical, respectively. The upper hull armor was also thick, being angled at 23 degrees from the vertical on the sides and 13 degrees from the vertical at the rear. The hull roof was The cast turret of the Hellcat was thick on the front (at a 23-degree angle from the vertical) and thick on the sides (also at a 23-degree angle from the vertical) and rear (angled at 9 degrees from the vertical) The front of the turret was further protected by a rounded cast gun mantlet which was thick.Hunnicutt, 1992 p489
Another disadvantage of the M18 was its very light armor protection and open-topped turret. In testing of the T70 for possible use as a light tank, 9 out of 30 armor-piercing .30 caliber bullets penetrated the side of the turret at a range of .Hunnicutt 1992 p315 The open-topped turret—a characteristic which it shared with all American tank destroyers—left the crew exposed to snipers, grenades, and shell fragments, however it gave the crew excellent visibility, which was of importance in the killing of tanks, the intent of tank destroyers being primarily ambush weapons. The doctrinal priority of high speed at the cost of armor protection thus led to a relatively unbalanced design.
Tactical mobility