The 76 mm Gun Motor Carriage (GMC) M18 was an American tank destroyer of World War II. The manufacturer, Buick, gave it the nickname "Hellcat" and it was the fastest tracked armored fighting vehicle during the war with a top speed up to 60 mph. Hellcat crews took advantage of the vehicle's speed to protect against hits to its thin armor. Many German Panther and Tiger tanks were destroyed because they could not turn their turrets fast enough to return fire.
From the “Buick at its Battle Stations” booklet. Post-production field test.
In December 1941, the Ordnance Corps issued a requirement for the design of a fast tank destroyer using a Christie suspension, the Wright/Continental R-975 engine, and a 37 mm gun.
In light of experience gained in North Africa, the 37 mm gun was found to be inadequate and the design was changed to use a British 57 mm gun. During the development process, the design was further upgunned to a 75 mm gun, and then finally to the 76 mm gun. The Christie suspension requirement was also dropped, and replaced with a torsion bar suspension. The design was standardized in February 1943 and production began in July 1943.
As a new design, the M18 incorporated several innovative maintenance features. The Wright R-975 engine was mounted on steel rollers, which permitted it to be disconnected from the transmission, rolled out onto the lowered engine rear cover, serviced and then reconnected to the vehicle. Similarly, the transmission could be removed and rolled out onto a front deck plate to allow inspection and repairs.
The T70 prototype for the M18 first saw combat at Anzio, Italy, and production versions of the M18 were used in North-West Europe and Italy from the summer of 1944 onwards.
In contrast to the M10 tank destroyer, which used the 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, and significantly faster, but carried the same gun as the Sherman 76 mm models. The M18 carried a five-man crew as well as 45 rounds of main gun ammunition, and an M2 Browning machine gun on a flexible ring mount for use against aircraft and infantry.
The main disadvantages of the M18 were its very light armor, and the inconsistent performance of its 76 mm gun against the frontal armor of later German designs such as the Tiger and Panther. The open-topped turret (a characteristic which it shared with the M10) left the crew exposed to snipers, grenades, and shell fragments. The doctrinal priority of high speed at the cost of armor protection thus led to an unbalanced design. The problem of the main gun performance was remedied with High Velocity Armor Piercing (HVAP) ammunition late in the war, which allowed the 76 mm gun to achieve greater frontal armor penetration, but this was never available in quantity.
While the M18 was capable of high road speeds this attribute was difficult to use successfully in combat, but along with the high top speed was a commensurate ability to accelerate rapidly and change direction rather quickly. Although sustained travel at road speeds was hardly ever used outside of the Allied response during the Battle of the Bulge, 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 a shot instead into the enemy's thinner side or rear armor. 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. This problem was not helped by the fact that the air-cooled engine pulled a percentage of its cooling air through the crew compartment, creating in effect, a large armour plated refrigerator. It was not designed to do so, but it proved impossible to seal off the crew compartment entirely from engine induced drafts.
The only M18 variant which was 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. 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. M39s saw combat during the Korean War, primarily as armored personnel carriers and munitions carriers, and were finally declared obsolete on February 14, 1957. About 100 M39s were transferred to the West German Bundeswehr in 1956, where they were used to train the reestablished Panzergrenadier armored infantry units. The M18 continued in production until October 1944, when the war was nearing its end. 2,507 had been produced by that time, at a unit cost of $57,500. Though all tank destroyer units were disbanded by the U.S. after the war, surplus M18s continued to see limited service.
Company A, 637th TD Battalion at Espiritu Santo, New Hebrides islands
The M18 served primarily in Western Europe, but was also present in the Pacific. Upon the American entry into the war in 1941, it had begun to supply China with AFVs including later the M18 Hellcats which along with M3 Stuarts, and M4 Shermans, trickled in through Burma and formed part of the several well-equipped, well-trained armies that the Chinese Nationalists could deploy. These units were responsible for stopping numerous Japanese attacks during the later phases of the war. However, due to the comparative rarity and poor quality of Japanese armor it was often used in a fire support role instead of as a tank destroyer. On September 19, 1944, in the Nancy Bridgehead near Arracourt, France, the 704th Tank Destroyer Battalion was attached to the 4th Armored Division. Lt. Edwin Leiper led one M18 platoon of C Company to Rechicourt-la-Petite, on the way to Moncourt. He saw a German tank gun muzzle appearing out of the fog 30 feet away, and deployed his platoon. In a five minute period, five German tanks of the 113 Panzer Brigade were knocked out for the loss of one M18. The platoon remained in their position and destroyed ten more German tanks, with the loss of another two M18s. One of the platoon's M18s, commanded by Sgt Henry R. Hartman, knocked out six of these and lived to fight another day. Most of these knocked-out German tanks were Panthers. The M18 Hellcat was a key element during World War II in the Battle of the Bulge. On December 19–20, the 1st Battalion of the 506th PIR, was ordered to support Team Desobry, a battalion-sized tank-infantry task force of the 10th Armored Division (United States) assigned to defend Noville located north-northeast of both Foy and of Bastogne just 4.36 miles (7 km) away. With just four M18 tank destroyers of the 705th Tank Destroyer Battalion to assist, the paratroopers attacked units of the 2nd Panzer Division, whose mission was to proceed by secondary roads via Monaville (just northwest of Bastogne) to seize a key highway and capture, among other objectives, fuel dumps—for the lack of which the overall German counter-offensive faltered and failed. Worried about the threat to its left flank in Bastogne, it organized a major joint arms attack to seize Noville. Team Desobry's high speed highway journey to reach the blocking position is one of the few documented cases wherein the legendary top speed of the M18 Hellcat (55 miles per hour (89 km/h), faster than today's M1A2 Abrams) was actually used to get ahead of an enemy force as envisioned by its specifications.
The attack of 1st Battalion and the M18 Hellcat tank destroyers of the 705th TD Battalion near Noville together destroyed at least 30 German tanks and inflicted 500 to 1000 casualties on the attacking forces, in what amounted to a spoiling attack. A Military Channel historian credited the M18 destroyers with 24 kills, including several Tiger tanks, and believes that in part, their ability to "shoot and scoot" at high speed and then reappear elsewhere on the battlefield and therefore appear to be another vehicle entirely played a large part in confusing and slowing the German attack, which subsequently stalled, leaving the Americans in possession of the town overnight. The Hellcat, due to its 76mm gun, had major difficulty penetrating the glacis of Panther tanks. Due to the almost absent armor of the Hellcat and its use of high flash powder the Hellcat made a relatively easy target for German tank crews. Tank destroyers, in contrast to the pre-war doctrine governing their deployment, came to attack enemy armour from long range from an ambush position, acting in essence like self-propelled anti-tank guns. The Hellcat had a gun that could penetrate roughly 88mm at 1000 m. The average combat range noted by the Americans for tank vs. tank action was around 800m to 900m. This was just enough to penetrate a Panzer IV frontally, a tank designed in 1939. If facing a Panther, a Hellcat would be facing a tank with roughly 140mm of line-of-sight armor frontally. Hypothetically, if the Soviet Union decided to invade the rest of Europe during the war, the Hellcat would face the IS-2 with a glacis of roughly 200mm line of sight armor.
Tank Destroyer Doctrine
American prewar armored doctrine was based on using tanks solely in a support and exploitation role, usually in conjunction with infantry. Tank destroyers, such as the Hellcat, were to be used against tanks. To this end the Hellcat was not intended to engage in protracted combat, and had light armor and extremely high speeds to quickly respond to breakthroughs in the line by German armor. In reality, the opposite was true, as the attacks with the Sherman ran into defending German tanks far more often than intended. In Italy, TDs compensated for a shortage of 155mm artillery ammunition by using their 3 inch or 76mm 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. The Hellcat was theoretically supposed to be used independently as a sort of mobile anti-tank gun, brought up from reserves to buffer an incoming armored thrust. In practice a TD battalion was assigned nearly permanently to a division. The doctrine of the time had Shermans acting in support of infantry to break enemy defenses, and then leading the attack with infantry in support during exploitation. Prewar expectation was that all anti-tank work was supposed to be done by tank-destroyer crews, because attacking tanks could concentrate against a small part of a defending line. Independent TDs groups were to counter concentrate, to stop enemy tanks from penetrating deeply. Speed was essential in order to bring the Hellcats from the rear to destroy incoming tanks. Obviously this would make it harder for an armored force to achieve a deep breakthrough, a main objective of armor, if the enemy had tanks. It would also be easier for an opposing armored force to achieve a local breakthrough against an American tank battalion which would not have all of its anti-tank assets at the front during the beginning of any attack. Thankfully, for Sherman crews, this doctrine was not entirely used as it would create a small window of time of weakness in the armored battalion until tank destroyers moved to the front. TD battalions assigned to front line divisions often split up to companies attached to regiments, and platoons attached to infantry battalions. When so attached, defending TD units supplemented organic antitank weapons (bazookas and 57mm towed guns).
Okinawa on the Shuri Line in May 1945. Note .30 cal
After World War II, many M18s were given 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 kept 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 Ekspres (Krajina Express).
Taiwan 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.
- 105 mm Howitzer Motor Carriage T88: M18 with the 76 mm gun replaced with a 105 mm T12 howitzer; canceled after the end of the war.
- 90 mm Cannon Motor Gun Carriage : M18 with the 76 mm gun replaced with a 90 mm Cannon; canceled after the end of the war
- 76 mm Gun Motor Carriage T86 (Amphibious): M18 with a specially-designed flotation hull, using its tracks for water propulsion.
- 76 mm Gun Motor Carriage T86E1 (Amphibious): Same as T86, but with the addition of propellers for propulsion.
- 105 mm Howitzer Motor Carriage T87 (Amphibious): This model had the 105 mm T12 howitzer of the T88, and like the T86, used its tracks for water propulsion.
All work on the three amphibious models was canceled after the end of the war.