Design
In February 1944, it was standardized as the M18 Gun Motor Carriage and nicknamed the Hellcat.1,6,7
The M18 Hellcat had three compartments: driver's, fighting, and engine.6
Driver's Compartment
The driver sat on the left and the assistant driver on the right.6 Located between them was the transmission with the shift levers on top.6 This allowed for either to shift the gears.6 Each had an accelerator pedal with the assistant driver having a lockout mechanism.6 Each had steering levers that would hang down, and the levers could be moved out of the way.6 On the left side of the driver was the instrument panel.6 There were adjustable seats for both.6 In the roof each had hinged double doors.6 These also had periscopes.6
Fighting Compartment
The turret basket contained three folding and adjustable seats.6 The commander was on the left side behind the gunner.6 The loader was located on the right side of the 76 mm gun.6
The gunner could use either a hydraulic or hand traverse control to rotate the turret.6 He also had an azimuth control.6 Periscopic and telescopic sights were used by the gunner.6
There was an auxiliary generator, located in the sub floor at the right front of the turret, that was used to recharge the batteries.6
The assistant driver had a portable fire extinguisher beside him.6
The turret bulge held the radio equipment.6
Engine Compartment
The fuel tanks were located on each side of the engine.6 The left held 75 gallons and the right 90 gallons.6
A propeller shaft went under the sub floor transmitted power from the engine to the transmission.6
The torqmatic transmission was mounted on extension rails so it could be slid out for repairs.6
There were controls inside and outside of the engine compartment that controlled its fire extinguisher.6
Starting with vehicle 1,700 it had blowers that would move hot air over the oil cooler, the battery, the engine, and to the crew.6 This equipment was sometimes installed at tank depots for earlier vehicles to winterize them.6
Main Armament
The M18 was armed with the 76 mm M1A1, M1A1C or M1A2 L/55 gun.6
The M1A1 had no muzzle brake.6 The M1A1C has threads for a muzzle brake and the barrel had rifling of one turn for each 40 calibers.6 M1A2 had threads for a muzzle brake and rifling of one turn for each 32 calibers.6
A travel lock was located in the turret for vehicles 1 - 1,857.6 This was a swinging lever with a pin.6 From vehicle 1,858 there was a spring loaded ball stud.6
Could penetrate 101 mm. Muzzle velocity 2,600 ft/sec.5 The AP shell weighed 15.4 lbs.5 It could fire a HE shell 14,000 yards.5
Prototype
On December 2, 1941, a memorandum from the G-3 to the G-4 of the General Staff recommended that a gun motor carriage be developed to carry a 37 mm M6 gun on a Christie type suspension.6 On April 1, 1942, the Ordnance Department suggested that a 57 mm M1 gun be used.6
On April 18, 1942, it was agreed to construct two prototypes that would be identified as the T49 Gun Motor Carriage.6 It was to be mobile, weigh around 12 tons, have a crew of five, reach a speed of 50 mph, armor was to be 7/8" on the turret, and 3/8" all around on the hull.6
The first prototype was tested and as a result the Tank Destroyer Command and the Ordnance Department wanted the second prototype to have a 75 mm M3 gun.6 This became the T67 Gun Motor Carriage.6 It was to have its armor increased to 1" in front.6 The side armor was reduced and the top and bottom was increased.6
The prototypes had helical coil suspensions.6 Two Buick 320 cubic inch gasoline engines produced a total of 330 HP.6
In late 1942, after the testing of a prototype at the Aberdeen Proving Ground, the Special Armored Vehicle Board recommended that the T67 have a standard engine, a 76 mm gun, and a torsion bar suspension.6 The Secretary of War approved the building of six prototypes.6 These would carry the 76 mm M1 in an open turret without a basket.6 The Continental R975-C1 engine was used.6 These prototypes were designated the T70 Gun Motor Carriage.6
Production
On January 7, 1943, without testing the T70 Gun Motor Carriage prototypes, the Army Service Forces ordered 1,000 to be constructed.6 The first first T70 was delivered in July 1943.6
* M18: 2,5073,6,7
o Manufacturer: Buick Motor Division (GMC)5
o Production: July 1943 - October 19447
* T41, T41E1 (M39): 6406
* T86: 3 prototypes6
o Manufacturer: Marmon-Herrington6
