The only armored vehicle purposefully designed as a tank destroyer from the ground up was the M18 Hellcat. Weighing less than twenty tons, the M18 combined fifty-mile-per-hour speed with an improved suspension. Crews roundly applauded the Hellcats maneuverability. Designers mounted a 76-millimeter, high-velocity gun in the open topped turret of the M18. Like the M10 and M36 tank destroyers, the M18 Hellcat left significant room for improvement in protection for the crews.
The M18 Hellcat tank destroyer was built on the chassis of the Sherman tank. It was the fastest armored vehicle of WW2 at 60 mph on roads. The M-18 was fielded in the latter part of WWII to replace the earlier M-10 Tank Destroyer. The primary mission of these vehicles was to serve in an anti-tank role. Although it appeared to have a turret like a regular tank, these Tank Destroyers had an open top to the turret fixture and they were not enclosed.
Notwithstanding the substitutes and expedients offered the Tank Destroyer Center and the Board, work continued on the development of the T49 which possessed the characteristics necessary for mobility andi maneuverabilty. The creation of the T49 offered definite indications of the basic characterisatics desired; but, in line with the need for a heavIer weapon, the Commanding General on 02 July 1942 recommended to the Commanding General, Army Ground Forces, a redesignation of the T49 and requested definitely that the 75mgun M2A3 be used in lieu of the 57mm.
A new 76mm gun embodied the same physical characteristics of the 75mm, and had the same breechblock, recoil and similar design of tube. Through a longer tube and the use of a larger cartridge case, a much higher muzzle velocity and a greater striking power were attained than with the 75mm gun, and that without on appreciable increase in weight. The 76mm gun was in reality a 76.2a caliber weapon, equivalent to the 3 inch but much lighter in weight than the 3 inch. It had the additional advantage over the 75mm of using the same projective as the 3 inch.
The 75mm gun motor carriage with Christie suspension was designated 19 November 1942 as the T67. Representatives of the Board met with the Ordnance Department and manufacturing representatives in Detroit on 22 and 23 December 1942 and the decision was made by General Bruce covering additional important characteristics which were to be incorporated in the GMC 76nm. One of the most important of these decisions was the use of the torsion bar in lieu of springs in the suspension system. On 7 January 1943, the T67 75=m gun motor carriage project was brought to a close and the Ordnance Department designated the 76.2mm gun motor carriage the T70.
The Tank Destroyer Board had adopted as a project the T70 on 31 December 1942 and from that time on until the middle of 1944, continued tests to achieve the perfection desired. Between 31 December 1942 and 23 November 1943, the Board made sixteen complete reports on tests of the T70 and the reconmendations covered in these reports were to effect 137 modifications and changes in the design of the vehicle. Subsequent to the preliminary reports, twelve production models of the T70 were received by the Board during the latter part of 1943 and after the Board had submitted the vehicles and the wea ons to every reasonable test, only fifteen modifications and changes were requested.
The Buick Division of General Motors Corporation had been making its own tests on the experimental models of the T67 and T70. The performance of the vehicle as such as to inspire the nickname "Hellcat", and it was thus named by them. The 76mm gun motor carriage T70 was standardized by 0CM Item 23202, as the M18, 16 March 1944.
During maneuvers, it was clearly discernable that the M-18 showed a tendency to out pace its own security section, and other tank destroyer vehicles. Upon a study by the Board of this problem it was found that by removing the gun and turret from the M18, a utility vehicle was quickly achieved with the necessary speed characteristics to stay up with the M-18. This adaptation of the M-18 was known as the T41 and recom- mended by the Board as an auxiliary vehicle for the tank destroyers. This adaptation of the M18 was designated by OCM Item 26106 on 1 February 1945.
The M18 was designed specifically as a highly mobile, self-propelled gun for action against enemy armor. Its development for this requirement was "from the ground up," - it was completely new. In its design, the 3" (76amm) was selected, with a 15 lb. projectile at 2760 ft/sec muzzle velocity and a penetrati.on of 4.5" of armor at 1000 yards. Since maximum gun power and high mobility were incompatible with armor of tank thickness, protection against small arms only was contemrpiated.
The 76mm (3") gun, utilizing a compact 75mm recoil mechanism, was mounted in an open turret for maximum vision. The direct sight of advanced design fetured an azimuth indicator and quadrant for indirect laying. With a maximum range of 14,500 yards, it was highly accurate. The projectile fit both 76m azd 3" guns and fixed in cartridge case, complete round being 5 lbs, lighter for 76mm than the 3". Since the cases differ, complete rounds are not interchangeable.
The chassis had 1/2" arnor basis. The improved radial engine of 480 hp powered newly designed track, steel, rubber-bushed, with slack compensation, capable of high speed and with life double that of tanks. The slope-cl*tabibg ability 60% or 50% with towed load of 8000 lbs. The fording depth was 48", and the top speed was 55 mph.
A highly mobile gun motor carriage with new suspension, having individually mounted wheels on trailing arms, torsion bar springing, new track, torqmatic transmissionn, engine and transmission differential removable on rails through doors.
The greatest single accomplishment of the Tank Destroyer Board was the development of the M-18, 76mm GMC Tank Destroyer and the M-39, an all-purpose vehicle. More than 157 modifications were effected on the original design of the M-18. In the development of the M-18, Mr. C. L. McCuen, Mr. W. J. Davidson, Mr. W. K. Haig, Mr. E, T. Ragsdale and other officials of General Motor Corporation deserves special mention for their cooperation and work with Ordnance and the Tank Destroyer Board. Major (later Lieutenant Colonel.) W. A. Wood, Jr., of the Tank Destroyer Board effected. many changes and modifications which resulted in superior performance by the vehicle. For his work in the development of the M-18, Colonel Wood was later awarded the Legion of Merit by the War Department.
The 827th Tank Destroyer Battalion moved to Europe in November 1944 after its scheduled shipment to the Pacific in the spring of that year had been cancelled because of training deficiencies. Other Negro tank destroyer battalions had been converted to service troops by the spring of 1944. Some of these were considered by the 827th to have been better units than itself. The battalion, whose training career had been analyzed and found wanting by previous commanders, had had about two and a half years of training in the United States, but under unusual circ*mstances. By the time it moved overseas, it had had eight different commanders, more than one of whom had recommended that the battalion be made a service unit. It had been organized and reorganized under four different tables of organization and equipment. It was re-equipped with primary weapons four times. Starting its career with towed 75-mm. tank destroyers, it changed successively to self-propelled M-10's, then to towed 3-inch destroyers, and finally to self-propelled M18's. These changes, normal as tank destroyer theories and weapons changed and improved, involved the disbandment and reconstitution of the battalion reconnaissance company, a unit which, in its final form, was looked upon by the battalion's officers as especially inefficient.
M39 Armored Utility Vehicle (AUV)
The first of the American fully tracked armoured personnel carriers, introduced soon after the Second World War, was of the Kangaroo type. Designated the M39, it consisted of the M18 tank destroyer minus its turret and armament. The Army removed the turret and put people inside an open top. The M39 Armored Utility Vehicle (AUV) was designed, and saw limited service prior to the end of World War II and during the Korean conflict. It suffered froin the same disadvantages as the Canadian and British Kangaroos and was still open-topped. One serious drawback was that it lacked overhead armor. With the introduction of the VT fuse to the battlefield this consideration was of prime importance. However, it was followed fairly closely by the 1A44 which had armor protection all round and which was designed from the start as an armored personnel carrier.
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