IMPROVING THE M18

Complaints from tank destroyer units about the vulnerability of the open turret design led to some belated efforts to develop a roof armor kit seen here during trials on a late production M18 (40146280). This came too late and was never authorized for production. (Patton Museum)

The Hellcat's 76mm gun often could not penetrate the thick sloped front armor of German Panther tanks. The Hellcat had very little armor, and the use of high flash powder made the Hellcat a highly visible and vulnerable target for German tank crews.

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 nearly all fully tracked American tank destroyers—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 a relatively 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. The HVAP ammunition made it possible for the 76mm to penetrate the frontal turret armor of Panther tanks are somewhat close range, giving it a possibility of taking on some of the heavier German tanks at close range.

Using the M62 APC round, the 76mm gun penetrated 109 mm (4.3 in) of armor at 1,000 m (3,300 ft), with a muzzle velocity of 792 m/s (2,600 ft/s). The HVAP round was able to penetrate 178 mm (7.0 in) at 1,000 m (3,300 ft), with a muzzle velocity of 1,036 m/s (3,400 ft/s). This was enough to penetrate a Panzer IV medium tank frontally, but not enough to deal with the newer Panther or Tiger from the front. The average combat range noted by the Americans for tank vs. tank action was around 800 to 900 m (2,600 to 3,000 ft). If facing a Panther, a Hellcat would be facing a tank with roughly 140 mm (5.5 in) of line-of-sight armor frontally that could not be pierced at any range, with a 100 mm (3.9 in) mantlet that could be pierced only at suicidally close range. However, the 76 mm gun could knock out the Panther with relative ease from the flanks and rear, as the turret and side armor was weak and the quality of German armor plate declined in the last two years of the war. While Panthers in defense were formidable, Panthers in the attack had great difficulty not exposing their vulnerable large side profiles. The tank destroyers often hid where only the commander was exposed over a ridge, who then had the gunner point the cannon in the general direction of the enemy before pulling forward. This allowed the tank destroyers to remain mostly behind cover. After emerging from the cover and engaging the enemy, the tank destroyer would then use its speed to change positions, while other tank destroyers took turns engaging the enemy with a nearly continuous bombardment.

There were a number of small changes introduced in the gunner's controls during the production run. For example, the later production vehicles had a M20 azimuth indicator added behind the hand-traversing mechanism which led to the repositioning of the turret lock further forward on the turret ring. One of the lingering problems with the M18 was the shock absorber configuration and in October 1944, there was yet another effort to introduce heavy-duty shock absorbers. The final production batch of M18 76mm GMC in the autumn of 1944 had the wider 21-in. T82 track, along with widened dust-guards and a new dustguard strip along the lower lip of the hull.

As to why no top armor? Two reasons:

1 ) It made them lighter. Automotive performance was a top priority. TDs were supposed to be a very mobile force, held in reserve for the anticipated armored break-through, and then rushed to the key sector to shoot up the panzers. They needed to be able to beat a panzer formation to the key terrain where they would lay their ambushes, to inflict casualties on to panzer formations, and then to withdraw in safety before getting shot up themselves, in order to repeat the process again and again until the panzers were beaten. To achieve the level of performance they wanted, anything armor beyond basic splinter-proof protection was left off as excess weight.

2 ) It improved situational awareness. One key to success in a TD was to see first, so you could shoot first. In a TD the whole turret crew was heads-out. With no roof armor, there were no hatches, so the commander, gunner AND loader all kept an eye on the surroundings when not working the gun on a target. This was considered an important edge in anti-tank combat. Interestingly, even when turret-roof armor kits were made for the M10 and M36, they were mounted on spacers that put a few inch high gap between turret sides and roof, so that the turret crews still kept their all-around visibility.

One unit to use it quite effectively was the 630rd Tank Destroyer Battalion in July 1944 reported destroying 53 Panthers & Tigers and 15 self propelled guns with only a loss of 17 of their M18s.

This tactic was effective enough that it is sometimes credited with allowing the Hellcat to become the most deadly tank killer of the war.

	Commander, driver, gun crew (3)
Physical Characteristics
Weight	40,000 lb
Length w/gun	21' 10"
Length w/o gun	17' 4"
Height	8' 5"
Width	9' 9"
Ground clearance	14"
Ground contact length	9' 8"
Ground pressure	12.5 psi
Turret ring diameter
Armament (mm)
Main	76 mm M1A1, M1A1C, or M1A2
Secondary
MG	M2 .50 cal Browning M1919A4, AA
Side arms	M3 .50 cal Tripod mount 5: M1 Carbines Grenades Smoke Pots
Quantity
Main	45 (HE, APC, HVAP, Canister, Smoke)
Secondary
MG	800
Side arms	450 12 4
Armor Thickness (mm)
Hull Front, Upper	12
Hull Front, Lower	12
Hull Sides, Upper	12
Hull Sides, Lower	12
Hull Rear	12
Hull Top	8
Hull Bottom	7
Turret Front	25
Turret Sides	12
Turret Rear	12
Turret Top
Engine (Make / Model)	Continental R-975 C1
Transmission	3 forward 1 reverse
Capacity	165 gallons
Performance
Traverse	360°
Max Speed	45-50 mph
Cross Country	20 mph
Road radius	150 miles
Turning Radius
Elevation Limits	+19.5° to -10°
Fording depth	4'
Trench crossing	6' 2"
Vertical Obstacle	3'
Suspension (Type)	Torsion bar
Wheels each side	5
Return rollers each side	4
Track length
Tires
Track width	14.25"
Track centers/tread	10' 10.25"
Production	Buick 1943-: 2,507