AU-1: U.S. Marines attack variant with extra armor to protect the pilot and fuel tank, and the oil coolers relocated inboard to reduce vulnerability to ground fire. The supercharger was simplified as the design was intended for low-altitude operation. Extra racks were also fitted. Fully loaded for combat the AU-1 weighed 20% more than a fully loaded F4U-4, and was capable of carrying 8,200 lb of bombs. The AU-1 had a maximum speed of 238 miles per hour at 9,500 ft, when loaded with 4,600 lb of bombs and a 150-gallon drop-tank. When loaded with eight rockets and two 150-gallon drop-tanks, maximum speed was 298 mph at 19,700 ft. When not carrying external loads, maximum speed was 389 mph at 14,000 ft. First produced in 1952 and used in Korea, and retired in 1957. Re-designated from F4U-6.