The Republic F-105 Thunderchief was the largest single-seat, single-engine combat aircraft to enter production. It was conceived as a supersonic low-altitude fighter-bomber capable of delivering a heavy nuclear payload. Although never used as intended, the Thunderchief did see extensive combat. The F-105 carried the brunt of the US bombing campaign during the early part of US involvement in Vietnam by completing more than 20,000 sorties. Although serviced and flown by brave and competent men, the extremely sophisticated Mach 2-capable F-105, or Thud as the type was nicknamed, was ill suited for repetitive missions in humid Southeast Asia. Maintenance requirements exceeded 100 man-hours labor per flight hour. Further, combat losses rose to almost half of the 833-unit production total. Ultimately, the F-105 was withdrawn from the theater.