Kingfisher B, or SWOD Mark 21, was of similar design to Kingfisher A, but was designed to be lightweight, the design calling for a weapon only 2/3 the weight of the Bat guided bomb. Its payload was intended to be a plunge bomb, an unguided projectile that upon release would glide briefly along a ballistic trajectory before sinking alongside a target and detonating underwater. Unpowered, it was also cancelled in 1947 as no longer meeting requirements. Kingfisher C Kingfisher C was the only member of the Kingfisher family to reach operational service. Developed from the SWOD Mark 15 airframe, and given the definitive designation AUM-N-2 Petrel, construction of the missile was contracted to the Guided Missiles Division of Fairchild Aircraft. As designed, Petrel was essentially a Mark 21 Mod 2 torpedo fitted with flying surfaces and a Fairchild J44 turbojet; the missile had a range of at Mach 0.5, and used semi-active radar homing for guidance. By the time Petrel entered operational service in 1956, the Mark 41 torpedo had been selected as its definitive warload; launched by the P2V Neptune patrol aircraft, Petrel was withdrawn from operational use by 1959, as it was useless against submerged submarines and the U.S. Navy placed a low priority on defense against surface vessels, considering them an insignificant threat by comparison. Surviving Petrels were repurposed as target drones, which were redesignated as AQM-41A shortly before leaving service in the early 1960s. Kingfisher D Kingfisher D was similar in concept to Kingfisher C, but was intended to utilize a new torpedo with a novel dual-purpose propulsion system; a solid-propellant rocket would provide thrust both during the flight of the missile and, following the release of the torpedo, underwater during its terminal run. Given the designation AUM-N-4 Diver, the torpedo's propulsion system proved too complex to successfully develop, and the missile was subsequently cancelled. Kingfisher E The only surface-launched variant of the Kingfisher family, Kingfisher E evolved into the SUM-N-2 Grebe anti-submarine missile, intended to deliver a Mark 41 torpedo at ranges of up to from the launching ship. Construction of Grebe was contracted to the Goodyear Aircraft Company; it was powered by a solid-propellant rocket in its base version, while pulsejet-powered variants were planned to extend the range of the weapon to . Flight testing of the missile began in 1950, but it was soon cancelled as sonar systems of the time were incapable