the U.S. and Canada, Douglas abandoned development in 1956. Canadair continued development until the Arrow was cancelled in 1959. Sparrow X A subvariant of the Sparrow I armed with the same nuclear warhead as the MB-1 Genie was proposed in 1958 but was cancelled shortly thereafter. Sparrow III Concurrently with the development of the Sparrow I, in 1951, Raytheon began work on the semi-active radar homing version of Sparrow family of missiles, the AAM-N-6 Sparrow III. The first of these weapons entered United States Navy service in 1958. The AAM-N-6a was similar to the -6, but used a new Thiokol liquid-fuel rocket engine for improved performance. It also included changes to the guidance electronics to make it effective at higher closing speeds. The -6a was also selected to arm the Air Force's F-110A Spectre (F-4 Phantom) fighters in 1962, known to them as the AIM-101. It entered production in 1959, with 7500 being built. Another upgrade reverted to a Rocketdyne solid-fuel motor for the AAM-N-6b, which started production in 1963. The new motor significantly increased the maximum range to for head-on attacks. During this year the Air Force and Navy agreed on standardized naming conventions for their missiles. The Sparrows became the AIM-7 series. The original Sparrow I and aborted Sparrow II became the AIM-7A and AIM-7B, despite both being out of service. The -6, -6a, and -6b became the AIM-7C, AIM-7D, and AIM-7E respectively. 25,000 AIM-7Es were produced and saw extensive use during the Vietnam War, where its performance was considered disappointing. The mixed results were a combination of reliability problems (exacerbated by the tropical climate), limited pilot training in fighter-to-fighter combat, and restrictive rules of engagement that generally prohibited BVR (beyond visual range) engagements. The Pk (kill probability) of the AIM-7E was less than 10%; US fighter pilots shot down 59 aircraft out of the 612 Sparrows fired. Of the 612 AIM-7D/E/E-2 missiles fired, 97 (or 15.8%) hit their targets, resulting in 56 (or 9.2%) kills. Two kills were obtained beyond visual range. In 1969 an improved version, the E-2, was introduced with clipped wings and various changes to the fuzing. Considered a "dogfight Sparrow", the AIM-7E-2 was intended to be used at shorter ranges where the missile was still travelling at high speeds, and in the head-on aspect, making it much more useful in the visual limitations imposed on the engagements. Even so, its kill rate was