The Sukhoi Su-15 (NATO reporting name: Flagon) is a twinjet supersonic interceptor aircraft developed by the Soviet Union. It entered service in 1965 and remained one of the front-line designs into the 1990s. The Su-15 was designed to replace the Sukhoi Su-11 and Sukhoi Su-9, which were becoming obsolete as NATO introduced newer and more capable strategic bombers. Development Recognizing the limitations of the earlier Su-9 and Su-11 in intercepting the new Boeing B-52 Stratofortress, particularly in terms of radar and aircraft performance, the Sukhoi OKB quickly began the development of a heavily revised and more capable aircraft. A variety of development aircraft evolved, including the Sukhoi T-49, which shared the fuselage of the Su-9 (including its single engine), but used cheek-mounted intakes to leave the nose clear for a large radome for the RP-22 Oryol-D ("Eagle") radar (NATO "Skip Spin"), and the T-5, essentially a heavily modified Su-11 with a widened rear fuselage containing two Tumansky R-11 engines. These led to the T-58, which combined the twin engines with a modified version of the T-49's nose, but with side inlets further back, behind the cockpit. It was approved for production on 5 February 1962, as the Su-15, and the prototype first flew on 30 May 1962. It entered service testing 5 August 1963, but its service entry was delayed by political infighting with the Yakovlev OKB over production line capacity in Novosibirsk, which was also building the Yak-28P. The Su-15 proved to be superior in most respects other than range, and it was officially commissioned on 3 April 1965. Series production began the following year, and it entered service with the PVO in 1967, replacing Su-9s, Su-11s, and Yakovlev Yak-25s. The initial Su-15 received the NATO reporting name "Flagon-A". A simplified trainer version, the Su-15UT (NATO "Flagon-C"), with no radar or combat capability, entered service in 1970. Initial delta-winged Su-15s had poor take-off and landing characteristics, and so Sukhoi investigated a new wing design with extended wingtips (increasing wing area) and boundary layer control. Su-15s with the new wing went into production in 1969. They were dubbed "Flagon-D" by NATO, although the Soviet designation was unchanged. Also in 1969 testing began of the upgraded Su-15T with the Volkov Taifun ("Typhoon") radar, which was based on the MiG-25's powerful RP-25 Smerch-A ("Tornado") radar (NATO "Foxfire"). The Taifun proved troublesome, however, and ceased production after only 10 aircraft had been built.