It was followed in December 1971 by the Su-15TM (NATO "Flagon-E"), with the improved Taifun-M radar (NATO "Twin Scan") and provision for UPK-23-250 gun pod or R-60 (AA-8 "Aphid") short-range air-to-air missiles. Aerodynamic demands forced a redesign of the radome with an ogival shape, earning a new NATO reporting name, "Flagon-F", although again the Soviet designation did not change. A comparable combat-capable trainer, the Su-15UM (NATO "Flagon-G"), followed from 1976. The final Su-15UMs, the last Su-15s produced, came off the line in 1979. Various OKB proposals for upgraded Su-15s with better engines and aerodynamics to satisfy a VVS requirement for a long-range tactical fighter were rejected in favour of the Mikoyan MiG-23 fighter. Design Although many components of the Su-15 were similar or identical to the previous Su-9 and Su-11 (NATO reporting name "Fishpot"), including Sukhoi's characteristic rear-fuselage air brake, the Su-15 abandoned the shock-inlet cone nose intake for side-mounted intake ramps with splitter plates feeding two powerful turbojet engines, initially the Tumansky R-11F2S-300. The change allowed room in the nose for a powerful search radar, initially the RP-22 Oryol-D (NATO 'Skip Spin'). The early Su-15 ("Flagon-A") had pure delta wings like its predecessors, but these were replaced from the 11th production series onward by a new double-delta wing of increased span and area, with a small wing fence above each outer pylon and blown flaps to improve landing characteristics. This was accompanied by a new tail of greater anhedral and a vertical fin of reduced height. The Su-15 had maximum speed of Mach 2.5 and a rate of climb of 228m/s (750 ft/s, 45,000 ft/min), a very important parameter for an interceptor aircraft. Take-off and landing speeds were comparatively high, with a take-off speed of for early delta-winged 'Flagon-A's and for the larger-winged 'Flagon-F'. While the controls were responsive and precise, the aircraft was unforgiving of pilot error. Despite its powerful radar, the Su-15, like most Soviet interceptors before the late 1980s, was heavily dependent on ground control interception (GCI), with aircraft vectored onto targets by ground radar stations. It was fitted with the Lazur-S datalink system, which transmitted instructions to the pilot to accomplish the interception. The later Su-15TM had a Vozdukh-1M datalink and SAU-58 (sistema automaticheskogo upravleniya, automatic control system) capable of carrying out completely automatic, 'hands-off' interceptions until the last moments of the interception. Primary armament of the Su-15 was the R-8/K-8 (AA-3 "Anab"; later R-98)