Su-24 FENCER (SUKHOI) - T-58M / T6-1 STOL Prototype
By the end of the 1950s the basis of Soviet tactical strike aircraft were obsolete bombers IL-28, gradually replacing the Yak-28 and Su-7B. The bomber Yak-28 was unsuccessful and did not receive wide distribution. The Su-7B fighter-bomber was undoubtedly one of the best front-line strike aircraft at the time, with combat capabilities are not inferior to the American F-105s. The timely launch of the Su-7B in the large-scale production significantly increased the combat potential of the Soviet Air Force.
However, in the early 1960s there were significant changes in the doctrine of the application of tactical strike aircraft. Under the changed requirements, the US firm General Dynamics is developing a fighter-bomber of the new generation - F-111. In the Soviet Union to begin planning attack aircraft design bureau PO Sukhoi. Work in the KB unfolded simultaneously in two directions: the modernization of the fighter-bomber Su-7B and the creation of a fundamentally new frontal bomber.
On 24 January 1961, the USSR Air Forces received the Su-7B fighter-bomber into service. At the insistence of the military, the Government's resolution making the aircraft operational also assigned the design bureau the task of developing a new all-weather version to enable destruction of small targets. The document laid the foundations for the effort of the P.O. Sukhoi Design Bureau to develop the Su-24.
The very first studies undertaken at the New Projects Department of the Sukhoi Design Bureau in 1961-62 under code names S-28 and S-32 made it obvious that it was not possible to do the job by upgrading the original Su-7B, seeing that the extent of the equipment to be installed on board the aircraft was far in excess of available space on the Su-7. A major problem in developing the new plane was to design the aiming and navigation system (ANS) to give the pilot as much automatic control over all major flight and tactical deployment procedures as possible. This Herculean task was given to Design Bureau OKB-794 (later known as Leninets SPA), with Ye.A. Zazorin appointed as chief designer of the system. The system itself became known under a code name of Puma.
Designing a new bomber, Sukhoi began development of two radically changed airplanes. In 1962, the Design Bureau began designing a radically new aeroplane, referred to as S-6. The aeroplane had a standard aerodynamic configuration with a thin tapered wing featuring moderate sweepback, with two R21F-300 type engines developed by N.G. Metsvarishvili, and lateral variable air intakes with horizontal airbrake. The crew of two (pilot and navigator/weapon systems officer) were seated in the cockpit in tandem, one behind the other. 1963 saw S-6 conceptual design and a full-scale mock-up produced. An Air Forces commission reviewed the materials and the mock-up, but further work on the project was suspended in connection with a lack of noticeable progress in the development of the Puma ANS.
According to the original project, which received the designation S-6 [C-6], the plane still had swept wings. The power plant was two turbojet P-21F-300 with afterburner thrust of 7,200 kg. In addition, to improve takeoff performance would be used solid rocket boosters. The crew (pilot and navigator-operator) were located in the cockpit in tandem. The bomber was equipped with a five-point suspension arms: one under the fuselage and four wing. To increase the duration of the flight could be used three external fuel tank. The aircraft was supposed to set for him developed by sighting and navigation system "Puma". The estimated maximum speed of the C-6 from the land -1400 km / h at high altitude - up to 2500 km / h. The estimated take-off weight - 20000 kg, the weight of payload - 3000 kg.
Starting 1964, the work to produce an attack aircraft at the Sukhoi Design Bureau was continued under a new code name, T-58M, to stand for a version of the Su-15 plane (factory code T-58). The Air Forces adjusted their performance requirements (PR) for the plane to position it now as a low-altitude attack aircraft with a shortened takeoff and landing run (STOL), a major requirement of the military being the ability to sustain a long supersonic flight at a low altitude to penetrate hostile airspace.
As the studies progressed, the design concept gradually became more focused. For example, in the summer of 1965, given an increased cross dimension of the Orion sighting station antenna in the nose section of the fuselage, it was decided to introduce a new solution for the cockpit layout, with the crew seated side by side. The powerplant consisted of two R-27F-300 type turbojet thrusters with afterburning developed by the Design Bureau of S.K. Tumansky, and four RD36-35 type lift booster engines by the Design Bureau of P.A. Kolesov for STOL performance. To try out the combined powerplant and lift-engine-boosted STOL design, the first Su-15 prototype aircraft (T58D-1) was used by the Design Bureau to build a flying laboratory, T-58VD, which was tested between 1966 and 1969, with the flights carried out by the design bureau's test pilot Ye.S. Solovyov.
Officially, the go-ahead for the development of the aircraft was given by a decree of the government on 24 August 1965. The Design Bureau assigned to the project the working name of T-6. In March 1966, the conceptual design and mock-up were approved, with detailed design completed at the end of 1966. Two prototypes were built concurrently: one for flight testing and the other for structural testing. The first flying prototype, T6-1, was completed in May 1967. On 29th June, it was brought to the FRI airfield and the very next day saw the design bureau's chief test pilot V.S. Ilyushin perform the first airfield run. On 1st July, a meeting of the FRI advisory board gave the green light for the factory tests to begin.
Just next day, 2nd July 1967, V.S.Ilyushin performed the plane's first flight. The urgency of the testing was due to the fact that T6-1 was entered for participation in the Domodedovo air display scheduled for 9th July. But on 4th July, the second test flight resulted in an accident: the left hinged canopy panel was torn off while the plane was in the air. The flight was brought to a successful conclusion, V.S. Ilyushin landing the T6-1 on the airfield. The plane's canopy design was immediately reengineered, with the next flight taking place the following day, but the T6-1's participation in pageant was cancelled. As a result, a presentation of the aircraft to western observers failed to take place in 1967.
Initially, the T6-1 was flight tested without the lift booster engines installed. These were fitted in the aeroplane during the engineering follow-up in October 1967, the R-27 type engines being at the same time replaced with regular AL-21F turbojets with afterburning developed by the Design Bureau of A.M. Lyulka. Following this the plane was tested as a STOL aircraft between November 1967 and January 1968. The testing findings confirmed the data obtained earlier in the course of testing the T-58VD. The improved takeoff and landing characteristics (TOLC) in no way compensated for the aeroplane's shortened range. This was due to the impaired fuel performance as a result of the weight of the lift engines.
The other drawbacks of "lifters" included:
- hazardous impact of the exhaust gases on the plane's structural members and landing gear,
- the impossibility of suspending external stores and ordnance from the bottom of the fuselage.
- the aeroplane's pronounced overbalancing in the forward-aft axis upon their activation.
The faults of the concept obviously overshadowing its advantages, it was necessary to look for alternative solutions to improve takeoff and landing characteristics (TOLC).
Despite the fact that the Soviet government at the time was well funded, getting the money for the design of the aircraft on its own initiative was not easy. Therefore, the management of OKB went on a trick: the C-6 in the outgoing documents had code T-58m. At that time the firm was working on twin-engined interceptor T-58 (the prototype of the Su-15). Presenting the new bomber as a deep modification of the interceptor OKB failed to obtain the necessary funds to continue the design. However, work on the C-6 were subsequently discontinued.
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