Authored By Dan Alex (Updated: 5/6/2016):
The Sukhoi Bureau and Their Su-7 "Fitter"
The Sukhoi design bureau had its origins in wartime 1939 and became something of a household name throughout World War 2. In the post-war world - 1959 to be exact - the firm delivered their first Su-7 "Fitter" jet-powered fighters to the ranks of the Frontal Aviation (Frontovaya Aviahtsiya), this being the tactical component of the Soviet Air Force. The initial Su-7 was a dedicated tactical air superiority fighter design while the follow-up Su-7B was introduced as a ground attack version in 1961. One of the pre-requisites of the Soviet Air Force's acceptance of the Su-7 into service was that the firm also develop a suitable all-weather strike variant to be tied with a new navigation/attack system known as the "Puma".
Sukhoi set about on some preliminary work in trying to utilize the existing Su-7 airframe for the all-weather strike task. It was quickly realized that the Su-7's airframe was ill-suited for the amount of additional avionics and equipment that would be required in making the Fitter a right "fit" for the role. As a result, other designs solutions were entertained.
While introduction of the Fitter did well to strengthen the Soviet Air Force's aging fleet of fighters, there was no true solution in beefing up her long-range bomber arm. In the inventory remained the aged Ilyushin IL-28, introduced in 1950 and categorized as a medium bomber. The Yakovlev Yak-28 "Bewer"/"Firebar"/"Maestro" series was unveiled in 1958 and first flew in 1960, intended to become the answer to the growing Soviet question. However, once in service, the Yak-28 was quickly noted for its disappointingly short operational ranges and small payload capabilities - two qualities critical to a successful bomber design in the Cold War. Additionally, weapons firing and general munitions delivery accuracy left something to be desired by the Soviet Air Force. Regardless, some 1,180 such systems were placed into circulation and the type was developed to fulfill a variety of battlefield functions.
The F-111 Outshines Them All
Soviet personnel were not blind to the aviation progress being made in the United States as per their USAF General Dynamics F-111 Aardvark twin-engine, swing-wing, two-seat fighter-bomber. The aircraft began its life in 1964 and was officially introduced in 1967, pioneering a variety of technological advancements in the realm. The F-111 featured a terrain following radar (as opposed to terrain avoidance radar), variable sweep of her swing-wing assemblies and an afterburning turbofan engine. Swing-wing capabilities allowed an aircraft to vary its drag during different phases of flight - landing, take-off, cruise.
While practical in both use and design, a "swing-wing" approach often involved a complicated set of internal working functions that had to be incorporated into an already complicated aircraft design. Only a few aircraft have ever fielded variable sweep wings and this included such notables as the Grumman F-14 Tomcat, Panavia Tornado, Rockwell B-1 Lancer and Tupolev Tu-160 "Blackjack" - all appearing some time later. Terrain following radars served the aircraft by allowing it to fly at supersonic speeds just underneath the perceived radar screen and deliver munitions from this low altitude with accuracy. It should be noted, however, that the F-111 itself overcame a variety of obstacles during her development - these being both political and technological in nature - to become the 560+ examples delivered to the USAF. Success, it seemed, came at a cost from any angle. On the other hand, the new breed of military aircraft being fielded by the Americans now outclassed just about any Soviet offering at the time.
Of similar note here is the technological advancements made in the world of surface-to-air detection and engagement - also playing a key role in the growing Soviet long-range bomber need. The current crop of Soviet bombers were ill-suited in their operations against such newer active defenses being fielded by NATO allies. For the Soviets, what once appeared to be a small hole in the dike initially had now exploded to become a bleeding chasm of desperation to some extent.
Sukhoi set to work to fulfill the new Soviet air arm requirement. They set their eyes on a long-range interdiction bomber design mimmicking the performance capabilities of the American F-111 without the complicated swing-wing functionality. The culmination of this early design work revealed the S-6, a mockup delta-wing model design fitting a pair of side-by-side Tumansky R-21F-300 turbojet engines. The aircraft would be piloted by a crew of two personnel seated in tandem, this helping to achieve a slimmer forward profile. While the design was ultimately reviewed, the progress was eventually nil as the "Puma" nav-attack system development was lagging sorely behind. Further tests of the mockup also revealed some key limitations that would ultimately doom the S-6 as a viable solution.
The S-58VD Flying Laboratory
1964 saw the Sukhoi firm press their attention onto the "S-58M", a modified form of their Su-15 "Flagon" interceptor product. By this time, the Soviet Air Force came back with a revision to their original requirement and sought an airframe with STOL (Short Take-Off and Landing) capability. This provided for something of a mechanical nightmare to the internal design layout of the new Sukhoi aircraft. Not only was the type to feature two afterburning turbojet engines for the supersonic speed requirement, she was now required to showcase STOL qualities through the implementation of four smaller turbojet engines for improved vertical ascension. The powerplants of choice became a pair of Tumansky R-27F-300 base engines and four Kolesov RD-36-25 series turbojets to fulfill the vertical thrust role. The radar of choice came under the "Orion" name. An existing Su-15 airframe was converted for the new engine arrangement to become the developmental "S-58VD". The new S-58VD was given a wider cross-section to make room for the crew of two seated side-by-side as well as the wider radar system to be mounted in the nose. Comparatively, the S-58VD shared a general overall external appearance to the Su-15 it was developed from (including the dual Tumansky engine layout and single vertical tail fin) but the wider forward profile quickly differentiated the type from the original.
The S-58VD featured a long, relatively flat nose cone ahead of the two-man cockpit. The pilot and co-pilot sat under a canopy not unlike that as featured in the F-111 Aardvark. Each professional sat in "zero-zero" Zvezda K-36D series ejection seats, allowing for ejections at any airspeed and at any altitude (in contrast to earlier seat designs that required certain speed and altitude minimums). The streamlined fuselage contoured rather elegantly into the squared-off rear body. Rectangular intakes were fitted to either side of the fuselage and this boxy shape carried on to the aft portion of the airframe. Wings were fixed and swept along their trailing edges in a sort of "double-delta" format, with each wing featuring a pair of underwing weapon stations. The tail section was dominated by the single vertical fin and applicable horizontal tail surfaces to each aft fuselage side (the latter all-moving surfaces). The undercarriage was conventional and made up of two double-tired main landing gear legs and a nose landing gear leg fitted a pair of smaller wheels. Of particular note in the S-58VD design were the four RD36-35 lift engines buried within the middle portion of the fuselage at about amidships. The upper panel of each intake sported doors meant to open and aspirate the engines when they were activated for STOL flight. The developmental aircraft was now assigned the more formal designation of "T-6" in 1965.
The S-58VD design itself would go on to become a flying laboratory of sorts, collecting STOL data from 1966 into 1969. Her captured information proved invaluable and showcased a loss of operational range over the gain of her STOL capability. Lift engines were always something of a peculiar love affair with aircraft engineers since the prospect of vertical flight was born with Leonardo Da Vinci. However, in 1960s terms, the devices required to achieve such flight were still rather large contraptions taking up vital internal space and making up critical weight increases for very little tactical gain. The areas within such an aircraft were best suited for the carrying of internal fuel stores to feed thirsty afterburning turbojet engines. Control of the S-58VD during the STOL process was also noted to be adequate at best, requiring much attention and direct pilot interference. True STOL operation would not be realized until the arrival of the subsonic British Harrier jump jet and perfected to an extend by the upcoming stealthy Lockheed F-35 Lightning II.
The T6-1 Prototype
The initial T6 prototype became the "T6-1", completed in May of 1967 and launched for the first time on July 2nd of that year. Initial evaluations were conducted sans the four lift jets to do away with early complexities during in-flight performance testing. The lift jets were not installed on the T6-1 until October of 1967 and the primary Tumansky R-27F-300 turbojet engines were later replaced in whole by a new pair of Lyulka-brand AL-21F series engines.
The F-111 Raises Eyebrows
The Soviet Air Force vision for the new aircraft was decidedly different than the T6-1 being offered from Sukhoi. Testing revealed that the design - in her current form - lacked the desired function sought. The 1967 display of the American F-111 Aardvark at the Paris Air Show only served to drive home the painful point - her variable wing, low-level flying was duly noted by aircraft firms all over the world - Sukhoi and her Soviet overseers being no exception. Her variable geometry wing design proved the swing-wing concept sound for a powerful mount such as the Aardvark. The Sukhoi firm finally steered itself in the direction of applying a similar swing-wing style function to their T6.
The T6-2I Prototype
Work on a new T6 prototype began on August 7th, 1968, producing the T6-2I. The T6-2I was essentially the T6-1 airframe mated to a new variable geometry wing system though the four lift jets were formally dropped. First flight was achieved on January 17th, 1970 and testing continued on until 1976, covering some 300 total flights. Seventy-three flights were made by T6-2I in 1971 and covered basic flight dynamics incorporating the various wing sweep settings. Only later were more complex systems thrown into the mix (including the automated flight system) and most evaluations were handled at low altitudes. A second swing-wing prototype, the T6-3I, was included beginning in late 1970. The T6-3I completed 90 flights in 1971 and contributed 300 flights of her own. A fourth swing-wing prototype, the T6-4I, began evaluation service on June 16th, 1971 but was lost to accident in 1973 after completing just 120 flights. Final evaluation of the T6 program concluded in 1976 with various landing scenarios to test out the airframe during rough-field, unpaved operations.
Further development of the T6 eventually included the incorporation of the "Puma" nav-attack system coupled with the "Orion-A" attack radar. Terrain avoidance was handled by the Relyef radar and featured an automatic flight guidance system. The design was furthered into production-standard forms within time. Even before the new aircraft had completed her various test regimes, however, the Soviet Air Force - happy with what it saw in its IL-28 replacement - quickly ordered the type for production under the designation of "Su-24".
Production was selected to begin at Factory No. 153 at Novosibirsk. The first production Su-24 went airborne on December 31st, 1971, with test pilot Vladimir Vylomov at the controls. Early Su-24s ("Fencer-A") were fielded in small numbers and this limited to trials units, leaving the Fencer-Bs as the true initial production face of the line. Fencer-C came online some time later and sported improved avionics. Though differences between the three were many, they were more-or-less minor enough to not warrant new designations from Sukhoi. Only NATO required the varying "Fencer" designations for their own nomenclature.
Her rush to delivery produced a few teething problems from the start, forcing several - though multiplying - modifications to be enacted (both to current and former aircraft) based on operational feedback from crews. Such changes included the enlarging of an internal fuel tank to increase the aircrafts range. Another included am aerodynamic revision of key rear fuselage surfaces in an effort to reduce airframe drag. The brake chute was relocated and a ram-air inlet was installed at the base of the tail fin. While early Su-24s sported variable intake ramps within their intake openings, this was later dropped to improve maintenance requirements and save on weight. The removal of these devices dropped the overall speed of the Su-24 from Mach 2.18 to Mach 1.35 - as devastating as this may have appeared, the expected low-level operations of the Su-24 suffered little in the move. The countermeasure capabilities of early Su-24s were progressively updated to include improved radar warning and missile launch warning coupled with integral onboard jamming equipment.
The Su-24 would not be formally accepted into service until February 6th, 1975. Her slab-sided fuselage eventually earned her the nickname of "Suitcase" from her crews. The West would not find out about the Su-24 until perhaps 1974 to which the incorrect designation of "Su-19" was assigned. Interestingly, this mistake would not be officially corrected until 1981. In true NATO fashion, the codename of "Fencer" was applied to the Su-24 - "F" being for "Fighter" ("B" was reserved for Bombers as in the "Brewer", "Bear", "Bison" etc...). It was not until the Soviet Air Force began arriving at East German air fields in 1979 did many of the fantastic Western estimates about the new Soviet aircraft finally dissolve.
The Su-24's external design owed much to both the T6 prototypes as well as the Su-15 "Flagon" from which it was directly developed from. She sported a long oblong fuselage with flat surfaces along all her sides save for the upper surface. The cockpit was situated aft of the nose assembly housing the important radar suite. The operators (pilot and systems officer) sat in a side-by-side arrangement under a glass canopy featuring relatively light, unobtrusive framing. There was a forward canopy portion split at the center and a main canopy portion that opened as individual assemblies, each hinged along their respective rear frames. The pilot and his radar operator entered/exited the aircraft by way of individual "clip-on" ladders to the left and right sides of the forward fuselage. This was a necessity for the Su-24 sat a good distance from the surface of the tarmac - herself measuring some 20-feet, 4-inches in overall height. By any measure she proved a very large design when viewed up close. As in her prototype, the air intakes were mounted along the sides of the fuselage and sported rectangular openings to aspirate their respective turbojet engines buried further aft in the fuselage. Of note here was use of splitter plates to divert "dead air" away from the intake mouth and fuselage sides. Interestingly, the rounded curves of the forward fuselage quickly gave way to the true Soviet-style Cold War-era design of sharp angles and flat, featureless surfaces found further aft. The tricycle undercarriage remained largely unchanged from the prototypes.