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The Viggen (Thunderbolt) was initially developed as a replacement for the Saab 32 Lansen in the attack role and later the Saab 35 Draken as a fighter. The first studies were carried out between 1952 and 1957 involving the legendary Finnish aircraft designer Aarne Lakomaa. Several different concepts were studied involving both single- and twin engines and also with separate lift engines, both simple and double delta wings and also with canards. Even VTOL designs were considered. The aim was to produce a robust aircraft with good short-runway performance that could be operated from numerous specially prepared roads and highways to reduce the vulnerability to attack in the event of war. Other requirements included supersonic ability at low level, Mach 2 performance at altitude, and the ability to make short landings at low angles of attack (to avoid damaging improvised runways).
To meet these design goals, Saab selected a radical configuration : a conventional delta wing with small, high-set canard wings. Canards have since become common in fighter aircraft, notably with the Eurofighter Typhoon and the IAI Kfir, but mainly for agility reasons rather than STOL capabilities. The final proposal was presented and accepted on 28 September 1962. Construction started in 1964, with a first prototype maiden flight on 8 February 1967.
The aircraft was also designed from the beginning to be easy to repair and service, even for personnel without much training.
In 1960 the U.S. National Security Council, led by President Eisenhower, formulated a military security guarantee for Sweden. The U.S. promised to help Sweden militarily in case of a Soviet attack against Sweden. As a consequence of this Sweden and the U.S. signed a military-technology agreement. In what was known as the "37-annex", Sweden was allowed access to advanced U.S. aeronautical technology which made it possible to design and produce the Saab 37 Viggen much faster and cheaper than would otherwise have been possible.
The reason for this officially unexplained U.S. support was, according to the doctoral research of Nils Bruzelius at the Swedish National Defence College, the need to protect U.S. Polaris submarines deployed just outside the Swedish west coast against the threat of Soviet anti-submarine aircraft.
The Viggen is powered by a single Svenska Flygmotor RM 8 turbofan. This is essentially a licence-built variant of the Pratt & Whitney JT8D engine that powered commercial airliners of the 60s, with an afterburner added for the Viggen. The engine also incorporates a thrust-reverser to use during landings and land manoeuvres, which, combined with the aircraft having flight capabilities approaching a limited STOL-like performance, enable operations from 500 m airstrips with minimal support. The thrust reverser can be pre-selected in air to engage when the nose-wheel strut is compressed after touchdown. Only the Viggen and the Panavia Tornado carry both afterburners and thrust-reversers.
The requirements from the Swedish Air Force dictated Mach 2 capability at high altitude and Mach 1 at low altitude. At the same time, short-field takeoff and landing performance was also required. Since the Viggen was developed initially as an attack aircraft instead of an interceptor (which role the Saab 35 Draken fulfilled), some emphasis was given to low fuel consumption at high subsonic speeds at low level for good range. With turbofan engines just emerging and indicating better fuel economy for cruise than turbojet engines, the former was favored, since the latter were mainly limited by metallurgy development resulting from limitations in turbine temperature. Mechanical simplicity was also favored, so the air intakes are simple D-section types with boundary layer splitter plates, while the fixed inlet has no adjustable geometry for improved pressure recovery. The disadvantage was that the required engine would be very large. In fact, at the time of introduction, it was the second largest fighter engine with a length of 6.1 m and 1.35 m diameter. Only the Tumansky R-15 was bigger.
SAAB had originally wanted the Pratt & Whitney TF30 as the Viggen powerplant. Since the engine was not finished in 1962 when the airframe vs. engine design size needed to be frozen, the civilian variant of the TF30 - the JT8D was chosen as the basis for a Volvo Aero modification instead. The RM 8 became the second operational afterburning turbofan in the world, and also the first equipped with a thrust reverser. It had a bypass ratio of around 1.07 :1 in the RM8A, which reduced to 0.97 :1 in the RM8B.
The AJ, SF, SH and SK 37 models had the first version RM 8A engine with uprated internal components from the JT8D it was based on. Thrust is 65.6 kN dry and 115.6 kN with afterburner. For the JA 37, the RM 8A was modified to an 8B by replacing one LP compressor stage with a fan stage and improved combustor, turbine and afterburner. Thrust is 72.1 kN dry and 125.0 kN with afterburner.
The engine was started via a small gas turbine, itself started by an electric motor. Standby power and cooling air for onboard avionics were supplied via an external cart. An internal battery permitted start of the starter turbine and main engine in absence of the standby power cart.
With the performance requirements to a large extent dictating the choice of the engine, the airframe turned out to be quite bulky compared to contemporary slimmer designs with turbojet engines. The first prototypes had a straight midsection fuselage that was later improved with a "hump" on the dorsal spine for reduced drag according to the area rule.
The wing had the shape of a double delta with a dogtooth added to improve longitudinal stability when carrying external stores.. Each dogtooth was also used as a fairing for a radar warning receiver (RWR) antenna.
The canards are positioned behind the inlets and placed slightly higher than the main wing, but are not movable as control surfaces. The purpose of the canard wings were to act as vortex generators for the main wing and therefore provide more lift. An added benefit was that they also improved roll stability in the transonic region around Mach 0.9. The canard flaps are deployed in conjunction with the landing gear to provide even more lift for takeoff and landing.
To withstand the stresses of no-flare landings, Saab made extensive use of titanium in the construction of the Viggen, especially in the fuselage; and incorporated an unusual arrangement for the main landing gear, in which the two wheels on each leg are placed in tandem. While such a layout is common in airliners and cargo planes, it is rare in fighters, but allows stowage in a thinner wing.
The tall single vertical stabilizer (45 degrees in the leading edge) is foldable to make it easier to store in hangars. After prototype testing of the SK, reduced longitudinal stability was discovered. To correct this, the vertical stabilizer was extended 10 cm and the pitot tube was moved from the top of the fin leading edge to about midpoint where a sawtooth was also incorporated. The JA model later used the same improvements.
The six tanks in the fuselage and wings held approximately 5,000 litres of fuel with an additional 1,500 litres in an external drop tank. The specific fuel consumption was only 0.63 for cruise speeds (fuel consumption was rated 18 mg/N dry and 71 with afterburner. The Viggen's consumption was around 15 kg/sec at maximum afterburner, which meant that the internal fuel was exhausted in just seven minutes due to the relative inefficiency of the turbofan over a turbojet at full afterburner. Performance comparisons with other aircraft from the same age are however slightly difficult, since no other fighter- or attack aircraft aside from the Harrier and Yak-38 were designed for STOL or VTOL capability.
The Viggen featured a Datasaab navigational computer, which by Swedish standards was a significant technological achievement for the time. It utilized the STRIL 60 system to be linked with the Swedish defence systems. The main sensor was an Ericcson PS-37 radar (I/J band) with several functions : air-to-ground and air-to-air telemetry, terrain avoidance, cartography, and radar navigation. There was also an RWR SATT system in the wings and the tail, a Decca Type 72 Doppler navigation radar, a series of electronic sub-systems, a landing-aid device (TILS) made by AIL, and a 200 MHz combat computer derived from the F-14 Tomcat. Externally there was also provision for BOZ and AQ series of ECM pods. In total, the electronics weighed 600 kg which was an impressive amount for a single-engine, late 1960s fighter.
A weapons load of up to 6,000 kg can be accommodated on seven hardpoints; including Rb 24, Rb 24J, Rb 28, Rb 71, Rb 74, Rb 99 air-to-air missiles and Rb 75, Rb 04, Rb 05A and Rb 15F air-to-ground missiles, four tube launchers for 135 mm rockets, and bombs included the 120 kg Virgo fragmentation type. Rockets are one of the more characteristic weapons of Flygvapnet and have warheads of several types : the 50 mm M56GP 4 kg armour-piercing, the M56B with 6.9 kg of HE, and the M70 with a 4.7 kg HEAT warhead.
Since a fighter variant was not initially considered, there was no internal gun on the first variants but two external gun pods could be carried each housing a 30 mm ADEN cannon with 150 rounds of ammunition. The later JA 37 fighter variant was equipped with an Oerlikon KCA 30mm internal cannon (126 rounds), in a conformal pod under the fuselage. The firing rate was selectable at 22 or 11 rounds/s, and the autopilot was slaved to the radar control to obtain the best precision when firing the cannon. The KCA cannon fired 50% heavier shells at higher velocity than the ADENs used in the attack Viggens, giving a much higher kinetic energy. This, in conjunction with the fire control system, allowed air-to-air engagements at longer range than other fighters.
One hundred and ten of the original, ground attack optimized variant, AJ 37 were built with the first operational squadron established in 1972 at Såtenäs.
A two-seat trainer was not initially planned since it was considered that new pilots could get enough experience with delta-winged aircraft on the SK 35 Draken trainer. Eventually, however, 18 SK 37 two-seat trainers were ordered and delivered in 1973. To make room for the second cockpit, one fuel tank and some avionics were deleted. The radar was also omitted limiting the weapons load to gun pods and unguided rockets.
A total of 26 of the SH 37 maritime reconnaissance and strike variant were built in 1974, replacing the S 32C Saab Lansen. Although fitted with radar and weaponry, the SH 37 Viggen could also undertake photographic missions with its single long-range camera, while external pods could carry a photographic day-set, a "Red Baron" IR set, an ELINT set, and AQ series ECM (made by SATT).
A further 26 of the SF 37 reconnaissance variant were also delivered to replace the S 35 Draken in 1975. These were recognizable by having an elongated nose, equipped with six cameras : two vertical for long-range acquisitions, four for low-level use, and an IR VKA 702 for night reconnaissance. Also, the "Red Baron" pod, with three IR cameras was widely used, as well as an ELINT set.
Although the Viggen was offered for sale worldwide, and regarded as a very competent aircraft, no export sales occurred. Reasons to explain Saab's failure to sell a competitively priced, highly advanced and well-respected aircraft include the Swedish government's relatively strict controls on arms exports to undemocratic countries, potential customers' doubts about continuity of support and supply of spare parts in the event of a conflict disapproved of by Sweden, and strong diplomatic pressure of larger nations. The United States blocked an export of Viggens to India in 1978 by not issuing an export license for the RM8/JT8D engine, forcing India to choose the SEPECAT Jaguar instead.
The Viggen saw initial service in natural metal, later receiving an extremely elaborate disruptive camouflage scheme for the AJ/SF/SH/SK variants and the first 27 JA aircraft. The 28th JA was painted in a gray tone that turned out too close to white. All latter JA aircraft were painted in a darker light/dark gray, appropriate for a high altitude fighter.
The final Viggen production variant was the JA 37 interceptor entering service in 1980. The last of 149 JA 37s was delivered in 1990. Differences from the previous models included an improved and more powerful engine, a new interception radar, new computers, HUD, ECM and some other subsystems were also fitted. The aircraft was also equipped with a new PS-46 interception radar (X band), with lookdown/shootdown capability up to 50 kilometres, and included a CW illuminator and many modes like TWS (Track While Scan). The MTBF is reported as 100 hours, a very high reliability level for that generation of avionics system.
Unusually for a 1970s fighter, three multi-purpose CRT display screens were fitted within the cockpit, in a system called AP-12, that also included a new model of HUD. The new radar was compatible with the Skyflash medium-range missiles, for the first time in a Swedish fighter. Two Skyflash missiles could be carried under the wings on hardpoints, as well as four Sidewinder J or L models. Another improvement was the addition of an Oerlikon KCA 30 mm cannon mounted internally, with 126 rounds of 360 g ammunition.
The structural strength was also improved, especially for the multi-sparred wings (initially Viggens had a high loss rate, with 21 aircraft lost in the early years).
Various upgrades have been performed over the years, mainly to cockpit equipment, weapons and sensor fit. In 1985, the "fighter link" went into service which permitted encrypted data communication between four fighters and ground radar based fighter command. This enabled one fighter to "paint" an airborne enemy with guidance radar for the Skyflash missiles of the three other fighters in the group while they had their search and guidance radar switched off. This system was operational ten years before any other country's.
Between 1998 and 2000, ten SK 37 trainers were converted to SK 37E electronic warfare trainers to replace the aging J 32E Lansen.
The Viggen has been phased out in favor of the advanced later generation JAS 39 Gripen.
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