SAAB AJS 37 VIGGEN

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The Saab 37 Viggen is a Swedish single-seat, single-engine, short-medium range combat aircraft, manufactured between 1970 and 1990. Several distinctive variants were produced to perform the roles of strike fighter (AJ37), aerial reconnaissance (SF 37), maritime surveillance (SH 37) and a two-seat trainer. In the late 1970s the all-weather fighter-interceptor JA 37 was added.

Development

The Viggen 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 Finnish aircraft designer Aarne Lakomaa. Several different concepts were studied involving both single- and twin engines, both simple and double delta wings and also with canard wings. Even VTOL designs were considered, with separate lift engines.

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). The aircraft was also designed from the beginning to be easy to repair and service, even for personnel without much training.

To meet these design goals, Saab selected a radical configuration: a conventional delta wing with a small, high-set canard foreplane. Canard aircraft have since become common in fighter aircraft, notably with the Eurofighter Typhoon, Dassault Rafale, Saab JAS 39 Gripen 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.

In 1960, the US National Security Council, led by President Eisenhower, formulated a military security guarantee for Sweden. The US promised to help the Swedish militarily in the event of a Soviet attack against Sweden; both countries signed a military-technology agreement. In what was known as the “37-annex“, Sweden was allowed access to advanced US aeronautical technology which made it possible to design and produce the Saab 37 Viggen much faster and more cheaply than would otherwise have been possible.

According to research by Nils Bruzelius at the Swedish National Defence College, the reason for this officially unexplained U.S. support was the need to protect U.S. Polaris submarines deployed just outside the Swedish west coast against the threat of Soviet anti-submarine aircraft. However, Bruzelius’ theory has been discredited by Simon Moores and Jerker Widén. The Polaris and Saab 37 connection also appears highly doubtful due to the time scale – the Saab 37 system only became operational in 1971 (strike version) and 1978 (fighter version) respectively, long after the Polaris system had been retired.

Propulsion

The Viggen was powered by a single Volvo RM8 turbofan. This was essentially a licence-built variant of the Pratt & Whitney JT8D engine that powered commercial airliners of the 1960s, with an afterburner added for the Viggen. The airframe also incorporated a thrust-reverser to use during landings and land manoeuvres, which, combined with the aircraft having flight capabilities approaching a limited STOL-like performance, enabled operations from 500 m airstrips with minimal support. The thrust reverser could be pre-selected in the air to engage when the nose-wheel strut was compressed after touchdown. The Viggen was the first aircraft to feature both afterburners and thrust-reversers. Only the Viggen, Concorde and the Panavia Tornado featured 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 (the Saab 35 Draken fulfilled this role), 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 favoured, since the latter were mainly limited by metallurgy development resulting from limitations in turbine temperature. Mechanical simplicity was also favoured, so the air intakes were simple D-section types with boundary layer splitter plates, while the fixed inlet had 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’s powerplant. Since the engine design had not been completed in 1962 when the airframe vs. engine design size needed to be frozen, the JT8D was chosen as the basis for 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 RM 8A, which reduced to 0.97:1 in the RM 8B.

The AJ, SF, SH and SK 37 models of the Viggen had the first version of the RM 8A engine with uprated internal components from the JT8D that it was based on. Thrust was 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.

  • Role: Attack Fighter, Reconnaissance