The JAS 39 Gripen entered service with the Swedish Air Force in June 1996 and is now the sole combat type in the Flygvapnet. Paul Stoddart compares this fourth generation aircraft with its ancestor, the portly yet effective, J 29 Tunnan which entered service 46 years earlier. 

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Two Western European nations have independently designed and built jet fighters of each generation since World War II. The leader, at least in terms of numbers exported, is France; Dassault’s highly successful Mirage series leading to the current Rafale. The other country is not, as you might expect, the UK but Sweden. A relatively modest nation, in GDP and population, Sweden has consistently punched well above its weight in the aerospace world. The Swedish Air Force (Flygvapnet) was the first West European air arm to take on charge a modern generation fighter, the Saab JAS 39 Gripen, ahead of both France’s Rafale (2001) and Europe’s Typhoon (2003). This is ‘gripen’ or griffon in English, is a mythological beast half eagle, half lion.

The prototype Gripen first flew on December 9th, 1988 and in October 1997 the first squadron was declared operational. Despite a population less than one sixth that of Britain or France, and with a correspondingly smaller economy, Sweden again led Western Europe in designing and building a state of the art combat aircraft. Saab’s track record in cutting edge jet fighters spans half a century. It began with the other half of this duo, the J 29 that first flew on September 1, 1948. Its stout fuselage gave rise to its nickname ‘Tunnan’, meaning barrel (and related to the English word ‘tun’ a barrel or cask and an Imperial measure of capacity equal to four hogsheads). Viewed from the rear, a taxying Tunnan resembled a waddling duck but its performance in the air bore comparison with any of its contemporaries.

When the North American F-86 Sabre entered USAF service in late 1948, it was the West’s first swept wing jet fighter to do so. Close on its heels came the USSR’s equivalent, the MiG-15 which reached front line squadrons during the winter of 1949-50. By contrast, the first British designed and built swept wing fighter, the Hawker Hunter F.1, did not enter service until July 31, 1954. As a stopgap, the Royal Air Force bought Canadair built North American F-86E Sabres, the first of which was handed over in January, 1953. By that time, the Tunnan had been in service for almost three years, deliveries to the Flygvapnet having begun in May, 1951. Saab might have lagged Britain in piston engined fighter design but it has been a worthy competitor in the jet age. The J 29 set the trend of Saab’s innovative design approach. It was not merely the first Western European jet fighter to have swept wings but also the first to have a flying tail (ie an all moving tailplane rather than an elevator hinged from the trailing edge of a fixed tail), automatic leading edge slats and full span aileron/flaps. Thus, for several years Saab’s Tunnan was the most advanced fighter in service in Western Europe. With Gripen, for its first years of service, Saab might justifiably make the same claim.

Ten most attractive Swedish aircraft here. 

Origins

Saab entered the jet age by way of an interim step. Sweden’s main fighter during the latter half of the 1940s was the somewhat bizarre piston-engined Saab J 21A. It was an unconventional design featuring a central ‘pod’ fuselage with the tail mounted on twin booms. The pod contained the cockpit and a rear mounted engine driving a pusher propeller. This feature prompted another Saab innovation; to ensure pilot clearance past the propeller on bailing-out, Saab fitted an ejection seat of its own design, the first example of such a system to reach front line use by some accounts (the German Heinkel He 280 featured an ejection seat, but never entered service) . Unorthodox the J 21A might have been, it was readily adapted to jet power as the J 21R. The V-12 Daimler Benz DB605B piston engine was replaced with a 3,000 lb (13.32 kN) thrust de Havilland DH Goblin turbojet, the same engine as used in the DH Vampire fighter. First flying in March 1947 and entering service in August 1949, the J 21R proved to be a manoeuvrable aircraft and an excellent weapons platform though it lacked performance for the fighter role. Recognising the Goblin’s lack of power, in December 1945 the Swedish Air Board directed Saab to base its new jet fighter project on the DH Ghost turbojet, which was rated at 5,000 lb (22.2 kN) thrust. Svenska Flygmotor built the engine under licence as the RM2. The fighter specification required the retention of the J 21A’s manoeuvrability and ability to operate under Sweden’s harsh conditions together with a maximum speed of Mach 0.85, a high service ceiling and the heavy armament of four 20 mm cannons.

Barrel role

Although the Tunnan was to become an effective ground attack aircraft, the original J 29 requirement was aimed primarily at the interceptor role. By contrast, the JAS 39 specification from the outset was for a multi-role capability so that a single Gripen variant could replace all versions of its predecessor the Saab 37 Viggen. (The two-seat JAS 39 is a trainer with full combat capability). JAS is an acronym for the three main Gripen roles: Jakt (fighter), Attack (attack), Spaning (reconnaissance). The Viggen was built for the Flygvapnet in four marks: AJ 37 attack, SH 37 sea surveillance, SF 37 photo-reconnaissance and JA 37 fighter. Those versions differed in the avionics systems fitted and the weapons carried; the JA 37 also featured a new pulse doppler radar and an extensively developed version of the Volvo RM8 turbofan engine. Gripen does more than replace the various Viggen variants with a single type. With its multi-mode radar, it is capable of swing role missions retaining considerable air-to-air capability while configured for an air-to-surface sortie.

The Gripen specification was very demanding in the multi-role requirement alone and the setting of a strict weight limit increased the challenge. All other things being equal (and they rarely are in aviation) aircraft cost is broadly proportional to weight; a shrinking defence budget dictated a firm price limit for the new fighter. Saab responded with its usual innovative approach, and bucked the trend of fighter weight increasing with each generation. Where the Viggen had an empty weight of around 23,100 lb (10,500 kg), the Gripen A tipped the scales at only 14,300 lb (6,500 kg), a reduction of 38%. This is actually extremely light for a modern combat aircraft. To put it in context, the Lockheed F-16A, which was specifically designed for the USAF’s Lightweight Fighter project of the early 1970s, weighed 16,234 lb (7,364 kg) empty, some 13% more. The fighter closest in weight and concept to the JAS 39 was the now defunct Northrop F-20 Tigershark. It had an empty weight of 15,060 lb (6,831 kg) and used a variant of the same engine used by the Gripen. Despite its high performance, the F-20 was not procured by any of the American forces and thereby lost in the export marketplace to the ubiquitous F-16. The JAS 39 is of a later generation than the Tigershark and is achieving impressive export success.

My flight in a Gripen here

Where to put the radar? 

Engine integration is a powerful design driver of a fighter’s airframe. The complete powerplant system comprises the intake and duct, the engine itself and the jet pipe plus nozzle; all these features affect the final form of the aircraft. Centrifugal compressor engines such as the DH Ghost are broad but relatively short and the 4 ft 5 in (1.35 m) diameter Ghost/RM2 was responsible for the stubby fuselage of the Tunnan. Axial flow engines are slimmer and produce leaner airframes but this can result in packaging problems for other systems. By contrast, the Tunnan fuselage had sufficient volume to stow (as well as the engine and cockpit) 308 Imp gallons (1,400 litres) of fuel, the undercarriage and the main armament. The air intake was placed centrally in the nose with a straight duct to the engine positioned in the rear fuselage, which aft of that was cut back below the tail to reduce the length of the jetpipe. Nose intakes were typical of the period (eg Sabre and MiG-15) and although the long ducts to a mid-fuselage located engine can cause relatively large pressure losses, they perform well through a wide range of angle of attack (AOA) and sideslip angles. They are also free from flow separation effects from other parts of the airframe. The main drawback of this approach is that it leaves little space to fit a radar.

Top 10 fighter radars here

By contrast Saab’s next military design, the A 32 Lansen, had twin lateral intakes leaving the nose free for its radar installation (the first Swedish aircraft with such a system). Similarly, the Gripen reserves its nose for the radar and uses twin lateral intakes offset from the fuselage sides, plus splitter plates, to avoid the stagnant boundary layer. The intakes themselves are of plain pitot design with no variable ramps or moving centre bodies for shockwave control. Although this may somewhat limit the Gripen’s maximum speed, it should be remembered that fighters spend a small fraction of their flight time supersonic. Pitot intakes proved entirely sufficient for the very successful F-16 and serve the JAS 39 well. They are also much cheaper than variable intakes and more reliable as there are no moving components to fail. The bifurcated intakes merge in the fuselage mid-section to form a single, circular section duct some distance upstream of the engine face. The aim is to allow the mingling airstreams to stabilise before entering the compressor.

At first sight, the Gripen’s intake location could appear less than ideal. An agile fighter might be expected to have its intakes under the fuselage (eg Eurofighter Typhoon) as this location is superior at high AOA to side intakes. Two points should be noted here: the Gripen is a small aircraft and its nose undercarriage is set forward of the intakes. Had the intakes been placed under the fuselage, they would have been very close to the ground with the consequent danger of ‘fod’ (foreign object damage) from the ingestion of debris thrown up by the nosewheels. Siting the Gripen’s intakes laterally on the fuselage much reduces this risk. There is also a school of thought that side location offers the best compromise of the conflicting demands of the aerodynamic, structural, weight and space requirements of intake design.

Saab has stuck to the single engine design philosophy from the Tunnan, through the Lansen, Draken and Viggen to the Gripen in order to minimise cost. Twin engine fighters are more expensive than the single engine equivalent and although a ‘spare’ engine is useful, twins do not have half the loss rate of singles due to engine problems.  (The single versus twin issue is worth an article itself).  The JAS 39 is powered by a Volvo Flygmotor RM12 turbofan, a development of the General Electric F404-GE-400 used in the Boeing F/A-18 C/D Hornet (the F-20 used the F404-GE-100). It is rated at 12,140 lb (54.0 kN) dry and 18,100 lb (80.5 kN) wet; these figures are some 15% greater than the original and are achieved from a 5% increased mass flow and higher turbine entry temperature (TET). This output is achieved from an engine of 2,325 lb (1,055 kg) dry weight representing a thrust to weight ratio of 7.8:1. The bypass ratio of 0.34:1 is fairly low and aimed at high performance at high altitude while retaining reasonable economy in lower level cruise. Of modular design, the RM12 has a three-stage fan and seven-stage high-pressure compressor, each driven by a single stage turbine. The first stage of the fan and the first two stages of the HP section have variable stators; the HP inlet guide vanes are also variable. All the turbine blades are of single crystal form with internal air-cooling to support the high TET. As the RM12 is used in a single installation compared to the twin engined Hornet, it also features a strengthened first stage fan for improved birdstrike resistance.

World speed record

The RM2 seems crude by comparison with its single stage compressor developed from those used in superchargers for aero piston engines. The dramatic extent of jet engine development since World War II is demonstrated clearly here. Technologies such as single crystal blades and internal blade cooling were simply not available to the RM2. At 5,000 lb (22.2 kN), the RM2’s thrust is only 41% of the RM12’s dry figure and 28% of its reheat output. The definitive Tunnan, the J 29E, with a loaded weight of 16,600 lb (7,530 kg) had a thrust to weight ratio on take off of 0.30:1. By contrast, the Gripen A at its maximum take off weight of 27,500 lb (12,500 kg) has figures of 0.44:1 in dry and 0.66:1 in reheat. The last Tunnan variant, the J 29F, had an afterburner (detailed below), which raised take-off thrust to weight ratio to 0.36:1. This was a 20% improvement on the E variant but still well below the Gripen’s figures. Nonetheless, the Tunnan was no slouch. A J 29B broke the world record for the 310 mile (500 km) closed course in May 1954. The record had been held by the F-86 Sabre at 590 mph (950 km/h) but the Tunnan raised it to 607 mph (977 km/h). In January 1955, two Tunnan S 29Cs gained the world record for the 620 mile (1,000 km) closed course achieving an average speed of 560 mph (901 km/h).

Paul Stoddart served in the Royal Air Force as an aerosystems engineer officer and now works for the Ministry of Defence.  His interests include air power and military aircraft from the 1940s onward.  He is a Fellow of the Royal Aeronautical Society.  He is giving a lecture at the headquarters of the Royal Aeronautical Society (4 Hamilton Place, London) at 18.00 on Thursday 1st June on the undeveloped potential of the Spitfire as an escort fighter.  It will last around 45 minutes with 15  minutes of Q&A.  

Part 2 coming soon. 

Notes: All information in this article is taken from public domain sources.

This article is Paul’s personal view of the development of the Tunnan in comparison with the Gripen A.  It contains no implication of Ministry of Defence policy nor should any be inferred.

If this article interests you, support Hush-Kit.net with a donation (buttons above and below). If this goes well we’ll be able to give you much more! Recommended donation £15. Many thanks for helping to keep us impartial and independent. 

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