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.
Part Onecan be read here.
Saab began its J 29 work by considering a straight wing design but in November 1945 obtained some German swept wing research data. The benefits of sweep back prompted a very rapid re-evaluation of the project and by February 1946, a 25-degree swept wing design had been selected for development. Automatic leading edge slats were fitted to prevent the airflow over the wings from separating in high AOA manoeuvres. At the transonic speeds achieved by post-war jets, shockwaves forming on the tailplane would render conventional (ie trailing edge mounted) elevators, downstream of the shock, ineffective. Those speeds also moved the mainplane centre of pressure rearwards resulting in pitch-down that the ineffective elevators struggled to correct. The solution was the flying tail in which the entire horizontal tailplane could move in pitch. Shockwaves still formed on a flying tail but its area ahead of the shock front would remain an effective control surface. As stated earlier, the Tunnan was the first Western European jet fighter to have this system and it has become standard for transonic and supersonic aircraft of conventional tail layout. Fighter agility depends, inter alia, on a rapid roll rate.
The J 29 prototype featured full span ailerons but these produced an excessive rate of roll of 180 degrees per second. They were superseded by ailerons of around 65% span with the remaining inboard section replaced by flaps.
From March 1953, the J 29B became the standard Tunnan version. The major change was the installation of internal wing tanks that added 154 Imperial gallons (700 litres) of fuel taking the internal total to 462 Imperial gallons (2,100 litres), a 50% increase. Twin 99 Imperial gallon (450 litre) drop tanks could also be carried so offering a total load of 660 Imperial gallons (3,000 litres). The tanks were fitted at roughly mid-span on the outer of the two main pylons with the inboard hardpoints retained for weapons. The original Gripen uses a 242 Imperial gallon (1,100 litres) drop tank that can be fitted on the centreline and inboard underwing pylons. With three tanks in place, total fuel carried is 1,386 Imp gallons (6,300 litres); the internal load being 660 Imp gallons (3,000 litres).
The Tunnan’s wing was shoulder mounted, meeting the fuselage somewhat above the centreline. The wing was a one-piece structure and ran straight through the capacious fuselage passing just behind the cockpit rear pressure bulkhead and above the intake duct. Gripen also has a shoulder-mounted wing, which is set roughly at the centreline of the slim fuselage. This is slightly below the level of the canards that in turn are mounted just below the upper surface of the intakes. Fuel tanks are fitted in the upper part of the fuselage middle section with the intake duct(s) and main undercarriage bays placed below.
A one-piece wing could have been mounted level with the fuselage upper or lower surfaces as per the Jaguar and Phantom respectively. A low mounted wing on the diminutive Gripen would have offered insufficient ground clearance for loading underwing stores without a longer and heavier undercarriage. To achieve favourable interaction, the canards have to be set above the level of the mainplanes. Lack of suitable alternative mounting points for the canards would rule out the high wing location option. Furthermore, in order to reduce drag, it is best to avoid forming acute angles at wing-fuselage fairings. A mid wing configuration arguably offers the best overall solution in this and the other respects; it is therefore an entirely reasonable design choice.
The Tunnan was of monocoque structure built from aluminium alloy. High strength and stiffness were required to withstand the loadings imposed by transonic flight and a very fine standard of surface finish was also achieved in order to reduce skin drag. In structural terms, the Gripen marked a major change for Saab with composite materials (carbon fibre, glass fibre and Aramid) accounting for 20% of the structure by weight. Fatigue life consumption is reduced by a gust alleviation system. Aircraft disturbance is sensed by the flight control system, which prompts control surface reaction to alleviate the loads imposed.
The most obvious difference between these aircraft is in their lifting and control surfaces. Although radical at its inception for its swept wing and flying tail, the J 29 was standard in being longitudinally stable with a conventionally sited tailplane. Such tailplanes apply a download to balance the mainplane’s lift (the mainplane’s centre of pressure being behind the centre of gravity). In turn, the mainplane must generate additional lift to counter the tail’s down force and as a result lift-induced drag is increased. The Tunnan’s primary flying controls were the tailplane for pitch and the ailerons for roll. By contrast, the Gripen controls pitch by the canard while the inboard and outboard elevons on the delta wing act in both pitch and roll. The canard applies a lifting force to balance the mainplane and this co-operative interaction reduces the overall lift-induced drag.
Saab originally reversed the traditional arrangement with the Viggen and adopted a tail-first or canard design although it retained natural longitudinal stability. With the JAS 39, the full potential of the canard was realised. Full time, full authority, digital, fly by wire flight control system (FCS) allowed the adoption of artificial stability in pitch with attendant gains in agility and aerodynamic efficiency. At supersonic speeds the centre of lift on all wings moves aft promoting a nose down moment. A conventional aircraft trims this by increasing the tailplane download whereas the opposite applies with the canard, a more efficient solution. An unstable canard design offers more lift during take-off and landing, better supersonic turning performance and lower supersonic drag. The FCS keeps the Gripen’s instability in check and allows the full envelope to be exploited without the risk of overstress or departure from controlled flight. This carefree handling facility enables the pilot to concentrate on the mission while the FCS controls the load factor, AOA, angle of sideslip and roll rate. Another function unavailable to the Tunnan is CG control. The fuel control system not only monitors the fuel remaining but also balances the amounts drawn from the various tanks to keep the CG within limits.
Weapons and Systems
The early swept wing combat aircraft were designed as gun-fighters and the Tunnan was no exception. Its original standard armament was a battery of four 20 mm Hispano cannons with 180 rounds per gun. Stemming from an original design of the early 1930s, the Hispano weighed 84 lb (34 kg) and fired a 4.88 oz (138 gram) round with a muzzle velocity of 2,880 ft/sec (878 m/sec). The muzzles were spaced circumferentially on the underside of the nose, a short distance back from the intake. Some jet fighters (eg the Hunter F.1 with its axial Rolls Royce Avon 100 series) experienced considerable engine problems through the ingestion of the shock waves of their cannon shells. A centrifugal compressor may be broader than its axial flow equivalent but it is inherently more tolerant of disturbed airflow. Cannon blast ingestion was inevitable on the Tunnan but no engine surge problems appear to have been experienced.
Two factors combined to make cannons alone an insufficient armament for air combat. The speed of jet fighters made for only fleeting firing opportunities while their stronger structures (for the high aerodynamic loads) could withstand cannon shell damage – up to a point. Cannons therefore required both a high rate of fire and a heavy high velocity round but these features tended to oppose each other. The more powerful cannons had a slow rate of fire and the faster models had less punch. One attempted solution to this problem was to fit fighters with batteries of unguided air-to-air rockets that would be fired in a barrage of a dozen or more in the hope of at least one hitting a fighter target or several hitting a bomber. The Tunnan was fitted with twenty-four 75 mm (2.95 in) diameter rockets carried in vertical stacks of four on three close-set hardpoints inboard of each wing drop tank. In the event, such rockets were to prove a blind alley and although adopted by several air forces they were superseded by guided missiles. From 1963, the J 29E could carry a pair of Sidewinder air-to-air missiles (AAM) termed Rb 24 in the Flygvapnet inventory. This addition helped keep the Tunnan viable until it went out of service in May 1967, the Saab Draken having progressively taken over the fighter role from 1959.
Proving to be a fine weapons platform during testing, the Tunnan J 29B was applied to the ground attack role with the new designation A 29B. The four Hispanos were retained and either fourteen 14.5 cm (5.7 in) anti-armour rockets or four 18 cm (7.1 in) rockets for hard, fixed targets could be carried. As mentioned above, the drop tanks were fitted to the outboard main pylons where they would have helped relieve the wing bending moment. Inboard of the tanks, there were up to four hardpoints under each wing. The 14.5 cm rockets were carried in six vertical pairs plus a single rocket on the furthest inboard position to avoid a clash with the undercarriage door. The 18 cm rockets had individual pylon mountings. All the Tunnan’s weapons bar the Rb 24 Sidewinder were unguided whereas the opposite obtains with the majority of the Gripen’s armament. Although guided weapon unit cost is greater than the ballistic equivalent, their higher accuracy makes for better cost-effectiveness. As fewer rounds and aircraft sorties are needed to destroy a target, the result is a lower overall cost in weapons plus likely lower aircraft and aircrew losses.
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The USAF had expended time, effort and lives but the B-36 was still just as v
The JAS 39’s interceptor predecessor, the JA 37 Viggen was equipped with the 30 mm Oerlikon KCA cannon, a weapon of exceptional performance. With the high muzzle velocity of 3,937 ft/sec (1,200 m/sec), it could engage targets at 1.25 miles (2 km). The KCA was simply too bulky for the diminutive Gripen and instead it has a single 27 mm Mauser BK27 cannon (as used in Eurofighter Typhoon) with 120 rounds. It is fitted in a semi-scabbed arrangement in the port lower fuselage behind the cockpit and with the muzzle emerging about 0.5 m ahead of the intake. Although a lighter weapon than the Oerlikon, it has a higher rate of fire, 1,700 rounds per minute compared to 1,320, so putting out 28% more shells in any burst. It fires the 9.2 ounce (260 gram) rounds at a muzzle velocity greater than 3,280 ft/sec (1,000 m/sec). There is a 300 rounds per minute option for air-to-surface targets; barrel life is extended in this mode. The cannon is deleted from the trainer in order to free internal volume for the second seat.
In the air-to-air role, the Gripen’s main weapons were originally the Rb 74 Sidewinder (the AIM-9L variant) and the RB 72 AIM-120 AMRAAM. Later, the IRIS-T (Rb. 98), A-Darter and MICA were cleared. One Rb 74 is carried on each wingtip; the four underwing hardpoints can each carry an Rb 72 although a typical fit might see the two inboard missiles replaced by drop tanks. The integration of radar guided AAMs in Saab aircraft began with the Draken, the Tunnan never gaining such a weapon. In the Gripen, the core of the system is the Ericsson Microwave Systems PS-05 pulse-Doppler radar that has look-down, shoot-down capability for the air defence mission. Moreover, it is a multi-mode radar with full, rather than secondary, attack and reconnaissance functions available in the one package. The PS-05 was based on the GEC-Marconi Blue Vixen (also the basis of the Typhoon’s CAPTOR radar) and was regarded as having similar performance to the Northrop Grumman AN/APG-66(V)2 radar of the F-16AM.
For air-to-surface, the Saab-Bofors Rbs 15F radar homing missile is the main anti-ship weapon with the infra-red imaging Rb 75 employed for high value land targets. The latter is the Swedish designation for the Raytheon AGM-65 Maverick; it is a fire-and-forget weapon with its own seeker image displayed in the cockpit before launch to allow target selection. For anti-ship missions, the radar is operated in search mode while the Rbs 15F has its own radar allowing the Gripen to turn away after launch. It is also fitted with a jet engine rather than a rocket motor so conferring greater range for a given size; rockets are not efficient at low altitude. The precision, long-range, Taurus KEPD 350 stand-off weapon has also been flown on the aircraft.
Other than its radar, Gripen also has several systems that were simply not available in the Tunnan’s day. Through its encrypted tactical datalink, a JAS 39 can receive updates on the battlespace from a ground controller, airborne early warning (AEW) aircraft or other Gripens. Its navigation suite is based on a ring-laser gyro integrated with GPS (global positioning system) satellite navigation. Head down information is presented on three screen displays while a head up display (HUD) provides the essential flight and aiming cues. By contrast, the Tunnan pilot had a map, compass and stopwatch for navigation and his head-up information was limited to a gyro gunsight. The main JAS 39 system and weapon functions are selected through HOTAS (hands on throttle and stick). Weapon selection is through stick-mounted switches. The throttle has two pistol grips; sensor controls (radar range and scan angle) are on the upper with the lower switches providing airbrake selection and certain navigation functions. The Litening G111 FLIR/LDP (forward looking infra-red/laser designator pod) has been integrated to support the use of the GBU-12 and GBU-16 LGB (laser guided bombs). Systems integration is achieved on Gripen through a MIL-STD 1553 databus, a massively more advanced technology than that of the J 29. The Swedish developed datalink of the JAS 39 A/B is not compatible with any NATO datalink, nor was the Swedish Identification Friend or Foe (IFF) compatible with the NATO equivalent. By contrast, the Gripen C/D is NATO compatible featuring a Link 16 datalink (essential for operating alongside US aircraft) and NATO IFF Mode 1-4.
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.
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