Sweden (1945)
Fighter / Attack Plane – 298 Built

The Saab J 21 is a peculiar airplane, not only because of its advanced features. This fighter in particular is one of the only two airplanes that were initially powered with a piston-propelled engine, then later modified to incorporate a jet engine using the same airframe and implementing very few modifications. This airplane is also the product of Swedish defence concerns and resourcefulness, as Sweden wanted to keep its neutrality and territorial integrity during WWII, electing to develop a domestic fighter program as access to foreign advanced technology was restricted.

A single-seat, single-engine airplane that later became one of the first-generation jet fighters. Its design is a twin-boom tail pusher configuration. It had two longitudinal booms, attached to the main wing but extending backwards from the middle section of each wing, with the main body placed in the middle. Similar to the Lockheed P-38. This design also implements a rear mounted piston engine, otherwise known as a pusher configuration, which made engine modifications easy.

The piston-propelled version, the J 21 – had the engine, a licensed Daimler-Benz DB 605B, simply fitted at the rear of the main body, behind the cockpit and between the longitudinal booms. The jet version – J 21R – was fitted with air in-takes at both sides of the fuselage, having the engine (a De Havilland Goblin 2 and later a Swedish-licensed version, a Flygmotor RM1) located on the same area as the piston-propelled version and elongating the main body. The fuselage section harbouring the engine was also widened. Another modification was that the elevator was placed at the upper area of the double tail plane. The wing in both configurations was a low-wing, being straight until it met the longitudinal booms, changing into a slightly swept wing from the longitudinal boom to the wing tip. It also received wingtip fuel tanks.

The J 21 could develop speeds up to 645 km/h (400,78 mph), while the jet propelled version could develop speeds up to 800 km/h (497 mph), being a fast aircraft in both configurations. Its firepower (J 21) was equally powerful, as it was armed with a 20 mm cannon, either a Bofors or a Hispano-Suiza HS.404, and 2X 13,2mm Bofors machineguns at the nose, with 2X 13,2mm Bofors machineguns in the wings. The J 21R received an even much more considerable firepower, as it featured a 20mm Bofors cannon, 4X 13.2mm M/39A heavy machine guns, a centreline pod with 8X 13.2mm M/39 machineguns, and wing racks for 10X 100mm, 5X 180mm, or 10X 80mm anti-armour rockets.

The J 21 was initially developed with the aim of providing Sweden with good air assets to defend its air space and neutrality, and also with the aim of replacing many of the existing airplanes development. The development began in 1939, under the lead of Frid Wänström, having as basis a Bristol Taurus as an engine, yet it fell into a momentary freeze until 1941, when it was resumed. This freeze was due to Saab’s concentration in the Saab B 17 and B 18 bombers. As the configuration resulted problematic for the pilot’s safety when bailing out, many proposed solutions came, such as blowing the propeller, blowing the entire engine or using a “bomb crutch” to throw the pilot away from the airframe. The solution came with the development and implementation of a Bofors ejection seat, which was tested first on ground and in-flight on a SAAB B 17. The nose landing gear wheel was tested on a steel platform attaching the three undercarriage components, with the structure being towed by a truck during the test programme. In 1943 the first flight of the prototype took place, with units entering in service with the Flygvapnet in 1945. Three prototypes were built during the development process. In 1947 the J 21 evolved into de J 21R when it received the De Havilland Goblin jet engine, but as the J 29 Tunnan was introduced, it replaced the J 21 as the main fighter, performing the J 21 instead ground attack missions, thus designated A 21R.

The J 21 was in service with the Flygvapnet from 1945 until 1954, with 298 fighters built from 1945 to 1949. The J 21R was in service with the Flygvapnet from 1950 until 1956, with 60 units built from 1950 to 1952. Three J 21 are preserved as static displays in museums.

Design

The basic design of the J 21/J 21R was a twin-boom tail pusher propeller, making it one of the most radical operational designs of those times. This scheme proved to be beneficial for two important aspects. First, it benefited both pilot view forward and allowed the armament to be concentrated on the nose, meaning that such combination provided a good firing scope and sight, let alone a good firepower and making maintenance services rather easy. Second, it made possible for the aircraft to be updated thus being able to install a jet engine using the same airframe of the piston-propeller engine version, which was basically the basic airframe.

The only drawback of this layout – mainly with the J 21 piston-propelled engine version – was the risk for the pilot to hit the blades when bailing out, as the engine was placed right behind the cockpit. The solution came with one of the first ejection seats in the world, developed by Saab in 1943, being tested on the ground and on-flight and being a SAAB B 17 the testing platform. Another drawback was that, similar as the earlier versions of the Mustang P 51, the rearward view was rather poor, which could be problematic in a dog-fight. The J 21 featured a characteristic wing, as it was roughly strait from the main fuselage to the tail twin-booms, then being slightly swept back from the tail booms to the wingtips.

The wing was purposed with acquiring laminar flow as far as possible. In regards to the aerodynamics, it was required the airframe to reduce minimum drag and engine cooling drag, so the oil and liquid coolers, along with the duct system for the engine, inside the airfoil contour between the fuselage and the tail booms. Considering the tail boom design, the horizontal elevator was placed between the tail sections, connecting them. The landing gear, meanwhile, was of tricycle configuration and long, which made the J 21 to be a tall aircraft so to keep the propeller away from the ground. The rear gear retracted into the tail booms behind the rear wing spar, but this forced the fuel tank to be placed in the wing centre section. The frontal wheel was located at the nose.

The engine was a Daimler-Benz DB 605B inverted V12 of 1475 hp, which gave the airplane speeds of maximum 645 km/h (400,78 mph), but as the engines were received in poor conditions, improvements and overhauling were required. As a result of the power provided by this engine and the aerodynamic characteristics, the J 21 was deemed good, as it had excellent handling, benign stall characteristics and tight turning circle. The armament also gave this fighter good firing power, with the Swedish pilots being able to compare it with the Mustang P 51D (Sweden received a good number of them) and considering they were a good match for it. But the main drawbacks were that at medium and high altitudes performance tended to decrease, the rearward view was poor, and the controls were heavy to operate, increasing tiring during combat.

The armament of the J 21 consisted of a 20 mm cannon, either a Bofors or a Hispano-Suiza HS.404, and 2X 13,2mm Bofors machineguns at the nose, with 2X 13,2mm Bofors machineguns in the wings. The J 21A-3 was able of carrying unguided rockets (2X 180mm or 8X 80/145mm) and bombs (600kg, 500kg, 250kg or 4X 50Kg).

After World War II, the jet engine technology was becoming the mainstream propulsion system, and the Flygvapnet wanted to catch up and incorporate such technology into its assets. As the development of a new jet propelled fighter would take some time, the J 21 was chosen to be the platform for using an airframe in use with the new technologies back then. As a result, the J 21R was developed and introduced, with the first prototype taking flight in 1947 and then entering service in 1950. This ‘new’ fighter required some structural changes so to cope with the new power plant, like up to 50% of its airframe. First, the main body was slightly prolonged ant widened, so to allow the De Havilland Goblin 2/Flygmotor RM1 engine (that allowed speeds of 800 km/h) and the air intakes, located at each side of the fuselage. In addition, the stabilizer was moved upwards top to the fin, so to allow the engine flow, requiring the tails to be redesigned. The wing leading edge was mover forward and made sharper. Airbrakes were introduced, one upward and other downward flaps placed on the outer wing’s trailing edge. Given the increased speed, the ejection seats were properly modified so to enable ejections at subsonic speeds. And as the propeller was removed, the landing gear was shortened in turn, reducing the height of the airplane. Fuel tanks were fitted in the middle wing and the wingtips, which increased the fuel volume.

The J 21R received an enhanced firing power, as the standard 20mm cannon/4X 13,2mm M/39A heavy machineguns set was added with a centreline external pod carrying 8 additional 13,2mm M/39 heavy machineguns. In addition, the J 21R was fitted with wing racks allowing the airplane to carry 10X 100mm or 5X 180 Bofors rockets, or 10X 80mm anti-armour rockets.

Materializing ‘Armed Neutrality’

The J 21 is, like the J 29 Tunnan, the product of Sweden’s concerns about its own security during WWII, especially in the light of Germany’s invasions of Norway and Denmark in 1940, which were neutral nations by the time. As Sweden considered that its existing air assets wouldn’t be able to successfully contribute to the defence, given their obsolete condition, it considered that new aircraft were necessary. As with the J 29, Sweden faced some problems when trying to acquire some technology due to the restrictions imposed by the conflict, although by sheer luck it was able to receive the Daimler-Benz DB 605B engine, as Germany was trying to hamper the delivery. These circumstances decided the Swedish government to undertake a local rearmament programme and implement a policy of ‘armed neutrality’ to secure the nation’s neutrality. The focus was placed on the development and fabrication of advanced aircraft. As the same concerns prevailed after World War II and into the very earlier days of the Cold War, it was deemed that the resulting technologies from the War needed to be exploited and incorporated, having in mind Sweden to catch up with the newly developed technologies, especially in regards of propulsion. The Saab J 21 became the platform for the Flygvapnet to make the transition from piston-propeller engine to jet engine, while at the same time providing the country with a locally built jet engine fighter, while newer and more advanced aircraft were put into service.

A feat of Swedish Nytänkande

The fact that the J 21 was used as a basis for an almost new jet powered engine fighter is a product of Sweden’s innovative thinking and also of its capacities – out of need, in part – of working with existing resources at the point of maximizing them. While the J 29 Tunnan has the honour of being the first jet fighter exclusively built for that purpose, it is the J 21 the very first jet engine fighter the Flygvapnet operated with, being amongst the very few designs, if not the only one, in being successfully modified as it received two different types of power plants. And while the J 29 Tunnan displaced the J 21 as a fighter, it was able to operate as a good ground attack aircraft until 1956, making this airplane born in the World War II, an early Cold Warrior and the basis for Sweden’s jet fighter industry and operationalization. It simply meant a huge step for the Swedish Air Industry, let alone its Air Force.

Variants of the J 21

• J 21A-1 – Fighter version and the very first production series of the J 21. It featured the armament configuration of the 20mm Hispano-Suiza HS.404 cannon and the 13,2mm Bofors/Colt heavy machine guns. In service until 1949. 54 delivered.
• J 21A-2 – Fighter version and the second and third production series, featuring enhanced avionics and incorporating a Bofors 20mm gun, with the other armament being the same. It was also equipped with further direction horizon instruments. In service until 1953-1954. 124 delivered.
• J/A 21A-3 – Fighter/fighter-bomber version based from modified J 21A-2 airframes. It was equipped with a SAAB BT9 bomb aiming sight and two RATO (Rocket-Assisted Take-off) devices, armed with unguided rockets (2X 180mm or 8X 80/145mm) and bombs (600kg, 500kg, 250kg or 4X 50Kg). 119 delivered.
• J 21B – A planned version to be armed with 3X 20mm guns at the nose, a radar in the starboard room, improved aerodynamics and better engines (A Daimler-Benz DB 605E/Rolls-Royce Griffon). It was also intended to feature a pressurized cockpit and a bubble canopy. Cancelled

Variants of the J 21R

• J 21RA / A 21RA – First production series powered by a De Havilland Goblin engine. Later reconfigured into ground attack airplanes (A 21RA). Fitted with wingtip fuel tanks to increase the operational range and endurance. Operated until 1953. 30 delivered.
• J 21RB / A 21RB – Second production series powered by a Swedish-license made De Havilland Goblin (RM1). It was also reconfigured later into a ground attack airplane (A 21RB), with the nose heavy machineguns changed to a 12.7mm caliber. Fitted with wingtip fuel tanks to increase the operational range and endurance. Operated until 1956. 30 delivered.

Operators

• Sweden -The Flygvapnet operated the J21 a time roughly after the end of World War II. It operated with 54 fighters of the J 21A-1 version, 124 The J 21 fighters of the J 21A-2 version, and 119 fighter/bombers of the J 21A-3 version. The J 21 was in service between 1945 and 1954, with X units: F9 Goteborg, F15 Soderhamn, F12 Kalmar, F6 Karlsborg and F7 Såtenäs. In addition, the Flygvapnet operated with 30 fighters of the J 21RA version, and 30 fighters of the J 21RB version. Both were later on modified into ground attack airplanes, being denominated as a result A 21RA and A 21RB. The J 21R was in service from 1950 to 1956, with three units: The F10 Ängelholm, the F7 Såtenäs, and the F17 Kallinge. Three J 21 remain today as museum exhibitions in Sweden.

Gallery

Sources

Chant, C. (2001). Aviones de la Segunda Guerra Mundial [Aircraft of World War II, Fabian Remo Tamayo & Fernando Tamayo, trans.]. Madrid, Spain: Editorial LIBSA, Sharpe, M. (2001). Jets de Ataque y Defensa [Attack and Interceptor Jets, Macarena Rojo, trans.]. Madrid, Spain: Editorial LIBSA, Goebel, G. (2014). The SAAB J 21 & J 21R. Air Vectors., Frederiksson, U. (2000). Saab J 21/A 21/A 21R. x-plane.org,  Aviastar.org (n.d.). Aircraft Profile #138. Saab J.21A & R., Aviastar.org (n.d.). Saab 21R. 1947., NGO valka.cz. (2015). Saab J 21R., Saab. (n.d.). 1940’s., SAAB 21. (2016, June 23). In Wikipedia, The Free Encyclopedia., Saab 21R. (2016, June 8). In Wikipedia, The Free Encyclopedia., Images: Converted J 21R, Converted J 21R 2, Saab A 21A-3 – Alan Wilson / CC BY-SA 2.0, Side Profile Views by Ed Jackson – Artbyedo.com