Saab 21R

During the early stages of the Second World War, the nation of Sweden was concerned that its neutrality and its independence could come under threat by one or more of the belligerent powers, enacted a series of emergency measures to increase its military's combat preparedness and deterrence value against would-be aggressors. During the period of 1939–1941, the Swedish Air Force committed itself to a major expansion programme, which included the procurement of large number of foreign-sourced and indigenously developed fighters.[2] As a consequence of the raging conflict across Europe, there were not many nations who possessed the available production capacity or a willingness to prioritise the supply of modern fighter aircraft to Sweden, being a relatively small neutral country; while Sweden's own domestic production capability had been deemed to be insufficient until at least 1943, it was recognised that it would be necessary for the nation to build up its own first-rate designs to meet its needs.[3][2]

During 1941, in response to demand from the Swedish Air Force, the Swedish aviation company SAAB commenced work upon bringing a radical new fighter design.[4] The company envisioned an unorthodox twin-boom pusher configuration fighter aircraft, featuring a low-mounted wing, a tricycle landing gear arrangement, and was furnished with heavy forward-firing armament.[5] It was powered by a piston engine, specifically a localised version of Germany's new Daimler-Benz DB 605B inline engine, which was refined and built by Swedish firm Svenska Flygmotor AB for the programme. Designated as the J 21 by the Swedish Air Force, the proposal was accepted and authorised to proceed to the prototype stage; on 30 July 1943, the first J 21 prototype, powered by the 605B engine, conducted its maiden flight, flown by SAAB test pilot Claes Smith.[6]

Throughout 1945, several different options were explored with the intention of improving the 21's performance.[7] Many of these speculative upgrades involved the substitution of the 605B engine with a more powerful alternative powerplant. During the first half of the year, the company had planned out a variant of the fighter that would be powered by a Rolls-Royce Griffon engine, capable of producing a maximum of 2,000 hp; according to estimate, the Griffon would have provided the 21 with a projected top speed of 416 mph. Other projects, such as the studied SAAB 27, were intended to use the Griffon as well; however, by the end of the year, all work on piston-engine design had been abandoned by the company in favour of another means of improvement.[7]

In parallel with the piston-engine studies, SAAB and several other Swedish companies had been evaluating the options for the adoption of an innovative, brand new type of engine - the jet engine. Two early studies, designated as RX 1 and RX 2, had been examined, both of which being twin-boom aircraft similar to the 21.[8] Recognising enthusiasm on the part of the Swedish Air Force for a jet-powered aircraft to be made available, during late 1945, SAAB resolved itself to producing a version of the 21 that would harness jet propulsion.[9] As such, Saab launched a design study with the aim of best determining how to power the J 21A with a jet engine. It was viewed as being highly desirable to 'catch up' with recent innovations in the field of fighter development, which was visibly advancing rapidly in nations such as the United Kingdom where, among others, British aircraft manufacturer de Havilland had already placed its own jet fighter, the de Havilland Vampire, into production.

For some months after starting work during late 1945, the design study proceeded without focusing on a specific engine, the selection of which was an open question for some time. As SAAB continued its work on the initiative, it became possible for Sweden to purchase the "Goblin 2", a turbojet engine, directly from de Havilland Engine Company, along with the license to manufacture it in Sweden. Acquired as per this manner, the engine became the first jet engine to be used by the Swedish Air Force, who designated it as the RM1. On 10 March 1947, the maiden flight of Saab's first jet aircraft, being a converted J 21, outfitted with a single Goblin 2 engine, took place. The results of the subsequent flight test programme was largely viewed as satisfactory.[10]

During 1947, at the direction of the Swedish Air Force, SAAB began to receive the service's piston-engined J 21s for the purpose of converting them to jet propulsion. This conversion work involved an extensive rebuilding programme, requiring each aircraft to be extensively modified. In all, in excess of 50 per cent of the airframe, tailplane and wing was changed and, most significantly, each of the fighters was furnished with a single British-sourced de Havilland Goblin turbojet engine, which took the place of the DB 605B and propeller unit. The resulting jet-propelled fighter was adopted by the Swedish Air Force and became the service's first jet aircraft. In light of its extensive changes and to differentiate them from its predecessors, rebuilt 21s were assigned the designation of J 21R.[11]

Many changes had to be made in order to accommodate the Goblin engine, which had considerably different properties and requirements than the earlier DB 605B unit. In order to sufficiently address the position of the new engine's exhaust, the stabilizer of the tailplane had to be raised from its former position, effectively requiring the tail section to be redesigned. Additionally, to sufficiently feed the hungry turbojet engine, the fighter's fuel volume was increased significantly via the installation of additional fuel tanks within the middle section of the wing and a pair of large wingtip tanks. At the same time, various measures to improve the fighter's aerodynamic properties were implemented during the rebuild process, leading to the adoption of curved glass in the forward section of the cockpit canopy, while the leading edge of the wing also received modifications. It was decided to implement air brakes in the form of an additional flap located on the outer wing's trailing edge. The ejection seat, which was a relatively new innovation, received various alterations and improvements that enabled it to correctly eject during higher speed flight.

The attack version, designated as the A 21R, saw the addition of 14.5 cm and 18 cm ground attack rockets, which could be installed underneath the middle wing. Furthermore, an alternative external gun pod called "Paddan", meaning the toad, could also be installed underneath the middle wing in place of the rockets. Paddan contained eight 8 mm ksp m/22 machine guns with 800 rounds of ammunition per gun. The fire control system of the plane allowed it to fire all guns and all its rockets or gunpod at the same time. This was an interesting "sensation" for the pilot as it made the airplane lose airspeed and almost stop mid air.

A production of 124 planes was planned originally, including four prototypes. When experience with the type as a fighter was gained with the F 10 wing it was concluded that all aircraft needed to be changed to attack aircraft. The production series was then reduced to 64 aircraft. Of these, 34 fighters were designated as the J 21RA (later re designated to A 21RA), powered by the Goblin 2 engine, known as the RM1 in Sweden, which was capable of generating a maximum of 1360kp of thrust. The other 30 were of the A 21RB standard, which was powered by a Swedish-made Goblin 3 engine, designated in Swedish Air Force service as the RM1A, capable of producing 1500kp of thrust.