The advent of the jet engine did more than offer a new form of propulsion. It opened the door to new and more exotic types of aircraft that fired the imagination and made the late 1940s and early 1950s a truly exciting time for aviation enthusiasts; one that perhaps will never be seen again.
One of the more unusual looking jet aircraft to take to British skies in this new golden age was the De Havilland Vampire. It looked nothing like the Spitfires and Hurricanes that came just five years before it. Instead of a long slender fuselage like those iconic fighters the Vampire seemed to be a set of wings, a cockpit and a double tail mounted on the end of two long booms thus producing what appeared to be a gap the aircraft. Naturally the aircraft and others like it adopted the term of “boom fighter” and it seemed to typify the future despite the fact that twin-boom aircraft were nothing new with piston engined fighters like the P-38 Lightning and P-61 Black Widow having served through the war along with the more traditional looking planes.
The Vampire very nearly didn’t make it in to service. Early jet engines were extremely underpowered and it was believed that at least two were needed to give an aircraft the power needed to fly and just as importantly for the RAF to fight. The RAF therefore backed Gloster’s Meteor design but De Havilland persisted with their single engine type and eventually convinced the RAF to invest in the Vampire. Despite this difficult birth the Vampire became a winner and achieved considerable export success. It also attained several accolades such as being the first jet aircraft to fly the Atlantic and the first jet to land and take off from a carrier. The twin-boom layout typified De Havilland’s combat aircraft of the 50s and 60s with the Vampire morphing in to the Venom before work began on the awe-inspiring De Havilland Sea Vixen all-weather naval fighter.
Another “boom fighter” that emerged in the early forties has gone almost forgotten outside of its home country. Sweden defended its neutrality fiercely before and during the war and to that extent went to the effort of attempting to build an air force based almost entirely on home-built designs. This freed them from being reliant on outside sources and so they were less likely to get dragged in to the war that tore Europe apart although the Swedes did operate both allied and German-designed aircraft as well. During this time the Swedish produced the extraordinary looking twin-boom Saab J.21 piston engine fighter. Powered by a license-built DB.605 engine (the same engine used in the superlative German Messerschmitt Bf.109) the J.21 also broke European trends for fighter design by adopting a pusher arrangement (propeller at the rear pushing the aircraft as opposed to one at the front pulling as in most types). Some aircraft even featured explosive bolts on the canopy and a primitive ejector seat designed to throw the pilot away from the aircraft and clear of the propeller at the back which made escape somewhat difficult otherwise.
Naturally such an advanced and unorthodox aircraft had a protracted development and it finally reached frontline units in December 1945 by which time its performance had proved wanting compared to other more mature designs such as the Supermarine Spitfire XIV which had almost 100mph over the aircraft’s top speed. The Saab J.21 therefore found itself used in the light attack role something for which it was well suited. Meanwhile the Swedish were looking for a new fighter and considered developing the J.21 in to a front engine puller version but the dawn of the jet age threatened to render that aircraft obsolete before work even began. What the Swedish needed was to develop a jet fighter and the willingness of the UK government to supply De Havilland Goblin jet engine gave them the opportunity to do just that.
Saab began drawing up plans for new jet fighters but in the interim they decided to take the J.21 and install the new jet engine in place of the DB.605. It was a logical decision since like the pusher arrangement the jet engine works by pushing the aircraft along. Saab needed to redesign the rear fuselage to include an exhaust and two side mounted intakes for the jet engine yet the resulting Saab J.21R still shared over 50% commonality with its piston engine predecessor. The Swedish government were taking no chances however and had already ordered the De Havilland Vampire as well.
In the end the Vampire served the Swedish for longer albeit mostly in a training role. They were both operated as fighter-bombers and the friendly rivalry between units was fierce and passionate.
So which was better?
For this comparison the De Havilland Vampire FB.5 will be compared to the Saab J.21R.
Both aircraft were powered by De Havilland Goblin Mk.II engine. In the Vampire this produced 3,100lbs of thrust that took the aircraft to a top speed of 540mph while a climb rate of around 4,800ft/min meant that it could reach its service ceiling of 42,000ft in a little under ten minutes. The Goblin Mk.II in the Vampire FB.5 gave the aircraft a maximum thrust-to-weight ratio of 0.46 which is slightly higher than that of the twin engine Gloster Meteor F.8. The Vampire was also an aerodynamically clean aircraft with the wing blending in to the air intakes which also generated lift.
Despite the J.21R being slightly lighter than the Vampire the low thrust of the Goblin Mk.II meant that it had a maximum thrust-to-weight ratio of 0.44. The engine took the J.21R to a speed of just 497mph most likely as a result of the higher drag the aircraft produced compared to the Vampire. The J.21R had a service ceiling of 39,000ft which gave the Vampire a 3,000ft advantage. Around 30 aircraft did feature the slightly more powerful Goblin Mk.III and this gave the aircraft a slightly better thrust-to-weight ratio but generally performance was not significantly improved. One significant criticism of the early J.21Rs were its low endurance with some flights barely lasting 40 minutes. Later models did feature increased fuel capacity but endurance remained quite limited.
Please note; maximum thrust-to-weight figures are determined by taking how much thrust is available compared to the empty weight. Internal fuel and adding ground attack weapons such as bombs and rockets decrease the thrust-to-weight ratio however as fuel is expended so the ratio becomes higher than it was just after take-off.
British aircraft designers in the mid-1940s benefited from experience gained in the early years of World War II in terms of gun armament. The Vampire was fitted with four Hispano Mk.V cannons, an arrangement that had quickly become standard on all British fighters of the period as it offered the best compromise between weight, ammunition capacity and of course hitting power. The weapon could hurl a 20mm shell at 840m/s and achieve a rate of fire of 750rds/min. Mounted close together in the nose meant that the pilot could bring all four guns to bear on a single spot on a target at longer ranges thus increasing their destructive power. The gun did have a somewhat chequered history however and the earlier versions of the weapon were quite prone to jamming. The Mk.V in the Vampire had largely resolved the problem but it would still jam if not properly maintained.
Main armament for the Saab J.21R was a single 20mm Bofors gun mounted in the nose. This was a 20mm development of the famous Bofors 40mm anti-aircraft gun and was selected for its hitting power against both air and ground targets but had a firing rate of just 360rds/min. This was backed up by four 13.2mm heavy machine guns that could churn out 900rds/min although a pod was available for an additional eight guns that could be put on the centreline pylon meaning this aircraft could effectively fly with thirteen guns (!) which must be something of a record for a single seat aircraft (a close runner up was the Hawker Hurricane IIB which had twelve .303 machine guns).
As its “FB” designation signified the Vampire FB.5 was a fighter-bomber and as such packed a hefty punch with an option to carry two 500lb bombs in place of the external fuel tanks. Alternatively the Vampire could carry a pair of launchers for a quartet of 60lb rockets that proved extremely useful in World War II against a wide variety of targets including tanks and ships. These rockets were quite heavy and had a very steep gravity-drop angle (the motor was not powerful enough to keep the rocket flying level after launch) which meant they were always launched in a steep dive towards the target.
The Saab J.21R had four underwing pylons and a single centreline pylon for the carriage of additional weapons. Common bomb sizes for the centreline pylon were 551lbs (250kg), 1102lbs (500kg) and 1323lbs (600kg). Alternatively, four 110lb (50kg) bombs could be carried on the four underwing pylons. The Saab J.21R had a wide variety of unguided rockets at its disposal. Typical loads were ten 80mm or 100mm rockets while alternatively up to five 180mm anti-tank rockets.
As day fighters then the Vampire held a better poker-hand than the J.21R. The Vampire was over 40mph faster, had slightly better acceleration and could attain a higher service ceiling thanks in no small part to its aerodynamic efficiency. The Vampire pilot also had much better all-round visibility compared to the Saab J.21R pilot who had to contend with the fuselage coming up behind him and a heavily framed canopy. By comparison the Vampire pilot had a two piece bubble canopy that protruded from atop the forward fuselage allowing him to take a good look around and above. The J.21R pilot did have higher cumulative hitting power in terms of his gun armament if he used both the machine guns and the bofors gun in conjunction. In the extremely unlikely event that he could bring his eight guns mounted in the external pod to bare as well then the Vampire would be torn to shreds if the J.21R pilot got the British jet in his sights.
In the ground attack role the Saab J.21R was a more rounded aircraft than the Vampire. It had more weapon options which it could tailor for specific target types whereas the Vampire was more heavy handed. The Vampire could fly further but for neutral Sweden who had a policy of defensive operations this was not so much of a concern. The Saab J.21R did serve a vital purpose in that it launched the Swedish aviation industry in to the jet age and over the next 60 years the company produced some of the finest fast jet types in Europe.