Battle of the Super Twins
Twin engine fighter designs met with mixed success during the Second World War. Some, like the P-38 Lightning or Bristol Beaufighter met with considerable success. Others however such as the Messerschmitt Bf110 or Westland Whirlwind didn’t enjoy as much at least in the daylight role. The concept was not lost entirely on aircraft manufacturers however who realized that in order to be successful against single engine types a twin engine fighter needed to be as light as possible or fitted with two powerful enough engines to compensate for the extra weight. The advantages to twin engine fighters was enough to spur this development. Twin engine fighters could carry more fuel making them excellent escort aircraft for bombers. They could also carry heavier armament and have radar installed for a night fighter capability.
Towards the end of the war the single engine types such as the Spitfire, P-51 Mustang and Fw190 were still the rulers of the sky but a steady stream of new twin engine fighters had started to make their mark also. In the UK the De Havilland Mosquito had proven a superlative weapon being able to adapt to numerous roles and excelling in nearly all of them. Encouraged by this success the engineers at De Havilland took the basic design and scaled down the fuselage in to an even lighter and more streamlined design for use by a single pilot and this produced the De Havilland Hornet fighter.
In Germany they had taken an entirely different approach however. As early as 1938 German engineers had realized that one of the biggest problems encountered by twin engine types was the increased drag from having the two engines on the wings and the weight penalty this incurred on agility. They therefore began experimenting with mounting the first engine in the forward fuselage and a second engine in the rear of the fuselage turning a pusher propeller. This had the advantage of creating a much more streamlined aircraft that had the power of a twin engine fighter but with the aerodynamic efficiency of a single engine type. Bizarrely, the German Air Ministry initially insisted this configuration be developed in to a bomber first but the project eventually collapsed before being revised again late in the war with the need for a new twin engine fighter. Fortunately for allied bomber crews the resulting Dornier Do.335A-1 Pfeil never entered service before the war ended.
For this comparison I am looking at the Hornet Mk.III (Hornet F.3) compared to the Do 335A-1 Pfeil. Neither of these aircraft reached frontline service before VE-Day and therefore it is a fair comparison of what they could have achieved against each other in combat. One important thing to note is that the Do 335A-1 was not as developed as the Hornet and therefore we can’t claim for certain that how the aircraft appeared in 1945 is not necessarily how the production version would have appeared. That being said the early plans for the Do 335A-1 were in this configuration so it is possible that this is how it would have gone in to combat at least initially.
The De Havilland Hornet was powered by a pair of Rolls-Royce Merlin 130 12-cylinder inline engines mounted in pods on the wings. These developed an impressive 2,080hp each and this hurtled the Hornet to a speed of 472mph at 22,000ft but this began to drop off considerably around the 30,000ft mark it reaching a speed in the region 340mph and further still when it reached its service ceiling of 35,000ft. These figures would be improved in later variants.
The Do 335A-1 was powered by two of the proven Daimler Benz DB-603A-2 inverted-V engines producing 1,750hp each; 3,500hp in total. The DB-600 series of engines, like the Rolls-Royce Merlin family for the Allies, were the engines that took the Luftwaffe fighter force through the war. They were tough, reliable and generally considered to be superior to the equivalent Merlin variant except in the later stages of the war when Allied engine technology caught up. These engines took the Do 335A-1 to a very respectable 478 mph at 28,000 feet in tests; something that most single seat fighters struggled to achieve even in 1945. The aircraft had a service ceiling of 37,400ft and could climb to its normal operating height of 26,000ft in just under 14 ½ minutes.
The De Havilland Hornet was armed with a quartet of 20mm Hispano cannons mounted in the forward fuselage. This arrangement was becoming standard practice for RAF fighters as it provided the best balance of hitting power and weight considerations on the aircraft. These offered an effective rate of fire of 6-700 rounds a minute with a muzzle velocity of around 860m/s which was enough tear a Messerschmitt Bf109F in half. Ground attack ordinance covered a wide array of bombs and unguided rockets. Up to two 1,000lb bombs could be carried on hardpoints under the wing. Alternatively, eight 60lb RP-3 rockets could be carried which was reported to give aircraft like the Hornet the same firepower as the broadside of a battleship armed with 5.5inch guns.
The Do 335-A1 sported a powerful MK-103 30mm cannon firing through the spinner of the forward propeller in a configuration proven by both the Bf109 and the Fw190 albeit with smaller calibre weapons to suit their size. The explosive warhead of this shell could inflict very heavy damage on an enemy aircraft and was optimized for bringing down the B-17 Flying Fortress and B-24 Liberator quickly before their defensive guns could be brought to bear on the attacking aircraft. It was capable of firing up to 380 rounds a minute and was ideal for quick slasher attacks on bombers. This weapon was backed up by a pair of 20mm MG151/20 cannons(see above image)mounted in the forward engine cowl. This was one of the great air-to-air weapons of World War II and saw use on a variety of aircraft ranging from the Bf109 to the Me262 jet fighter. Depending on the type of shell being fired the weapon had a rate of fire between 600 and 750 rounds a minute and was the weapon of choice for engaging enemy fighters. For a projected ground attack role the Do-335A-1 was able to carry up to 1,000lbs of ground attack ordinance.
The Hornet suffered from the same problem that all multi-engined aircraft that house their engines on the wings suffer; heavy rotational inertia. In layman’s terms this is having to overcome the weight of the engines being mounted further away from the aircraft’s centre of gravity than on an equivalent single engine type to such an extent that roll rate falls off substantially. To reduce this as much as possible designers tried to place their engines as close to the fuselage as possible but in piston engine aircraft this was made difficult by the diameter of the propeller. Thanks to the unique construction methods employed by De Havilland, namely of the balsa-ply configuration, the aircraft remained very light despite its size compared to single seat types and with its two Rolls-Royce Merlin engines it achieved 400hp per ton.
The disadvantages of having its two engines mounted in pods on the wings was totally negated by the Do335A-1. It therefore enjoyed a high rate of roll, superior to that of the Hornet and other similarly configured twin engine types, but the weight of the second engine in the tail slowed it down in the latitudinal plane particularly when in the nose down attitude when the aircraft was working against its lift forces to achieve flight. It was still a heavy aircraft and its two engines developed approximately 363hp per ton.
Without question the Hornet pilot enjoyed one of the best views out of the cockpit compared to many other types thanks to its teardrop shaped canopy. As proven by its Mosquito forebear it was also relatively easy to produce compared to all-metal fighters thanks to its wood-based construction of the fuselage and wings. This also made repairing it considerably easier.
The Do.335A-1 pilot had a much poorer view from his cockpit than the Hornet and many contemporary fighters. This was especially true in the aft most quadrants and was largely a result of the positioning of the rear engine and fuel tanks. This would probably have been addressed in later models but the urgency with which the aircraft was needed meant that it work had to go ahead with getting the aircraft in to service. Construction of the Do335A-1 was no easy feat as there had been no other aircraft like it before and so converting existing production facilities to accommodate the new type was wrought with difficulty. This was made immeasurably worse by destruction of many of those facilities by allied bombing raids.
The Hornet was an exceptional aircraft and proved most adept at the ground attack role (ironically the original role envisioned for the Do335) but in the air the Dornier “wonder weapon” was the superior aircraft enjoying a higher speed at altitude and a superior roll rate. It’s worth noting that the differences between the two aircraft’s performances are of a similar percentage to those between the Supermarine Spitfire V and the Focke Wulf Fw190A and in that instance the Fw190A’s advantages inflicted a heavy toll on the Spitfire V. Therefore there is the highest probability that in combat with one another the Do335A-1 would better the Hornet.
The Hornet had the bigger punch however thanks to its four 20mm guns which when total firepower is considered out-punched the Do335A-1’s mixed calibre arrangement. Also the close coupling of all the Hornet’s guns made training them all on to a single target significantly easier as opposed to the cowl and spinner mounted weapons of the Do335A-1. The Hornet pilot also had superior vision which was crucial in a dogfight.
There is one final note worth mentioning about the differing configurations of these two aircraft. The Do335A-1’s inline arrangement of its two radial engines meant that an attacking enemy fighter could lay down a stream of bullets along the fuselage and potentially knock out both engines in a single pass especially in an attack from the forward hemisphere. The Hornet on the other hand with its two engines mounted much wider apart would be a much more difficult aircraft to knock out in a single pass.