Several German aircraft makers of World War 2 were convinced of the merits inherent in joining two aircraft by way of a common center wing section. This was most famously attempted by Heinkel with their He 111 medium bomber to produce the He 111Z "Zwilling" ("Twin") and the idea found even more success in America with the post-war North American F-82 "Twin Mustang" which saw combat service in the Korean War. The Dornier Do 635 existed as another short-lived attempt by German engineers to create a new "super aircraft" from an existing design.
The Do 635 was proposed during 1944 and the idea was simple enough on paper - join two examples of the promising Do 335 heavy fighter with a common center wing section while retaining all other original aspects of the high-performance fighter. The Do 335 was a unique fighter design in the grand scope of World War 2 as it showcased a pair of engines seated in line across a well-streamlined, cylindrical airframe. One engine was installed at the nose in a traditional "puller" arrangement and the other was fitted in the aft section of the fuselage in a "pusher" arrangement (behind the pilot's position). This combination of power made the Do 335 the fastest German prop-driven aircraft of the war but delays in acquiring the necessary engines led to fewer than forty examples delivered before the end of the conflict in Europe in May of 1945.
In the Do 335 "Zwilling" design, designated as Do 635, the aircraft was to retain the services of all four engines in their respective locations about the airframes. Each engine outputted 1,777 horsepower for a combined output of 7,108 horsepower. A crew of two was natural as each airframe was designed to hold one under their respective framed canopies. The combined fuselages generated a wingspan of 90 feet but more or less retained the same overall length (45.4 feet) and height (16.4 feet) of the original heavy fighter. Gross weight was estimated to be 72,532 pounds and performance specs included a top speed of 447 miles per hour with an impressive range out to 4,722 miles.
The unique arrangement of the aircraft provided generous operational ranges as output power and fuel stores were shared across both airframes. This played well into the intended role of long-range reconnaissance platform and onboard would be 2 x Rb 50 series cameras fitted to the portside fuselage. In addition to this, five target marker bombs would be carried to designate targets of opportunity. Due to the hefty weight of the aircraft, it was proposed to fit it with 2 x Walter rocket units for Rocket Assisted Take-Off (RATO). Landing would be conventionally accomplished as each airframe would retain its respective tricycle undercarriage.
Beyond a wind tunnel model, nothing else came of the Do 635 initiative as it was cancelled in February of 1945 amidst the worsening war situation for Germany. It is unknown what value this large aircraft would have had considering the Germans were embroiled in what was largely a defensive-minded war by this point and fighters /interceptors / night-fighters were the call of the day.
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