One novel method of reaching the United States from Germany was the proposal for a hybrid of two planes. A Heinkel He-177 would be used to transport a Dornier Do-217 bomber equipped with an extra Lorin-Staustrahltriebwerk ramjet engine until the planes were sufficiently close to the United States for the Do-217 to be released and fly on towards the target. The plane would deliver its bomb to United States territory and then be ditched in the Western Atlantic, where the pilot would be recovered by a German submarine. The design could not be realized as the distances proved to be insurmountable, so the idea was soon abandoned.
This was not the novel idea it might seem. The first planes to be carried as ‘parasites’ were small planes that could be released from giant airships in the 1920s. During the 1930s the Soviets experimented with the idea of carrying fighters aboard larger aircraft. The Zveno series became increasingly complex, until a Tupolev TB-3 carried three Polikarpov I-5 fighters: one mounted on each wing, and a third above the cockpit. By 1935 the Aviamatka had been flown, a TB-3/AM-34 which carried five small fighters. Some were deployed just once during the war years; in mid-1941, a Zveno flight from the Black Sea was dispatched to attack the Negru Voda bridge in Romania.
The idea of carrying a plane aboard a larger transporter also re-emerged elsewhere in Germany during World War II. The principle was nicknamed Mistel (Mistletoe) because of the way that a small plane could be carried, like the parasitic plant, safely attached to a larger one. The concept was that the ‘parent’ plane could carry the attack aircraft towards the target, so that the smaller aircraft could carry out its attack and still have a full fuel tank with which to return to base. The first of these trials earned the nickname Huckepack (Pick-a-Back, or Father-and-Son). The attack aircraft was to be a Focke-Wulfe Fw-190 Würger (Strangle) fighter that had been designed by Kurt Tank in 1938. As a fighter, this small plane became well known as a routine strike aircraft and a ground-attack plane. It would be carried aloft mounted above a Ju-88 Mark 4, a bomber based on the design of the successful Ju-88 but with more powerful engines. It flew for the first time in early 1940. The Mark 4 of the Ju-88 had a wingspan of 70ft (20m) compared to the 65ft 10.5in (18m) of the previous models, with improved streamlining and a somewhat larger cockpit offering improved visibility.
The first five of these combined Mistel aircraft were ready early in 1944 and training began in Nordhausen. Both upper and lower aircraft were initially crewed, but it was envisaged that the Ju-88 could eventually be controlled from the smaller fighter and released as an unmanned ‘flying bomb’. Once the Allied invasion of the Normandy coast began, the unit moved from Nordhausen to St Dizier with a squadron of 12 of the combined aircraft. They were used with some effect; the crashing of an unmanned fully laden Ju-88 was extremely unnerving for the British, and this curious combined aircraft went on to fly further successful missions.
For extreme long-distance raids, there remained one top-secret German proposal of immense potential. It was for the ‘antipodal bomber’, a stratospheric aircraft that could span the world. It was the brainchild of one of the greatest German visionaries, yet also one of the least well known. This was Eugen Sänger, born in Pressnitz, now in the Czech Republic, who had studied civil engineering at the Technical Universities of Graz and Vienna. As a student, Sänger had been captivated by Hermann Oberth’s book Die Rakete zu den Planetenräumen (The Rocket into Planetary Space) and indeed we will come across both Oberth and Sänger again later in this book when we look more specifically at rocket-powered weapons. It was Oberth’s book that inspired Sänger to change from studying civil engineering to a career in aeronautics and as a first step he joined the Society for Space Travel in order to learn more from Oberth. The subject of Sänger’s thesis was originally intended to be rocket-powered flight, but this was rejected by the university as being unrealistic. Instead he was instructed to submit a relatively mundane report on the statics of wing trusses. His original work was not lost, however; and Sänger went on to publish it under the title Raketenflugtechnik (Rocket Flight Technology) in 1933, and followed this with several magazine articles on rocket-powered flight. They appeared in an Austrian magazine Flug (Flight) and soon came to the attention of the German Air Ministry, who could envisage Sänger’s ideas as one way to build a bomber that could attack the United States from an airfield in Germany.
One of Sänger’s dreams was for his great antipodal bomber — a long-range sub-orbital aircraft, launched by rockets, that could travel around the world through the stratosphere and deliver a payload to the opposite ends of the earth. Sänger called his craft the Silbervogel (Silver Bird) and worked on the design with a brilliant young mathematician named Irene Bredt, of Vienna, Austria, whom he later married. Sänger also designed the rockets so they could generate a thrust of 225,000lb-ft (0.3 meganewton metres). The silver bird was known as the RaBo (Raketenbomber or Rocket Bomber) and it would be launched by rocket sled from a 2-mile (3km) launch-track at about 1,200mph (2,000km/h). It would then fire its own onboard rocket motors and climb to an altitude of 90 miles (145km) reaching a velocity of 13,700mph (22,100km/h). The craft would then slowly descend in a sub-orbital trajectory until it entered the upper atmosphere and, as its wings and body generated lift, it would bounce back into space before slowly descending again. After several ‘hops’, it would be at the opposite pole of the earth. It was calculated to be able to deliver a 8,800lb (4,000kg) bomb to the United States and then fly on to land in the Pacific where the Japanese would ensure it was recovered for the Germans. The complete flight would be 12–15,000 miles (19–24,000km) in length.
It was an adventurous and futuristic scheme — too futuristic for the Luftwaffe, who brought development to a halt as early as 1942. As we have seen, they preferred to invest in proven aircraft technology. Sänger was assigned to carry on research at the German Gliding Research Institute (Deutsche Forschungsanstalt für Segelflug) where he made important developments in ramjet design. He remained an authority on rockets, and came to prominence again when German rocketry was reaching its height.
The end of the project was not the end of the idea. After the war, calculations proved that the antipodal bomber would not have worked — the heat generated by the ‘hops’ into the upper atmosphere would have caused far greater heating than the Germans had realized, destroying the craft. Even so, the Soviets tried to tempt Sänger to move to Russia and continue his research there, but he declined the offer and instead continued his research in France, where he founded the French Astronautical Federation (Fédération Astronautique) in 1949. He spent his later years back in Germany, working on a ramjet-powered spacecraft that was never realized. He also pioneered the idea of using photons from the sun for long-distance space propulsion, and introduced the still-popular concept of the solar sail.