Silbervogel (“silver bird”) was a super-revolutionary design (actually more concept than a full-fledged design). It was decades (if not a hundred years) ahead of its time because even today’s technologies are not advanced enough to produce this weapon and make it operational.
Silbervogel was designed in the late 1930s by Eugen Sänger (Austrian aerospace engineer) and Irene Bredt (German engineer, mathematician and physicist) as rocket-powered sub-orbital intercontinental bomber. It was one of a number of designs considered for the Amerika Bomber project.
When Walter Dornberger attempted to create interest in military spaceplanes in the United States after World War II, he chose the more diplomatic term antipodal bomber when describing the Silbervogel project.
Although it was deemed impossible to produce as the just about all necessary technologies (propulsion system, navigation, protection from heat, etc.) simply did not exist (actually, were decades away), it still was an influential design.
Silbervogel was the first spaceplane design that incorporated liquid-fuel rocket technology, and the principle of the lifting body, foreshadowing future development of operational winged spacecraft such as the X-20 Dyna-Soar of the and the famous Space Shuttle (and its Soviet clone Buran).
The Silbervogel was intended to fly long distances in a series of short hops. The aircraft was to have begun its mission propelled along a 3 km long rail track by a large rocket-powered sled to about 800 km/h. Once airborne, it was to fire its own rocket engine and continue to climb to an altitude of 145 km, at which point it would be travelling at some 5,000 km/h.
It would then gradually descend into the stratosphere, where the increasing air density would generate lift against the flat underside of the aircraft, eventually causing it to “bounce” and gain altitude again, where this pattern would be repeated.
Because of aerodynamic drag, each bounce would be shallower than the preceding one, but it was still calculated that the Silbervogel would be able to cross the Atlantic, deliver a 4,000 kg [nuclear] bomb to the continental United States, and then continue its flight to a landing site somewhere in the Empire of Japan–held Pacific, a total journey of 19,000 to 24,000 km.
Postwar analysis of the Silbervogel design involving a mathematical control analysis unearthed a computational error and it turned out that the heat flow during the initial atmospheric re-entry would have been far higher than originally calculated by Sänger and Bredt.
Consequently, had the Silbervogel been constructed according to their flawed calculations the craft would have been destroyed during re-entry. The problem could have been solved by augmenting the heat shield, but this would have reduced the craft’s already small payload capacity (or would have required far more powerful rocket engines).
On 3 December 1941 Sänger sent his initial proposal for a suborbital glider to the Reichs Air Ministry (RLM) The 900-page proposal was regarded with disfavor at the RLM due to its size and complexity and was promptly rejected.
Professor Walter Gregorii had Sänger rework his report and a greatly reduced version was submitted to the RLM in September 1944. It was the first serious proposal for a vehicle which could carry a pilot and payload to the lower edge of space.
Two manned and one unmanned version were proposed: the Antipodenferngleiter (antipodal long-range glider) and the Interglobalferngleiter (intercontinental long-range glider).
Both were to be launched from a rocket-powered sled. The two manned versions were identical except in payload. The Antipodenferngleiter was to be launched at a very steep angle (which would shorten the range) and after dropping its bomb load on New York City was to land at a Japanese base in the Pacific.
However by that time RLM had much more pressing needs than the development of an intercontinental Amerika Bomber (which without a nuclear bomb was useless anyway). So the project was rejected again.
After the war ended, Sänger and Bredt worked for the French government and in 1949 founded the Fédération Astronautique. Whilst in France, Sänger was the subject of a botched attempt by Soviet agents to win him over.
Joseph Stalin had become intrigued by reports of the Silbervogel design and sent his son, Vasily, and scientist Grigori Tokaty to kidnap Sänger and Bredt and bring them to the USSR.
When this plan failed, a new design bureau was set up by Mstislav Vsevolodovich Keldysh in 1946 to research the idea. A new version powered by ramjets instead of a rocket engine was developed, usually known as the Keldysh bomber, but not produced. The design, however, formed the basis for a number of additional cruise missile designs right into the early 1960s, none of which were ever produced.
In the US, a similar project, the X-20 Dyna-Soar, was to be launched on a Titan II booster. As the manned space role moved to NASA and unmanned reconnaissance satellites were thought to be capable of all required missions, the United States Air Force gradually withdrew from manned space flight and Dyna-Soar was cancelled.
One lasting legacy of the Silverbird design is the “regenerative cooling—regenerative engine” design, in which fuel or oxidizer is run in tubes around the engine bell in order to both cool the bell and pressurize the fluid. Almost all modern rocket engines use this design today and some sources still refer to it as the Sänger-Bredt design.