What it was: A boost-glide intercontinental spaceplane. It would reach space, if not orbit due to lack of speed, but manage to get all the way around Earth once by repeatedly skipping off the upper atmosphere to gain more altitude. During World War II it was positioned as an extreme long-distance bomber (capable of, for example, carrying a 3600-kilogram bomb to New York City from a launch site in Germany), but it also would have made an interesting surveillance vehicle—utterly immune to being shot down and the next best thing to a spy satellite.
Details: Ever since space travel became even marginally possible doing so has been torn between two approaches. One is to stick a one-shot capsule of some sort on top of a rocket and then let it return ballistically after the mission is over; the other is to build a spaceplane which either gets to space under its own power or is launched on a rocket, and then is capable of gliding back to Earth. Theoretically planes are cheaper because of their reusability while capsules are easier to build. In practice, though, no-one’s ever been able to develop a spaceplane that could undercut a capsule because the added complexity of the plane adds back on to the saved costs. As a result, with the exception of the Americans’ long excursion into the Space Shuttle program, all spacecraft that were successful for more than one or two flights have been capsules.
Both approaches date back to the first time and place that had any chance at all of putting something into space, which is to say Germany in the 1940s. Wernher von Braun’s ballistic rocket approach has been the one followed by the USSR and China, while the United States used it into the 1970s and is returning to it now with the upcoming Orion MPCV.
Less well-known is Eugen Sänger and Irene Bredt’s Silbervogel (“Silverbird”) which was the first serious attempt at building a spaceplane, work on which contributed to the success of several other later spaceplanes that flew, and which itself was refactored and raised as a possibility as late as the 1980s.
Sänger began work on the concept in his original engineering thesis for the Vienna Polytechnic Institute. When it was rejected as too radical in 1931, he submitted a second, more acceptable thesis on a different subject, but arranged for the original to be published by a different route in 1933. At the same time he perfected a regeneratively-cooled rocket engine (which is to say that it used the expansion of the rocket fuel’s gases to carry away heat and keep the engine from overheating). His research couldn’t secure funding in his native Austria, but an article in the journal Flug (“Flight”) in 1935 attracted the attention of the Luftwaffe in Germany. He was invited to set up a research facility there, which he did in 1936 and then the real work on Silbervogel began.
By 1942 he had advanced the rocket engine which would power the craft, worked on the rocket sled and track which would be used for its initial boost launch, and worked out the aerodynamics of a plane that would be both subsonic and supersonic as well as flying in the near-vacuum of space.
The Silbervogel would have been a two-part ship. The spacecraft itself was to have been a 10-ton, streamlined plane with two stubby wings and two tailfins, both raked upwards at about ten degrees. Four fuel tanks took up most of the fuselage and contained liquid oxygen and kerosene which would burn in a single rocket engine over the course of 168 seconds. On the ground the plane would be mated with a rocket sled which would give it an initial boost from behind along a rail track for a mere ten seconds but with nearly five times the thrust as the spaceplane’s engine.
Once the Silbervogel completed both burns it would be moving at a minimum of Mach 13 (15,926 km/h) and as much as Mach 20 depending on its mission and payload, and reach a maximum altitude of anywhere from 31 to 121.5 kilometers, the latter value being well into space. Just to put this in perspective, the air speed record in 1944 was 1130 kilometers per hour (Mach 0.92), while the altitude record in an aircraft was 17.3 kilometers. Sänger and Bredt did not think small.
The Silbervogel would then begin a roller-coaster-like ride up and down into the Earth’s atmosphere, using its wings and angle of attack to skip off the denser air at about 20 kilometers up and regain altitude for another distance-eating hop. An example diagram in the 1944 paper discussed below shows no less than eight such skips before settling into a steady flight at 20 kilometers and a return to base after a complete trip around the world.
What happened to make it fail: It was too advanced for the time, and even Sänger (who underestimated the technical difficulties of the heat Silbervogel would have to endure when skipping into the atmosphere) thought that it would not fly for many years. As World War II heated up, the Nazi government officially put the program on hold in 1942 to save money and resources for weapon systems that could be used before the end of the ongoing fighting. Oddly enough, despite the stop Sänger was still assigned to it and continued work on it until 1944, as the Nazis looked at several possibilities for being able to bomb the United States from the Azores if fascist Spain and Portugal could be brought into the Axis.
In that year he and Bredt published their final version of their research, which was published as Über einen Racketantrieb für Fernbomber (translated after the war as A Rocket Drive for Long Range Bombers, a copy of which can be downloaded as a PDF). This remarkable document outlines how the Silbervogel would have looked and worked, as well as how it might have been used in a variety of ways—for example avoiding the difficulty of having to go the whole way around the Earth by setting up a second Silbervogel landing and launching base in the Japanese Marianas Islands or, better, in the occupied territory in California which the Japanese would helpfully conquer for the Nazis. A cheerful diagram showing the complete destruction of Manhattan from roughly Union Square north to the corner of 27th Street and Broadway and south to Houston Street is included, as this would be possible with a mere 84 sorties with 3600-kilogram bombs. Note that the Space Shuttle Discovery holds the record for the most flights above 100 kilometers by any one spaceplane, 39, racked up over the course of 27 years.
What was necessary for it to succeed: Under any reasonable circumstances, it wasn’t going to work as initially designed. The design was simply too far advanced for the time, and Germany couldn’t come up with the physical resources or money to build one.
That said, if there had been no war, and if the Germans had had access to high melting-point molybdenum for its belly (or developed heat-resistant ceramic tiles as would be used on the US’s Space Shuttle), and if there had been the political will to spend those marks and metals—and that’s an awful lot of “ifs”—something like the Silbervogel could have flown around 1960. It likely would have been heavily redesigned by then.