In December 1941, a German engineer handed the Reich Air Ministry a several-hundred-page report describing a way to bomb New York from the edge of space. The weapon would be launched off a two-mile rail, rocket to the fringe of the atmosphere at some 24 times the speed of sound, then skip across the sky like a stone flung across a pond — all the way to America and on to a landing in the Japanese-held Pacific.

It was called the Silbervogel, the Silver Bird. It was never built, its central sum was wrong, and it was decades ahead of anything that could actually fly. It is also one of the most influential aircraft that never existed.

The man who wouldn’t give up

Eugen Sanger had been chasing rocket planes since the 1920s. His doctoral thesis on rocket-powered flight was rejected by his Vienna university as too fanciful; a professor told him bluntly that spaceflight was no way to earn a degree.

He switched topics, got his doctorate, and kept working on rockets anyway. In 1935 he lost his job and went into debt to publish his own book on the subject. Undeterred, he wrote a line that became his motto.

Skipping across the sky

With Bredt handling the mathematics, Sanger worked out the concept in detail. A rocket-powered sled would fling the flat-bottomed craft down a two-mile track. Its own engine would then boost it to the rim of space at close to orbital speed. As it fell back toward the denser atmosphere, its wings would generate so much lift that it would bounce back upward — skipping in and out of the air, shedding a little energy each time, until it glided down onto its target.

Sanger calculated that roughly three skips would carry it from Germany to the eastern United States, where it would drop a four-tonne bomb before flying on to a friendly landing site. Along the way he made a genuine engineering leap: he was among the first to propose cooling a rocket engine by circulating its own fuel around the nozzle — a technique real rockets still use today.

The fatal miscalculation

The Silbervogel was one of several ideas floated for the Nazi “Amerika Bomber” project, but the Air Ministry backed proven technology instead. Sanger and Bredt got no further than wind-tunnel tests before the war ended. Their report was published in 1944 as a secret command document, in fewer than a hundred copies.

It was just as well for any pilot. Postwar analysts rechecking the mathematics found an error: the heat load during that first, fastest skip back into the atmosphere would have been far higher than Sanger and Bredt calculated. Built to their figures, the Silver Bird would have burned up on its very first bounce.

And yet the idea would not die. Sanger’s skip-glide concept fed directly into America’s Dyna-Soar spaceplane project, fascinated Stalin enough to order his own study of it, and lent its designer’s name to a 1980s German space-shuttle proposal. The Silver Bird never flew — but the thinking behind it helped carry others into space.

Sources: GlobalSecurity.org; T. A. Heppenheimer, “Facing the Heat Barrier: A History of Hypersonics” (NASA); Wikipedia.