The Nazi Space Shuttle

On December 8, 1941, the day after the surprise attack on Pearl Harbour, the United States declared war on the Empire of Japan. Three days later on December 11, Japan’s ally Nazi Germany declared war on the United States. This was to prove a major strategic blunder for the Nazis, for not only was the United States the greatest industrial power in the world, but its geographic isolation from both Germany and Japan made it nearly invulnerable to enemy attack. While the U-boats of the German Kriegsmarine scored impressive victories against Allied shipping early in the war, by 1943 the sheer volume of merchant vessels being churned out by American shipyards plus advancements in antisubmarine technology and tactics had all but secured Allied supremacy over the Atlantic. After that, the only way America could be attacked was from the air – something no existing German aircraft was capable of. This, however, did not stop the Nazis from trying, and throughout the war German engineers came up with a variety of ambitious designs for a so-called Amerikabomber – an aircraft capable of flying 6,000 kilometres from Germany to New York City and taking the fight to the enemy. While most of these designs consisted of giant, multi-engined bombers, one proposal stood out among the rest. Impressively prophetic, this design was so fantastically advanced that a vehicle of its type would not be successfully flown for another four decades. This is the story of Silbervogel, the Nazi space shuttle that never was.

Since the earliest days of science fiction, most writers assumed that future spacecraft would be similar to sailing ships or other aircraft: large reusable vehicles that could fly to and from space multiple times before needing to be repaired or overhauled. Though theoretically this approach permits enormous cost savings over single-use spacecraft, in practice the added complexity of designing a reusable spaceplane nearly always outweighs these savings. Consequently, over the past 60 years, most astronauts have travelled into space aboard simple, single-use ballistic capsules. However, this has not deterred designers from trying to crack the problem of reusable spacecraft, and among the first to tackle this challenge was a German engineer named Eugen Sänger.

Born 1905 in the town of Pressnitz in what is now the Czech Republic, Sänger initially pursued studies in civil engineering at the Technical Universities of Graz and Vienna in Austria. However, during his studies Sänger read the influential book By Rocket to Planetary Space by pioneering rocketry theorist Hermann Oberth and, like many young engineers of his day, was inspired to change his field of study to aeronautics. Around this time he also joined the Society for Space Travel, which counted among its members Oberth and a young Wernher von Braun – future father of the V2 ballistic missile and the Saturn V moon rocket.

Befitting his new-found passion, Sänger attempted to write his thesis on the subject of rocket flight, but this was rejected by the University as too fanciful. He finally graduated in 1929 after submitting a more mundane thesis on the statics of wing trusses. In 1936, Sänger published his rejected thesis as a book titled Rocket Flight Engineering, in which he laid out a unique concept for achieving high-speed, long-distance flight using a rocket-powered spaceplane. Rather than entering orbit, Sänger’s proposed spaceplane would climb up to just above the atmosphere and descend on a ballistic trajectory. The flat fuselage of the craft would be specially designed to generate lift, causing it to “skip” off the atmosphere like a stone skipping across a pond. Due to atmospheric drag, each skip would be progressively shorter than the last, but if the velocity and and reentry angle were properly calculated, such a craft could theoretically travel halfway around the world in mere minutes. Today, this type of flight profile is referred to as non-ballistic atmospheric entry or boost-glide.

These ideas caught the attention of the Reich Air Ministry, who saw Sänger’s rocket glider as a potential means of attacking the continental United States from Germany. In 1936, the Ministry established a laboratory near Braunschweig in Lower Saxony where Sänger could develop the 100-tonne thrust rocket engine needed to make his futuristic spacecraft possible. Here he worked alongside a team of technicians and mathematician Irene Bredt, whom Sänger would later marry in 1951. Together, Sänger and Bredt worked out many of the theoretical problems with boost-glide flight, including the ideal shape for the spacecraft’s lifting-body fuselage and the massive frictional and compression heating the vehicle would encounter as it reentered the atmosphere. They also pioneered one of the key developments in rocket engine design: regenerative cooling. The burning propellants in a rocket engine produce extremely high temperatures which easily can melt ordinary aerospace materials like aluminium. While combustion chambers and nozzles can be built of more heat-resistant materials like steel, these add weight to the engine’s structure, reducing its performance. Sanger and Bredt’s solution was to circulate the fuel through a double-walled chamber and nozzle wall prior to injecting it into the engine. This simultaneously wicked away combustion heat and pre-pressurized the fuel, allowing the engine to be be made far lighter and more efficient. This basic design is used in nearly all liquid-fuelled rocket engines to this day.

On December 3, 1941, Sänger presented his final proposal to the Air Ministry. Dubbed Silbervogel or “Silver Bird”, Sänger’s space plane looked like something straight out of science fiction. 28 metres long and weighing 10 tonnes, the craft featured a wide, streamlined hull with a flat bottom for skipping over the atmosphere, stubby wings, and a small twin tail. The majority of the internal volume was devoted to tanks of kerosene and liquid oxygen propellant to feed the single rocket engine, leading only minimal space for the pilot and weapons payload. The craft was to be launched horizontally on a 3-kilometre track using a rocket sled, whose cluster of six liquid-fuelled engines would accelerate it to nearly 2,000 kilometres per hour in only 10 seconds. Once airborne, Silbervogel would fire its own engine for 168 seconds, reaching a speed of 21,800 kilometres per hour – nearly 18 times the speed of sound – and an altitude of 145 kilometres, well above where most consider space to have begun. The aircraft would then enter the boost-glide phase, skipping its way across the Atlantic Ocean towards the United States. Once over New York or another target city, the pilot would open the bomb bay doors and release his 4,000 kilogram payload, his extreme speed and altitude making him invulnerable to enemy aircraft. As Nazi Germany never came close to developing an atomic bomb, for maximum destructive effect this payload would likely have been a “dirty bomb” composed of radioactive material wrapped around a conventional explosive core – and for more on this, please check out our previous video How Close Did the Nazis Actually Come to Building an Atomic Bomb?

As there was no way for Silbervogel to turn around and head back to Germany, Sänger proposed that it continue gliding over the North American continent to the Pacific Ocean where it would land on an island occupied by Germany’s ally Japan – a total journey of up to 24,000 kilometres.

Though decades ahead of its time, Sänger’s technically-dense, 900-page proposal failed to impress the Air Ministry, who filed it away and decided to focus on simpler, more conventional technology for the Amerikabomber. The Braunschweig team was disbanded, and Sänger was reassigned to the German Gliding Research Institute in Darmstadt. There he carried out research on ramjets engines and worked on the design of the Skoda-Kauba P14 ramjet interceptor, one of many hastily-conceived “emergency fighters” developed in the final days of the war in a desperate attempt to stem the tide of Allied bombers. In 1944 Sänger and Bredt submitted an edited version of the 1941 Silbervogel proposal to the Air Ministry under the title A Rocket Drive for Long Range Bombers, but once again it was rejected.

In the end, four major designs were submitted to the Amerikabomber program, all conventional multi-engined bombers: the Messerschmitt Me 264, the Focke-Wulf Ta 400, the Junkers Ju 390, and the Heinkel He 277. Of these, only the Messerschmitt and Junkers entries were actually built, with the Junkers Ju 390 ultimately being selected for production. However, only two prototypes were ever completed, and none entered service before the end of the war.

Several other schemes were proposed to attack America from Germany, including the unusual Huckepack or “piggyback” concept. This involved mounting a modified Dornier Do-217 light bomber on the back of a Heinkel He 117 heavy bomber, which would carry its passenger as far as possible before releasing it and returning to Germany. The light bomber would then fly the rest of the way to the target then ditch in the ocean, where a U-boat would be waiting to rescue the crew. While the plan was seriously considered, a lack of fuel and the loss of the Luftwaffe air base at Bordeaux prevented a full-scale test flight. Furthermore, the Kriegsmarine refused to spare any U-boats, effectively killing the project.

Meanwhile, Wernher von Braun and his team of rocket engineers at Peenemünde in northern Germany proposed a supersized version of their infamous V2 ballistic missile that would be capable of reaching North America. Dubbed the A10 or Amerika Rakete, the rocket would have measured 14 metres tall, weighed over 14 tons, and been able to cover a distance of 5,000 kilometres in around 35 minutes.
As guidance systems at the time were not sophisticated enough to provide the desired accuracy at such ranges, the craft was designed to be manned, with the unfortunate pilot using radio beacons mounted aboard U-boats to guide the 1-tonne warhead on its final, suicidal descent. In 1943, Otto Lafferenz, director of the Deutsche Arbeitsfront labour organization, also submitted a proposal for launching ordinary V2s from U-boats off the American coast. Codenamed Projekt Schwimmweste or “Project Life Jacket”, Lafferenz’s proposal involved the construction of a 500-ton watertight canister to be towed behind a U-boat, which would contain one rocket and tanks for its alcohol and liquid oxygen propellants. When close to the target, the U-boat crew would flood compartments in the rear of the canister to make it float upright, then fuel and launch the rocket. The canister would then be scuttled, and the U-boat would return to base. Though one prototype launch canister was completed, the concept, like the A10 Amerika Rakete, and Eugen Sänger’s Silbervogel, was never tested before the war came to an end. Nonetheless, these concepts proved prophetic, presaging the Intercontinental Ballistic Missiles and Submarine Launched Ballistic Missiles of today.

But for all the advanced concepts it incorporated, Silbervogel would most likely never have worked as-designed. Post-war analysis revealed that Sänger and Bredt severely underestimated the heat load their craft would encounter on reentry, meaning that Silbervogel – and its unfortunate test pilot – would likely have burned up on the first atmospheric skip. The problem could theoretically have been solved by thickening the heat shield, but this would have reduced the useful bomb load to practically nothing, rendering the aircraft useless as a weapon. Nonetheless, the design would prove hugely influential on the field of spacecraft design.

After the war, Eugen Sänger and Irene Bredt moved to France, where in 1949 they founded the Fédération Astronautique. In 1954 they returned to Germany, where Sänger directed jet engine research in Stuttgart and consulted for Junkers aircraft before dying in 1964 at the age of 59. It was while living in France that the Sängers survived a botched kidnapping attempt by Soviet agents. After occupying the Peenemünde research facility, Red Army troops discovered a copy of Sänger and Bredt’s Silbervogel proposal. The idea intrigued Soviet leader Josef Stalin, who dispatched his son Vasily and engineer Grigori Tokaty to convince the Sängers to come to the Soviet Union. However, the two would-be agents were unsuccessful – as were NKVD agents sent to take the couple by force.

Despite lacking Sänger’s expertise, in 1946 the Soviets set up a design bureau under the direction of mathematician Mstislav Keldysh to develop their own spaceplane. Realizing that the sheer volume of propellant needed to feed a liquid rocket engine would have given Silbervogel a negligible useful bomb load, Keldysh instead fitted his design with more efficient ramjets, creating a vehicle capable of carrying a practical nuclear weapon over a range of 12,000 kilometres. However, Keldysh estimated that the design would not be ready until the late 1950s, by which point it would likely be rendered obsolete by simpler intercontinental ballistic missiles. As a result, the “Keldysh Bomber” was quietly shelved.

Meanwhile, across the Atlantic, Sänger’s ideas were gaining traction thanks to one Major-General Walter Dornberger. The military head of the V2 ballistic missile program, after the war Dornberger, along with many German rocket scientists, was brought to the United States as part of Operation Paperclip. After developing ballistic missiles for the United States Air Force, Dornberger worked in the private aerospace sector, becoming Vice President of Bell Aircraft and consulting for North American Aircraft and Boeing. Throughout the 1950s and 60s, Dornberger attempted to sell the United States military on the concept of a boost-glide spaceplane like Sänger’s Silbervogel, which he dubbed the “Antipodal Bomber”. Following the launch of Sputnik 1, the world’s first artificial satellite, on October 4, 1957, the Air Force finally approved Dornberger’s concept under the designation X-20 Dyna-Soar – short for “Dynamic Soarer.” Other names included “Rocket Bomber” or “RoBo”, “Bomber Missile” or “BoMi”, “Brass Bell”, and “Hypersonic Strategic Weapons System.”

Measuring 11 metres long with a 6 metre wingspan, the dart-shaped, flat-bottomed Dyna-Soar was designed to be launched into space atop a modified Titan II ballistic missile. While boost-glide missions were possible, the craft was also capable of reaching orbit like the later Space Shuttle. Once in orbit, the single pilot could carry out a variety of missions, such as orbital bombardment of enemy targets, orbital reconnaissance with cameras and other sensors stored in a small equipment bay, or rendezvousing with and repairing satellites in orbit. Uniquely among spacecraft of the era, the Dyna-Soar’s aerodynamic, lifting-body shape would have allowed it to very efficiently change its orbital inclination by briefly dipping into the atmosphere. To protect the craft against the heat of reentry, it was built of high-temperature nickel superalloys, while its stubby wings and wire-brush skids would have allowed it to glide to a runway landing like a regular aircraft.

In April 1960, seven astronauts were secretly selected for the Dyna-Soar program – including one Neil Armstrong, later the first man to walk on the moon. However, the project soon began running into problems – the biggest being what the X-20 was even for. As rocket and guidance technology continued to improve, it quickly became clear that nuclear bombardment could be more easily and cheaply carried out using ballistic missiles – and orbital reconnaissance using unmanned spy satellites – than with a complex manned spaceplane. This, combined with confusion over what booster to use, Department of Defense budget cuts, and a government decree placing all manned spaceflight under the purview of NASA, led to the project being cancelled in 1963. It was not until April 12, 1981 that the first successful winged spacecraft – the Space Shuttle Columbia – was launched into orbit, finally making Eugen Sänger’s prophetic vision a reality. But it was only a partial realization, for due to a long list of design compromises the Space Shuttle was never able to achieve the massive cost savings promised by reusable spacecraft. Thus, following the Shuttle’s retirement in 2011, the world returned to using reliable, relatively inexpensive ballistic capsules to get into space. But as the ongoing development of new spaceplanes like the Boeing X-37 and the SNC Dream Chaser demonstrate, the dream of reliable, reusable, winged spaceflight is still alive and well, and future astronauts may yet glide-rather than plummet – back to earth.

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