Silverbird - Bredt-Sanger Antipodal bomber
Designed by Eugen Sanger and Irene Bredt in the late 1930s, the rocket-powered Silbervogel(silver bird), conceived as a long-range bomber, never went into production. It was too complex and expensive for the time, but with elements like a flat bottom and low-aspect-ratio wings, the aircraft's sleek, advanced design did foreshadow later hypersonic craft. During World War II, the Germans developed the V-2 rocket and worked on winged missiles such as A-9, A-10, and A-4. Sanger took note of these developments and continued to refine his Silver Bird [Silbervogel] concept accordingly after the War.
Individuals such as H.G. Wells, Jules Verne, Konstantin Tsiolkovsky, Robert Goddard, Sergei Korolev, Wemher von Braun, and Eugen Sanger helped to lay the theoretical and practical foundations of spaceflight. Since the early twentieth century, aeronautical engineers dreamed of developing an airplane that could fly into Earth orbit by taking off and landing horizontally on a runway. The German rocketeer Max Valier had suggested simply adding rockets under the wings of conventional airplanes such as the Junkers G-23 transport. Valier was conducting research on rocket-propelled gliders in the years leading to his accidental death in 1930.
Eugen Sanger first proposed the development of winged hypersonic vehicles, starting in the late 1920s, continuing with his antipodal aircraft studies of the 1930s and 1940s, and this interest helped stimulate a climate that resulted in the first attempt at a high supersonic winged vehicle, the A-4b of 1945. This "winged V-2," according to the Sanger-Bredt study, would skip in and out of the atmosphere to drop its payload and land halfway around the world.
Eugen Sanger, whom some have called the “father of the reusable space transporter,” was trained as an aeronautical engineer and got his inspiration from similar work done by Franz von Hoeff, Valier, and other earlier European figures. In the 1930's Sanger came up with the idea of a reusable rocket plane that could carry people. Sanger, a specialist in aeronautics and propulsion, received a doctorate at the Technische Hochschule in Vienna and stayed on to pursue research on rocket engines. Sänger made rocket-powered flight the subject of his 1933 thesis, but it was rejected by the university as too fanciful.
Born in 1905, he was of the generation that came of age as ideas of space flight were beginning to germinate. Sänger’s own thoughts began to take shape while he was still in grammar school. His physics teacher gave him, as a Christmas present, a copy of a science-fiction novel, Auf Zwei Planeten (“On Two Planets”). “I was about 16 years old,” Sänger later recalled. “Naturally I read this novel avidly, and thereafter dreamed of doing something like this in my own lifetime.” He soon broadened his readings with the classic work of Hermann Oberth. “I had to pass my examination in mechanics,” he continued, “and had, therefore, made a particular study of this and related subjects. Then I also started to check and recalculate in detail everything in Oberth’s book, and I became convinced that here was something that one could take seriously.”
He then attended the Technische Hochschule in Vienna, where he tried to win a doctoral degree in 1928 by submitting a dissertation on the subject of rocket-powered aircraft. He did not get very far, later recalling that his professor told him, “If you try, today, to take your doctor degree in spaceflight, you will most probably be an old man with a long beard before you have succeeded in obtaining it.” He turned his attention to a more conventional topic, the structural design of wings for aircraft, and won his degree a year later. But his initial attempt at a dissertation had introduced him to the line of study that he pursued during the next decade and then during the war.
In 1933 he turned this dissertation into a book, Raketenflugtechnik, (Rocket Flight Engineering) [a translation of which was published by NASA in 1965]. It was the first text in this new field. He wrote of a rocket plane burning liquid oxygen and petrol, which was to reach Mach 10 along with altitudes of 60 to 70 miles. This concept was significant at the time, for the turbojet engine had not yet been invented, and futurists, such as Aldous Huxley who wrote Brave New World, envisioned rockets as the key to high-speed flight in centuries to come.
In 1933 Sanger proposed achieving velocities as high as Mach 10, along with altitudes of up to 70 kilometers. He wrote "The main object of this book is to direct discussion of rocket flight into serious channels and to free it from Its former fantastic notions, -which have, understandably but technically imdesirably, prevented sober consideration by engineers busy with majay other matters.... Perfomance presently attainable leads us to expect that rocket vehicles excel ordinary propeller aircraft by about a factor twenty as regards maximum speed and crusing speed, and by about a factor five as regards ceiling height.... The process of flight by a rocket aircraft is externally exactly the same as that of an ordinary aircraft.... Flight in the upper reaches of the stratosphere at very high speeds cannot employ the presently usual propulsion system with airbreathing engine and air-screw; even the solution of the supercharger and airscrew problems becomes impracticable for the air densities in question...
The most outstanding superiority of the rocket airciraft over propeller aircraft lies in the flight speed.... The rocket aircraft altitudes required for the transportation purposes are associated in the first instance with the attainable peak velocities, and therefore in turn with the length of the flight. Nevertheless, the range of altitudes is a relatively narrow one. Since the aircraft attains the region of pure supersonic speeds only at altitudes from 35 to 40 km, the flight altitudes will definitely have to be above that n-umber. But since the aerodynamic forces increase only very slovly, with speed in the pure supersonic region, altitudes higher than 50 km will hardly he required for the peak velocities attainable within a foreseeable span of time..... "
Sänger’s altitudes became those of the X-15, a generation later. The speed of his concept was markedly higher. He included a three-view drawing. Its wings were substantially larger than those of eventual high-performance aircraft, although these wings gave his plane plenty of lift at low speed, during takeoff and landing. Its tail surfaces also were far smaller than those of the X-15, for he did not know about the stability problems that loomed in supersonic flight. Still, he clearly had a concept that he could modify through further study. In 1934, writing in the magazine Flug (“Flight”), he used an exhaust velocity of 3,700 meters per second and gave a velocity at a cutoff of Mach 13. His Silbervogel, Silver Bird, now was a boost-glide vehicle, entering a steady glide at Mach 3.5 and covering 5,000 kilometers downrange while descending from 60 to 40 kilometers in altitude.
He stayed on at the Hochschule and conducted rocket research. Then in 1935, amid the Depression, he lost his job. He was in debt to the tune of DM 2,000, which he had incurred for the purpose of publishing his book, but he remained defiant as he wrote, “Nevertheless, my silver birds will fly!” Fortunately for him, at that time Hitler’s Luftwaffe was taking shape, and was beginning to support a research establishment. Sänger joined the DVL, the German Experimental Institute for Aeronautics, where he worked as technical director of rocket research. He did not go to Peenemunde and did not deal with the V-2, which was in the hands of the Wehrmacht, not the Luftwaffe. But once again he was employed, and he soon was out of debt.
He also began collaborating with the mathematician Irene Bredt, whom he later married. His Silbervogel remained on his mind as he conducted performance studies with help from Bredt, hoping that this rocket plane might evolve into an Amerika- Bomber. He was aware that when transitioning from an initial ballistic trajectory into a glide, the craft was to re-enter the atmosphere at a shallow angle. He then wondered what would happen if the angle was too steep.
In 1935 Sanger published articles on rocket-powered flight for the Austrian journal Flug (Flying). These attracted the attention of the Reichsluftfahrtministerium (Reich Aviation Ministry / German Air Ministry / German Aviation Administration), which saw Sänger's ideas as a potential way to accomplish the goal of building a bomber that could strike the United States.
In Sanger’s concept, a launch vehicle would propel itself to the upper atmosphere then glide with no power until it hit denser air. It then would use kinetic energy to skip off the atmosphere back up to higher altitudes, similar to a stone skipping along water. Each skip reduces the available energy until it glides toward its target. Sänger also designed the rocket motors that the space-plane would use, which would need to generate 1 MN of thrust. In this design, he was the first to suggest using the rocket's fuel as a way of cooling the engine, by circulating it around the rocket nozzle before burning it in the engine.
Sanger calculated that a missile launched from Nazi Germany would require three skips to strike a target in the eastern United States. Each skip would be shallower than the preceding one, but it was calculated that the vehicle could cross the Atlantic, deliver a 4,000 kg (8,800 lb) bomb to the continental US, and then continue its flight to a landing site somewhere in the Japanese held Pacific, a total journey of 19,000 to 24,000 km (12,000 to 15,000 miles).
As a global rocket bomber carrying a one ton warhead -- RABO (for Raketenbomber), Sanger-Bredt offered their design to Nazi Germany in 1941. Sanger-Bredt proposed launching their hypersonic boost-glider with a rocket-powered sled. After releasing the sled, the laundry-iron-shaped craft would coast upwards until the pilot ignited the "silver bird's" (the nickname of the Stnger-Bredt glider) rocket engine, boosting it into space at Mach 24. The vehicle would then reenter the atmosphere like a stone skipping off the water, until it entered a final supersonic glide just before landing.
Wunderwaffe is German for "wonder-weapon", and was a term assigned by the German propaganda ministry to a number of revolutionary "superweapons". By 1936 Ernst Udet, a flying ace fromt he Great War who lacked any technical capability, used political connections to gain command of the T-Amt (the development wing of the Reichsluftfahrtministerium) (Reich Air Ministry). Udet insisted that the Luftwaffe have its own rocket development center and hired Eugene Sanger to head the “Aircraft Testing Center.” Sänger agreed to lead a rocket development team in the Lüneburger Heide region in 1936.
Dr. Wernher Magnus Maximilian von Braun was the Superstar of Space of his day. Under the fascist dictatorship, Wernher von Braun was a member of the Nazi party, having waited almost five years after Hitler came to power before signing up. Under heavy pressure, he later accepted an officer’s commission—essentially honorary—in Heinrich Himmler’s brutal SS corps. He had been named the civilian chief of the German Army’s rocket program two years earlier, before Adolf Hitler gained power. By age twenty-five he was the civilian technical director of the Wehrmacht side of the Peenemünde rocket R&D base on the Baltic seacoast.
The Luftwaffe ran the other side. "Vengeance Weapon No. 1" - V-1 - was a subsonic guided missile powered by a pulsejet engine, developed by the German Air Force. Sanger’s program essentially duplicated the existing Army missile program and top experts considered it a waste of effort and resources. Udet later became Generalluftzeugmeister (Luftwaffe Director-General of Equipment) in February 1939. Due in some measure to Udet’s lack of vision, the joint character of Peenemunde had collapsed in less than a year. Udet eventually committed suicide in 1941.
During World War II, Sanger made a further contribution, showing how the addition of wings could greatly extend a rocket's range. Initially, a winged rocket would fly to modest range, along an arcing trajectory like that of an artillery shell. Upon reentering the atmosphere, however, the lift generated by the rocket's wings would carry it upward, causing it to skip off the atmosphere like a flat stone skipping over water. Sanger calculated that with a launch speed considerably less than orbital velocity, such a craft could circle the globe and return to its launch site.
Sanger's "Uber einen Raketenantrieb fur Fernbomber," which, though produced in 1940, contains handwritten notes which postdate 1940. In the LaRC Technical Library is an English translation of this paper, "Work on Rocket Drive for Long-Range Bomber and Skip Re-Entry," done later by the Central Intelligence Agency [Eugen Sanger, Rocket Flight Engineering, NASA TT F-223 (Washington, 1965)].
Sänger wrote up their findings in a document of several hundred pages, with the title (in English) of “On a Rocket Propulsion for Long Distance Bombers.” In December 1941 he submitted it for publication—and won a flat rejection the following March. This launched him into a long struggle with the Nazi bureaucracy, as he sought to get his thoughts into print.
His rocket craft continued to show a clear resemblance to his Silbervogel of the previous decade, for he kept the basic twin-tailed layout even as he widened the fuselage and reduced the size of the wings. Its bottom was flat to produce more lift, and his colleagues called it the Platteisen, the Flatiron. But its design proved to be patentable, and in June 1942 he received a piece of bright news as the government awarded him a Reichspatent concerning “Gliding Bodies for Flight Velocities Above Mach 5.” As he continued to seek publication, he won support from an influential professor, Walter Georgii. He cut the length of his manuscript in half. Finally, in September 1944 he learned that his document would be published as a Secret Command Report.
The print run came to fewer than a hundred copies, but they went to the people who counted. These included the atomic-energy specialist Werner Heisenberg, the planebuilder Willy Messerschmitt, the chief designer Kurt Tank at Focke-Wulf, Ernst Heinkel of Heinkel Aircraft, Ludwig Prandtl who still was active, as well as Wernher von Braun and his boss, General Dornberger.
The addition of wings could greatly extend a rocket's range. Initially, a winged rocket would fly to modest range, along an arcing trajectory like that of an artillery shell. Upon reentering the atmosphere, however, the lift generated by the rocket's wings would carry it upward, causing it to skip off the atmosphere like a flat stone skipping over water. Sanger calculated that with a launch speed considerably less than orbital velocity, such a craft could circle the globe and return to its launch site.
Sanger and Bredt found that rather than enter a glide, the vehicle might develop so much lift that it would fly back to space on a new ballistic arc, as if bouncing off the atmosphere. Stones skipping over water typically make several such skips, and Sänger found that his winged craft would do this as well. With a peak speed of 3.73 miles per second, compared with 4.9 miles per second as the Earth’s orbital velocity, it could fly halfway around the world and land in Japan, Germany’s wartime ally. At 4.4 miles per second, the craft could fly completely around the world and land in Germany.
The winged-rocket was to have a length of 92 feet, a span of 50 feet, and a takeoff weight of 110 tons. Unlike von Braun, Sanger preferred horizontal launch. For 11 seconds, a rocket sled would propel the bomber along tracks, two miles in length, until a takeoff velocity of 1640 feet per second was attained. Under power of its own rocket engine, the vehicle would then climb to an altitude varying from 30 to 60 miles. At the end of ascent, the bomber would proceed in an oscillating glide flight, conceivably circum-navigating the Earth.
While the turbojet engine was unknown at that time, it was this engine, rather than the rocket, that would offer the true path to routine high performance. Given that a turbojet uses air from the atmosphere, its aircraft need carry fuel only and is, therefore, able to carry more fuel and, thus, maintain longer flight times. By contrast, a rocket must carry oxygen as well as fuel, and thus, while capable of high speeds, lacks endurance. After World War II, rocket airplanes as experimental aircraft went on to reach speeds and altitudes far exceeding those of jets. Jet planes, however, took over the military and, later, the commercial realms.
Sanger was intent on explaining the military value of his proposed system and detailed possible modes of attack. To achieve a strike on aspecific point, the vehicle would be accelerated only until it acquiredengh velocity to reach the target. After releasing its bomb, the vehicle would turn at the lowest possible speed, ignite its engine, and then return to its original base. For greater distances and bomb loads, the possession of an auxiliary landing site near the target was necessary. If such a site were not available, the rocket bomber would have to be sacrificed. An attack on a larger area, however, did not necessitate a low velocity over the target, and, consequently, there was more likelihood that the bomber could circumnavigate the globe.
It was to be boosted by a supersonic rocket sled, which would have been both difficult to build and vulnerable to attack. Even then, and with help from its skipping entry, it would have been a single-stage craft attaining near-orbital velocity. The RaBo (Raketenbomber or "rocket bomber") was one of a number of designs considered for the 'Amerika Bomber Project', which started out in the spring of 1942. But the RLM cancelled this project along with other more ambitious and theoretical designs in favor of concentrating on proven technologies. The idea of an antipodal bomber that would shp off Earth’s atmosphere to acheve intercontinental range was considered in 1943 by von Braun and General Walter Dornberger at Peenemunde. In August 1944 Eugen Sanger and Irene Bredt of the German air ministry completed their calculations for a manned, rocket bomber.
Some accounts claim the Luftwaffe lost interest as early as 1942, while others claim that as late as 1944 the Antipodal Bomber became one of chosen concepts to bring a "Miracle Weapon" to the USA (it was only one of the Amerika Bomber projects). In 1944, a report was published Zenger and Bredt, which justified the possibility of creating a long-range "antipode" bomber. In 1944, Sänger and Bredt were moved to an isolated laboratory complex in the mountains near Lofer, Austria, where Sänger's main known project centered on development of a high-speed, ramjet powered manned bomber interceptor that resembled a stubby missile.
No one then, 75 years ago, knew how to build such a thing. Its rocket engine lay well beyond the state of the art. Sänger projected a mass-ratio, or ratio of fueled to empty weight, of 10—with the empty weight including that of the wings, crew compartment, landing gear, and bomb load. Structural specialists did not like that. They also did not like the severe loads that skipping entry would impose.
The drawbacks to Sanger's proposal were obvious, and, consequently, the German military did not give serious consideration to the rocket bomber. The difficulties inherent in turning the rocket bomber at hypersonio speeds only increased the desirability for an antipodal landing site. To depend on the possibility of possessing friendly landing areas so near a target was unrealistic. Even if a fleet of rocket bombers could circle the Earth, a bomb capacity of about 8,000 pounds per vehicle as estimated by Sanger, could not have changed the course of the conflict.
To support their R&D during the war, Germany built no less than fourteen supersonic wind tunnels, including Mach 3.3 and 4.4 tunnels at a laboratory at Kochel, Bavaria. At war's end, a Mach 10 hypersonic tunnel with a 1-meter-by-1-meter test section was under construction at the same site [a similar Mach 10 wind tunnel would not emerge in the United States until the Arnold Engineering Development Center (AEDC) placed its fifty inch tunnel "C" into service in 1961].
While Sanger and Bredt had accomplished wind tunnel testing, their design never attained the hardware stage like Dornberger's A-4b. Embittered, Sanger stopped work on the project. Still, as early as 1945, the report was translated into French, Russian, and English, generating considerable interest for the potential of hypersonic flight in postwar aeronautical communities.
Because of the tremendous thermodynamic loads Sanger expected the vehicle to endure as it skipped through its reentry his ideas earned a cool reception from the Third Reich. 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 re-entry would have been far higher than originally calculated by Sänger and Bredt.
General Walter Dornberger worked for Bell aircraft after the war and was a tireless advocate for Dyna-Soar. In 1952, the US Air Force decided to sponsor a study of Dornberger's manned hypersonic rocket-launched glider concept at Bell (Project BOMI). This study advanced and improved the Sanger-Bredt concept by developing, for the first time, a detailed "hot structures" concept. Non-load-bearing flexible metallic radiative heat shields ("shingles") and water-cooled leading-edge structures were to protect the wings while passive and active cooling systems would keep cabin temperature within human tolerance.
The boost-glide concept was refined by Dr. Qian Xuesen, the father of China’s space and missile program, while at the U.S. Jet Propulsion Laboratory (JPL) in 1951. The “skipping” also could involve energy management or “phugoid” porpoise-like measures in which the missile pitches up and climbs then pitches down and descends.
The Soviets recovered copies of Sänger's RaBo reports and were so fascinated with the concept (particularly Stalin, who seems to have been riveted by its implications) that they dedicated a great deal of effort to designing an updated RaBo equipped with huge ramjet engines for boost and cruise propulsion. The Soviet version of the Saenger antipodal bomber was intensely studied on Stalin's direct orders in 1946-1947. The final study concluded that, given the fuel consumption of foreseeable rocket engines, the design would only be feasible using ramjet engines and greatly advanced materials. This meant that development could only begin in the late 1950's, when such technologies were available. By that time the design had been superseded by more advanced concepts.
In 1958, an article which appeared in a Soviet aviation journal referred to a Russian glide-bombing system, capable of attaining an altitude of 295,000 feet and striking a target at a distance of 3,500 nautical miles. Later, an American aviation periodical reported that Russian scientists were developing an antipodal glide-missile, designated the T-4A. By March 1960, the Assistant Chief of Staff for Intelligence, USAF headquarters estimated that the Soviets were at least conducting research directed towards the development of a boost-glide vehicle.
Irene Sanger-Bredt also contended that her husband and other designers were certainly aware of the ballistic capsule approaches that Robert Goddard, Hermann Oberth, and Konstantin Tsiolkovski, the three giants of early rocketry, envisioned. But certainly if it were ever possible to create a viable space plane that could take off and land like a conventional airplane and go into Earth orbit, this could be much more economical, and thus Sanger took that line.
Sänger, for his part, remained actively involved with his rocket airplane. He succeeded in publishing some of the material from his initial report that he had had to delete. He also won professional recognition, being chosen in 1951 as the first president of the new International Astronautical Federation. He died in 1964, not yet 60.
Irene Sanger-Bredt, his colleague and wife, wondered why “manned spaceflight did not evolve gradually and consistently from aviation”. She concluded that the main factor pushing the development of ballistic capsules was the World War II legacy of military missiles. Sanger also contended that her husband and other designers were certainly aware of the ballistic capsule approaches that Robert Goddard, Hermann Oberth, and Konstantin Tsiolkovski, the three giants of early rocketry, envisioned. But if it were ever possible to create a viable space plane that could take off and land like a conventional airplane and go into Earth orbit, this could be much more economical, and thus Sanger took that line.
Sänger gave a specific design concept for his rocket craft, presenting it in sufficient detail that other engineers could critique it. Most importantly, his skipping entry represented a new method by which wings might increase the effectiveness of a rocket engine. This contribution did not go away. The train of thought that led to the Air Force’s Dyna-Soar program, around 1960, clearly reflected Sänger’s influence. In addition, during the 1980s the German firm of Messerschmitt-Boel- kow-Blohm conducted studies of a reusable wing craft that was to fly to orbit as a prospective replacement for America’s space shuttle. The name of this two-stage vehicle was Sänger.
The idea of flying to and from space held special appeal to the test pilots who derided the capsule approach to manned spaceflight as “Spam in a can". The psychological aspects of the institutional preference ofthe Air Force for piloted spaceflight and the disdainful attitude of USAF test pilots towards the capsule approach to manned spaceflight are best captured in Tom Wolfe, The Right Stuff.
|Total length of the plane, m||28|
|Range, km||25000 - 40000|
|flight speed, km / h||10000|
|Full starting weight, t||100|
|Dry weight. m.||20|
|Weight bomb load||300|
|Thrust rocket engine aircraft, t||100|
|Fuel capacity, t.||80|
|Total thrust rocket engine starting||600|
|Flight altitude, km.||50 - 300|
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