In 1933 Austrian-German aerospace engineer Eugen Sänger published Raketenflugtechnik. This treatise on rocket flight engineering was Sänger's thesis for a degree in engineering, which had been rejected by the Technical University of Vienna as "too imaginative." Sänger was allowed to graduate when he submitted a more mundane thesis on the statistics of wing trusses. Raketenflugtechnik was the first study leading to the eventual development of a reusable human-piloted rocket-powered space plane, a concept which evolved into the X-planes and the space shuttle.
Sänger introduced his goals and purposes for the book as follows:
“By rocket flight is meant here the motion of such a vehicle within the general air space, the propulsive force being provided by a rocket motor.
“Rocket flight in the narrow sense is taken to be motion in the upper levels of the stratosphere with a speed such that inertial forces arising from the curvature of the path have a marked effect on the lift.
“This type of rocket flight is the next major development from trophospheric flight, which has been the product of the last thirty years; it is also the forerunner of space travel, the greatest technical problem of the present time.
“This forerunner and the installation of a space station* are the noblest tasks of rocketry, but for the present they are still not realizable.
“There are also several directly practical purposes to be served. Rocket flight should especially:
"1. Provide rapid intercontinental travel around the globe with the highest possible terrestrial speeds.
"2. Advance scientific research in certain fields, especially geophysics and astrophysics.
"3. If necessary provide a war weapon of exceptional power.
“These three purposes can now be reckoned as in part technically feasible. The present book is concerned with the technical basis of the realization of this first stage of rocket flight.
“* In cosmonauts’ plans this is a vehicle that revolves around the Earth outside the sensible atmosphere with a speed such that the weight is balanced by the centripetal force. The space station would serve as starting point for flights to even greater heights” (Sänger, Rocket Flight Engineering. Nasa Technical Translation F-223  3).
Sänger and his associate, Irene Bredt, who later became his wife, intended to publish their continuing researches as a second volume of Raketenflugtechnik. However, with the advent of World War II, their space vehicle project had to be repurposed for military use if it was to survive. A 900-page report on space vehicles, prepared by the two in 1941, was rejected by the German Research Institute for Aviation due to its size and complexity; Sänger and Bredt reworked this into a shorter 376-page secret report on a long range bomber with a rocket engine, intended to drop a dirty bomb on a U.S. city, issued as the GRIA’s “Secret Command Report” UM 3538. The report entitled Über einen Raketenantrieb für Fernbomber was issued in a highly-controlled edition of 100 copies for the Nazi German State Ministry for Aviation in 1944. In 2011 three copies of this original report were recorded worldwide in OCLC, one in the United States.
The Sänger-Bredt Silverbird (Silbervogel), the designs for which were described in the secret report, was a reusable winged vehicle “propelled by a rocket engine burning liquid oxygen and kerosene, capable of reaching Mach 10.0 at altitudes in excess of 100 miles” (Jenkins, Space Shuttle, p. 1). The Sänger/Bredt report was "the first serious proposal for a vehicle which could carry a pilot and payload to the lower edge of space" (Wikipedia article on Silbervogel).
In order to realize his concept of a reusable rocket engine, Sänger had to solve the major problem of how to cool the engine. “Between 1932 and 1934, [Sänger] performed a series of pioneering experiments with reinforced cooled liquid rocket motors capable of burning mixtures of gas-oil and liquid oxygen (LOX), achieving thrust levels up to 30kp, pressures up to 50 bars, and exhaust velocities of about 3,000 m/s” (Sänger & Szames, “From the Silverbird to interstellar voyages,” 2).
In 1934 Sänger published these studies in "Neuere Ergebnisse der Raketenflugtechnik," Flug: Zeitschr. f. d. gesamte Gebiet der Luftfahrt, Sonderheft 1. This paper contained the results of Sänger’s extensive tests of various rocket engine models in 1933 and 1934, leading up to his 1935 patent for regenerative forced-flow cooling of rocket engines. This he accomplished by designing a “regeneratively cooled” engine cooled by its own fuel circulating around the combustion chamber. This rocket engine was a lasting feature of the Silverbird design. "Almost all modern rocket engines use this design today and some sources still refer to it as the Sänger-Bredt design" (Wikipedia article on Silbervogel).
“Sänger’s former rocket-powered civilian space transport airplane project now evolved into an Earth-orbiting, single-stage, rocket-powered intercontinental bombing machine with a launch weight of 100 tons . . . It would be propelled by a rocket engine using highly efficient fuels with liquid oxygen used as an oxidizer in a combustion chamber at a pressure of 100 atmospheres and creating 100 tons of thrust” (Myrha, p. 78).
This rocket-powered bomber was designed to attack strategic targets in the United States: New York City, Washington DC, Chicago and the steel-refining plants in Pittsburgh. Page 339 of Sänger and Bredt’s report shows a map of lower Manhattan superimposed with a bull’s-eye and containing calculations of the expected destruction pattern.
After World War II Sänger emigrated from Germany to France where he worked for the Arsénal de l’Aéronautique. During his time in France “he was the subject of a botched attempt by Soviet agents to win him over. Joseph Stalin had become intrigued by reports of the Silvervogel design and sent his son, Vasily, and scientist Grigori Tokaty to convince [Sänger] to come to the Soviet Union, but they failed to do so. It has also been reported that Stalin instructed the NKVD to kidnap him” (Wikipedia). In 1954 Sänger returned to Germany, where he founded a research center in Stuttgart and earned unwelcome notoriety through his involvement with Egypt’s military buildup in the early 1960s. From 1963 until his death, he was a professor of astronautic technologies at the technical university in Berlin.
An English translation of the Sänger-Bredt report, prepared by the Technical Information Branch of U.S. Navy’s Bureau of Aeronautics in 1946, was also limited to a small number of copies. A condensed version of the translation was published in 1952. The work was also studied in Russia where a Russian translation was published.
Sänger-Bredt, “The Silver Bird story: A memoir,” in Hall, ed., Essays on the History of Rocketry and Astronautics, vol. 1 (1977), pp. 195-228. Sänger-Bredt & Engel, “The development of regeneratively cooled liquid rocket engines in Austria and Germany, 1926-42,” Durant & James, eds., First Steps toward Space, 217-46. Myrha, Sänger: Germany’s Orbital Rocket Bomber in World War II (2002).