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©Juhani Westman 2005, 2006, 2008, 2011.jw 12 dec-05, Updated 25 oct-11.

Global Bounce.
   In the history of controlled re-entries there is a startling plan for utilizing the enormous kinetic energy embodied in a rocket returning into the atmosphere from a ballistic high flight. In 1944 fairly little was known of the re-entry problem. Thus it seemed that the enormous kinetic energy in a rocket descending in a ballistic trajectory could be put to use to gain substantial increases in range. Among several schemes the most ambitious was the one on a Rocket Glider with Antipodal Reach, put forward in 1944 by scientist Eugen Sänger and matematician Irene Bredt. After World War II both the Western Powers and the leadership in the Soviet Union showed a marked interest in the projected Sänger-Bredt Antipodal Glider".

   

In a report, innocuously titled "Über einen Raketenantrieb für Fernbomber", (UM 3536, Aindring 1944, 376 pages) stamped "Secret" by the nazi authorities. The Austrian theoretician and rocket practitioner, Dr.Ing. Eugen Sänger (1905-1964) and his assistant, later wife, the matematician Irene Bredt, sketched a rocket glider to reach antipodal distances, and, ultimately, circle the world.
The propulsion system envisioned was, for it´s day, impressive, and the layout and mass allotment clearly way beyond state of the art, but the details of the craft and the propulsion system was, seen in retrospect, not all that important. What mattered was the aerodynamic principle: Any craft that could convert the kinetic energy it had acquired during boost and ballistic flight into aerodynamic lift, could use this for trajectory shaping and, as the end result, get an enormously increased range.

   This principle was, of course, apparent to practitioners like Wernher von Braun and Walther Dornberger in Peenemünde, who argued that their A4, supplied with wings, could attain more than double the ballistic range. Using a booster stage, the A10, the glide range would span the Atlantic.
    Eugen Sänger was going one better. Sänger´s fame rested on a book, published in 1933 with the all-explanatory name "Raketenflugtechnik" (Technology of Rocket Flight), where the "flight" was to be read as "flight like an aeroplane", albeit a rather specialized one. The flight velocity would rise to Mach 10, and part of the coasting and gliding post-boost flight would reach altitudes of 60...70 kilometres, velocities and altitudes unheard of in 1933.

Sänger Rocket Plane 1933, copyright: Irene Sänger-Bredt/ref 7

The Rocket-Glider by Dr.ing. Eugen Sänger as presented in the book "Raketenflugtechnik" 1933, © Dr.Ing. Irene Sänger-Bredt, and ref 7.).

   Eugen Sänger became a doctor in engineering sciences in 1929 with a dissertation on aircraft wing structures, his suggestion of a disseration on rocket-propelled flight having been refused. The roots of the Rocket Glider idea is to be found in this refused doctoral thesis of Sänger, which he enlarged and published in 1933, calling it "Raketenflugtechnik". The book was published after some delay by the same Publisher who had taken on Hermann Oberths books in 1923 and 1929. It was considered equal to Oberths seminal works by the then expertise.

Sänger was along with his theoretical acumen a skilled practitioner, who, concurrent with preparing his enlarged thesis for publication, with his own hands built the at the time most advanced rocket motor using fluid propellants. Thus it was natural that he would gravitate into Germany, where he was working for the Deutsche Versuchsanstalt für Luftfahrt (German Test Center for Air Traffic). In 1937 he was charged with comissioning a Rocket Flight Research Center at Trauen in the Lüneburger Moor. It was at Trauen that the highpressure rocket motor technology was developed and the theortical work on a long range rocket propelled glider also was advanced.

1 Mp test in Trauen, copyright: Irene Sänger-Bredt/ref7

Test of 10 kilonewton (1 tonne) thrust high pressure rocket engine in Trauen,
Propellents: Oxydator: Liquid oxygen and fuel: Diesel oil. Specific Impuls close to 300 lbs/lb, or 3 000 Ns/kg. Sänger also tested metal dispersions in diesel oil in this motor.
© Dr.ing. Irene Sänger-Bredt, and ref 1.).

   In 1933 the sketched rocket glider was of a very simple shape: an ogival-cylindrical body, with a pair of broad supersonic profile wings, and a tail fin above the blunt tail end of the fuselage needed to accomodate the nozzle of the high-pressure rocket engine. By the end of the thirties the shape had progressed to a flat-bottomed shape, ironically christened "the Flatiron".
    The plane also acquired a more distinguished sounding name: Silbervogel, the Silver Bird.

Sänger-Bredt Antipod Rocket Bomber 1944, copyright: Dr Irene Sänger-Bredt
   "Silbervogel" or "RaBo", Antipodal or Global Reach Rocket-Glider by Eugen Sänger and Irene Bredt. Length 28 metres, wing span 15 metres. Dry mass 10 metric tons, maximal propellant load 90 ton, propellants offloaded in proportion to bomb load, maximum load 30 ton, minimum 0,3 ton. (Sänger-Bredt: "Über einen Raketenantrieb für Fernbomber", secret report UM 3536, Aindring 1944, 376 pages. Picture Captions translated in /Ref 9/1952. © Irene Sänger-Bredt).

   The idea of a Rocket Glider was contained in what Sänger and Bredt called "a Sequel to the treatise on Rocket-powered Flight by the senior Author", i.e "Raketenflugtechnik, Part Two", writ larger by a decade of practical experience by Sänger himself and by the aeronautical research community in Germany. Amongst other things, the problems of aerodynamics of supersonic flight was being solved in wind tunnels, and the re-entry of the A4-rockets gave some hands-on-experience of flight at hypersonic velocities.     Eugen Sänger and his assistant Irene Bredt had during the years before the war been working on the theory and practices of a rocket-powered glider, but in 1940 the authorities asked Sänger to shift his priorities towards a more immediate goal. He chose to develop a ramjet for a fighter aircraft to be able to intercept all current allied bombers and also anything that could be counted on in the foreseeable future. The ramjet work put everything else on a backburner at Trauen.

Sänger ramjet fighter, copyright: NACA

The planned ramjet-powered Sänger Fighter. The ramjet was supposed to yield propulsive power equal to 60 000 horsepower, it was tested in flight, carried piggyback on a twin-engined Do-217, but the design of the fighter around it never matured to manufacturing stage.
© NACA.

Nevertheless, Sänger and Bredt continued their studies on the rocket powered glider, and they finalized a report in the summer of 1944.

For many years, the details of the contents of the Report were sketchy, but even so interesting enough. Only lately the full content has been available in the open literature, which alters the perception of the ideas put forth considerably. In the following I shall first relate the impressions given in literature, ref 1...8, and then complete the exposition with some glimpses from the Report itself.

The 100-ton "Rabo"-vehicle (RAketenBOmber)was to be launched from a catapult rail by a rocket-driven sled.

Sänger Rocket Bomber on Launcher Sled 1944, copyright: Smithsonian

The Rocket-Glider Bomber "Silbervogel"(2) with Booster Sled(3) on Launching Track(1). Thrust of Booster Sled Rocket(4) around 6 000 kN (600 Mp).
© Smithsonian.

When reaching about 500 m/s, 1 800 kph, it would take to the air on it´s wings and rise to around 12 kilometres. During this 25 second phase the pilot would veer the craft into the correct azimuth heading for the mission in hand. Ignition would follow at a velocity of 285 m/s. Propelled by it´s 100 tonne engine, using 90 tonnes of liquid oxygen and diesel oil in a 100 atmosphere thrust chamber, giving an exhaust velocity of 3 000 metres per second, the vehicle would attain a velocity of 6 000 metres per second, enough to cause it to rise to an altitude of 260 kilometers and attain a range of 4 500 kilometres.

Trajectory of Sänger-Bredt Antipodal Rocket Bomber 1944, copyright: Irene Sänger-Bredt/ref 7
   Trajectory of Sänger-Bredt Antipodal Rocket Glider. (The Sänger-Bredt Report, August 1944, © Irene Sänger-Bredt, and Ref 7.).

Using the enormous amount of kinetic energy bestowed upon it during the acceleration and ballistic phase, the vehicle then would re-enter the atmosphere and bounce off again from 40 kilometres, going up to altitudes of 125 km, 120 km, 90 km, 82 km and so on, for nine times, until a level glide slowly descending from 40 kilometres would carry the craft to some landing site, 23 500 from the starting point. For all this the bomb load carried would be rather ridiculous in terms of 1944: 300 kilogrammes. A year later such a payload did not seem so ridiculous, which supposedly explained the enormous interest in the Sänger-Bredt Report.
   In the referred versions of the Report there was a sequel to the Antipodal Glider, a World-Circling Glider. The specific impulse of 3 000 Ns/kg was high for those times, but something Sänger already had attained with one of his high-pressure experimental motors of 10 kN at his test center at Trauen. Now he and Bredt suggested raising the Isp to 4 000Ns/kg. This, they believed, would be possible by using metal dispersion in the diesel oil fuel.
   Sänger had considerable experience with dispersions of various metals in fuel oil, which also was referred to in the Report. Aluminium, magnesium and beryllium were among the metals tested. Using high technology in preparing the metal powders, for example grinding under a nitrogen atmosphere, in the final stage by ultrasound, and raising the viscosity of the oil, he could add up to 30 % metals in the mixture and still get a storable slurry, whick did not clog the valves and injector orifices or damaged the pumps./8/ He believed with some reason that the rise in temperature from burning the metal would translate into a heightened In this case the end velocity could be raised to 7 000 metres per second, not far from circular velocity, and the range in a "bouncing" flight would be streched accordingly. The first high would be 290 km, the first range 6 750km, level glide would commence at a distance of 27 500 kilometres and end at the starting point after 3 hours and 40 minutes. Also, the required mass ratio would be less than in the original, some 7,4 : 1 instead of 10:1, which meant that a payload of some 4 tonnes could be carried.
   In fact, all that much of specific impulse would not be needed. It is easy to calculate, that with the given mass ratio, 10:1, the construction ratio, 0,108 times the propellent mass, and the payload of 300 kilogrammes were to be retained, then an Isp = 3 434 kN/s would suffice to give the craft a Round-the-Earth capability.

Trajectory of Sänger-Bredt Global Rocket Bomber 1944, copyright: Dr.ing. Irene Sänger-Bredt/ref7
   Trajectory of Global Reach Sänger-Bredt Rocket Glider-Bomber, from Sänger-Bredt-report august 1944, © Dr.Ing. Irene Sänger-Bredt, and Ref 7).

   Soon after the war, however, a mathematical control analysis unearthed a computational error. It transpired that the heat flow during the first part of re-entry would be considerably higher than the heat flow calculated by Eugen Sänger and Irene Bredt. Had the craft been built according to their calculation, the first flight would have ended with the craft being destroyed during re-entry. The problem would have been curable by adding considerably to the heat shield capability, and eating into the already small margin for payload.

So far the published facts in ref 1...8 jibe with the text in the Report, but obviously the referents have only seen an incomplete digest of it, curtailed, for obvious reasons of military secrecy. The Report itself /ref 9/ has of late been available in an English translation, here referred to, Mark Wade has also published it as i pdf-file on his "Encyclopedia Astronautica" on the Web. It makes for interesting reading. The weapons system envisioned is serious enough "as is", even without nuclear warheads. The knowledge in the open literature hitherto consists of only the most mariginal parts of the prestanda envelope - lowest specific impulse and longest flight distance, with accordingly minimal bomb load.

It´s also clear that the claim in the Foreword concerning it being a sequel to the 1933 book, is borne out. It is not a ready-to-be-implemented plan, but more of an exposé of what has been done in theory and practice so far, what will be possible after a fair amount of further research and development work is done, and as a conclusion, what research should be done to make the possibility a reality.
    The Report consists of three parts. Part One deals with the theory and practice of high pressure rocket propulsion, handling of cryogenics and metal slurries, as experienced during the years at Trauen. The Rocket Drive for the Long-distance Bomber is here sketched out. Then follows Part Two: the aerodynamic principles concerning "the Oscillating Supersonic Gliding Flight", i.e the jumping re-entry leading to the extreme ranges. The Bomber itself with it´s Catapult is sketched out, and aerodynamic Models have been constructed and tested in sub- and supersonic wind tunnels. Clearly all this is given as basis for the computed examples which follow in the Third Part. The Third Part is aimed at those authorities who allocate funds for development work. By the examples Sänger and Bredt tries to show, that a fleet of Rocket Propelled Long Distance Bombers would be a valuable adjunct to any military air power.

The Bomber and it´s catapult, as sketched out, has a dry weight of 10 metric tonnes, consisting of airframe, 3,25 ton, cabin for the pilot, 0,3 ton, wings, 2,5 ton, rocket engine 2,5 ton, empennage, landing gear, bomb bay structure 1,25 ton and nonspecified 0,2 ton. A maximum of 90 ton propellants may be carried bomb load permitting. We learn, that the Bomb Bay is to be able to take up to 30 metric tons of bombs - with corresponding offloading of propellants, as the Catapult and wing loading during launch and liftoff is sized for a liftoff weight of 100 tons. Of course a heavier bomb load leads to correspondingly shorter ranges.
   Attack modes are either Precision Point bombing, or Area Bombing. Attack flights may be made with return flight, dogleg flight after the bombing attack to a remote landing base, or a straight on flight to maximal distance. Only this last attack version is usually referred to.

The examples are worked out for three values of Specific Impulse: 3 000 Ns/kg, 4 000 Ns/kg, and 5 000 Ns/kg. The attainability of those were covered in the First Part of the Report. Results of the calculations arepresented in curves. We lift some data out of the diagrammes and learn that the maximum range attainable with 30 metric tonnes of bombs are 850 km, 2 200 km and 3 500 km respectively for the 3 values of Isp. With a load of 5 tons the range becomes 2 700 km 5 500 km and 10 100 km. And so on.
   Attack flights may be made with two propulsive phases: One example: With a Specific Impulse of 4 000 Ns/kg the Panama Canal ( range from Germany 9 450 km) can be precision-attacked with 2 tons of bombs after a 69 minute flight, and after the attack the Bomber ignites it´s motor for a second time and flies on for 3 200 km, landing at some clandestine base off South America after a total of 12 650 km and 113,5 minutes flight time.
   The precision bombing attacks are made at the end of the subsonic glide period which follow the firstly oscillating and then level supersonic glide, and thus presuppose a 2-phase fight plan. The bomb is then aimed according to "Stuka" divebombing principles, and the motor ignited at the pullup, when the pilot is aiming into the escape azimuth.

Area Bombing is done with the highest practical airspeed at release. A bomb travelling at multiple Mach Numbers will add a considerable amount of kinetic energy to the explosive energy in the bomb itself: Sänger and Bredt calculates the effect gain as more than ten times the HE yield. Bomb Aiming would be an extremely demanding exercise for the pilot, who would use star sightings and horizon measuring as complementary sighting aids. The area to be hit being a city of sizable proportions, and the immensely K.E.-enhanced yields of the bombs would ensure telling effect on the target.
   An example: A fleet of 70 bombers - remember, those were the days of air armadas of WW2! - could flatten the inner city of Sydney with 3 tons of bombs each, after a flight of 15 400 kilometres from Germany. 3 hours 38 milutes after launch the attackers would land in the wicinity of the launch area.

In the last month of war, the debris in Peenemünde was sifted by Soviet intelligence, among them rocket motor constructor Alexey Isayev, who found a copy of the "Report". Soon enough a translation cirkulated among the Soviet rocket specialists, and a digest found it´s way to the desk of generalissimo Josef Stalin himself. In all certainty this digest contained the ranges and bomb loads which were kept out of the open literature for decades. Stalin was, by all accounts, visibly impressed, and as a begining, ordered the bringing of Sänger and Bredt to the Soviet Union, a matter singularly ineptly handled.
   The mathematician Mstislav Keldysh got the task of planning for a Soviet version. Keldysh dispensed with both the launch rack and the skipping trajectory and gave his version antipodal range using two ramjets at the wingtips. Later on, the plans shifted from manned bomber to unmanned cruise missile. Interesting enough, the US missile "Navaho" started it´s planning stage life wiht a similar propulsion system.

Keldysh Global Ramjet Rocket Bomber 1947, copyright: Steven Zaloga ja Asif Siddiqi
   The Soviet version of the Sänger-Bredt Antipodal Bomber, developed by Mstislav Keldysh 1947. The bomber was to ude the big ramjets to attain global range.
© Steven Zaloga ja Asif Siddiqi.

Navaho, copyright: Smithsonian
   "Son of RaBo"(?), the US supersonig global reach missile Navaho in an early shape. The later version actually realized, had the rocket propulsion in a separate unit from the missile.
© Smithsonian Institute.

TSTO named after Sänger copyright Raumfahrtgeschichte.de
A Two Stage To Orbit Launch Vehicle, named after Sänger, which was studied by German space Agency DASA in the late 1980-ies and 1990-ies. The First Stage, the "Sänger" proper, was to be propelled by airbreathing engines vith multi Mach range capability, the Second Stage, named "Horus", would use rocket power for ascent to orbit and subsequent return.
© www.raumfahrtgeschichte.de, 2001

   After the war Eugen Sänger got warning of the great interest Josef Stalin in person had for him and Irene Bredt, and to forestall attempts of being kidnapped to the Soviet Union, Sänger and Bredt were invited to, and moved to France, started working for the French and got married. The French were interested in both the rocket and the ramjet technology. In the fifties there actually existed a french fighter plane with rocket-ramjet propulsion, called "Trident".
   Later on they returned to the then West-Germany.
   Eugen Sänger, remembering the happy days of his youth when everybody in the field knew or at least knew about everybody else, later in life was instrumental in fostering the international cooperation between practitioners and enthusiasts of Space Flight. When the International Astronautical Federation, consisting of Societies in Europe and the US, came into being during the London Conference in 1951, Sänger served as the first President of the Federation.     At the end of his life Sänger was a Professor at the Berlin Technical University. He died "in his boots", by cardiac arrest while lecturing on space flight theory.

The skip-skip-skip-bouncing re-entry was never put into use as such, but the use of aerodynamic forces during re-entry belongs to actual practice. Both the Soviet Circumlunar craft L1-"Zond" and the Apollo Command Module did a Bounce-Once during their re-entries. So does the US Shuttle. For the Apollo astronauts it was a matter of easing the rigours of re-entry deceleration. For the "Zond" crafts, the bounce guaranteed that the landing would take place on Soviet territory.


Notes and Bibliography:

Notes on Sources.
1. Eugen Sänger und die Raumtransporter von Übermorgen   http://rover.wiesbaden.netsurf.de/~sgrm/an/an/raumfahr/singer.htm
2. Silverbird, Absolute Astronomy Reference   http://www.absoluteastronomy.com/encyclopedia/s/si/silverbird.htm
3. Hermann Oberth: "Wege zur Raumschiffahrt", Verlag R. Oldenbourg, München und Berlin 1929, pp. 261 ff.
4. Willy Ley:  "Rockets, Missiles and Men in Space",  Signet/NAL New York 1969, pp 511...514.
5. Kenneth Gatland, Anthony Kunesh:"Space Travel", Allan Wingate Ltd London 1953, pp 58...61.
6. Werner Buedeler: "Geschichte der Raumfahrt", 1979 Sigloch Edition, Künzelsau, Thalwil, Strassburg, Salzburg, pp 275...277.
7. Jack Hagerty, John C.Rogers: "Spaceship Handbok, ARA Press, Livermore CA 2001, s. 20..22
8. Josef Stemmer: "Raketenantriebe", Schweizer Druck- und Verlagshaus AG, Zürich 1952, pp. 25...27; 82...24
9. E Sänger, I Bredt: "A Rocket Drive for Long Range Bombers", Transl M Hamermesh, BUAER, Navy Dept, Santa Barbara 1952, orig: E Sänger, I Bredt: "Über einen Raketenantrieb für Fernbomber" Deutsche Luftfahrtsforschung UM 3538, Aindring, aug. 1944.

General:

H.H.Koelle(ed)"Handbook of Astronautical Engineering", McGraw-Hill, New York 1961 H. Ulv Mai: "Rakettitekniikan Perusteet", PIK 1967.
George P Sutton: "Rocket propulsion Elements, 6th Ed.", John Wiley&Sons, New York 1992
Michael Rycroft (ed) "The Cambridge Encyclopedia of Space" Cambridge University Press, Cambridge 1990
Eugen Sänger: "Raumfahrt, Heute - Morgen - Übermorgen" Econ-Verlag, Düsseldorf und Wien 1963


Links:

Eugen Sänger und die Raumtransporter von Übermorgen
Silverbird, Absolute Astronomy Reference
Mark Wade's Encyclopedia Astronautica
European Space Agency,   ESA.
National Aerodynamics and Space Administration,    NASA.
Tähtitieteellinen seura    Ursa.
Suomen Avaruustutkimus-Seura - Sällskapet för Astronautisk Forskning i Finland - Finnish Astronautical Society     SATS - SAFF


e-mail your Comments to: juhaniwestman@gmail.com or juhani.westman@pp.inet.fi


Read more about the prehistory of Space Flight in Six steps to Orbit  The BIS Launchers,   and
Was there a Nazi Space Program?

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