EUROSPACE and the European spaceplane (part 1)by Hans Dolfing
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| The fledging European space industry realized that there might be an opportunity. While the United States concentrated on single-use, expendable, vertical launch for the next decade, maybe it was time to develop something more future-proof? |
The US effort on reusable boosters was one victim of the budget and priorities in the early 1960s. They were conceived to be launched horizontally or vertically. Initially conceived as a single-stage-to-orbit (SSTO) system, it was a step too far for the technology at the time. A two-stage-to-orbit (TSTO) alternative was far more realistic. There was significant work by leading US aircraft companies, such as Douglas and Aerojet-General Corp., on the “Aerospaceplane”, “Astro”, and “Astrorocket”. These were typically two-stage concepts. Some launched vertically but landing was often planned in a horizontal, aircraft-like way. [28]
The fledging European space industry realized that there might be an opportunity. While the United States concentrated on single-use, expendable, vertical launch for the next decade, maybe it was time to develop something more future-proof? With a start around 1960, there was a push that European space effort could progress via two different organizations, one for science and one for engineering. As such, the European Space Research Organisation (ESRO) was created along the lines of the nuclear research at CERN. It was separate from any launcher development in the European Launcher Development Organisation (ELDO). ELDO was aimed at the development of a European launcher, the later “Europa” rocket. ELDO was firmly focused on vertically launched ballistic rockets to launch European satellites in space. This left open the question how Europe would develop more future-proof, reusable launchers. [2,3]
The non-profit organization of EUROSPACE, or Eurospace, was founded on September 21, 1961, as a union of European companies to encourage aerospace R&D plus a comprehensive European space program. This was not a cross-governmental European organization like ELDO and ESRO but a non-profit, industry organization. Based in Paris and separate from the European ELDO and ESRO, there were initially 47 associate members from seven European countries, three honorary members, plus five corresponding members in the USA. The level of institutional knowledge was high as demonstrated via the honorary members which included Theodore von Kármán and Pierre Auger. [1,8.i ]
 Figure 2. Left: Pierre Auger © ESA, Middle: Theodore von Kármán, Right: Eugen Sänger. |
Pierre Auger was the first Director General of ESRO, serving from 1964 until 1967, and also active in the creation of the European Organization for Nuclear Research (CERN) created 1954 in Switzerland. Theodore von Kármán was a director at the California Institute of Technology (Caltech) from 1930 for the Guggenheim Aeronautical Laboratory (GALCIT) and helped to create the Jet Propulsion Laboratory (JPL). [25,26]
Other leading Eurospace figures included Jean Delorme, its first director and Chairman and General Manager of the company “L’air Liquide”, who saw an opportunity to expand cryogenics into space, as well as Michael N. Golovine, a director of one of the Hawker Siddeley groups in England.
With five working groups, group III of Eurospace was responsible for “Technical studies” that included the study and evaluation of space, communication, and navigation satellites, as well as launcher studies. Initially led by the British Alexander D. Baxter and later by the French former ONERA president Maurice Roy. [6] The group was off to a flying start with 14 subgroups and 15 preliminary reports. [1,8]
There was never a goal to provide one single European program but rather proposals, studies, and starting points to coordinate short- and long-term European space programs. Though there were clearly competing business and political interests, the Atlantic partnership with the United States was encouraged based on the view of long-term stability and US technological advances. As such, US firms were invited to join Eurospace. [8]
As one study stated:
“It is not very easy to try to define a European point of view on recoverable and reusable spaceflight systems, even if such an attempt does not aspire at representing anything like the European point of view. There is no such thing as the European point of view on the subject, and in present circumstances there cannot be. Any attempt to arrive at a consensus of opinion would at this time have to take into account far too many opinions, views and notions in too many quarters, i.e. authorities, science, industry, to have any chance of success.”. [13]
And then there was Eugen Sänger, who became head of the technical subgroup “Aerospace transporter” as part of the EUROSPACE group III technical study groups from 1962 to 1964. Born 1905, and with a rejected PhD Thesis titled “Raketenflugtechnik” from 1933, he is one of the leading engineers in this story.
| “It can be imagined what the effect on the economy of present day air transport would be if, for instance, at the end of every transatlantic flight passengers and crew had to be dropped by parachute and recovered, and the jet aircraft abandoned in the sea,” Sänger wrote in 1964. |
He was from the so-called “school of Vienna” where they researched aircraft capable of reaching orbit as well as rocket planes. A contemporary of Wernher von Braun, and a member of the NSDAP, he was recruited to Nazi Germany’s aerospace department and given his own research institute in Germany during World War II. He researched the “Silbervogel” (Silverbird) antipodal bomber with the aim of bombing New York from Germany during the war. Sänger and von Braun knew each other and met twice during the war. After World War II, Wernher von Braun came to America via “Operation Paperclip” but Eugen Sänger refused to go to America. A deal was negotiated for him to move to France and work there. [17-24]
In an amazing 1947 story, worthy of a “James Bond” movie, Josef Stalin, the General Secretary of the Communist Party in the Soviet Union, read Sänger’s works and was so captivated by the ideas that he ordered his son to recruit Sänger to the Soviet Union, either voluntarily or by kidnapping. The plot failed. Sänger relocated back to Stuttgart, Germany, in 1954 when aeronautic research was reauthorized after World War II. [17]
In another twist in 1960, Sänger was paid a very large consulting fee to teach rocket science in Egypt, presumably to build sounding rockets. Egypt was in the middle of a pan-Arabic cause under President Nasser. The obvious dual-use technology to fire rockets at the newly founded country of Israel lead to an enormous diplomatic affair involving France, Germany, and Israel, and Sänger’s consulting contracts were ultimately disbanded. [17]
To develop a future, reusable launcher system, Eurospace performed studies starting in 1962. Conferences were held and reports written. With Sänger’s gifted engineering talents plus a life-long interest in “flying into space,” he became the lead of the Eurospace technical study group in 1962 until his untimely death in 1964. This subgroup was part of group III. He suffered a heart attack on February 10, 1964, while lecturing at the TU Berlin on his favorite topic of the Aerospace Transporter. [22]
In recent years, companies such as SpaceX also argue that rockets need to be reused as you do not throw away airplanes after one flight either. However, this is not a new perspective. More than 60 years ago, in 1964, Sänger offered a similar perspective:
“It can be imagined what the effect on the economy of present day air transport would be if, for instance, at the end of every transatlantic flight passengers and crew had to be dropped by parachute and recovered, and the jet aircraft abandoned in the sea. Such a philosophy seems grotesque and ridiculous, but in space flight today it is still accepted. In the interests of the reliability and profitability of space flight, this stage must, of course, be passed through as quickly as possible.”
As such, Eurospace and Sänger were big proponents of reusable space launchers which included a big increase in reliability to make spaceflight more economically viable. [8.vi]
The Eurospace “Aerospace Transporter” study group coordinated concepts from individual European companies and countries. This included French work by Dassault, Nord-Aviation, Sud-Aviation, and SNECMA. German work on the transporter came from Junkers, Bölkow, Dornier, and ERNO. British work included the British Aircraft Corporation, Hawker Siddeley Aviation Ltd., and Bristol Aircraft Company work. Several other European countries and companies contributed. [8, 9.i,10,13,14 ]
The documents
In the course of recent historical research in England, several surviving copies of “Eurospace memoranda” came to light. This was a surprise as a fire destroyed much of the Eurospace archives in 2002. Figure 3 and the references below show what has been found and what proves elusive. [8]
 Figure 3. Examples of Eurospace memoranda and reports. [8] |
Based on the located documents, the earliest mention of a “Space Transporter” is in “Eurospace: Proposals for a European Space Programme” from 1963. [8.i ] This was followed by the “Aerospace transporter”, October 1964, with 59 pages tha- includes the seminal ten page essay “The Historical Background and Motivation for a European Aerospace Transporter Proposal” authored by Eugen Sänger. [8.vi,9.xiv] The historical and engineering essay was included by Eurospace in the October 1964 report to honor Sänger, who died unexpectedly earlier that year, depriving the working group of its leader.
Sänger observes, “A quarter of a century ago, when space flight first became a technical possibility, there appeared to be two feasible approaches to the problem of imparting the velocities required to enter space, either the development of a ballistic, missile-like space vehicle, as proposed by Ziolkowsky, Goddard, Oberth and Esnault-Pelterie or the progressive development of aircraft.” which is followed by five chapters on scope, missions and requirements, technical aspects, R&D, and conclusions. [8.vi,9.xiv]
In the spirit of Sänger’s vision of “flying into space,” the Eurospace concept for an Aerospace Transporter concentrated on reusable, two-stage, horizontal take-off and landing (HTOHL) concepts. Vertical take-off and horizontal landing (VTOHL) were in the minority. Figure 4 shows some of the concepts. [8.xvi ] The annual “Le Bourget” industry fair in Paris in 1965 was an opportunity to present these results and discuss the future. The later 1966 Memorandum “Towards a European Space Programme” also has a detailed discussion of the status of the Aerospace Transporter. [8,8.xvi]
 Figure 4. Several Aerospace Transporter concepts from 1964. [8.xvi] |
The “Aerospace Transporter” document from October 1964 starts with a rationale that Europe needs to have a cooperative industrial approach for a cohesive space program. The state of the art and recommendations were summarized. The task of building an Aerospace Transporter would be economically too large for one country on its own. It is acknowledged that in this very early stage on the Aerospace Transporter, many detailed facts were not available yet. The transporter was seen as a tool to go to space, not the end result. Vertical launchers like Atlas and Soyuz were in progress in the United States and Soviet Union, which is why Europe looked for a different approach, something with many spinoffs in aerospace, hypersonics, and R&D.
The Concorde supersonic plane project was in full swing and planes to go faster and fly to space seemed like a good option. [8.vi]
The Aerospace Transporter concept was a development on the 1930s “Boost Glider” German bomber concept but in a modern world. A crew of two was to fly reasonably comfortably and avoid more than 2.5 Gs. As one study explained: “The Aerospace Transporter is essentially a type of Earth Surface-to-Orbit vehicle designed to carry reasonably physically fit personnel e.g. engineers, construction crews, scientists (e.g. astronomers), etc., without subjecting them to excessive strain and fatigue which would impair their efficient operation.” [8.vi]
| As the United States had different, more immediate goals with Apollo, and to be more of an innovator than a follower, there was much discussion in Europe to prioritize something like the Aerospace Transporter. |
Example missions such as logistics, orbital resupply, and orbital servicing were discussed. On the requirement side, the useful load and satellite rendezvous for various orbits was an important topic. In addition, the crewed aspects meant that mission duration and its consequences needed to be considered. Regarding mass to orbit, an initial estimate on 2.5 tons of useful load was considered, which included 1.5 tons of transferable cargo and 1 ton of fuel to an orbit of 200 miles (320 kilometers) high. All stages were planned to be recoverable and the returning stage from orbit should have some aerodynamic capability to glide and land. Finally, landing locations versus specific orbits were considered.
As a first approximation, the vehicle velocity requirement was summarized as 30,000 feet per second (about 10 kilometers per second) to reach an orbit at 200 to 300 miles (320 to 480 kilometers) high. With chemical propulsion, a preference was identified for liquid hydrogen and oxygen (LH2/LOX) propulsion over LOX/RP-1 as the latter, less energetic solution would only allow an estimated payload fraction of 1%. Nuclear stages were considered unlikely.
Takeoff weight for the full system was estimated between 70 and 200 tons. With one or more reusable booster stages, a payload fraction of 5% was envisioned for LH2/LOX propulsion with a structural, dry mass fraction of 15% and specific impulse greater than 430 seconds. It was acknowledged that an SSTO approach was technologically not achievable at the time which is why most studies focused on TSTO. Another variant aimed at a dry mass fraction of 20% and a payload fraction of 7%. For reference, today’s typical rocket launchers have payload fractions of 3% to 6%.
Earlier European TSTO studies had already established that a catapult start could be useful and a payload fraction of 8% might be achievable. Even air-breathing boosters were considered but the additional structural weight made their application doubtful.
On booster recovery, passive and active recovery was considered. [30] Passive recovery in the form of parachutes or balloons and active recovery as crewed flyback. While the latter would be more difficult to develop, the European factors such as geography and economics made it desirable.
The discussion on horizontal versus vertical takeoff was extensive. In principle, both would work. It was observed that the maximum payload mass for a vertical ballistic rocket system could be much higher compared to a horizontal takeoff system, which maxed out at about 500 tons takeoff weight. However, a horizontal takeoff system with a takeoff weight comparable to a Boeing 747 could launch and recover at many airports worldwide. If both booster stages of the TSTO could fly back to an airport, then aircraft-like operations were within grasp. A TSTO staging speed of Mach 5 to 7 was considered a good start for the studies. Note that ballistic launchers were already studied at ELDO and the Aerospace Transporter studies aimed to bring something different to the table.
The October 1964 Aerospace Transporter document ends with a recommendation for a two-year feasibility study. In summary, it seemed that the project best suited to European conditions would appear to be the development of a multi-stage space transporter with first stage propulsion of either air-breathing engines, possibly with velocities up to Mach 6, or rocket engines. As it states: “At the end of this 2 year period, the feasibility studies should indicate the Aerospace Transporter missions, the different Aerospace Transporter systems and the characteristics of the most appropriate type of vehicle. Governments should then be in a position to decide whether the continuing development of a definite Aerospace Transporter system is in the European interest.” [8.vi]
Conclusion
As the United States had different, more immediate goals with Apollo, and to be more of an innovator than a follower, there was much discussion in Europe to prioritize something like the Aerospace Transporter. In proper European fashion, these concepts were named in English “Aerospace transporter”, in German “Raumflugtransporter”, and in French “Transporter Aérospatial”. Despite the many languages and countries, a close partnership with the US was seen as much preferable to an isolated development, which shows in the Eurospace member lists and conferences. [13]
From the other side of the Atlantic, Wernher von Braun pitched in and said, “I cannot see why an important target such as the aerospace transporter should not be attainable if the joint capabilities of the European aerospace and electronic industries are systematically aimed towards that goal.” [8.xvi] Therefore, this short period in time was seen in Europe as an opportunity to innovate and develop the spaceflight industry while avoiding plain copies of American and Soviet rockets.
All of the major United States aerospace companies had presented proposals to the public including concepts like the Aerospaceplane. These were based on considerably higher payloads, usually about 13 tons, the systems having one to three stages. The propulsion systems sometimes included very innovative engine types. The European proposals were intentionally concentrated on a smaller payload of about 2.5 tons, which for the majority of missions is adequate and avoided duplication of effort.
In 1965, at the second US-European Conference in Philadelphia, there was a surprise in the European-American spaceflight discussion. For some reason, the “Aerospace Transporter” as a spaceplane was the topic of least interest to the United States. There was speculation that the large American effort on the Aerospaceplane with SSTO and TSTO concepts, as well as the concentrated Apollo effort to go to the Moon, was to blame. [eurospace_list, eurospace1964b] Alternatively, there might simply have been a United States policy shift as happened a few times with the Intelsat saga and the European Space Tug. [4,29]
History should be kept in mind for today’s situation as well. While partnerships are important, Europe can innovate and drive spaceflight innovation on its own. While Airbus and Boeing compete to build the best planes, they still build Ferraris in Italy while the US is more fond on pickup trucks. With a look at ENTRAIN and VORTEX projects, it might be a good time to double down on the European space program with innovative variants to interest the likes of Frank Sinatra. [6,31-33] This will be discussed in part two.
References
[1] “European launcher development organisation preparatory group EUROSPACE”, ELDO/PG/Secr. 69, 39 pages, London, May 31, 1962.
[2] “Commemorating 60 years since the ELDO and ESRO Conventions”, March 2024.
[3] J. Krige, A. Russo, “A History of the European Space Agency 1958-1987”, Volume 1, “The story of ESRO and ELDO”, ESA SP–1235, April 2000.
[4] S.J. Dick, R.D. Launius, “Critical issues in the history of spaceflight”, 674 pages, pp168-, NASA SP-2006-4702, 2006.
[5] “International Cooperation and Organization for Outer Space”, “Staff report prepared for the Committee on Aeronautical and Space Sciences United States Senate”, 89th Congress, Document No. 56, 600 pages, pp 123-128, August 12, 1965.
[6] J.C. Carbonel, “French secret projects 3 : French and European spaceplane designs 1964-1994”, ISBN 9781910809914, 232 pages, 2021
[7] “ASD-EUROSPACE”
[8] Chronological list of Eurospace reports, conferences and memoranda. Those with a star (*) have not been located yet. NTIS accession numbers via STAR.
i. “Eurospace: Proposals for a European Space Programme”, by EUROSPACE , N63-46770, at National Aerospace Library, Farnborough (UK), CID 56419, 171 pages, May 1963.
ii. N65-23968#, “PROCEEDINGS OF THE U.S.-EUROPEAN CONFERENCE HELD IN ROME ON THE 22-24 JUNE 1964”, Eurospace, Paris (France), 47p. (*)
iii. N65-21439#, “EUROSPACE Memorandum No. 1”, “Laboratory Test Methods and Facilities”, June 1964, 21p. (*)
iv. N64-31350#, “EUROSPACE Memorandum No. 2”, “Reliability”, June 1964, 45 pages (*)
v. “EUROSPACE Memorandum No. 3”, “Fundamental Studies”, September 1964. (*)
vi. Eugen Sänger, N64-32878#, “Aerospace Transporter”, by EUROSPACE Working Group III, “Technical Studies” at National Aerospace Library, Farnborough (UK), CID 59959, 58 pages, October 1964.
vii. N65-32889#, “EUROSPACE Memorandum No. 4”, “Launching from Aircraft”, January 1965.
viii. “Memorandum: A proposal for a Feasibility Study of an Aerospace Transporter System”, by EUROSPACE Working Group III, “Technical Studies”, March 1965. (*)
ix. “Evaluation of European Industrial Potential with respect to Satellite Telecommunication Systems”, April 1965. (*)
x. “EUROSPACE: SUGGESTIONS FOR COOPERATION OF THE US AND EUROPEAN INDUSTRIES IN SPACE PROJECTS.”, April-May 1965.
xi. “EUROSPACE: EUROPEAN SPACE POLICY”, April-May 1965.
xii. “EUROSPACE: PROBLEMS AFFECTING US-EUROPEAN INDUSTRIAL COOPERATION”, April-May 1965.
xiii. “EUROSPACE: PROPOSALS FROM EUROPEAN INDUSTRY FOR SPACE PROJECTS”, April-May 1965.
xiv. N65-36385, “Proceedings of the Eurospace US European Conference held in the USA”, at National Aerospace Library, Farnborough (UK), CID 71177, Report Eurospace-9961, 62 pages, Philadelphia, PA, April-May 1965.
xv. “Memorandum: The urgent measures required for the implementation of a European Space Programme”, March 1966.
xvi. “Memorandum: Towards a European Space Program”, at National Aerospace Library, Farnborough (UK), CID 57305, 125 pages, May 1966.
xvii. N69-11405, N69-11756, “PROCEEDINGS OF THE THIRD EUROSPACE U.S. - EUROPEAN CONFERENCE”, Munich, Germany, 18-21 June 1968. “PROCEEDINGS OF THE FOURTH EUROSPACE U.S.-EUROPEAN CONFERENCE”, Venice, Italy, 22-25 September 1970.
xviii. N71-17159, SALVADOR, J. C., “Introduction to the themes of the 4th Eurospace US-European Conference” (*)
[9] The Eurospace Conference on the Space Transporter, 23-24 January 1964, Brussels, Belgium. All of these were presented at the conference. NTIS accession numbers via STAR but no copies located yet except xvi.
i. N65-23958#, Maurice Roy, “Minutes of the Technical Meetings held during the Eurospace conference in Brussels on the Space Transporter”, also called “Aerospace plane transporter systems engineering, structural designs, and cost estimates”, Eurospace Report-6525, Eurospace, Paris (France), 32p.
ii. N65-23959#, M. Kaufmann,”HIGH PRESSURE ROCKET POWER UNITS FOR SPACE TRANSPORTERS”, Bölkow Entwicklungen K. G.. Munich. (West Germany), 14p.
iii. N65-23960#, D.G. Thomas,”PERSONNEL SUB SYSTEMS”, Martin Co., Baltimore, Md., 23p.
iv. N65-23961#, M.B. Dunn, “STRUCTURES PROBLEMS OF SPACE SYSTEMS”, Boeing Co. Seattle Wash, Aero-Space Div, 27p.
v. N65-23962#, T.W. Smith, “ENGINEERING PROBLEMS OF NEAR FUTURE HYPERSONIC VEHICLES”, British Aircraft Corp., London, England, Preston Div, 18p.
vi. N65-23963#, J.C. Peters, “SPACE LAUNCH VEHICLE COST CONSIDERATIONS”, United Aircraft Corp.. Farmington, Conn. Corporate Systems Center, 17p.
vii. N65-23964#, M. A. Hauzeur, “THE SPACE TRANSPORTER--GENERAL MISSION ANALYSIS”, Sociétés Anonyme Belge de Constructions Aéronautiques (SABCA), 8p.
viii. N65-23965# G. Mounis, “OUTLINE OF METHODS AND DATA REQUIRED FOR HEAT SHIELD CALCULATIONS”, Sud-Aviation, Paris (France), 31p.
ix. N65-24024# R.J. Lane, C.J. Austin, and M. J. Welch, “COMPARATIVE METHODS OF SPACE BOOSTING”, Bristol Siddeley Engines, Ltd. (England). Advanced Propulsion Research Group, 25p.
x. N65-24027# J. Tubeuf and J. Bedel, “CONSIDERATIONS ON ROCKET PROPULSION FOR AN AEROSPACE VEHICLE”, 29p.
xi. N65-24031# J. Lambrecht, “COMPARED COSTS OF SPACE TRANSPORTERS AND OF BOOSTER ROCKETS”, Junkers Flugzeug- und Motorenwerke A. G.. Munich (W. Germany), 15p.
xii. N65-22883#, P.O. Hawkins, “ELECTRONIC ASPECTS OF SPACE TRANSPORTER”, Elliot Bros. Ltd., London (England), 8p.
xiii. N65-23324# H. Deplante and P. Perrier, “ABOUT A CONCEPT OF AN AEROSPACE TRANSPORTER”, Dassault (Marcel) Aeronautique-Electronique (GAMD), France, 10p.
xiv. E. Sänger, “The Historical Background and Motivation for a European Aerospace Transporter Proposal”, 10p.
[10] The Eurospace Conference held in Rome, Italy, on June 22-24 1964. All of these were presented at the U.S.-European Conf. NTIS accession numbers via STAR but no copies located yet.
i. N65-23968#, “PROCEEDINGS OF THE U.S.-EUROPEAN CONFERENCE HELD IN ROME ON THE 22-24 JUNE 1964”, Eurospace, Paris (France), 47p.
ii. N65-23966#, Hilliard W. Paige, “RELIABILITY AND ENVIRONMENTAL TESTING OF SPACECRAFT”, General Electric Co., Philadelphia, Pa. Missile and Space Div., 16p.
iii. N65-23967#, Carl Kober, “PRINCIPLES OF INTERFACE MANAGEMENT IN ADVANCED PROJECTS”, Martin Co., Baltimore, Md., 29p.
iv. N65-23969#, E.P. Wheaton, “TESTING OF BOOSTER UPPER STAGES”, Lockheed Aircraft Corp., Burbank, Calif., 18p.
v. N65-23970#, Harold A. Rosen, “AN INTERNATIONAL COMMERCIAL COMMUNICATION SATELLITE SYSTEM”, Hughes Aircraft Co., El Segundo. Calif. Communications Satellite Lab., 18p.
vi. N65-23971#, H. K. Hebeler, “THE INTEGRATION TOOLS OF SYSTEM MANAGEMENT”, Boeing Co.. Seattle, Wash., 46p.
vii. N65-24030#, T. J. Gordon, “COST EFFECTIVENESS IN A DEVELOPMENT PROGRAMME”, Douglas Aircraft Co. Inc. Santa Monica, 14p.
[11] J. Lambrecht, “Probleme bei der Verwirklichung eines einstufigen Raumtransporters”, presented at “3rd European space flight symposium”, Stuttgart, Germany, 1963, in English “Design problems for a one-stage transporter”, 20 pages, N71-71498, NASA TT-8573, NTRS 19710066184, November 1963.
[12] H. Billig, “Forschungsprojekt 623, Raumtransporter”, ERNO, 124 pages, Bundesarchiv BArch B 228/13832, December 31, 1962.
[13] A. Cejka, “A European Point of View on Recoverable and Reusable Spaceflight Systems (Aerospace Transporter)”, Space Technology Conference, SAE 670374, pp 3-8, February 1967.
[14] H. Tolle, “Review of European Aerospace Transporter Studies”, Space Technology Conference, SAE 670385, pp 120-128, February 1967.
[15] P. Coué, M. Rigault, “Dassault Aviation’s Aerospace Transporter an historical perspective”, 6 pages, id 7024, IAC-10.E4.3.7, 44th History of Astronautics Symposium, IAC 2010, September 2010.
[16] P. Coué, “Dassault Aviation, designer of spaceplanes”, 8 pages, IAC-22-E4.3.69225, 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022.
[17] H. E. Sänger, A. D. Szames, “From the Silverbird to Interstellar Voyages”, IAC-03-IAA.2.4.a.07, 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law, 29 September 2003, Bremen, Germany.
[18] Wikipedia, “Eugen Sänger”.
[19] New Mexico Museum of Space History, “Eugen Sänger”.
[20] Deutsches Museum, “Nachlass NL 230 Eugen Sänger”.
[21] Der Spiegel, “Raumgleiter Heißer Schlauch”.
[22] “Obituary Eugen Sänger”, NY Times.
[24] “Liste von NSDAP-Parteimitgliedsnummern”, Nr. 1.303.775, Eugen Sänger.
[25] JPL, “Dr. Theodore von Kármán (1881-1963)”.
[26] ESA, “Pierre Auger was the first Director General of ESRO, serving from 1964 until 1967. He passed away in 1993.”
[27] P. Bono, K. Gatland, “Frontiers of Space”, 279 pages, ISBN 978-0025428102, 1976.
[28] “REUSABLE LAUNCH SYSTEMS”, Astronautics & Aeronautics, January 1964
[29] H. Dolfing, “The European Space Tug 1970-1972”, 2025.
[30] T. Moulin, “L’avion lanceur de satellites”, Icare, n.50, pp 56-68, 1969.
[31] D. Ashford, “An aviation approach to space transportation”, The Aeronautical Journal, Vol. 113, Iss. 1146, pp 499-515, August 2009.
[32] S. Stappert et al, “European Next Reusable Ariane (ENTRAIN): A Multidisciplinary Study on a VTVL and a VTHL Booster Stage”, DLR, 70th International Astronautical Congress (IAC), Washington DC, United States, IAC-19-D2.4.2, 17 pages, 21-25 October 2019.
[33] “VORTEX (VÉHICULE ORBITAL RÉUTILISABLE DE TRANSPORT ET D’EXPLORATION)”, 2025.
[34] D. Myhra ,”Sänger: Germany’s Orbital Rocket Bomber in World War II”, 176 pages, Schiffer Pub. Ltd., 2002/
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