In 1913 French aeronautics engineer, pilot, and theoretician of space flight Robert Esnault-Pelterie published "Considérations sur les résultats d’un allégement indéfini des moteurs," Journal de physique théorique et appliqué, cinquième série, 3 (1913) 218-230.
Esnault-Pelterie’s lecture on “the unlimited lightening of engines,” delivered in 1912 in both St. Petersburg, Russia, and Paris, was the first European work to demonstrate theoretically that space travel was possible.
“The lecture contains all the theoretical bases of self-propulsion, destroying the myth that rockets need atmospheric support and giving the real equation of motion. Anticipated is the use of auxiliary propulsion for guidance and complete maneuverability of rockets. Also contained are calculations of the escape velocity, the phases of a round-trip voyage to the Moon, and the times, velocities, and durations, of trips to the Moon, Mar s, and Venus, as well as thermal problems related notably to the surface facing the sun . . . . (Blosset, 9).
As noted above, the use of rockets for space travel had been discussed by the Russian scientist Konstantin Tsiolkovsky in his Exploration of Cosmic Space by Means of Reactive Devices (1903, 1911-12). "Tsiolkovsky had grasped the principle of reaction flight as early as 1883, and his 'Exploration of Space Using Reactive Devices' contained the first mathematical exposition of the reaction principle operating in space. In ‘Issledovanie mirovykh prostranstv reaktivnymi priborami’ . . . Tsiolkovsky set forth his theory of the motion of rockets, established the possibility of space travel by means of rockets, and adduced the fundamental flight formulas" (Dictionary of Scientific Biography).
Tsiolkovsky’s work was published only in Russian, and remained little known to Western scientists until the 1920s. Whether Esnault-Pelterie (known as REP to friends and colleagues) knew of Tsiolkovsky's work before he wrote his 1912 paper is unclear. However, considering that he had published little up to this time, one wonders how he would have been invited to speak in Russia if he had not been in communication on these topics with people in Russia before this date. This leaves open the possibility that he may have had access to Tsiolkovsky's work in some form prior writing his paper. REP did not refer to Tsiolkovsky’s work in his 1912 paper-- at least not in the abridged form it which it was published-- but at the very minimum he must have been informed of Tsiolkovsky's work during his trip to Russia, as by this time Tsiolkovsky's paper had been published twice in Russian. What sort of reception his speech received seems also to be unknown. In his L’Astronautique (1930) Esnault-Pelterie mentioned that his 1912 speech was never published in Russia. He also acknowledged Tsiolkovsky's contributions in print for the first time when he mentioned Tsiolkovsky's papers in the historical introduction (pp. 17-38) of his L’Astronautique.
Esnault-Pelterie’s 1912 lecture first appeared in print in the Journal de physique théorique et appliqué, but in abridged form, due to both space considerations and the trepidations of the Journal’s editor, who was shocked by Esnault-Pelterie’s ideas on space travel.
“REP deplored the exaggerated condensation of the lecture, which was the cause for an apparent divergence between Goddard’s and his own opinions concerning the possibility at the time of building vehicles capable of escaping from the earth’s gravitation. In fact, Goddard wanted only to send a projectile loaded with powder to the moon and observe its arrival by telescope. REP considered the conditions necessary for transporting living beings from one celestial body to another and returning them to the earth; his more pessimistic conclusions were based on considerations of the substantial initial mass required for a rather small final mass, in view of the limited means available at the time” (Blosset, “Robert Esnault-Pelterie: Space pioneer,” in Durant and James, First Steps toward Space  5-31; pp. 23-31 contain an English translation of the unabridged lecture).
Fourteen years after his initial publication on space travel, on June 8, 1927, REP gave a lecture at the Sorbonne before the Société Astronomique de France on rocket exploration of the upper atmosphere and the possibility of interplanetary travel, in which he communicated the results of his continuing theoretical research in astronautics; this lecture was published the following year under the title "L’Exploration par fusées de la très haute atmosphère et la possibilité des voyages interplanétaires." In his lecture Esnault-Pelterie devoted special attention to the problem of escape velocity necessary to overcome the earth’s gravitational pull, estimating this at 10,000 meters / second (22,369 mph); the accepted figure at present is c. 25,000 mph. This paper was published as a supplement to the March 1928 issue of the Bulletin de la Société Astronomique de France.
Continuing to research rocketry and space travel, in 1930 REP published his most extensive work on the subject, entitled L'Astronautique. L’Astronautique was the first work to popularize the word astronautics among the scientific community. The book encompassed all that was then known about rocketry and space flight. The work was
"a veritable treatise on space vehicles that served as a basis for all later works on this subject. It is a very profound theoretical study based on the thorough knowledge of celestial mechanics, astrophysics, and ballistics, as well as physical chemistry and physiology. Nothing in it has yet been invalidated.
"This book is a basic text for all interested in astronautics. One needs only to scan the chapter titles to see that it is both a scientific and technical document and an encyclopedia of precious practical knowledge:
-Rocket Motion in Vacuum and Air
-Density and Composition of the Very High Atmosphere //-Expansion of Combination Gases through a Nozzle
-Combustion in a Chamber
-Possible Use of Rockets (high altitude exploration, launching projectiles to the moon, high-speed travel around the earth, and travel through the atmosphere)
-Interplanetary Travel (with sections on the conditions under which trips around the moon will be carried out, the design of the spaceship, guidance, navigation and piloting devices, the conditions for habitation).
"For these last points, [Esnault-Pelterie] states that the spaceship could be filled with pure oxygen, which would reduce the pressure to about a tenth that of the atmosphere . . . [He] also suggests that the spaceship, for its return to earth, be turned and braked first by its own engines (today’s retrorockets) and then by the use of a parachute" (Durant and James, First Steps toward Space, pp. 11-12).
In 1934 REP published L'Astronautique complément “in which he presented the practical conditions and the advantages of interplanetary trips” (Durant and James, p. 12). The work included studies of rocket motion, combustion gas expansion nozzles and combustion thermodynamics, as well as prophetic considerations of nuclear propulsion and the use of radioactive elements in rocketry.
Von Braun & Ordway, History of Rocketry and Space Travel, 74-75.