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Early rocketry research

In the fall of 1914 Goddard's health had improved, and he accepted a part-time position as an instructor and research fellow at Clark University. His position at Clark allowed him to further his rocketry research. He ordered numerous supplies that could be used to build rocket prototypes for launch and spent much of 1915 in preparation for his first tests. Goddard's first test launch of a powder rocket came on an early evening in 1915 following his daytime classes at Clark. The launch was loud and bright enough to arouse the alarm of the campus janitor, and Goddard had to reassure him that his experiments, while being serious study, were also quite harmless. After this incident Goddard took his experiments inside the physics lab in order to limit any disturbance.

At the Clark physics lab Goddard conducted static tests of powder rockets to measure their thrust and efficiency. He found his earlier estimates to be verified; powder rockets were converting only about two percent of the thermal energy in their fuel into thrust and kinetic energy. At this point he applied de Laval nozzles, which were generally used with steam turbine engines, and these greatly improved efficiency. (Of the several definitions of rocket efficiency, Goddard measured in his laboratory what is today called the internal efficiency of the engine: the ratio of the kinetic energy of the exhaust gases to the available thermal energy of combustion, expressed as a percentage.) By mid-summer of 1915 Goddard had obtained an average efficiency of 40 percent with a nozzle exit velocity of 6,728 feet (2,051 meters) per second. Connecting a combustion chamber full of gunpowder to various converging-diverging expansion (de Laval) nozzles, Goddard was able in static tests to achieve engine efficiencies of more than 63% and exhaust velocities of over 7,000 feet (2,134 meters) per second.

Few would recognize it at the time, but this little engine was a major breakthrough. These experiments suggested that rockets could be made powerful enough to escape Earth and travel into space. This engine and subsequent experiments sponsored by the Smithsonian Institution were the beginning of modern rocketry and, ultimately, space exploration. Goddard realized, however, that it would take the more efficient liquid propellants to reach space.

Later that year, Goddard designed an elaborate experiment at the Clark physics lab and proved that a rocket would perform in a vacuum such as that in space. He believed it would, but many other scientists were not yet convinced. His experiment demonstrated that a rocket's performance actually decreases under atmospheric pressure.

In September 1906 he wrote in his notebook about using the repulsion of electrically charged particles (ions) to produce thrust. From 1916 to 1917, Goddard built and tested the first known experimental ion thrusters, which he thought might be used for propulsion in the near-vacuum conditions of outer space. The small glass engines he built were tested at atmospheric pressure, where they generated a stream of ionized air.