The first use of hydrogen in aeronautics was for the inflation of balloons. Hydrogen was first employed when, in France, a small silk balloon was constructed by the Roberts brothers, under the direction of physicist J. A. C. Charles. The Charles/Roberts hydrogen balloon was made of silk covered with a thin layer of crude rubber. Hydrogen gas from the turbine exhaust passed through injectors into an annular combustion chamber where it mixed with air discharged from the fan and burned at about 2000°F. In 1955, a report by Silverstein and Hall of the NACA-Lewis Flight Propulsion Laboratory was published in which the potential of liquid hydrogen as a fuel for use in both subsonic and supersonic aircraft was explored. In the NACA hydrogen flight test program, the converted B-57 aircraft took off and climbed to the altitude and speed specified for the test, typically 50,000 ft and Mach 0.75 over Lake Erie, using conventional JP fuel in both engines.