Up to 1945, gaseous hydrogen had been considered many times as a fuel for internal combustion engines, particularly for dirigible engines. It caused engine knock (detonation) and was limited to experimental investigations or as a component in a gaseous fuel mixture.
Three early rocket pioneers-Tsiolkovskiy, Goddard, and Oberth-all proposed to use liquid hydrogen with oxygen in a rocket engine for space travel, but none tried it experimentally. The Germans, who made the greatest advances in rocketry up to 1945, experimented with liquid hydrogen in a small rocket engine prior to World War II, but numerous leaks and other problems made the fuel appear impractical.*
The development of jet engines and rockets during World War II opened up a new line of propulsion systems and with them, new considerations of fuels. The Germans, following the lead of Goddard, showed that a cryogenic fluid-liquid oxygen-could be used in a practical propulsion system, the V-2. From this cryogenic fluid to another-liquid hydrogen-was a big step, but it was inevitable that propulsion engineers would take another look at hydrogen's possibilities. That step came a month after the close of World War II in Europe, when the Air Force contracted for a general investigation of liquid hydrogen as a fuel for aircraft and rockets. The Navy was not far behind in becoming interested in liquid hydrogen, but contracted for a specific application. These military contracts resulted in the first experiments in the United States with liquid hydrogen as a propulsion fuel and were responsible for advancing the technology of liquid hydrogen during the second half of the 1940s.
* For a summary of hydrogen properties and technology through World War II. see appendix A.