Part II : 1950-1957
 During the period 1954-1957, the NACA-Lewis Flight Propulsion Laboratory at Cleveland investigated liquid hydrogen as a fuel for high-altitude aircraft and missiles. The experiments began in 1954 with an investigation of low-pressure combustion in a single turbojet combustor, extended to other components (tanks, pumps, heat exchangers, controls) and complete turbojet engine systems, and culminated in the first (and only) flight experiments. Among the many contributions:
(1) Gaseous hydrogen burns well at low pressures in a turbojet combustor.
 (2) Promotion of hydrogen as a turbojet fuel; especially the concept that high-altitude, low-speed flight using turbojet engines demands efficient combustion at low pressure, best provided by hydrogen; and, at the same time, aircraft configurations for that flight regime favor large-volume aircraft which alleviates the disadvantage of hydrogen's low density.
(3) Lightweight, low-loss liquid hydrogen tanks are feasible.
(4) Liquid hydrogen can be pumped satisfactorily for turbojet engine conditions.
(5) Hydrogen requires less combustion volume than hydrocarbons, making possible shorter and lighter engines.
(6) A complete turbojet engine for subsonic flight can be operated with hydrogen at higher altitudes and with less fuel consumption (mass basis) than the same engine using hydrocarbon fuels.
(7) Existing turbojet engines can be easily adapted to use hydrogen.
(8) Flight demonstrations that liquid hydrogen can be handled safely in ground operations and in flight.
(9) Liquid hydrogen is an excellent heat-sink for very high-speed flight where air friction heats the vehicle surfaces.
(10) Turbojets using hydrogen give good performance at flight speeds of Mach 4 and ramjets for flight speeds of Mach 7, with much higher speeds feasible with the latter.
All these advantages made hydrogen appear to be the fuel of the future for advanced air-breathing engines; but, in fact, its prospects were already being tested, as we will see.