[1] Starting in 1956, the United States Air Force funded a highly secretive program of research and development on high-speed aircraft for long-range reconnaissance.
This was done to ensure that the liquid hydrogen, which was stored in the fuselage, would have to pass through the aircraft's hot wings (up to 436 Kelvin) to reach the engines; a vacuum sealed, insulated line was provided to do this.
Techniques for handling LH2 on the ground, including making the fuel, transporting and rapid refuelling systems, were all developed as part of the program.
Ultimately, budgetary pressures and difficulty achieving sufficient range, plus the fact that an LH2-powered aircraft was considered too dangerous[3] and expensive to maintain, led to the project's cancellation.
The existence of the CL-400 was not fully disclosed to the public until the 1970s, when Lockheed discussed the possibility of using hydrogen as an alternative fuel for future aircraft.