Suntan was the code-name of a prototypereconnaissanceaircraft program, with the goal of creating a much faster and higher-altitude successor to the U-2, enabled by the use of liquid hydrogen (LH2) as fuel. From 1956 to 1958, the United States Air Force funded a highly secretive program of research and development on the aircraft (the CL-400, designed at the LockheedSkunk Works) and engine design, and made significant investments in large-scale LH2 production. In addition to Lockheed, Pratt and Whitney played a major part. Program successes included the concept design of a Mach 2.5 aircraft capable of flying at 30,000 meters, and successful conversion of an existing turbojet engine to run on liquid hydrogen, as well as 25+ hours of testing on a customized LH2 engine design.
Ultimately, budgetary pressures and difficulty achieving sufficient range, plus the fact that an LH2-powered aircraft was considered too dangerous and expensive to maintain led to the project's cancellation. In addition, the unusual fuel would have meant that existing airbases would have needed extensive facilities to handle the aircraft.
However, the aircraft research was redirected to more conventionally fueled designs and resulted in the successful SR-71. By advancing the state of the art in LH2 propulsion, and by establishing an industrial infrastructure for high-volume hydrogen production, the groundwork was laid for successful use of liquid hydrogen as a rocket fuel for the Apollo program and the Space Shuttle.
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.