Suntan was the

code-name A code name, call sign or cryptonym is a code word or name used, sometimes clandestinely, to refer to another name, word, project, or person. Code names are often used for military purposes, or in espionage. They may also be used in industrial c ...

of a

prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...

reconnaissance In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities. Examples of reconnaissance include patrolling by troops (skirmisher ...

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

program, with the goal of creating a much faster and higher-altitude successor to the U-2, enabled by the use of

liquid hydrogen Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully li ...

(LH2) as

aircraft fuel Aviation fuels are petroleum-based fuels, or petroleum and synthetic fuel blends, used to power aircraft. They have more stringent requirements than fuels used for ground use, such as heating and road transport, and contain additives to enhance ...

. The program was cancelled in 1958. Starting in 1956, the

United States Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Signal ...

funded a highly secretive program of research and development on high-speed aircraft for long-range reconnaissance. Lockheed's

Skunk Works Skunk Works is an official pseudonym for Lockheed Martin's Advanced Development Programs (ADP), formerly called Lockheed Advanced Development Projects. It is responsible for a number of aircraft designs, beginning with the P-38 Lightning in 1 ...

was the natural partner for this work, having successfully delivered the U-2 and having all the required secrecy and security arrangements to keep the program secret. Lockheed settled on the CL-400 design, which looked like a greatly scaled-up

Lockheed F-104 Starfighter The Lockheed F-104 Starfighter is an American single-engine, supersonic air superiority fighter which was extensively deployed as a fighter-bomber during the Cold War. Created as a day fighter by Lockheed as one of the "Century Series" of fi ...

. The main change to the layout was the twin engines, mounted on the wing tips. 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. LH2 was selected due to its predictable burning characteristics and the fuel's light weight, although the low density required the enormous fuselage to hold the required fuel load. In addition to Lockheed,

Pratt and Whitney Pratt & Whitney is an American aerospace manufacturer with global service operations. It is a subsidiary of Raytheon Technologies. Pratt & Whitney's aircraft engines are widely used in both civil aviation (especially airlines) and military av ...

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 The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...

engine to run on liquid hydrogen, as well as 25+ hours of testing on a customized LH2 engine design. 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 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

liquid rocket propellant The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...

for the Apollo program and the

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...

. 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 Alternative fuel, known as non-conventional and advanced fuels, are any materials or substances that can be used as fuels, other than conventional fuels like; '' fossil fuels'' (petroleum (oil), coal, and natural gas), as well as nuclear mat ...

for future aircraft.

See also

References

Further reading