Typical CL-400 mission included a cruise to target speed of Mach 2.5 at an altitude of 90,000 feet. These were impressive figures for the late-1950s

In 1956, the United States Air Force was unpleasantly surprised to learn that the Soviets were able to track U-2s during their high-altitude overflights, in spite of intelligence estimates suggesting Soviet radar wasn’t up to the task. This development, alongside a number of research and development studies the USAF was conducting into liquid hydrogen as a fuel source, led the legendary Kelly Johnson of Lockheed’s Skunk Works to propose the CL-400, often referred to as “Suntan,” the name of the USAF request to which Skunk Works was responding.

Johnson proposed an airplane that could fly at an altitude of 100,000 feet (30,500 meters) and a speed of Mach 2.5, with a range of more than 2,500 miles (4,000 kilometers). Johnson also promised that he could deliver the first of two flyable prototypes in just eighteen months. The airplane was to be powered by Pratt & Whitney engines designated 304-2, engines that Pratt & Whitney did eventually produce and test. Lockheed’s design was understandably sleek, looking at least superficially like a big F-104 Starfighter, though the tip-mounted engines were a notable difference. Later iterations moved the engines to varying locations under the wings and added a small canard toward the front of the fuselage.

By 1957, Johnson had drastically reduced his predictions for the aircraft’s range, becoming convinced, based on the efficiency of liquid hydrogen, that the airplane was only likely to manage about 1,300 miles (2,100 kilometers) without refueling— and refueling wasn’t an option. His counterparts were convinced that the design could fly up to 1,700 miles, (2,700 kilometers)—and that it would have to in order to be considered a success. Both sides maintained their position until, with costs exceeding $100 million ($250 million according to some), the project was cancelled in 1958. Johnson didn’t protest, having recommended cancellation himself, telling the Secretary of the Air Force “I’m afraid I’m building you a dog.” Most reports indicate that Lockheed returned the bulk of the funding to the USAF.

Following the initial phase of study and experimentation, Suntan proceeded on schedule. About $95 million had been allocated by the Air Force, and construction of miscellaneous components had been initiated by the Skunk Works. Lockheed in fact had ordered no less than 2-1/2 miles of aluminum extrusion; Pratt & Whitney was moving ahead with construction and static testing of various Model 304 engines; the Massachusetts Institute of Technology was working on an inertial guidance system; and Air Products Corporation was moving ahead with construction of the large hydrogen liquefaction plant in Florida.

By early-1957, the technological problems lurking in Suntan’s background had begun to haunt it. Within six months of its formal approval by the Air Force, a difference of technical opinion over achievable range had surfaced as an item of considerable contention between the Air Force and Johnson. Surprisingly, Johnson-the man who had sold the aircraft to the Air Force in the first place- after careful analysis of the airframe, the powerplant, the fuel, and the proposed mission requirements had concluded that severe range limitations could not be overcome with extant technology… and that the program should be terminated in favor of a more conventional hydrocarbon-fueled aircraft. During a March 1957 meeting with James Douglas, Jr., then Secretary of the Air Force, and Lt. Gen. Irvine, Johnson had told them bluntly, “we have crammed the maximum amount of hydrogen in the fuselage that it can hold. You do not carry hydrogen in the flat surfaces of the wing…” and he noted that the range growth potential by adding more fuel was only 3%.

Air Force reaction to the meeting was mixed. Suntan proponents, particularly Appold and Seaberg, continued to support Suntan even in light of Johnson’s negative assessment. By late 1958, however, it was apparent the program was in deep trouble.

Johnson apparently had held strong reservations concerning liquid hydrogen almost from the beginning. During Suntan’s first six months of development, he had concluded a range of 2,500 miles was the maximum that could be expected of a hydrogen-fueled aircraft of this type. Air Force engineers at Wright Field had generated considerably more optimistic figures and had concluded 3,500 miles, if not greater, to be a more realistic range limit. Because of his intimacy with the actual aircraft, however, Johnson became increasingly convinced the CL-400 would not be able to achieve the Air Force’s optimistic projections. By mid-1958, others involved in the aircraft’s development had reached similar conclusions. During February of 1959, upon Johnson’s insistence, the program was terminated… even though national security issues, such as a U-2 successor, remained unresolved.

Approximately a year prior to the actual demise of the CL-400, the Air Force approved a series of follow-on studies to explore potential performance improvements. Boeing, Convair, and North American were invited to participate and the resulting design exercises provided some credibility to Air Force claims. Fourteen configurations were generated by the Skunk Works team in response to the study initiative, but they generated little in the way of strong support, other than to verify Johnson’s contention that hydro-carbon fuels were considerably more practical.

In the end, Suntan and the CL-400 died because of a combination of several factors, not the least of which was Johnson’s concerns about range; the logistics of processing, transporting and handling liquid hydrogen; excessive program costs (estimated by some to have required expenditures in excess of $250 million); and other, more practical intelligence gathering options.

In addition to the design of the aircraft itself, significant effort went into developing procedures to deal with the production and handling of liquid hydrogen, research that provided direct value to the burgeoning US space program, just as research on the CL-400 itself contributed directly to the A-12 and SR-71.



ENGINE: 1x liquid hydrogen-powered Pratt & Whitney 304-2


Empty: 178,500 lb (80,966 kg)

Maximum: 358,500 lb (162,613 kg)

LENGTH: 290 ft (88.4 m)

WINGSPAN: 98 ft (29.9 m)

HEIGHT: 52 ft 3 in (15.9 m)


Maximum: Undetermined

Cruise: Mach 2.5 (1,700 mph, 2,750 kph) at 100,000 ft (30 km)

CEILING: 100,000 ft (30,480 m)

RANGE: 1,300 mi (2,092 km)


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