Three-view drawing of the Lockheed CL-400 Suntan liquid hydrogen reconnaissance aircraft

Lockheed CL-400 Suntan: Kelly Johnson’s Hydrogen-Fuelled Spy Plane

by Max Grünwald | Oct 8, 2026 | Historia y leyendas, Aviación militar | 0 comments

In January 1956, Kelly Johnson walked into the Pentagon with an offer the Air Force could not refuse. The man who had just built the U-2 proposed a successor that would cruise at Mach 2.5 and nearly 100,000 feet, and he promised the first of two prototypes would fly within 18 months. The fuel would be liquid hydrogen, at minus 253 degrees Celsius.

The project that followed was called Suntan, and even the name was classified. According to NASA's official history of liquid hydrogen as a fuel, its security rating sat above Top Secret and only about 25 people had full access. It spent at least 100 million dollars, perhaps as much as 250 million, and it produced no aircraft at all. What it did produce changed spaceflight.

Datos rápidos

Aeronave: Lockheed CL-400, project code name Suntan
Diseñador: Clarence "Kelly" Johnson, Lockheed Skunk Works
Role: high-altitude reconnaissance, successor to the U-2
Combustible: liquid hydrogen, about 9,740 kg carried in the fuselage
Design performance: Mach 2.5 at about 30,000 m; radius about 2,000 km
Tamaño: about 49 m long, wingspan 83 ft 9 in, two crew
Motores: two Pratt & Whitney 304-2, mounted on the wingtips
Programa: 1956 to 1959; CL-400 development cancelled in October 1957

A U-2 Successor Burning Hydrogen

The Air Force had missed the chance to buy Johnson's original Lockheed U-2 proposal, which ended up with the CIA. It was not going to miss the next one. At a meeting on 18 January 1956, Lt. Gen. Donald Putt, the Air Force's deputy chief of staff for development, made it clear he wanted a new high-altitude aircraft within two or three years. Pratt & Whitney was picked to build the engine, and by the spring both companies were at work.

The design that emerged, the CL-400, looked like a stretched Caza estelar F-104 blown up to bomber size. Hydrogen is extraordinarily light, but it is also extraordinarily bulky, so the fuselage had to be about three metres across and around 49 metres long just to hold roughly 9,740 kilograms of fuel. The two engines sat at the wingtips, which meant the super-cold hydrogen had to travel through vacuum-jacketed lines inside wings heated to well over 400 Kelvin by air friction.

The CL-400 mission profile: climb to about 30,000 metres, cruise at Mach 2.5, a radius of 2,037 km. Drawing from Ben Rich’s 1973 paper, reproduced in NASA SP-4404

Ben Rich, the Skunk Works engineer Johnson put in charge of propulsion and hydrogen handling, started from almost nothing. NASA's history records that the standard engineering handbook he checked described liquid hydrogen as impractical, a laboratory curiosity. The Skunk Works built its own test site, nicknamed Fort Robertson, in an old wartime bomb-shelter revetment next to the Burbank works, and deliberately ruptured tanks of liquid hydrogen to see what would happen. Often the gas simply escaped without igniting.

The Range That Killed It

The fatal problem was not danger. It was distance. The Air Force insisted on a radius to target of at least 2,800 kilometres. Johnson came to believe that about 2,000 kilometres was the best the CL-400 could ever do, and the two sides never agreed. Johnson, who had sold the idea, became one of its main opponents.

Asked by Air Force Secretary James Douglas how much room for growth was left in the design, Johnson answered: practically none. Adding fuel would buy perhaps three per cent more range, and Pratt & Whitney's Perry Pratt expected only five or six per cent better fuel consumption over five years.

“…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.”

Kelly Johnson — Lockheed Skunk Works, on the CL-400’s range, as quoted in NASA SP-4404

“How do you justify hauling enough LH2 around the world to exploit a short-range airplane?”

Ben Rich — Skunk Works engineer, as quoted in NASA SP-4404, “Liquid Hydrogen as a Propulsion Fuel, 1945–1959”

Ben Rich saw the same problem from the ground: a short-range aircraft that needed a worldwide hydrogen supply chain made little sense. The money ran out next. When the Suntan team briefed Gen. Curtis LeMay, then the Air Force vice chief of staff, in August 1957, his reaction is recorded in the NASA history in one line.

“What, put my pilots up there with a … bomb?”

Gen. Curtis E. LeMay — Air Force vice chief of staff, at his first full Suntan briefing in 1957, as quoted in NASA SP-4404

Kelly Johnson standing beside a Lockheed U-2 — Kelly Johnson with a U-2. He sold Suntan to the Air Force as the U-2’s successor, then concluded it could never fly far enough. U.S. Air Force photo

In September 1957 the Air Force released only 32.3 million of the 95 million dollars approved for Suntan that year. In October, development of the CL-400 itself was cancelled, although Pratt & Whitney kept working on the 304 engine and Lockheed continued fuel system tests. The project lingered, quietly, until the management team asked for it to be ended in February 1959. True to its name, as NASA's historian put it, Suntan simply faded away.

What Suntan Left Behind

The aircraft never flew, but the hydrogen did. Suntan funded the first large-scale production, transport and handling of liquid hydrogen, and Pratt & Whitney's engineers learned to make engines run on it. NASA's history states that the project led directly to the first rocket engine that flew using hydrogen: Pratt & Whitney's RL10, which powered the Centaur upper stage and, in modern versions, is still flying.

Pratt and Whitney RL10 liquid hydrogen rocket engine — A Pratt & Whitney RL10, the first rocket engine to fly on liquid hydrogen. NASA’s history traces it directly to Suntan. Photo: Steve Jurvetson / CC BY 2.0

The airframe lesson mattered too. Having shown that a hydrogen reconnaissance aircraft could not reach far enough, the Skunk Works went back to conventional fuel and a new approach. The result was the A-12 and, from it, the SR-71 Blackbird. The CL-400 itself stayed secret until 1973, when Ben Rich finally described it in a paper at NASA's Langley Research Center.

Curious Droid's documentary below tells the full story of Project Suntan.

Sources: John L. Sloop, Liquid Hydrogen as a Propulsion Fuel, 1945–1959 (NASA SP-4404, 1978); Wikipedia; Bill Sweetman, Aurora (1993), as cited by Wikipedia

Preguntas frecuentes

What was the Lockheed CL-400 Suntan?

The Lockheed CL-400 was a hydrogen-fuelled reconnaissance aircraft designed by Kelly Johnson’s Skunk Works under the secret Air Force project Suntan, begun in 1956. It was meant to succeed the U-2, cruising at Mach 2.5 at about 30,000 metres. It was never built.

Why did the Lockheed CL-400 use liquid hydrogen?

Liquid hydrogen carries far more energy per kilogram than jet fuel, which promised very high altitude and speed. Its drawback is very low density, so the CL-400 needed a fuselage about three metres across and 49 metres long to carry roughly 9,740 kilograms of hydrogen.

Why was Project Suntan cancelled?

Project Suntan was undone mainly by range. The Air Force wanted a radius of at least 2,800 kilometres, but Kelly Johnson concluded that about 2,000 kilometres was the best the CL-400 could achieve. Funding was cut in 1957, CL-400 development was cancelled in October 1957, and the project ended in February 1959.

How much did Project Suntan cost?

According to NASA’s history of liquid hydrogen as a propulsion fuel, at least 100 million dollars was spent on Project Suntan, and project manager Norman Appold believed the total was closer to 250 million dollars. Because funds were deliberately routed through different channels, the exact figure is unknown.

What did Project Suntan lead to?

Project Suntan built the first large-scale liquid hydrogen production and handling capability in the United States. NASA’s history states it led directly to the first rocket engine to fly on hydrogen, the Pratt & Whitney RL10. Lockheed then turned to conventional fuel, leading to the A-12 and SR-71.

Can I fly in an F-104 Starfighter?

Yes. The CL-400 looked like a scaled-up F-104 Starfighter, and the F-104 itself still flies. MiGFlug offers F-104 Starfighter flights in Florida, USA.