Project Suntan - The Lockheed CL-400

Project Suntan – The Lockheed CL-400

In Planes, Videos by Paul ShillitoLeave a Comment


The SR-71 was a groundbreaking aircraft designed to take the place of the U2 spy plane but in the time between the U2 and before the SR-71, another aircraft was proposed and designed by the same man Clarence “Kelly” Johnson,  this would be so radical that it had the classification of “above top secret” and its existence would only become known some 15 years later in 1973 and even now few know about it.

Though it never came to fruition, the work done would go on to influence not only SR-71 itself but also the space industry, Apollo and the Space Shuttle, this is the story of Project Suntan and the Lockheed CL-400.

After world war 2, relations between the Soviet Union and the west deteriorated rapidly. In 1949 and to everyone’s surprise, the Soviets exploded their own atomic bomb and the closed Soviet state became a very real threat, so there was an urgent need for the US to know just what they were up to.

The soviets defended their borders and airspace aggressively, attacking anything that came close and the existing US reconnaissance aircraft which were just converted bombers made an easy target for the Soviet air defences.

The US needed an aircraft that could fly over otherwise out of reach areas to take reconnaissance photos but one that could also avoid the MiG-17 fighters and surface to air missiles. That aircraft would be the U-2 spy plane, It could fly at 70,000 ft or 21,300 meters out of the range of Soviets and although it was operated by the USAF it was controlled by the CIA.

Now whilst things seemed to be going well the USAF resented the fact that they were subservient to the CIA and were looking to not only take over the U-2 operations, they also wanted to develop their own successor that would outperform the U-2.

Meanwhile, Clarence “Kelly” Johnson the designer of the Lockheed U-2 was also concerned that at the very high altitudes of 70,000 ft were the aircraft operated. There was the very real possibility of the engines flaming out as they were at the operational limits of the specially formulated JPTS jet fuel. If this happened the U-2 would have to descend to around 9000 meters or 30,000 ft to relight its engines and at this height, it would be sitting duck. Losing a U-2 in Soviet airspace would have huge political ramifications, something which would be borne out in 1960 when Francis Gary Powers U-2 spy plane was downed by a surface to air missile over the Soviet Union.

By 1955 the USAF was conducting a number of R&D projects looking in using hydrogen to power both existing turbojet engines and new hydrogen optimised engines. Data from tests showed that a hydrogen-powered engine could operate at altitudes of up to 100,000 ft or 30,500 meters, some 30,000 ft higher than the U-2.

Meanwhile, Lockheeds Skunk Works had finished building the U-2 prototypes and was also working on a study of hydrogen-fueled aircraft for the Garrett corporation who had bought the rights to a hydrogen powered engine called the Rex III.

Johnson, the chief designer for the Skunk works saw an opportunity to take forward the work they had done with the U-2 and combine it with hydrogen-powered engines to achieve something new and make a supersonic aircraft that would fly at 100,000 ft.

The Airforce who had missed out on the U-2, jumped the chance to have one up on the CIA and wanted the new plane within two the three years whilst glossing over the considerable difficulties that working with hydrogen still presented.

Because the U-2 was classified and the airforces new plane would be superior, it was given the classification of “above top secret” and project Suntan was born.

Within weeks of the contract to build two prototypes and six further aircraft, Lockheed gave the aircraft the designation of CL-400 and Pratt & Whitney were contacted the build the engines which would be known as the 304-2, a hydrogen optimised engine based on a simplified version of the Rex III engine.

The CL-400 would have a top speed of Mach 2.5 and altitude ceiling of 30,300 meters or 99,409 ft. It would have a fuselage diameter of 3 meters and be 49 meters long with engines mounted on the ends of the wings and in some ways it looked like an oversized Lockheed F-104 starfighter.

Although hydrogen has 3 times the energy of kerosene-based jet fuel, because of its low density it takes up 4 times the volume. This had to stored in cryogenic tanks in the fuselage as the wings would get too hot at supersonic speed to place tanks into.

The typical mission profile would need a range of 4070 km meaning that the target would have to be within 2000km of the takeoff and landing bases. The only way to increase range would be to dramatically increase the size of the aircraft itself to hold more hydrogen.

This raised another issue in the availability of hydrogen and transporting it around the world to bases where the plane would operate from. If this was to be successful then it would have to be produced in quantity and shipped and handled like gasoline.

The only large scale production of hydrogen at the time was for the Hydrogen bombs tests and just getting enough to test the engines and pumps posed a problem. Initially, it was produced in an old bomb shelter at the skunk works which was named Fort Robertson after the man who ran the operation.

However, if the CL-400 was to go into service it would require a huge amount of liquid hydrogen and eventually a series of 3 plants were built with increasing capacities by APIX or Air Products Incorporated Experimental, opposite the Pratt & Whitney test centre in Florida and called by the suntan team as Baby Bear, Mama bear and Papa bear with Papa bears capacity alone capable of producing 27,200 kilograms of liquid hydrogen per day. This would not be ready until 1959 and after project Suntan had ended but it did become a major supplier to the space program that followed.

Testing of the engines by Pratt & Whiney was also progressing, there had been some failures but nothing more than would have been expected with new and untested technology. By the time the project ended, they had run for 25-1/2 hours and development was proceeding satisfactorily.

However, problems with the range were causing concern with the top brass in the airforce and the misconception about hydrogen as being a dangerous fuel didn’t help even though the Suntan team had proved it was much less damaging in a fire compared to gasoline and hydrogen was difficult to explode unless it was mixed in equal amounts of pure oxygen, something that didn’t really occur until the space program.

When General Curtis LeMay formerly of the Strategic Air Command who had moved up to vice chief of staff received a briefing about the project his reaction was along the line of “What, put my pilots up there with a bomb”, which was rather ironic as he had dozens of B-52s flying 24-7 with multiple hydrogen bombs on board all of them.

But the demise of project suntan really came about when Kelly Johnson changed is mind about the whole feasibility of the hydrogen-fuelled-aircraft after about 6 months of experimentation and study.

The Airforce insisted that the minimum distance to a target should be 2800km through Johnson thought 2000km was the best that could be achieved.

In the end, funds which had been allocated to the project were diverted to other more projects and just like a real suntan, project Suntan faded away without any real conclusion.

The costs involved are, because of the top-secret nature still not known though its believed to be between $100 and $250 million dollars.

Kelly Johnson’s fear of a downed U-2 became real in 1960 but by 1957 Lockheed had been approached by the CIA to build an undetectable supersonic successor to the U-2 even if it couldn’t fly at the 100,000ft of the CL-400. This was initially known as the A-10 which was developed in the A-12 and then of course, the now-iconic SR-71.

But the work on the production, handling, transportation and power of hydrogen wasn’t to be lost and found its way in the space program of the 1960’s including Apollo, the space shuttle and space projects right up until the present day.

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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