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.
This video is sponsored by nord vpn 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.
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