SUNTAN - Hydrogen Aircraft
The initial contract with Lockheed called for two prototype reconnaissance aircraft, with the first to fly in 18 months. Hard on its heels, also in 1956, Lockheed received a contract for six of the aircraft. The design Lockheed selected was designated CL-400 and was capable of a speed of Mach 2.5 at an altitude of 30 300 meters. The CL-400 was described openly for the first time in 1973 by Ben Rich at a symposium on hydrogen-fueled aircraft at the NASA Langley Research Center. The CL-400 had a fuselage diameter of three meters and a length of 49 meters to accommodate the 9740 kilograms of liquid hydrogen. The retractable ventral (bottom) fin improved directional stability at supersonic speeds.
The engines, designated 304-2, were to be supplied by Pratt & Whitney and will be described later. Each weighed 2850 kilograms, provided 42 kilonewtons at sea-level, and 27 at Mach 2.5 and 29000 meters altitude.
The mission profile range was 4070 kilometers and could be extended only by a considerable increase in airplane size. Airplane sizes with lengths as long as a football field, as well as other variables, were studied at the Skunk Works. The relatively short radius of 2000 kilometers was later to become a matter of great concern.
Kelly Johnson saw his task as much more than designing and building a hydrogen-fueled airplane. He was also concerned about its operation, for if it was to be successful, liquid hydrogen had to be produced and shipped in quantity and be handled like gasoline. On 16 March 1956, he and his staff met with representatives of J.H. Pomeroy and Company of Los Angeles, a consulting engineering firm. Johnson wanted Pomeroy to study the engineering feasibility and cost of producing parahydrogen in quantity, and he was interested in three production rates-45 000, 135 000, and 225 000 kilograms per day. He wanted the plant location to be in the Antelope Valley of California. Pomeroy agreed to undertake the study, and ten days later Johnson sent them a letter of intent with ground rules.
At the outset of the project, Johnson assigned one of his assistants, Ben Rich, a thermodynamics and heat transfer expert, the dual responsibilities of propulsion and the handling of hydrogen. Rich, who knew little about liquid hydrogen at the time, checked Mark's Mechanical Engineering Handbook which stated that liquid hydrogen was an impractical fluid and only a laboratory curiosity. He was to understand why in his subsequent visits to laboratories and firms working with liquid hydrogen. Among those contacted were Professor William Giauque, University of California at Berkeley, and Russell B. Scott at the Cryogenic Laboratory of the U.S. Bureau of Standards at Boulder.
Rich concluded that liquid hydrogen was mostly in the hands of highly skilled scientists, and few of them appreciated the practical problems he saw in adapting liquid hydrogen to routine use as an airplane fuel. In that application, a temperature range from the boiling point of liquid hydrogen, 20.3 K, to the frictional temperature of the airplane skin at Mach 2.5, about 670 K, had to be handled with designs and materials dictated by volume and weight restrictions. The earthbound design and construction methods used with liquid hydrogen generally were unsuitable. Moreover, Rich found that he was thinking of far greater quantities of liquid hydrogen than others; he used the unit "acre-feet" to emphasize his point.
All these considerations made it obvious that the Skunk Works staff had to learn how to handle liquid hydrogen and to adapt it to the particular application. This required a liquid hydrogen test facility. During World War II, a bomb shelter revetment had been built adjacent to the Skunk Works, and it was selected as the site of the hydrogen facility. It was named "Fort Robertson" after the man who was in charge of the test operations. A Collins cryostat, capable of producing nine liters of liquid hydrogen per hour, was installed to test materials, bearings, seals, and small components. When larger quantities were needed for tank flow and spill tests, liquid hydrogen was obtained from the Bureau of Standards Cryogenic Laboratory at Boulder and stored in a 2200 liter refrigerated dewar built by the Air Force for the hydrogen bomb program. The Skunk Works also relied heavily on the experts at the NBS Cryogenic Laboratory, particularly Russell Scott, regarded as "Mr. Hydrogen," who became a consultant.
On 1 October 1956, the J.H. Pomeroy Company reported on hydrogen liquefaction plants. The report is an excellent summary of the state-of-the-art, and cites 52 references.13 An entire plant was planned-from incoming natural gas for producing gaseous hydrogen to underground storage of liquid hydrogen. A plant of 45 000 kilograms per day capacity was studied in detail, as well as multiples of it-well above the size of the Boulder installation, which had the largest capacity in existence in the U.S. Pomeroy considered the 45 000 kilogram per day capacity to be about the largest practical size. Construction cost was estimated at $45 million and operating costs at $0.386 per kilogram. A million cubic meters of natural gas per day would be required. Pomeroy discussed an expansion engine process that would. with some additional R&D, be available. With catalysts, it would permit continuous liquefaction of para-hydrogen.
