Part II : 1950 -1957
Mobile Liquid Hydrogen Equipment, 1952-1954
 The Air Force worked closely with the Atomic Energy Commission in hydrogen bomb development and as part of its responsibilitv. contracted for the development of mobile equipment. This work was cancelled in 1954, but not before some remarkable equipment had been built, which later became available for rocket research by the National Advisory Committee for Aeronautics.
One piece of equipment the Air Force developed was the air-transportable dewar for carrying liquid hydrogen or deuterium in the B-36 or B-47. The National Bureau of Standards and H. L. Johnston, Inc.. both developed tactical dewars for the Air Force. and the vessels were described in a 1954 cryogenic engineering conference.17 Essentially they utilized the same thermal insulation method as the familiar dewar vacuum flask. but the design was much more elaborate and complex in order to store liquid hydrogen at 20 K. The design minimized heat transfer by its three modes: conduction,  convection, and radiation. The liquid hydrogen was held in an inner tank. Surrounding it was a space evacuated to a high vacuum to minimize heat transfer bv conduction and convection. The wall on the other side of the space was maintained at the temperature of liquid nitrogen, 77 K, which minimized heat transfer by radiation. The radiation shield-the walls of a liquid nitrogen container-was itself insulated from the outer shell of the dewar by another vacuum space.
The air tactical dewars held 750 liters of liquid hydrogen. The heat flow to the liquid hydrogen shell was slight, amounting only to about 4 watts, liquid hydrogen boll-off was about 7.5 liters per day, or I percent of rated capacity. The dewars were equipped with an array of valves, instruments, and a vacuum pump.
The tactical dewars (fig. 13) had to be built for rough treatment. The Johnston design employed hardened stainless steel rods to suspend the inner tanks and minimize conduction. These rods had to be tuned to an exact frequency to meet vibration and shock load specifications. Howard Altman solved this problem in an ingenious fashion typical of Johnston's operation. He calculated the required frequency and pitch and brought his violin to work one night to tune the rods; the Johnston dewar then passed its Air Force test. Several years later it passed another- unplanned -test. Altman was helping to unload it from a truck at the NACA Lewis Laboratory in Cleveland when it slipped and dropped four feet to the ground. He heard the rods vibrating sweetly, and the dewar survived undamaged.18
Another type of transportable dewar was fabricated by the Cambridge Corporation for the Air Force, using a design by the Arthur D. Little Company. Called the....
 ....Refrigerated Transport Dewar, it used a closed cycle helium refrigerator, and the 2000 liters of liquid hydrogen could be stored or transported indefinitely with no loss as long as the refrigerator was operated. It was a large piece of equipment, occupying all of a 10.7-meter semi-trailer, and weighed 18.1 metric tons, including a diesel generator for refrigerator operation away from electric power lines.19
The third type of mobile equipment developed for the Air Force in the 1952-1954 period was a mobile hydrogen liquefier. again built by H. L. Johnston, Inc. It was mounted on three semi-trailers and was capable of producing 100 liters per hour of 45 percent liquid parahydrogen. Two of the trailers housed huge Norwalk horizontal compressors. The trailers also contained a gas holder and auxiliary equipment for the compressors. The third trailer housed the complete hydrogen purification and liquefaction equipment (fig.14). All three trailers were capable of highway transport at 89 kilometers per hour. Gross weight was about 25 metric tons, and they required 105 kilowatts of electric power for operation.20 The author remembers inspecting these trailers when they were loaned by the Air Force to the NACA (about 1956) and marvelling at how much equipment had been packed into such a small space. Particularly impressive were the big compressors with their large flywheels. Johnston's students had designed the layout of these trailers using cardboard cutouts to arrange the equipment. In their first operation at Kirkland Air Force Base, the whole 25-
 -metric-ton trailer began to bounce and "walk," moving 8 to 10 centimeters forward with each bounce-quite an awesome sight. The problem was solved by raising the trailer off its tires on large jacks.21