Part II : 1950-1957
New Interest in Hydrogen
 Beginning in February 1954 and extending through March, the fuels and propulsion panel of the Scientific Advisory Board met three times, in an exhaustive survey of the major aspects of the propulsion program of the Air Force.8 Although no mention was made of hydrogen in the minutes, the panel was greatly interested in high-energy fuels and the Air Force program on them. On the same day as the last meeting (24 March 1954), Randolph S. Rae visited Wright Field with a proposal to use hydrogen in a high-altitude aircraft powered by a unique engine called Rex I. By all indications his visit touched off a strong renewal of interest in liquid hydrogen for aircraft, which will be described in the following chapter.
The origin of interest within NACA to use hydrogen as an aviation fuel has not been fully established, but experiments began in 1954. Several events apparently contributed to the NACA interest. In Washington, A. M. Rothrock, chief of propulsion research, completed a comprehensive survey and analysis of turbojet propulsion and its effect on airplane performance in August; it was published seven months later. Rothrock discussed seven major propulsion factors and the state-of-the-art concerning them. One was the heat of combustion of the fuel-where, of course, hydrogen excels. Rothrock's favorite way of beginning such a discussion was to show a plot of heat of combustion as a function of atomic number, and hydrogen was higher than the upper limit of his scale. Despite this, Rothrock's discussion of hydrogen revolved more around hydrogen as an element in fuel molecules than as a fuel per se. He acknowledged interest in hydrogen mainly in focusing on the current favorite fuels, the boron hydrides, and did not mention hydrogen in his conclusions. A month after completing his report, Rothrock attended a meeting of the fuels and propulsion panel of the Scientific Advisory Board when Rae's Rex engine using hydrogen was discussed.9 Apparently Rothrock was not sufficiently impressed with the idea of using low-density hydrogen in volume-limited aircraft to change his report, which was still in the process of publication; but he may have passed word on the hydrogen proposal to the Lewis laboratory.
In 1954, current turbojet engines could operate at altitudes of 13 700 meters without serious loss of combustion efficiency. Under the direction of Olson and Childs, a group  of researchers at the Lewis laboratory was engaged in a series of experiments to relate the effect of fuel characteristics. combustor design, and altitude operation on combustion efficiency. From this research, the altitude limits for good combustion had been extended to 21 000 meters, but the goal was 30 500 meters. As part of this research, Jonash, Arthur Smith, and Vincent Hlavin turned to gaseous hydrogen in 1954 and were not disappointed.* In their report completed two days before Christmas, 1954 (published five months later), they indicated that hydrogen burned well in a single turbojet combustor at pressures as low as 1/10 atmosphere; at 1/4 atmosphere, combustion efficiency was above 90 percent. These results were within the combustion pressure range for turbojet engines operating at 30500 meters altitude. The authors believed that they could attain 100 percent combustion efficiency with better mixing of hydrogen and air. Propane was investigated briefly and found to be greatly inferior to hydrogen, with the difference attributed to hydrogen's higher flame speed and wider flammability limits.10
Sometime during 1954 or early 1955, Abe Silverstein, the associate director of the Lewis laboratory, was struck with an idea concerning hydrogen. Well aware of high-altitude flight objectives and well versed in aircraft design principles, he suddenly saw a way of using hydrogen's superior combustion characteristics and coping with its principal disadvantage, low density. At high altitudes and low speeds, large wings are needed and these call for a proportionately large fuselage. Under these flight conditions, the drag of the airframe is low. The large volumes available in the wings and fuselage favored the use of low-density liquid hydrogen, provided lightweight hydrogen tanks proved feasible.
As was his custom when struck with a new idea, Silverstein made some approximate or "back of the envelope" calculations. He became so enthused over the results that he went to Washington to discuss them with Hugh Dryden, NACA director of research. Dryden, too, was impressed, and the two discussed the idea with Air Force officials. Silverstein was convinced that he had something good, but needed more detailed calculations to back it up.11
* Whether the Lewis group thought of using hydrogen independently or as a result of a suggestion by the Air Force or by Rothrock has not been established.