Part II : 1950-1957

6. NACA Research on Hydrogen for High-Altitude Aircraft



[95] Hydrogen was considered as an aviation fuel by P. Meyer in 1918 (p.12); Tsiolkovskly considered and rejected it for a rocket-powered airplane in 1935 (p.256). In 1939, George W. Lewis, director of research for the National Advisory Committee for Aeronautics (NACA) was talking about using liquid hydrogen with atmospheric air, presumably for aircraft propulsion (p.73). During World War II, F. Simon, a respected physicist in England, nearly confounded the practical fuel experts in the United States by suggesting that liquid hydrogen be used to increase aircraft range (pp.11-12). Ople Chenoweth, Robert Kerley, John Duckworth. and their associates at Wright Field's power plant laboratory contracted with Ohio State University in 1945 to investigate the application of liquid hydrogen to aircraft and rockets (p.18). None of these. however, got very far, principally because hydrogen's very low density made its application in volume-limited airplanes appear totally impractical. If this was not enough, opponents to hydrogen clinched their case by citing its very low availability as a liquid and its handling hazards.


Beginning in the 1950s. however, several factors combined to make liquid hydrogen appear exceedingly attractive as an aviation fuel. Among them: incentives to operate airplanes at very high altitudes, advances in liquid-hydrogen technology, and experiments showing that hydrogen burned readily at low pressures.


One of the places where an intense interest in hydrogen for aircraft developed during the 1950s was the NACA Lewis Flight Propulsion Laboratory in Cleveland, where it was pushed hard by the associate director, Abe Silverstein. NACA involvement with hydrogen for this application, however, had its roots in earlier work in fuels and combustion.


One of the initial facilities built at the NACA Cleveland laboratory in the early 1940s was a well equipped chemical laboratory for fuels and lubricants. New fuels or blends for piston engines could be synthesized, and during the war, for example, the laboratory studied alternate high-octane fuels such as the aromatic amines. With the switch from piston to jet engines after the war, the type and characteristics of desired fuels also shifted. The amount of heat obtainable per unit mass and volume became of great importance. Research involved not only the theoretical energy content of fuels, but how to release and harness that energy over a range of operating conditions. How [96] well did the fuel mix in an air stream? Would the fuel ignite and propagate over a range of combustible mixtures? How efficient was the combustion process over a range of operating conditions, particularly at the reduced pressures of high altitude? Such questions became important for research to answer.


In 1948, the Lewis laboratory presented its research on fuels at a conference; six of the nine papers were on fuels for turbojets and ramjets.1 Melvin Gerstein discussed powdered metallic fuels such as aluminum and beryllium, which had heats of combustion per unit volume up to four times greater than gasoline. Gerstein also discussed diborane, reporting that its flame speed was fifty times greater than that of hydrocarbons. It was part of the great love affair with diborane and pentaborane by the laboratory and others which extended beyond the mid-1950s.


In 1950, the uneasy international situation, and especially the outbreak of the Korean war, led to an acceleration of aeronautical research and development. One goal was aircraft capable of operating at very high altitudes, and one obstacle in doing this was described by Walter T. Olson, J. Howard Childs, and Edmund R. Jonash of the Lewis laboratory in 1950:


Experience has shown that, as operating altitudes are progressively increased beyond 25000 feet [7600 m], the effects of altitude on combustion efficiency ultimately result in severe penalties in thrust and specific fuel consumption. The problem of maintaining high combustion efficiency is one of the most important problems of altitude operation.2


The investigators found that combustion efficiency increased with fuel volatility, with greater hydrocarbon content as compared to aromatics, and with more straight-chain and fewer branched-chain hydrocarbons.


The following year, Olson and Louis Gibbons surveyed fuels suitable for ramjets and summarized results achieved by several organizations, including the experiments on liquid hydrogen at Ohio State University. Although Olson and Gibbons included liquid hydrogen among the fuels of interest, they were more interested in investigating diborane, pentaborane, and slurries of magnesium and aluminum.3 The same year, Benson E. Gammon examined the performance of liquid hydrogen and two other fuels for ramjets, finding hydrogen superior per unit mass but inferior per unit volume.4 Another Lewis laboratory analyst, Hugh M. Henneberry, considered fuels for aircraft during 1951 and concluded that:


neither the very high nor very low fuel densities have any advantage for long-range flight ... the practical difficulties associated with the use of liquid hydrogen cannot be justified on a range basis, but if tactical considerations predicate flight at extremely high altitudes, liquid hydrogen must be considered as a possible fuel [emphasis added].5


There it was-the advantage of hydrogen for attaining extremely high altitudes-but Henneberry, like others at Lewis, was impressed by the potential of another high-energy fuel, diborane, and consideration of hydrogen went no further at that time.*


[97] The military services and their advisors also showed little or no interest in hydrogen for aircraft prior to 1954. The Navy had embarked on a massive investigation of boronhydride fuels for jet engines and was joined in this effort by the Air Force and NACA.6 The fuels and propulsion panel of the USAF Scientific Advisory Board, in considering high-energy fuels at its April 1952 meeting,** noted that rockets favored fuels with combustion products of minimum molecular mass but that "this condition is irrelevant in a turbojet."7 This indifferent attitude towards hydrogen appeared to prevail generally for two more years until a series of events, starting in 1954, swept it aside like fog before the wind.


* For flight at an altitude of 21000 m and speed of Mach 3.6, Henneberry concluded that diborane had a 59 percent greater range than hydrocarbon fuels.
** The chairman was Prof. C. Richard Soderberg. M.I.T. Other members: Louis G. Dunn, William M. Holladay, Andrew Kaletensky, and W. D. Rannie. A. M. Rothrock of NACA attended the meeting as a guest and was later a member, 1953 1955.