Part II : 1950-1957

6. NACA Research on Hydrogen for High-Altitude Aircraft



Silverstein-Hall Report


[98] When Silverstein returned from Washington, he asked Eldon Hall, one of the laboratory's top analysts, to assist him in refining his analysis on using hydrogen for high-altitude aircraft. While this was under way, the fuels and propulsion panel met in March 1955 to discuss high-energy fuels.* The panel was very impressed with the potential of liquid hydrogen and boron hydrides.12 The work of Jonash, Smith. and Hlavin was described, as well as current work by Thalne W. Reynolds of the Lewis laboratory. Reynolds, who was assisting Hall in the analysis for Silverstein. was studying lightweight tanks for hydrogen and was convinced that they were feasible.


[99] The fuels and propulsion panel suggested that the Air Force begin work on hydrogen fuel systems, hydrogen-fueled engines. and preliminary designs of hydrogen-carrying aircraft. This meeting apparently spurred the Lewis analysts to faster action. for Silverstein and Hall completed their report on 1 April 1955 and published it two weeks later-a near record for fast NACA publication and an indication of the importance Silverstein attached to the subject.


In their introduction, Silverstein and Hall noted that despite hydrogen's high heating value and good combustion characteristics, it had received only casual attention. They acknowledged the deterrents of low density, low availability, and difficult handling, but made a case for considering hydrogen based on four points: a military need that could not be met in any other way, advantages of hydrogen for high-altitude flight, improvements in jet engines that indicated their mass could be halved for the same power, and large wing and fuselage requirements for high-altitude flight. The first two points were based on hydrogen's unique properties. The third favored light weight, and the fourth high volumes, to overcome hydrogen's disadvantage of low density. As for availability and handling, Silverstein and Hall cited past experiences, implying that if the flight problems could be solved, so could those on the ground.13


Of the flight problems, the authors singled out hydrogen tankage as a major problem. They drew on the technology of long-range missiles, particularly the Atlas, and suggested that liquid hydrogen tanks be constructed as cylindrical balloons of light-gage metal, depending upon internal pressure to maintain shape (fig.19). This, of course, was the same idea proposed by Oberth in the 1920s and Martin and North American engineers in the 1940s, and being used for the first time on the Atlas ICBM amid some skepticism.


cut away diagram of proposed storage tank

Fig. 19. Liquid-hydrogen tank suitable for aircraft as envisioned by Abe Silverstein and Eldon hall. "Liquid Hydrogen as a Jet Fuel for High Altitude Aircraft." NACA RM E55C28a. 15 Apr. 1955. Of light-gage metal that depended on internal pressure to maintain its shape, the tank was 25 m long, 3 m in diameter with a volume of 175 m3. Liquid-hydrogen capacity was 11300 kg. The estimated tank mass was 10 percent of the fuel mass.


[100] Using the basic hydrogen-tank design, Silverstein and Hall analyzed the use of liquid hydrogen for a subsonic bomber, subsonic reconnaissance airplane, and supersonic fighter. Of these, the reconnaissance type will be described as a typical example and for its relationship to later events.


The subsonic reconnaissance airplane had a gross mass of 40 000 kilograms and carried hydrogen tanks in wings and fuselage, as well as optional drop tanks for additional range (fig. 20). It operated at an altitude of 24 000 meters and could make observations 13 500 kilometers from its base. A supersonic version was about 1/4 lighter, operated at the same altitude at a speed 3 times faster, but had a range less than 1/5 the subsonic type.


The subsonic version was powered by advanced turbojet engines weighing about half those in current use. The supersonic type also used an advanced turbojet that was equipped with an afterburner. Additional data on the airplanes and engines are given in table 2.


Silverstein and Hall concluded that "within the state of the art and the progress anticipated, aircraft designed for liquid-hydrogen fuel may perform several important missions that comparable aircraft using hydrocarbon (JP-4) fuel cannot accomplish." They also concluded that "substantial applied research and development effort will be required in many technical fields to achieve the goal outlined."14 It was a convincing case for hydrogen if the assumptions were accepted. Silverstein, as the chief research executive of the Lewis laboratory, thereupon initiated a massive research pogra on hydrogen to given substance to his assumptions.


cross-sectional drawing of proposed liquid hydrogen fueled plane


Fig. 20. High-altitude, subsonic reconnaissance airplane using liquid hydrogen as fuel. The liquid hydrogen tanks are in both fuselage and wings. Flight Mach number, 0.75; altitude 24400 m. From Silverstein and Hall, "Liquid Hydrogen as a Jet Fuel," 1955.



[101] Table 2. -Characteristics and Performance of Reconnaissance Airplane and Engine Designs


Reconnaissance Airplane



Cruise Mach No



Cruise altitude, m

20 000

22 000

Target altitude, m

24 000

24 000

Gross mass, kg

39 800

34 000

Fixed (instruments, cameras, controls)

2 268

2 268


13 000

13 000


6 328

6 169

Fuel tank

2 372

1 610


15 760

10 730


Area, m2



Sweep angle, deg



Aspect ratio



Average section thickness ratio



Taper ratio



Empennage ration, m2




Length, m



Diameter, m



Lift coefficient, initial cruise



Lift/Drag ratio (airplane less engine nacelles, initial cruise)



Radius, km

13 500

2 490

Engines (turbojet)




Compressor diameter, m



Sea-level thrust N

64 050

72 500


(14 000)

(16 300)

Cruise specific consumption, kg/hr/N



From: Abe Silverstein and Eldon Hall, " Liquid Hydrogen as a Jet Fuel for High-Altitude Aircraft," NACA RM E55C282.
15 Apr. 1955, p.21.


A key assumption of the Silverstein-Hall analysis was the feasibility of lightweight, insulated flight tanks suitable for liquid hydrogen. Reynolds continued his investigation and reported the results in August 1955. Table 3, taken from the report, summarizes the results. Reynolds concluded that it was feasible to design a tank that had a mass less than 15 percent of the liquid hydrogen it contained. Estimated hydrogen vaporization rates were less than 30 percent of hydrogen consumption during cruise, and prior to flight, the tank could be held in stand-by condition and readied for flight in a short time.15


Following, the completion of the report with Hall on flying aircraft fueled with hydrogen, Silverstein again visited the Air Force with missionary zeal. He also set in motion a great wave of research related to hydrogen's use in aircraft at the Lewis laboratory. This included properties, combustion, materials, tankage, bearings, pumpings, controls, and complete engines. In 30 months, the investigations led to three dozen reports and were climaxed by a research conference in November 1957.


In September 1955, Jerrold D. Wear and Arthur L. Smith completed an investigation of six types of injectors for burning gaseous hydrogen in a turbojet...




[102] Table 3. Flight Type Liquid - Hydrogen Tank Design.


Diameter, m


Length, m


Volume, m3


Surface area, m2


Working pressure, atm


Styrofoamb insulation:

Thickness, cm


Density, kg/m3


Mass of tank:

Shell, kg


Insulation, kg


Covering, kg


Allowance for baffles and stiffeners, kg


Approximate total mass, kg


Estimated performance with ambient temperature at:

300 K

218 K

Outer surface temperature, K



Heat-leak rate, W

25 770

14 500

Hydrogen-vaporization rate, kg/hr



No-loss time on ground, min


a Holds 11 340 kg liquid hydrogen with 9 percent expansion volume.
b Covered with layer of Mylar-aluminum foil.
c About 14 percent of fuel mass.
d For a tank with 5.7 cm insulation, precooled with liquid nitrogen. No-loss time is the time for heat-leaking into the tank to vaporize enough hydrogen to raise the pressure to the working pressure (2 atm).
From: T.W. Reynolds, "Aircraft-Fuel-Tank Design for Liquid Hydrogen," NACA E55F22, 9 Aug. 1955, p.9.



....combustor.16 They found that at conditions simulating full power, all six injectors gave high combustion efficiency-an indication of the ease of burning hydrogen. Some relatively low combustion efficiencies were obtained, but these were at conditions where ordinary jet fuel would not burn. These experiments were followed by others as the laboratory probed deeper and deeper into the combustion of hydrogen under a variety of conditions.


* Mark M. Mills had succeeded Soderberg as chairman. Other members: W. D. Rannie.E.S. Taylor, Gale Young, and A. M. Rothrock.