Hydrogen In Aircraft

As valuable as hydrogen is for an automobile fuel its advantages as an aircraft fuel are enormously greater. When taking off and flying, an airplane must lift its own weight, the weight of its payload, and the weight of the fuel it needs to reach its destination. Hydrogen produces over two and one half more energy per unit weight than jet fuel. This translates into a much lighter fuel load for an airplane. The light fuel load allows a higher payload and/or greater range.215 In the late fifties and early sixties, the United States Air Force examined the use of hydrogen for military aircraft. They concluded that there were significant advantages in lower lift off weight, greater payload and range. The lack of a liquid hydrogen supply infrastructure prevented the Air Force from capturing these advantages.216

In the mid-seventies, G. D. Brewer and R. E. Morris of the Lockheed California Company performed a study for NASA Langley Research Center to define the characteristics of a hydrogen fueled commercial airliner. This comparison was performed for airplanes with short, medium, and long ranges, carrying 130, 200, and 400 passengers respectively. Table 7.3 shows the results of this study.

Range

Short

Medium

Long

Kilometers

780

5560

9265

Passengers

130

200

400

Type

lh2

Jet A

lh2

Jet A

lh2

Jet A

Gross Weight

44,600

49,300

81,400

98,400

266,400

450,000

Total Fuel Wt.

3,360

8,940

9,480

27,720

68,000

238,000

Empty Weight

28,300

27,400

51,900

50,700

158,100

172,600

Thrust/Weight

3.43

3.43

3.33

2.75

2.65

1.96

No. of Engines

2

2

4

4

4

4

Thrust/Engine

75,600

84,100

66,700

68,100

175,300

221,100

Wing Area

84.7

86.3

148.8

154.6

466

662

Wing Span

29.3

30.8

37.5

38.7

68.3

85.3

Aircraft Length

42.7

34.4

52.7

44.2

77.4

68.6

Take-off Distance

2,410

2,430

1,640

2,432

2,106

3,650

Price per Aircraft

7.85

7.51

13.95

13.33

38.90

40.00

Energy Utilization

763

734

631

876

950

1,210

Noise

Sideline

86

86

86

86

94

93

Flyover

79

79

82

86

93

100

Table 7.3 LH2 Fueled Passenger Transport Aircraft

Weight in Kilograms, Thrust in Newtons, Area in meters2, Length in Meters, Cost in $ Millions, Noise in dB, Energy Utilization in Kilojoules per Seat Kilometer, LH2 is Cryogenic Liquid Hydrogen, Jet A is standard commercial jet engine fuel

This study was quite thorough. Hundreds of factors were evaluated to determine which combination of airframe, engine and fuel system would be most suitable for the hydrogen-fueled plane. When the optimum combination was found it was compared to a jet fueled aircraft of the same payload and range capability. The design utilized in this study was relatively conventional and followed practices used

215 Brewer, G. D., "The case for Hydrogen Fueled Transport Aircraft", American Institute of Aeronautics and Astronautics, Paper No. 73-1323 November 7, 1973

216 Schalit, L. M., and Read, H. E., "Military Applications of Liquid Hydrogen Fueled Aircraft", report AFFDL-TR-74-102, Air Force Flight Dynamics Laboratory, Wright Patterson Air Force Base, Ohio 45433

for aircraft such as the Lockheed L 1011, the MacDonald Douglas DC 10, the Boeing 757 and the European A 300. The design incorporated a wide cylindrical body with two levels for passengers. Full body size liquid hydrogen tanks were placed ahead and behind the passengers. Two tanks are necessary to provide proper center of gravity control as the fuel is used. The only outward difference in the appearance of the aircraft is a section with no windows behind the cockpit and behind the passenger section.217

Examination of the data developed during this study indicates the liquid hydrogen fueled aircraft has advantages in almost every category. It is lighter, has shorter wings, uses smaller engines, has a more favorable thrust to weight ratio, uses about the same or less runway for take off, is slightly quieter, and utilizes energy more efficiently. The only parameters where the Jet A fueled aircraft has the advantage is in empty weight for the shorter-range aircraft. The short-range hydrogen fueled aircraft is estimated to be slightly more expensive than the Jet A fueled equivalent. Clearly, if the cost of liquid hydrogen can be reduced to about the same cost per unit of energy, the liquid hydrogen aircraft will by highly desirable.2I8'219

The insulation of the fuel tanks must be adequate to prevent excessive fuel boil-off and must prevent the outer surface of the tank from becoming cold enough for frost or ice to form. There is a trade off in the technique of insulating the tank. Vacuum multilayer insulation, of the type suggested for the automobile tank, has a low heat leak, but is relatively heavy, possibly too heavy for use in aircraft. Internal foam insulation is much lighter. Unfortunately, it provides less thermal protection. Light foam insulation, similar to the type used on the space shuttle liquid hydrogen drop tank, is adequate for most operational regimes. The Lockheed study assumed foam insulation placed on the inside of the tank. The heat leakage through this insulation allows hydrogen boil-off. During most operational conditions, the rate of boil-off provides approximately the proper amount of gas for pressurization of the tank for fuel expulsion. Only when the aircraft is allowed to stand on the runway for a long period with engines idling does the pressure rise exceed the desired limit. As with the automobile it will be necessary to develop a safe method of venting hydrogen to prevent excessive pressure buildup during unusual operating conditions.

The Japanese have evaluated the use of hydrogen in aircraft in their studies of how to reduce their dependence on imported oil. European aircraft manufacturers have also studied the use of hydrogen. 220 In both of these cases, the studies indicated that hydrogen offered great advantages over the continued use of fossil based jet fuel, but the lack of a hydrogen infrastructure has inhibited development of actual aircraft.

217 Brewer, D. G., "Study of LH2 Fueled Subsonic Transport Aircraft", NASA Langley Research Center, contract NAS1-12972, January 1976

218 Brewer, D. G., "Liquid Hydrogen Airport Requirements Study", National Aeronautics and Space Administration, NASA CR-2700, October 1976

219 Johnson, John E., "The Economics of Liquid Hydrogen Supply for Air Transportation", Union Carbide Corporation, Linde Division, New York, N.Y. (Presented at the Cryogenic Engineering Conference, August 10,1973 Atlanta, Georgia)

220 Editors, "MBB Proposes A300 Capable of using Liquid Hydrogen Fuel", Aviation Week and Space Technology, July 3, 1989, Page 57

BLENDED WING BODY (BWB)

Nasa Electric Low Drag Airframe Research

Figure 7.12 A NASA Advanced Concept Hydrogen Fueled Airplane

Picture supplied by David Ercegovic, NASA Glenn Center

Figure 7.12 A NASA Advanced Concept Hydrogen Fueled Airplane

Picture supplied by David Ercegovic, NASA Glenn Center

The NASA Glenn Center at Cleveland Ohio is studying low emission airplanes. Hydrogen fuel is an integral part of their program. They are also investigating advance airframe concepts to achieve high performance in the new low emission aircraft. Figure 7.12 show a drawing of a very low drag aircraft that is being evaluated for use with hydrogen as the fuel. This work is being performed under the "Zero CO2 Emissions Technologies" program, managed by David Ercegovic at NASA Glenn Center, Cleveland, Ohio.

In routine operations, refueling the liquid hydrogen fueled aircraft will be similar to refueling an aircraft with Jet-A fuel. The aircraft will land and taxi to the passenger terminal. A liquid hydrogen truck will arrive, the fill lines will be attached, the liquid hydrogen will be transferred, the lines will be detached and the plane will depart for the next leg of its journey. As with the automobile, the transfer will be performed without allowing contact of air with liquid hydrogen. Any boil-off occurring during refueling will be recondensed in the truck and no gaseous fuel venting will be permitted. Airline operators like to keep their planes flying. As a result, most of the time the tanks will remain chilled.

For maintenance and repair, the aircraft fuel tanks may be allowed to warm to ambient temperatures. In these circumstances, a different operational sequence will be required to recharge the tanks with liquid hydrogen. When the aircraft tanks are warm, all of the initial liquid hydrogen charge will boil because of the relatively high temperature of the tank. As the liquid boils, it cools the tank; ultimately it becomes cold enough that hydrogen no longer boils. A significant amount of liquid is vaporized during the chill down process. A separate facility for chilling warm tanks and liquefaction of the hydrogen will be required. Recovery and liquefaction of the vapor that boils out of a warm tank will be essential to prevent wasteful fuel loss.

The hydrogen-fueled aircraft, like the hydrogen-fueled automobile, will have a low impact on the environment. The only pollutants possible are the nitrogen oxides. In the automobile engine it will be possible to eliminate nitrogen oxides by adjusting the combustion mixture ratio slightly fuel rich. This technique is not possible with the turbojet engine. In these engines, the combustion is continuous and always fuel lean. In this type of combustion, there is always excess oxygen and nitrogen that can react to form nitrogen oxides. Reduction of nitrogen oxides is achieved by careful design of the combustion chambers and mixing process so that there are no hot spots at a high enough temperature for their formation. The maximum temperature allowed for the metal parts of the engine is in the range of 1,000 Kelvin. The temperature required for significant production of nitrogen oxides 500 Kelvin higher. At the points of fuel injection, it is possible to have gas combustion zones hotter than the combustion chamber walls. The walls are protected from the higher temperatures by the flow on the incoming air. Careful design of the fuel injectors and routing the airflow can decrease the temperatures in the gas. If the design of the combustion chamber limits the gas hot spots to temperatures only about 200 Kelvin above the maximum allowed for the metal parts, the production of nitrogen oxides is low.

As in automobiles, hydrogen will be as safe an aircraft fuel as current fuels under all normal operating conditions. The safety considerations are different only in a crash with sufficient violence to rupture the fuel tank. Crash safety is extremely complicated to evaluate. On the advantage side for hydrogen is its great speed of dissipation, low luminosity of the flame, lack of toxic gas on combustion, and total lack of toxicity of the hydrogen. The low temperature of the liquid is a safety disadvantage. If passengers were exposed to the liquid during a crash, there is the possibility of frostbite and death by freezing. With proper attention to design details it should be possible to field a liquid hydrogen fueled aircraft as safe as the current generation of Jet-A fueled commercial passenger aircraft.

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