This Month in Aviation History

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Aircraft of the Month for March 2001

Alternate Fuels in Aviation

by Alex Stoll

The Zero CO₂ Emissions Technology Project
In the fall of 1999, NASA launched the Zero CO₂ Emissions Technology Project, a three-year program to explore the prospects of zero-emission vehicle (ZEV) aircraft that would use hydrogen as fuel for jet engines and gas turbines to eliminate carbon dioxide from civil aircraft and for the possible development of advanced ultra-lightweight fuel cells as powerplants at least initially for small four to six passenger aircraft. David Ercegovic from the NASA Glenn Research Center manages the program.
A British study by the Institute for Public Policy Research in London, dated August 2000, concluded that “aviation is the fastest-growing source of transport greenhouse gases, although it is still small in proportion to others.” It also stated that the International Civil Aviation Organization is presently developing policy options for decreasing these emissions and expects to report to the United Nations Framework Convention on Climate Change in the fall of 2001. It implied that carbon dioxide emissions from international civil aviation ought to be incorporated in the global emission trading scheme recommended under the Kyoto Protocol. The study also suggests that the International Civil Aviation Organization and the Intergovernmental Panel on Climate Change should work together to set up an international task force “with the major aircraft and engine manufactures to find alternative designs for passenger aircraft capable of radical reductions in greenhouse gas emissions.”
Aside from the X-30 National Aerospace Plane, which was to be powered by slush hydrogen but was never built due to technical obstacles, the last NASA hydrogen aircraft was in 1956 when a Martin B-57 jet bomber flew with liquid hydrogen powering one of its two engines. The Soviet Union experimented with hydrogen for jet fuel when it flew a modified Tupolev Tu-154 airliner with hydrogen fueling one of its three engines in 1988. This aircraft was flown multiple times with both liquid hydrogen and liquid methane as fuels.
Seven full-time scientists staff NASA’s new effort, but it is budgeted for the equivalent of ten full time employees for fiscal years 2000-2002. The total budget available for those three years for in-house research is around seven million dollars. The main investigated area include Requirements and Concept Analysis, Hydrogen Combustion and Emissions, Air-Breathing Fuel Cells, YSZ Fracture Toughness (tougher electrolyte materials are needed in the higher-pressure air cycles envisioned for solid oxide fuel cell systems), and Liquid Hydrogen Tankage, Transfer, and Storage. In 1999, the project was started to support NASA’s stated objectives laid down by the Office of Aero-Space Technology in 1995 to decrease nitrogen oxide emissions of future aircraft by a factor of three within ten years, and reduce carbon dioxide emissions by twenty-five percent within ten years and by fifty percent within twenty-five years.
The team is also studying the permeability of composite fuel tanks to liquid hydrogen. Aside from aiding liquid hydrogen-powered aircraft, this technology could help reduce the cost of ferrying materials to low-earth orbit to about one-tenth of the current $22,000 per kilogram in the next ten years and to around one-tenth of that within twenty-five years. A summary paper by the team stated that while hydrogen fuel can decrease carbon dioxide emissions, it also has the potential to produce “copious amounts” of nitrous oxide in advanced high-pressure high-temperature gas turbines. “The ability to develop fuel injection technology that controls this ‘prompt’ NOX production is a necessity for the successful use of this fuel,” the document asserted. Development of these technologies, coupled with efficient, low permeability, lightweight polymer matrix composite liquid hydrogen tanks “may allow the introduction of hydrogen fueled gas turbine subsonic aircraft in the future.” The team will burn hydrogen in a flame tube combustor simulating gas turbine engine operations to collect more data.
Fuel cells have never been used as powerplants for aircraft before. “A thorough systems analysis of exotic fuel cell electric hybrid and advanced open cycle gas turbine systems optimized to fully exploit the beneficial physical properties of liquid hydrogen will be conducted," the document added. "These analyses will determine the feasibility of far-term application of liquid hydrogen as a fuel for both hybrid fuel cell aircraft propulsion and optimized subsonic gas turbine propulsion systems."
Both proton-exchange membrane and solid oxide fuel cells are under consideration by the team. In a high-pressure air cycle, these systems “may significantly enhance fuel cell performance as measured in watts per pound.” Together with lightweight polymer matrix composite liquid hydrogen fuel tanks, this “may enable a revolutionary application of electric propulsion to subsonic aircraft with the benefit of zero carbon dioxide and nitrous oxide emissions.”
It is not clear whether the weight of a fuel cell and an electric motor to drive a propeller or fan would be too much for aircraft. “We need to put a stake in the ground and know whether a Cessna 172 class aircraft needs fuel cell energy density two or five times that of the current state of the art,” said Ercegovic. The analysts will also have to determine whether regional jets will require two or five times the energy density of the Cessna and whether a Boeing 737-sized aircraft will require two or five times the energy density of a regional jet. If the 737-size aircraft will require a fuel cell that is 125 times as efficient as state-of-the-air fuel cells, systems such as these will be impractical. However, if the Cessna type aircraft will require a fuel cell with an energy density of only twice to three times state of the art, Ercegovic says “we can continue to pursue a reasonable project to develop these technologies.”
The team has developed a few preliminary estimates of energy density sensitivities for a viable flight vehicle, but verification is required. “I am willing to say that a Cessna 172 class aircraft test bed on hydrogen and a fuel cell is not out of the question in the next five to ten years. Of course, we have not gotten a good estimate yet on the support hardware for the LH₂ tank and the power controllers for the ‘engine/motor/fuel cell.’ That can be a show stopper and is the subject of our next two years of systems analysis work.” No fuel cell manufacturers or aircraft manufacturers are actively involved in this study.
The team plans to obtain fuel cells during the next fiscal year from contractors experienced in fuel cell production and “test them to the extreme to determine scaling parameters,” Ercegovic says. “Our current goal is not develop a flight system but determine scaling parameters to get a much better handle on whether it is reasonable to pursue a development program.”

Pulse Detonation Wave Engines
Pulse Detonation Wave engines have no moving parts. Laser ignition works well for this application - the location where detonation is started can be chosen freely; detonation can even be started on the surface of the aircraft. Possible fuels include acetylene, hydrogen, ethane, and others. FOA, Sweden’s defense research establishment has been studying Pulse Detonation Wave engines since 1993. The first test rig was run in the spring of 1994 and it was later demonstrated in the United States. Since Pulse Detonation Wave engines are very compact, they would be good for use in missiles, drones, and decoys. Additionally, they cost about one fifth as much as conventional jet engines with the same thrust.