Project Cryoplane

Air Safety Week, July 12, 2004

To develop an airplane safely powered by renewable hydrogen fuel, an active research program currently is under way, working under the aegis of the European Union and involving some 35 partner companies and government agencies from 11 countries. The project goes by the moniker Cryoplane, to signify that the hydrogen fuel will be carried in liquefied form at a temperature of -253[degrees] C (-423[degrees] F).

Hydrogen fuel was the most promising renewable fuel alternative to mineral kerosene in a recent study by London's Imperial College Center for Energy Policy and Technology (see ASW, May 24).

There are literally dozens of papers, outlines, briefs, commentaries and other materials available on the Internet. Via Google or Yahoo, type "Cryoplane" and numerous links will come up. From such a search, and by way of follow-up, herewith some additional considerations:

Hydrogen characteristics:

* Hydrogen offers 2.8 times the energy content per unit of mass of kerosene.

* The major disadvantage is that hydrogen has a low ratio of energy to volume, and for aviation use has to be cooled to cryogenic temperatures to reach its liquid state (LH2).

Design considerations:

* Storing supercooled hydrogen in a liquid state on an airplane will require well-insulated tanks, pipes and pumps.

* The tanks will have to be spherical or cylindrical to minimize structural weight. While kerosene-fuelled aircraft carry much of their fuel in wing tanks, this will not be possible with liquefied hydrogen, because of the volume and insulation required.

* The tanks will have to be pressurized (1 bar) to avoid ingress of oxygen.

* Ideally, tanks should not be located in the disk burst area associated with an uncontained engine failure.

* With minimal changes to current kerosene fuelled aircraft, LH2 tanks could be located in large pods under the wing. This concept complicates insulation and keeping the fuel cooled to a liquid state. Alternatively, the tanks could be located above the passenger cabin. This approach eases somewhat the insulation challenge but adds to the length of piping required to provide fuel to the engines. For long-range widebody aircraft, LH2 tanks could be placed in the fuselage, front and the rear, without the bulbous extension for tanks topside. This can be done while preserving a cabin-to-cockpit connection via a narrow aisle adjacent to the forward fuel tank. It should be noted that if the LH2 tank completely fills the forward area, the cockpit could be entered via an external door or hatch. Indeed, separate cockpit entry might serve as an antihijacking feature.

For jumbo-size jets that must fit within the 80-meter-by-80-meter "box" needed for compatibility with current airport layouts, the larger volume needed for hydrogen fuel tends to force the design of a widebody fuselage into a multiple deck layout. For narrower single-aisle aircraft requiring a cabin-to-cockpit connection, the tank-on-top layout results.

Safety considerations:

Those involved in the Cryoplane concept believe that safety will be equal to, or even better, than that of kerosene-fuelled aircraft. However, according to the Swedish Defense Research Agency, "the specific characteristics of the new fuel need careful attention in choosing aircraft overall configuration as well as in detail design and operation." (See www.hynet/info/regions/sweden/demos/proj004.html). As an example, the impact of uncontained engine failures, and the resulting spray of shrapnel into the airplane, and into fuel tanks, is one such matter of "detail design."

Dr. Heinz Klug, project manager for Airbus, said in an interview:

"It is, however, possible that hydrogen [fuel] tanks create less problems than traditional kerosene tanks. With the traditional tank, we have a mixture of kerosene and air in danger of reacting with each other. With hydrogen, this danger is non-existent. The tank only contains liquid hydrogen and hydrogen gas. The potential damage during an accident might therefore even be smaller." (See www.freesen/de/h2report/iss0101.htm)

Liquid hydrogen burns much faster than kerosene, but with lower radiation heat and without making a fire carpet, as does kerosene. LH2 does not explode in open air, only in a confined space, so special attention will have to be paid to the design of fuel system vents, valves, etc., to prevent leakage into the passenger cabin.

Studies reveal that passengers would have a greater probability of survival after a crash if they remained in the cabin and waited for the fuel to burn outside the aircraft. This characteristic of hydrogen fuel will require changes to airworthiness requirements and, concomitantly, to emergency evacuation procedures.

Environmental considerations:

Hydrogen emits 2.5 times more water than kerosene.

Although no condensation nuclei are emitted (e.g., soot), the water vapor in the exhaust is conducive to contrail formation. The contrails could contribute to the phenomenon of "radiative forcing" (RF). That is, a sky filled with contrails could reflect heat back to the ground, contributing to global warming. According to one abstract, which assumed the entire worldwide fleet of kerosene-powered airplanes would be replaced with cryoplanes in 2015: