Science: The Exotic Fuels

Question: How to make an airplane or missile fly farther and faster. Answer: Give it fuel that packs more punch.

Last week a super-secret conference met at Dayton’s Wright-Patterson Air Force Base to discuss high-energy “exotic” fuels, the hottest subject in the air propulsion business. The conference issued no public report, but it is well known that all major engine and rocket-motor manufacturers are experimenting with the new fuels, and that chemical plants are being built to produce them in quantity (TIME, March 18).

High B.T.U. When engineers speak of fuel, they mean any material that yields energy as a result of chemical reaction. This definition excludes nuclear “fuels.” e.g., U-235, but it includes thousands of lesser energy-yielders. Petroleum hydrocarbons (gasoline, kerosene, etc.) are the commonest aviation fuels only because they are plentiful, convenient and relatively cheap. Many other chemicals yield more energy. Hydrogen has the highest heat of combustion (52,000 B.T.U.* per lb.), but carbon is rather low (14.500 B.T.U. per lb.). Hydrocarbons, which contain both carbon and hydrogen, are therefore intermediate. Kerosene burned in jet engines yields only about 18,500 B.T.U. per lb.

It would be nice to use pure hydrogen, but this is impractical because hydrogen is a gas that cannot be kept in the liquid state without extreme difficulty. Next best is to “liquefy” hydrogen by making it combine with some other element to form a conveniently liquid compound. Kerosene is such a liquid, but it contains too much low-energy carbon.

Fortunately, nature has provided a chemical element, boron, which can be forced with some difficulty to improve on carbon’s performance. Boron has a high heat of combustion (25,000 B.T.U. per lb.), and it forms compounds that contain more energy-rich hydrogen than most hydrocarbons do. The heat of combustion of diborane (B2H6), for instance, is 31,000B.T.U. per lb., almost twice as good as kerosene.

Cloud of Poison. But boron hydrides are bad actors. Besides being poisonous, they have a reputation for exploding spontaneously for no apparent reason. This disadvantage may have been overcome, but it is more likely that the best boron-containing fuels are compounds of boron with carbon, hydrogen and perhaps other elements. There is a long list of such compounds to choose from. A boron-carbon-hydrogen compound would not be quite so powerful as a straight boron hydride, but it might be a pleasanter playmate.

Boron fuels, with their higher energy and higher temperature of combustion, will come close to doubling the range of airplanes. In long-range missiles, they may make the difference between success and failure. They will be expensive (at least $1 per lb.) and probably highly toxic. They will burn with a bright green flame, and their exhaust, a white cloud of solid boron compounds, will be so poisonous to vegetation that tests will have to be run in deserts.

Boron compounds do not end the list of possible exotic fuels. Paintlike slurries of powdered aluminum or magnesium, suspended in some combustible liquid, contain a lot of energy. In the case of rocket motors, which do not depend on atmospheric oxygen, both the fuel and the oxidizer material with which the fuel combines can be varied. Nitric acid is popular because it is a convenient form of oxygen and yields additional energy when it decomposes. Liquid fluorine is theoretically the best oxidizer, but it is fantastically corrosive and hard to handle. Some material may be discovered that yields fluorine conveniently in the way that nitric acid yields oxygen.

*British Thermal Unit—the amount of heat necessary to raise the temperature of i lb. of water one degree F.