Hydrogen Harnessed

The dream fuel of rocket engineers is liquid hydrogen. It develops more thrust per lb. than kerosene, the standard ingredient for liquid-fuel rockets. But because of its low boiling point ( -- 423DEG F.), it is hard to keep in liquid form, has to date proved too tricky for use in major rocketry.

Last week the Pratt & Whitney division of United Aircraft Corp. announced the development of a new engine in which balky liquid hydrogen is successfully harnessed. Under test, it ran dependably under simulated outer-space conditions, and developed a thrust of 15,000 lbs.

When hydrogen combines with oxygen, its hot exhaust is mostly molecules of water, which are much lighter (molecular weight 18) than the carbon dioxide molecules (weight 44) in the exhaust of a kerosene-oxygen engine. Since light molecules move faster at a given temperature than heavy molecules, they carry more energy per pound. In rocket language. the hydrogen engine produces a "specific impulse" 30% greater than its kerosene-burning rivals.

Liquid hydrogen's bad habit of flashing suddenly into vapor has been turned by Pratt & Whitney engineers into a virtue. Most liquid-fuel rocket engines require a secondary combustion chamber to which some fuel is diverted; the exhaust gas is shot through a turbine to pump fuel and oxygen into the main chamber. In the Pratt & Whitney engine there is only a single combustion chamber. The liquid hydrogen is pumped first through a cooling jacket surrounding the chamber. As the hydrogen flows through, it cools the chamber, is in turn vaporized and heated. The heated, expanding hydrogen gas spins a turbine, and the turbine drives the main fuel pumps. Then, having pumped itself to its fate, the hydrogen is burned in the combustion chamber. This "bootstrap" trick contributes importantly to the engine's efficiency.

When the hydrogen engine matures into a production model, it is slated to push the final stage of multistage space vehicles, including Centaur and Saturn. Pratt & Whitney claims that in some cases the payloads can be increased by more than three times above present design levels.

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