Death of the Contrail

The white trails left by high-flying airplanes may be a pretty sight in a blue sky, but bomber crews dislike them for good reason: an airplane may look from the ground like an almost invisible speck, but the condensation trail behind it is a gigantic chalk mark shouting "Here I am!'' to enemy attackers. Last week the Air Force announced that it has found a way to eliminate the familiar, death-bringing contrail.

When aircraft fuel burns, the hydrogen in it combines with oxygen to form more than the fuel's weight of water. If the air is cold enough, which it generally is above 30,000 ft., the water condenses to droplets which immediately freeze into highly visible ice particles. When a big bomber flies at top speed, it marks the sky each minute with 150 lbs. of gossamer advertising.

When scientists at Cornell Aeronautical Laboratory started studying contrails for the Air Force and Navy, they found that contrail ice particles are one five-thousandth to two five-thousandths of an inch in diameter. This is small but not small enough. One way to suppress contrails, the scientists reasoned, would be to make their water form particles smaller than a wave length of light. Then they would not reflect light whitely like clouds or snow.

Much of the actual experimental work was done in the Cornell Laboratory's high-altitude chamber. Researchers dressed in Arctic clothing were sealed inside while much of the air was pumped out and the remainder chilled as low as--85DEG F. Then they lit a small blowtorch with jet engine fuel and studied the captive contrail that it created in the cold, thin air.

What they were searching for was a material to feed into the incipient trail and make its water form ice particles too small to reflect light. Hundreds of materials were tried. At last a hygroscopic (water-attracting) powder was found that promised to do the trick and meet practical requirements. The laboratory built a mechanism to shoot it into the exhaust of one engine of a B-47. It reduced the contrail to a thin wisp a few hundred feet long. Later improvements made the trail completely invisible.

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