Fast & Hot

Supersonic aircraft now on the drawing boards will soon be moving at more than 1,500 m.p.h. At such high speed, say aeronautical engineers, friction between air and airplane will build a wall of heat--a "thermal barrier"--that will grow worse as planes fly faster. Their metal may soften like the wax in the wings of Icarus when he flew too near the sun.

Only a few years ago, designers thought that at the speed of sound, turbulent shock waves would pound a plane to bits. But when jets pushed aircraft up to the sonic barrier, it turned out to be nothing worse than a bump in the road. Plane after plane passed over into the exhilarating calm of supersonic flight. In the current issue of Skyline magazine, Vice President Ray Rice of North American Aviation, Inc. explains why the thermal barrier can only be pushed ahead, never completely overcome.

"Even the air at the icy regions of 40,000 ft. does not help the engineer with his problem," says Rice. "At the speeds contemplated for the future, aluminum will relax and lose much of its strength. Canopies of today's materials will soften like putty and pull from their foundations. Radar equipment may give the wrong message . . . And the pilot would simmer like beef stew without refrigeration."

Even today, fighter planes fly at such searing speeds that their engines must suck in some 18 tons of air an hour in order to keep cool. Pilots already have their own refrigeration system. Soon, says Rice, refrigeration systems will be needed for electronic and hydraulic equipment. Engineers are already searching for new oils that can withstand high temperature.

To find the practical limits of the thermal barrier, says Rice, "take a hypothetical missile flying at 2,000 m.p.h. at 60,000 ft." If it were no bigger than a home refrigerator it would need the power of 20 such refrigerators to keep it cool enough. The real barrier "may be the point at which the power required to cool the equipment equals the power of the airplane or missile."

Engineers are learning to air-condition cockpits, insulate electronic gear and use tough metals like titanium in high-speed planes, but another hope for dealing with the problem of high-speed heating lies in speed itself. When high speeds are reached. Rice points out, there is a certain time lag before the airplane's structure heats to the danger point. Future military planes may be fast enough to accomplish their missions and slow down again before they begin to melt.

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