Science: Supersonic Nemesis

Until recently most airmen believed that the problem of supersonic flight was tough, but eventually crackable. Last week they were muttering doubts. U.S. Army Air Forces at Muroc Dry Lake, Calif, had postponed their scheduled attempt to break the British-held speed record (616 m.p.h.). The British themselves were poking into hedgerows, looking for further bits of Geoffrey de Havilland’s Swallow, which mysteriously came apart in mid-air (TIME, Oct. 7). Unofficial reports indicated that the Swallow had reached 650 m.p.h. in level flight before it disintegrated. This figure, many airmen now feared, might be close to the permanent speed record for anything resembling an airplane.

Invisible Demon. The obstacle to higher speeds was “compressibility,” that invisible, ravening demon formed in the air by the plane’s own motion. Long before an airplane itself reaches the speed of sound (770 m.p.h. at 68° F. at sea level), the air, which is speeded up by passing over its surfaces, touches locally that critical velocity. When it does, a “standing sound wave” may set up such vibration that the plane flies to pieces.

So far, the remedy has been to refine the plane’s lines, polish the skin. Such tricks push the speed limit upward. But they succeed at a fearful price. Above 600 m.p.h. the slightest irregularity may beckon a fatal wave out of the speeding air.

The jet-propelled P-80, for instance, was designed as a fighter. But when its guns were first fired at 600 m.p.h., their bullets and gas blasts interrupted the smooth flow of air over the polished skin. The dread sound wave snapped its teeth, and the P-80 disintegrated.

Recently the Air Forces removed all paint from its P-80s. The tiniest chip or crack might endanger the plane by roughening the air flow. In a test flight, according to one group of experts, a gnat squashed against the leading edge of a P-80’s wing. It stuck, and behind it a sound wave hammered perilous dimples in the skin.

The most dangerous speed zone lies between 85% and 120% of the speed of sound (roughly 650 to 900 m.p.h.). Flying in delicate balance, a plane may enter the zone, but it cannot maneuver, fire its guns, or open its bomb bay without inviting destruction.

Peaceful Heights. Above the danger zone, most experts hope, lies an unknown but safer range. Airplanes that could accelerate (and decelerate) fast enough to pass safely through the enormous supersonic room would have to behave like bullets or shells, leaving harmlessly behind them the sound waves their motion creates. But no one knows at present how to get over even the perilous threshold without suffering disaster.

Pessimists believe that ultra-high speed belongs to unmanned, guided missiles. Like the V-2 rockets, they will slam through the danger zone as quickly as possible, and come under radio control on the peaceful upper side.