Supersonic Raindrops

Physicists have long known that raindrops--at high speeds--pack a dangerous wallop. But their effect on aircraft remained mainly a theoretical problem until jets started flying at the speed of sound. After passing through rain squalls at a supersonic clip, new jet fighters returned to base peppered with pits and abrasions. U.S. Air Force engineers have now begun to reckon with rain.

Last year Convair's propulsion-engineering laboratory was assigned to test the raindrop effect. On a U.S. Navy firing range outside San Diego, Convair's engineers developed a simple but effective experiment. To approximate supersonic flight, test pellets of aircraft materials (e.g., light metals, plastics, fabrics) were fired from a standard 20-mm. cannon through a "rainstorm" produced by a 500-ft. series of sprinklers. The pellets' speed was kept constant--1,520 m.p.h.--and a parachute, timed to open after 1,500 ft., brought the projectile to earth.

After six months of shooting, the engineers found that very soft metals showed erosion after one flight, harder metals were pitted only after repeated firings (the equivalent of prolonged flight through rain squalls). For the Air Force, the evidence is sufficient warning that, with individual plane parts worth as much as $20,000, rain damage may ground a supersonic jet for costly repairs.

More important, say Convair's experts, is the possibility that raindrops can puncture a jet's fuselage or cockpit blister, causing the pressurized cabin to explode at high altitudes. At 1,520 m.p.h. (Mach 2), a raindrop smashes into a plane with a force of 70,000 Ibs. per sq. in. At higher speeds, raindrops may be as deadly as enemy bullets.

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