Blunt v. Ablative

The body of a long-range missile lives only for its nose. Once shot into space, the nose, with its payload of thermonuclear explosive, speeds on alone, and its problem becomes re-entry into the atmosphere. U.S. missilemen need nose cones that will not burn up from friction as they plummet earthward in a long arc at up to 16,000 m.p.h.

This month, when the Air Force's Atlas sped 2,500 miles over the Atlantic, pictures of its virtually blunt nose seemed strange to the streamline-minded. But current Atlas and Thor noses are likely to stay blunt for good reason. Developed by General Electric, they are made mainly of heavy copper, which helpfully spreads and diffuses the heat. But the main design trick is to keep the nose from ever getting too hot. The bluntness creates a shielding shock wave out front that cuts the velocity of the air actually hitting the nose to subsonic speed, then slows the missile to around 500 m.p.h. Instead of evaporating in more than 10,000DEG re-entry heat, as a sharp-nosed metal warhead might, it descends at a cool 2,000DEG.

But this tidy re-entry solution may soon be due for obsolescence. The trouble is that a blunt nose falls too slowly to evade sophisticated countermissiles, and even gusty winds can mess up its accuracy. Future nose cones will have to fall much faster.

Despite its proneness to bluntness, the Air Force last month successfully fired a Thor-Able missile with a faster-flying new nose of the type developed by the Army on its Jupiter IRBM. This one is more classically sharp. Instead of absorbing and avoiding heat, it removes heat by "ablation." Technique: coating the nose surface with materials that melt or vaporize while absorbing heat, yet leave the material underneath cool and undamaged. The best materials seem to be polymer plastics, mixed with fibers of glass.

Last week the Air Force was highly enthusiastic about this concept. The beauty of ablating materials is the lightness that they allow in a nose cone. A solid-fuel missile like the projected Air Force Minuteman ICBM (due in 1963) would be badly overloaded with a heavy copper nose. Now the Minuteman will reportedly get a sharper, ablative nose, as may later advanced versions of the liquid-fuel Atlas and Titan, thus returning advanced missilery to orthodox streamlining.

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