When an experimental NASA aircraft streaked across the sky last weekend at Mach 7 -- seven times the speed of sound -- those of us watching it on television worldwide probably all had the same response: We did some quick math.

The data were impressive. For about 11 seconds, NASA's 3.7-meter-long unmanned X-43A aircraft traveled at about 8,000 km per hour. To put that in perspective, NASA engineer Lawrence Huebner invoked the Wright brothers: "A little over 100 years ago a couple of guys from Ohio flew for 40 meters in the first controlled powered flight," Huebner said. "Today, we did something very similar in the same amount of time, but our vehicle . . . went over 24 kilometers."

Even we armchair-bound nonengineers could figure out that at such a speed it would be possible to fly from London to Sydney, one of the world's longest passenger routes, in about two hours. Better yet for Japanese viewers was the prospect of a Tokyo-New York hop of just 70 minutes. Of course, the ticket would cost millions, the fuselage would be so hot that the plane couldn't have any windows, making the flight a bit claustrophobic, and that plunge into the ocean was disturbing, but . . . a traveler can dream, right? Unfortunately, dreaming of warp speed is about all we ordinary folk can do, at least for the foreseeable future. Despite predictions last week that hypersonic flight will revolutionize both commercial and space travel, the reality, according to scientists and defense and industry consultants, is that it is likely to prove useful mainly for military purposes. The sad, short history of the Concorde bears this out: If supersonic passenger planes were unsustainable, what are the chances that an infinitely more expensive and complicated hypersonic version can succeed commercially?