X-51A Waverider

Beyond supersonic speeds lies the realm of the hypersonic--faster than Mach 5.0, or in somewhat more down-to-earth terms, in excess of just under 4,000 miles per hour. On Tuesday, the U.S. Air Force plans to venture into that realm with the test flight of a chisel-shaped aircraft called the X-51A Waverider.

The test won't last long. The sleek, 14-foot-long X-51A will zip through the air for only about five minutes, but in that time it could hit a speed of Mach 6, or six times the speed of sound. It'll be pushed to that blink-of-an-eye tempo by what's known as a scramjet engine, "scramjet" being the compression of the phrase "supersonic combustion ramjet." This kind of engine is designed to burn oxygen from the atmosphere, unlike conventional rocket engines whose functioning requires a combination of oxidizers and big fuel tanks.

Update, Wednesday 7:42 a.m. PDT: Tuesday's planned flight of the X-51A was postponed by a day because of freighter traffic in the Pacific Ocean test area.

Update, Thursday 12:26 p.m. PDT: The X-51A flew later Wednesday morning. To find out how well it performed, see "X-51A races to hypersonic record."