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An experimental unmanned jet called the X-43A is being readied for a dash at the edge of space on Saturday that could see it reach a speed of 5,000 miles per hour, seven times the speed of sound, in testing the concept of a scramjet engine. If testing is successful, NASA's 12-foot-long X-43A will become the first nonrocket, air-breathing plane to reach hypersonic speeds, a development engineers hope could lead to sending payloads into space much more cheaply or to aircraft that could whisk people to any point in the world within two hours.

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The flight plan calls for a modified NASA B-52 bomber to drop the 2,700-pound X-43A, attached to a booster rocket, from 40,000 feet above the Navy's Pacific test range off the California coast at 4 p.m. The rocket is to accelerate the craft, made of steel and aluminum alloys, to 100,000 feet and Mach 7 before the vehicles separate. Seconds later, the scramjet is to fire for 7 to 10 seconds and propel the X-43A about 5,000 m.p.h., after which the craft will conduct a series of high-speed maneuvers before crashing into the sea. There are no plans to recover the X-43A because of the expense that would be involved.

Researchers have tested scramjet, or supersonic-combustion ramjet, engines in laboratories for decades, but none have been flown successfully. Conventional turbojets work by concentrating air with fan-like blades in a compressor, combining it with fuel and burning the mixture to produce thrust. Faster speeds can be attained using ramjets, which forgo the compressor and use a specially shaped inlet to concentrate air for burning when moving at high speed.

Airplanes powered by scramjets, engineers believe, could travel at thousands of miles per hour. And space rockets, instead of carrying heavy liquid oxygen to burn with their fuel, could use scramjets to scoop it out of the atmosphere and carry more cargo into orbit.

The NASA factsheet on the program is here.