By WARREN E. LEARY The New York Times.

ASHINGTON, May 21 2004 — After more than four decades of promise and speculation, a new type of jet engine is about to power a small experimental plane at speeds previously reserved for rockets.

Early next month, the unpiloted plane called X-43A is to be shot to the edge of space on the nose of a rocket before cutting loose for a short dash on its own off the Pacific Coast at almost 5,000 miles per hour, seven times the speed of sound.

If successful, the flight will be the first of an air-breathing, no rocket plane at hypersonic speeds. This could lead to aircraft that can take people anywhere in the world within two hours or help boost cargoes into space at significantly lower costs, proponents say.

To reach such speeds, the X-43A uses an engine called a scramjet, which combines features of a conventional turbojet with those of a rocket. While the design and materials used to make regular jets limit the speed of aircraft to three or four times the speed of sound (2,000 to 3,000 m.p.h.), scramjets theoretically may push planes to speeds of 18,000 m.p.h.

If scramjets work as engineers predict, proponents say, it could bring an advance in aircraft propulsion equal to that of jet engines over motors driving propellers.

"This flight will make aviation history," said Joel Sitz, X-43 project manager at NASA's Dryden Flight Research Center in California, which is in charge of the test flights.

Three X-43A's are to make hypersonic test flights over 18 months, starting on June 2, from Edwards Air Force Base, Calif. Hypersonic speeds are those above Mach 5, or five times the speed of sound. Mach 5 is about a mile per second, or 3,600 m.p.h. at sea level. The first two X- 43A's are to try for Mach 7 and the last, Mach 10, about 7,000 m.p.h.

The aircraft are part of a six-year, $185 million program of the National Aeronautics and Space Administration called Hyper-X, intended to refine hypersonic design and ground testing and validate the results with flights. The program is being conducted jointly by NASA's Langley Research Center in Hampton, Va., which is in charge of design and ground testing, and Dryden.

The first hypersonic aircraft was the manned X-15, a rocket-powered craft that broke speed and altitude records more than 30 years ago. The air-breathing X-43A hopes to break the aircraft speed record of Mach 6.7 set by the X-15 in October 1967. The fastest air-breathing plane is the SR- 71 "Blackbird" jet, slightly faster than Mach 3, or 2,100 m.p.h.

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 slow and concentrate air. But ramjets, which have been used in military missiles, do not work unless the aircraft is already moving at high speed, usually with the initial assistance of a rocket. Ramjets are also limited to about Mach 6 because their combustion chambers overheat at higher speeds.

Scramjets, or supersonic-combustion ramjets, can attain higher speeds by reducing airflow compression at the entrance of the engine and letting it pass through at supersonic speeds. This reduces the temperature buildup in the combustion chamber, overcoming the limits of regular jets but requires a rapid and tricky mixing and burning of fuel and air.

Charles R. McClinton, technology manager for the Hyper-X program at Langley, said researchers have worked on scramjets for more than 40 years, building mountains of data from wind-tunnel and ground tests. Some early trials with limited prototypes led some people to believe that hypersonic engines would not produce enough thrust to overcome the atmospheric drag on the plane, Mr. McClinton said. "The X-43 flight is to prove scramjets, once and for all, will work and will move an airplane."

Scramjet-powered craft are also different from other airplanes because the engine and vehicle are integrated as one unit. The craft must be designed to capture large amounts of thin air in the upper atmosphere, Mr. McClinton said, and the shape of the vehicle must work like a giant air scoop.

The shock wave produced by the fast-moving aircraft helps guide the air into the engine, and high pressure, trapped by the shock, on the bottom of the vehicle provides lift, engineers said.

It has taken so long to develop scramjets because "the Apollo program came along, and there was a shift to rocket technology," said Griff Corpening, chief engineer for the X-43 at Dryden.

There was a resurgence of interest in scramjets when President Ronald Reagan announced the X-30 National Aero Space Plane, or NASP, project in 1986, intended to produce a scramjet-powered craft that would revolutionize air travel and go into space at 25 times the speed of sound.

NASP never flew because it tried to combine too many untried technologies into a test vehicle, Mr. Sitz and other experts said.

"We have taken NASP and chopped it up into more manageable chunks," Mr. Sitz said. "This gets us a reasonable, mature technology base and gives us more confidence to step up to a larger vehicle."

The X-43A is a 12-foot-long craft shaped like a flat blade with the engine sculpted into a smooth pod on its bottom. The 2,700-pound vehicle, made by Micro Craft of Tullahoma, Tenn., is 5 feet wide across its tail fins and made of aluminum and steel alloys, with a special heat-resistant carbon material on its leading edges to withstand temperatures expected to reach 2,200 degrees Fahrenheit.

The research craft is attached to a modified, air-launched Pegasus rocket booster made by Orbital Sciences of Dulles, Va. The flight plan calls for a modified B-52 bomber to drop the X-43A from 24,000 feet above the Navy's Pacific test range. The rocket is to accelerate the craft to 95,000 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 4,700 m.p.h.

Although engine burns of a few seconds may not seem significant, project engineers say the data will be vastly superior to any from wind tunnels and will show how the engine works under real conditions. More than 500 sensors will provide information about almost every aspect of the flight, and chase aircraft also will be collecting data, officials said.

Because of cost and complexity, there are no plans to recover any of the X-43's, which will be maneuvered to crash into the ocean.