U.S. Developing Jets That Fly Five Times the Speed of Sound

John Roach
for National Geographic News
March 14, 2007
The U.S. Air Force is preparing to test a new vehicle that could make missiles—and someday, jets—travel ten times faster than those flown today, military officials say.

The research vehicle, known as the X-51A, will be able reach hypersonic speeds when it is tested in 2009.

Hypersonic speeds are above Mach 5—faster than five times the speed of sound.

"This could significantly change an operation's tempo," said Bob Mercier, deputy for technology in the aerospace propulsion division at the Air Force Research Laboratory in Ohio.

A cruise missile today takes about 90 minutes to reach a target located 600 nautical miles (1,100 kilometers) away. A hypersonic cruise missile using the X-51A would reach its target in 10 minutes.

"The military obviously has a need for speed," said Paul Reukauf, a hypersonic technology expert at NASA's Dryden Flight Research Center in Edwards, California.

Hypersonic Speed

Flight engineers define three categories of speed: subsonic, supersonic, and hypersonic. The way air flows around the aircraft distinguishes the categories, Reukauf explained.

At subsonic speed, which is below the speed of sound, shock waves are absent. At supersonic speed, shock waves form on the aircraft as it flies through the air. As the air pressure rises through these waves, a sonic boom is generated.

(See a photo of a jet's "sonic boom cloud.")

At hypersonic speeds, the shock waves form very close to the aircraft, and engineers are developing ways to harness the power of these waves.

"The lift and drag and performance of the airplane can essentially be explained … by the resulting forces of the molecules of the air hitting the airplane," Reukauf said.

Rocket-propelled aircraft, such as the space shuttle, routinely reach hypersonic speeds on return from space.

Reukauf and his colleagues built the first demonstration aircraft that reached hypersonic speeds within the Earth's atmosphere using an engine called a scramjet.

A scramjet, or supersonic combustion ramjet, uses the shock waves generated in hypersonic flight to compress air into the plane's engine.

"You then combust the fuel and use the whole afterbody of the airplane to act as the nozzle to expand the exhaust and create thrust," Reukauf said.

This process requires no moving parts, which is important since at speeds greater than Mach 5, temperatures reach a scorching 4,000 to 5,000 degrees Fahrenheit [2,200 to 2,760 degrees Celsius].

Hypersonic technology allows a less expensive, more efficient way to get into space, Reukauf noted.

Rocket-propelled spacecraft must carry their own fuel and oxidizer—which releases oxygen to burn the fuel—onboard, he explained.

The space shuttle, for example, carries about eight pounds (3.8 kilograms) of oxidizer for every pound (0.5 kilogram) of hydrogen fuel.

If a shuttle could instead get the oxygen from the air as the scramjet does, the shuttle would only need to carry enough oxidizer for the final push out of Earth's atmosphere.

"That would allow you to carry a larger payload or consequently make a smaller vehicle for the same size payload," Reukauf said.

(See a photo gallery about the future of flight.)

Technology Transfer

Reukauf and his NASA colleagues twice successfully flight tested an unmanned scramjet called the X-43A in 2004, setting a world speed record.

The experimental craft was carried to 40,000 feet (12,000 meters) aboard a B-52. A booster rocket then propelled the scramjet to 95,000 feet (29,000 meters).

The scramjet separated from the booster and flew for about 10 seconds at Mach 7—4,800 miles (7,700 kilometers) an hour.

In the second test, the plane topped out at nearly Mach 10—7,000 miles (11,000 kilometers) an hour and is noted in the 2006 edition of the Guinness Book of World Records.

Mercier, of the U.S Air Force Research Laboratory, said the military is using the tools and techniques from these tests to develop the X-51A and other hypersonic technologies.

The main difference in the new vehicle will be the fuel, he said.

NASA uses highly combustible hydrogen fuel. The military is developing a scramjet that uses hydrocarbon fuels, which are easier and safer to package but burn less efficiently.

"Hydrogen would not be a fuel of choice for a weapon to go on the wing of a fighter jet that's going into a combat situation," Mercier said.

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