They call it a "scramjet," an engine so blindingly fast that it could carry an airplane from San Francisco to Washington, D.C., in about 20 minutes -- or even quicker. So fast it could put satellites in space. So fast it could drop a cruise missile on an enemy target, almost like shooting a rifle.
Next week, NASA plans to break the aircraft speed record for the second time in 71/2 months by flying its rocket-assisted X-43A scramjet craft 110,000 feet above the Pacific Ocean at speeds close to Mach 10 -- about 7,200 mph, or 10 times the speed of sound.
The flight will last perhaps 10 seconds and end with the pilotless aircraft plunging to a watery grave 850 miles off the California coast. But even if the X-43A doesn't set the record, it has already proved that the 40-year-old dream of "hypersonic" flight -- using air-breathing engines to reach speeds above Mach 5 (3,800 mph) -- has become reality.
Unlike rockets, which must carry oxygen with them as a "combustor" to ignite the fuel supply, scramjets take oxygen from the atmosphere, offering a huge savings in aircraft weight, and researchers around the world would like to take advantage.
In northeast Australia, a scramjet team funded by the U.S. and Australian armed forces will try for Mach 10 next year as a first step in using a scramjet to put satellites in space. The U.S. Air Force hopes to demonstrate within five years a scramjet-driven cruise missile fast enough to drive explosives deep into hardened targets. Other projects are moving forward in France and Japan.
Under NASA's $250 million Hyper-X program, engineers at Langley Research Center here and the Dryden Flight Research Center in Edwards, Calif., designed and built three aluminum scramjet aircraft, each one 12 feet long and weighing about 2,800 pounds. Controllers aborted the first test flight in 2001 after the rocket booster malfunctioned.
But the second, on March 24, reached Mach 6.83 (5,200 mph), shattering the world speed record for air-breathing, non-rocket aircraft, previously held by a jet-powered missile. The highest speeds by manned aircraft were achieved by SR-71, the U.S. spy plane known as the "Blackbird," capable of flying in excess of Mach 3 (2,300 mph).
"The idea was to demonstrate these technologies," said Luat T. Nguyen, deputy manager for the program that designed X-43A. "We've done that. This is the first scramjet to work, and it is the only one at this point."
Next week's third flight will test the limits of the X-43A. Temperatures will be significantly higher: The leading edge of the aircraft's nose will reach about 3,700 degrees Fahrenheit, 1,600 degrees hotter than during the March flight.
Also, Nguyen added, "it's more of a challenge to get it to operate at Mach 10 rather than Mach 7. You want it to be robust enough to give us the level of performance we're looking for, and at Mach 10, the constraints are a little narrower."
Regardless of the outcome, however, the third flight will be the last. The Hyper-X program ends, and there are no plans to replace it. The next steps are largely up to researchers elsewhere.
"What they have done is lay the foundation on how to go further," Allan Paull, of Australia's University of Queensland, the leader of the U.S.-Australia "HyShot" scramjet project, said of NASA. "We live in a society of high tech that's built over time. We went from the horse and cart to scramjets in 200 years."
Barring mechanical glitches or bad weather, the X-43A and its Pegasus rocket booster will leave Dryden between 9:30 a.m. and 2 p.m. Pacific time, slung below the belly of a B-52B launch aircraft.
About 50 miles off the California coast, the B-52 will drop the craft at an altitude of 40,000 feet. The booster rocket will ignite and bring the X-43A's speed close to Mach 10 at an altitude of 110,000 feet. At that point, controllers will fire two small pistons to jettison the rocket. Then they will open the cowl covering the X-43A's air intake and light the engine.
Scramjets work on the same principle as all jet engines -- igniting fuel in compressed air and aiming the expanding gases to the rear to propel the aircraft forward. Standard turbojets use fans to compress the air and can reach speeds of about Mach 2.2 (1,600 mph).
"Ramjets" can reach supersonic speeds of perhaps Mach 6 (4,600 mph) by using the plane's forward motion alone to bring air into the combustion chamber. But the air must be slowed to subsonic speed for ignition.
Scramjets (short for "supersonic combustion ramjets") are ramjets that ignite fuel in air traveling at supersonic speeds, a feat that NASA compares to "lighting a match in a hurricane." For this to work, virtually the entire aircraft becomes an enormous scoop, opening to receive the air and compressing it before injecting a chemical called silane, which ignites in the presence of air. The hydrogen fuel is added once the flame is lighted.
Neither a ramjet nor a scramjet can operate from a standing start. The Blackbird used a turbojet to reach high enough speeds for its ramjet to work. The X-43A uses the rocket, and Nguyen said Langley engineers predict the X-43A will reach a peak speed of Mach 9.6 or Mach 9.7 before it burns all its liquid hydrogen fuel and glides into the sea.
The X-43A will leave behind both a body of data and a practical demonstration of an idea that aeronautical engineers have worked on by fits and starts, through good and bad funding years, for more than four decades.
"They put together a well-thought-out experimental process, including ground tests, wind tunnel tests and flight," said Charlie Brink, scramjet program manager for the Air Force Research Laboratory at Ohio's Wright-Patterson Air Force Base. "The coordination of all this [ground and flight] data to see how it matched was spectacular. It provided a fundamental baseline."
The Air Force's cruise missile program, known as "HyTech," is developing a scramjet that burns hydrocarbon fuels -- easier to handle than liquid hydrogen. "The scramjet can travel hundreds of miles in minutes to defeat time-critical targets," said Bob Mercier, deputy for technology in the Air Force laboratory's aerospace propulsion division. "In addition, the high speed could improve penetration of hardened and deeply buried targets."
Australia's Paull said in a telephone interview that HyShot hopes to use scramjets to launch small satellites cheaply, inserting them as the second stage of three-stage launch vehicles. A rocket would get the spacecraft to scramjet speed, and a third rocket stage would propel it once it gets above Earth's atmosphere.
"Putting something into space and making it stay there are two different things," Paull said.
It takes a speed of 25,000 mph to escape the pull of Earth's gravity and get into orbit, "and we'd like to get 18,000 [mph] from a scramjet," he added. "Can we do it? I don't know the answer. If it doesn't work out, we'll just say, 'A rocket's the best you can do, mate,' and pack it up."