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.
NASA hopes to break the aircraft speed record again with its 12-foot-long X-43A.
How the Scramjet Works: Supersonic air compression in the Scramjet engine boosts thrust exponentially over conventional jet engines.
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."