An unmanned supersonic jet shattered the speed record for plane flight by flying at more than seven times the speed of sound on Saturday.
NASA's X-43A plane flew high above the Pacific for 11 seconds using an engine which expels hot exhausts to propel it forward like a rocket, before gliding for several minutes and plunging into the ocean. It was the first successful demonstration of acceleration by a supersonic combustion ramjet or "scramjet" engine.
"The scramjet is the holy grail of aeronautics" because it promises the most efficient path from the ground to space, says Joel Sitz, HyperX project manager at the NASA Dryden Flight Research Center in Edwards, California.
Unlike a rocket engine which carries its own oxygen as well as hydrogen fuel, the scramjet engine carries hydrogen fuel only and can scoop oxygen up from the air - making it potentially highly efficient.
For the test flight, the 3.7 metre X-43A was mounted on a Pegasus rocket which was carried to 12 kilometres by a B-52 bomber. At this altitude the Pegasus ignited and soared to 27 kilometres, where it released the X-43A at hypersonic velocity.
Vents on the X-43A opened, scooping up air and for 11 seconds the engine burned hydrogen fuel. Although most of the craft's speed came from the Pegasus, preliminary data shows that its own engine accelerated it further - a key goal. The announcement prompted a whoop of glee from NASA staff at a briefing two hours after the flight.
The test was "simple in concept, but very very challenging in its implementation," flight chief engineer Griff Corpening of NASA Dryden said at a preflight briefing.
The first flight in the $230 million HyperX program failed in June 2001 when the aerodynamic load from the test vehicle sent the Pegasus carrying it off-course as it passed through the sound barrier.
This time engineers were delighted as the flight went without a hitch, says mission controller Brad Neal of Dryden. "We've been rehearsing this for three years," he says.
The concept of a scramjet engine is 40 years old. It a variation on the ramjet, which scoops up oxygen out of the air to burn fuel and is pushed by hot gases expanding out the exhaust. The key difference is that gas flow is slowed to subsonic in a ramjet, but remains supersonic inside the scramjet.
Scramjet engines have been tested before, but only in wind tunnels where engines could burn for only milliseconds, and were not integrated with a vehicle. Although the X-43A burned only eleven seconds, that brief interval generated enormous amounts of aerodynamic data and validated six years of work on the ground.
NASA has one more test vehicle, which it plans to fly at ten times the speed of sound. But Vincent Rausch, program manager at NASA Langley, says that scramjets able to carry crews are 20 to 25 years away. That is too far away for the space agency's plans to go to the Moon and Mars, so plans to build a larger X-43C test vehicle were scrapped on 18 March.
"We are looking at what to do next," Rausch said at a preflight press conference. The US Air Force has its own hypersonic program, but whatever NASA can salvage is likely to stay on the ground.
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