The X-51A "Waverider" scramjet will fly at last next Tuesday, it has been announced. It had been expected that the radical engine - capable of Mach 6 velocities while running on fairly ordinary jet fuel - would fly last December, but budgetary and scheduling difficulties have delayed the project.

The X-51A is being run by the Air Force Research Laboratory and built by Boeing and Pratt & Whitney Rocketdyne. It will be dropped from a modified B-52 bomber operated by NASA next week, and a booster rocket will accelerate it to ignition speed (ramjets can't even fire up until they're going fast enough to cram suitable amounts of air into their front ends).

Unlike well-understood ordinary ramjets, which slow the air passing through them to subsonic speeds, the X-51 is intended to maintain combustion in a supersonic internal airflow - hence the name scramjet, for supersonic combustion ramjet - a feat often likened to "striking a match in a hurricane".

Having ignited, its designers hope that the Waverider will accelerate further to speeds of Mach 6 and fly for as long as five minutes before making its final dive into the Pacific. There are no plans to retrieve the aircraft: all data from the test will be gathered remotely.

"In those 300 seconds, we hope to learn more about hypersonic flight with a practical scramjet engine than all previous flight tests combined," says Charlie Brink, X-51 honcho at the Air Force Lab.

The previous longest scramjet flight, by NASA's X-43 in 2004, lasted only ten seconds. The X-43, too, burned hydrogen, which is not a good feature in aircraft as it requires enormous tanks for storage. Hydrogen scramjets offer higher speeds (Mach 10 has been achieved) but the X-51's ability to run on ordinary hydrocarbons has long been sought. Previous attempts along these lines haven't gone well, but the X-51 represents a new approach.

Brink said in a statement released yesterday that delays to the project had resulted from budgetary and scheduling problems - in particular the difficulty of getting a time slot on the Pacific range and a booking of NASA's B-52. Rather than an initial shot last December followed by three more this year, there will now be only one test before the new fiscal year.

"This is an experimental X-plane and it's a complicated test," says Brink. "We knew the original schedule was aggressive and we would need to be flexible," said Brink. "It's also expensive to keep a staff of engineers and support staff at the ready and then not be able to fly when supporting assets aren’t available. So we elected to make only one hypersonic try this spring and then pause for a few months to conserve funding."

If successful, the X-51 is expected to lead in the short term to new missiles able to strike faraway targets quickly and hit so fast as to be extremely difficult to defend against.

In the longer run, scramjet tech could be a key factor in access to orbit via reusable runway-takeoff spaceplanes. Rockets struggle to achieve this because they must carry oxidiser as well as fuel, reducing what they can lift, but a spaceplane which could burn air for part of its ascent would have improved performance.

Brink has said before that the X-51 project could allow for tests to see what the minimum ignition speed of the scramjet might be.

“The lower it can go, the lower the stress on a turbine,” he said last year, alluding to the possibility of an ordinary jet being used to shove the scramjet up to ignition speed - so opening the door to craft which could take off from a runway standing start without rocket boost. Ordinary turbojets are limited to around Mach 2.5 in normal use, though they can beat Mach 3 if the user doesn't mind replacing them afterwards.

"If the test flight meets even a subset of our expectations, the leap in engine technology will be the equivalent to the post-World War II leap from propellers to jet engines. It will be a historic event," enthused Boeing veep Alex Lopez. ®