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X-51 ordinary-fuel scramjet to fly in December

Could offer scope for combo turbo/scram runway jets

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Flight tests of a powerful US military scramjet prototype able to exceed Mach 4 while burning ordinary jet fuel have been scheduled for December, according to reports.

Aviation Week reports today that the X-51 "WaveRider" scramjet will now fly in December. The X-51 will be released from a B-52 bomber mothership operated by NASA above southern California. A modified missile booster rocket will accelerate the test stack past Mach 4, after which the 25-foot-long X-51 will ignite using ethylene lighter fluid, easier to get burning in a very fast airstream.

The purpose of scramjets is to beat the speed limitations of ordinary ramjets, which slow their internal air intake stream down to subsonic speeds. Unfortunately this produces unacceptable levels of drag and fuel consumption above Mach 3 to 4, hence the desire for a supersonic-combustion ramjet - a scramjet.

To date, most successful scramjet tests have been carried out using hydrogen fuel, which is relatively easy to keep burning in a supersonic airflow. But hydrogen is expensive and dangerous to handle in large quantities, besides taking up enormous storage volumes - any hydrogen fuelled aircraft or missile would be mainly fuel tank.

In the X-51, regular JP-7 jet fuel will swiftly replace the initial ignition stream of burning ethylene. This will permit WaveRider type engines to propel much more practical aerospace craft or missiles, if it works. After flying for several minutes at a speed of almost 5000 mph (in excess of Mach 6), the X-51 test vehicles will splash down into the Pacific.

According to Charlie Brink of the US Air Force Research Laboratory, quoted by Av Week, the planned four X-51 test shots could allow scope for trying to ignite the scramjet at lower speeds and temperatures.

“The lower it can go, the lower the stress on a turbine,” said Brink.

A major aspiration of scramjet designers is to achieve a design which could be brought up to ignition speed by an ordinary jet engine rather than a booster rocket. This would potentially allow the construction of reusable aircraft able to take off and land from runways in normal turbojet mode, then accelerate to hypersonic speed once airborne. Quite apart from military uses - perhaps a successor to the legendary Mach-3.5 Cold War turbo/ramjet "Blackbird" spyplane - such craft might develop into much cheaper access to space.

The Pentagon formerly had an ambitious plan for a "Blackswift" runway plane able to do a barrel roll at Mach 6, but it was cancelled last year. Now the hopes of hypersonics fanciers are riding mainly on the X-51 and - perhaps in future - the "Project Vulcan" bomb/jet scheme. ®

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