In the last week of May, thousands of square miles of airspace above the Pacific Ocean will be cleared to make way for a skinny, shark-nosed aircraft called the X-51.
The 4-metre-long prototype will drop from beneath the wing of a bomber and attempt to become the first scramjet to punch through the atmosphere at hypersonic speeds for minutes, not seconds.
Like an airliner鈥檚 jet engines, supersonic combustion ramjets 鈥� or scramjets 鈥� work by compressing air enough to ignite fuel which drives air out of the back of the engine to provide thrust. It is designed to work at hypersonic speeds 鈥� above about 5 times the speed of sound.
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A handful of experimental scramjets have flown successfully, reaching speeds as high as Mach聽10, but not for long. 鈥淣o one has successfully flown a vehicle of this nature for more than a few seconds,鈥� says Joe Vogel, X-51 programme manager at Boeing. 鈥淥ur goal is about 300聽seconds of powered flight.鈥�
The project is a collaboration between several US military agencies and private firms like Boeing that have ideas about how to solve the problems with heat and manoeuvrability that have limited previous scramjet flights.
Hot stuff
At the test鈥檚 top speed of Mach聽6, the temperature of X-51鈥檚 nose will reach 1480聽掳C, says Vogel. To handle the heat, the vehicle鈥檚 fuel is piped through tubes around the surface of the engine. Not only does that draw off heat to prevent the engine from melting, it also helps warm the fuel to the temperature needed to ignite it.
The US air force hopes to conduct as many as four tests of the X-51 this year. In each, a vehicle will be dropped from beneath a B-52 bomber some 15 kilometres above the Pacific Ocean. A solid rocket booster at the back of the vehicle will ignite and accelerate the X-51 to 4.7 times the speed of sound.
That鈥檚 fast enough for the craft鈥檚 scramjet to kick in. The booster will fall away to let the X-51 fly under its own power until it runs out of fuel. 鈥淚f we could recover it, the engine would basically be pristine and reusable,鈥� Vogel says.
Short hop
The last US hypersonic scramjet to fly successfully was NASA鈥檚 X-43, a hydrogen-powered engine that flew twice in 2004, managing only 10 seconds of powered flight. Unlike its predecessor, the X-51鈥檚 engine uses novel active cooling systems and uses standard jet fuel.
Scramjets are touted as a cheap way to get most of the way into space because they don鈥檛 require the bulky oxidisers needed to get conventional rockets into orbit. The X-51 is likely to advance that dream, says of the University of Maryland in College Park. 鈥淚t solves a lot of the practical issues you need to solve in order to make a real space vehicle.鈥�
Significant hurdles remain, though, not least that to reach orbit a scramjet would have to operate at roughly 25 times the speed of sound. In the short term, the X-51 is likely to pave the way for fast reconnaissance planes and mid-range missiles, Lewis says.
