— The X-51A, run by the US Air Force and DARPA, pushes air through its engine vertically (Illustration: Boeing)
Ultrafast missiles designed to strike targets around the globe will be the first technologies to use hypersonic flight, with early prototypes set to begin flight tests this year, according to plans announced by the US and Australia.
For decades, researchers have dreamed of building hypersonic aircraft that could whip around the world in just a few hours. But the first fruits of their labour will instead be compact, long-range missiles that can be launched at enemy targets at the same high speeds.
Missiles are easier to develop because "you don't need them to come back" and because they are about 10 times smaller than hypersonic aircraft would have to be, says Kevin Bowcutt, a senior hypersonics specialist at the US aerospace firm Boeing Phantom Works.
NASA set the record for hypersonic flight in 2004 with its X-43A test vehicle, which reached 9.6 times the speed of sound, or Mach 9.6. However, by then the agency had already pulled the plug on its planned follow-on vehicle so it could focus on reaching the Moon and Mars.
Now, the militaries of the US and Australia are leading the development of the high-speed technology. The projects include a test vehicle called the X-51A, run jointly by the US Air Force and the US Defense Advanced Research Projects Agency (DARPA), and a separate DARPA-led vehicle called Falcon. In addition, a joint US-Australian programme called HIFiRE (Hypersonic International Flight Research Experimentation) has just announced plans for 10 test flights over the next five years.
All are based on so-called scramjets (supersonic combustion ramjets), which mix fuel with air and ignite the mixture as it flows through the engine at supersonic speeds. This means the vehicles do not need to carry oxygen to ignite their fuel, as rockets do. And unlike jet plane engines, which use fans to compress air, they use no moving parts the shape of their air intakes sucks in and compresses air at supersonic speeds.
The engine for the X-51A uses the gas flow system tested in the X-43A and is being built by US firm Pratt and Whitney. That design uses a rectangular intake, which compresses the air and generates a shock wave that pushes the air through the engine vertically.
Boeing is building the vehicle, which will be mounted on a solid rocket booster and fired from a B-52 bomber flying at an altitude of about 10 kilometres. The scramjet will kick in after the rocket reaches Mach 4.5, boosting its speed to nearly Mach 7. The first flight tests are planned for mid- to late 2007.
The US Air Force and the Australian Defence Science and Technology Organisation are pushing a more advanced, 3-D gas flow system in their USD$54 million HIFiRe project. The goal is to compress the gas in three dimensions sideways as well as vertically using a round intake.
The extra dimension of compression allows "more shock waves and more efficient compression of the flow with lower losses", says Bill Lyons, manager of international research analysis at Boeing Phantom Works. The design should decrease atmospheric drag, allowing the vehicle to increase its range or the weight of its payload.
Ten tests of HIFiRE are planned to take place beginning late this year on the Woomera range in South Australia. Boeing, a partner in the project, announced on Wednesday that it would run three of the tests, beginning in early 2009. The programme hopes to demonstrate advanced scramjets capable of sustained flight faster than Mach 8.
DARPA's Falcon project is even more ambitious, with plans to reach Mach 15 to 20 speeds needed to reach and return from orbit. Its first test flights are scheduled for December 2008.
Still, practical hypersonic flight remains a long way off. The first US Air Force missiles are estimated to become operational in 2035, and unmanned aerial vehicles that could fly as far as 3000 kilometres are even further off. "We're pretty close to the point where we could design a missile, but we're not there," Lyons says. In the longer term, air-breathing hypersonic craft could cut the cost of reaching space.