A new arms race is underway between the US, China and Russia, aimed at developing hypersonic aircraft and missiles. Hypersonic vehicles are able to reach five times the speed of sound, or Mach 5, which is around 3,800mph.
Four flight tests of the X-51 WaveRider, the US's prototype hypersonic aircraft, have taken place. During the last one the aircraft flew more than 230 nautical miles at Mach 5. It was launched from under the wing of a B-52 bomber and travelled at a height of 70,000 ft.
Scientists at the Wright-Patterson base in Dayton, Ohio, believe that controllable hypersonic flight is now achievable. A senior US Air Force official says that America could have hypersonic fighter aircraft operating by 2040. "It's game-changing technology," said the official.
Meanwhile, both China and Russia are developing hypersonic weapons programmes of their own. China's vehicle has been labelled the Wu-14 by the US.
The Wu-14 has had four test flights to date, with the latest earlier this month. The last test took place amid ongoing territorial tensions between the US and China in the South China Sea.
The Russian variant, Yu-71, also had a test flight earlier this month – though that was unsuccessful according to Jane's Intelligence Review.
China and Russia are both attempting to develop missiles which can penetrate the US's ballistic missile defence systems. The missile defence systems are geared to intercept rockets in space, while hypersonic weapons would be able to evade them by travelling through the atmosphere with the capability to change course.
The US's own hypersonic project is motivated by the need to hit targets more quickly. Current weapons technology is relatively slow. The Tomahawk cruise missile is subsonic, travelling at 550mph and even the present generation of supersonic fighters, such as the F-22 Raptor, also lack the speed military chiefs want for rapid strikes.
The X-51 WaveRider and other hypersonic prototypes use Supersonic Combustion Ramjet Engines (Scramjet) technology. This means that their engines take oxygen directly from the atmosphere, rather than using a heavy on-board tank.