X-51 Wave Rider Unmanned Scramjet Aircraft, USA

The X-51 is an unmanned scramjet demonstration aircraft designed and developed by the consortium of Defence Advanced Research Project Agency (DARPA), Boeing Integrated Defence Systems, Pratt & Whitney Rocketdyne and Nasa. The aircraft was developed to meet the requirements of the US Air Force as part of the WaveRider programme. The programme envisages design and development of demonstrator aircraft for the US Air Force.

The X-51 is principally used to demonstrate the speeds and various aspects of hypersonic flight. It is developed to demonstrate a reliable system adequate to jet fuel and accelerate through multiple Mach numbers. The maiden flight of the aircraft is expected to take place in February 2010.

Fuelled by military fuel JP-7, the scramjet engine of X-51 is designed to run for 5min (300s), while the unmanned experimental supersonic aircraft X-43 is fuelled by hydrogen and runs for just 100s.

Scramjet demonstration aircraft programme

The US Air Force Research Laboratory (AFRL) launched the endothermically fuelled scramjet engine flight demonstrator (EFSEFD) programme in early 2003. The programme envisages the development of demonstrator aircraft series X-43 and X-51. It was later renamed scramjet engine demonstrator – WaveRider (SED-WR) and the vehicle was officially named X-51 in September 2005.

Researching the air-breathing system-level potential of scramjets through multiple flight tests will cost around $7.7m.

X-51 demonstration aircraft development

The X-51 is being developed to operate at speeds of more than Mach 7 (8,575km/h) at an altitude of 50,000ft. The number of flight tests was increased by AFRL from four to six, with flights at four to six-week intervals, assuming no failures.

On 14 December 2009, the first captive flight was carried on the B-52 bomber aircraft from Edwards Air Force Base with the unmanned craft snugged under a wing. X-51's next flight test is scheduled for January 2010, during which only telemetry support and systems will be examined.

During the third flight test, scheduled for February 2010, the B-52 bomber aircraft will carry the Scramjet demonstration aircraft to 50,000ft above the Pacific Ocean, from where it will be released. An army tactical missile system (ATACMS) solid-rocket booster will then burn and propel the X-51 to achieve Mach 4.5 speed.

Following this, the booster will be thrown away and the X-51A's SJY61 supersonic combustion ramjet propulsion system will ignite the ramjet engine and impel the vehicle to fly at speeds of Mach 6.

Wind tunnel tests and modelling will be performed during this test to assure the safe separation of the vehicle from B-52.

The on-board sensors equipped in the vehicle transmit data to ground systems will be located at the Edwards Air Force base before being immersed into the Pacific Ocean.

X-51 scramjet aircraft design

The X-51 is designed as a missile-size demonstration aircraft to showcase scramjet operation from Mach 4.5 to Mach 6. The SED-WR programme successfully completed the preliminary design review (PDR) in December 2004, and in January 2005, a contract was awarded to execute the detailed design phase of the X-51. The critical design review (CDR) of the vehicle, completed in January 2007, cost around $70m.

The lifting body design provides the vehicle with a substantial amount of lift for flights without being dependent on the wings.

The aircraft weighs about 1,814kg and is designed to be fully controlled in supersonic speeds, even when gliding without propulsion. Upon completion of the test, it will crash in the Pacific Ocean and will not be recovered.

When the X-51 travels at high speeds, the increased heat can damage the metal portions of the airframe by melting them. In order to avoid damage, the hypersonic combustion is used to produce cycling water behind the engine cowl and sidewall edges, thereby cooling the surfaces. As a result, the engine is maintained at the required operating temperature.

X-51 turbine engine

The X-51 is powered by Pratt & Whitney Rocketdyne's hydrocarbon (JP-7) fuel-cooled or SJY61-2 scramjet engine. The conventional turbine engines are used to achieving high-speed flight research at supersonic speeds.

When the engine is ignited, a mixture of both ethylene and JP-7 is burnt before switching solely to JP-7 fuel.