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Scramjets - Boeing's X-51A rides the supersonic wave

15 August 2012




Successful tests of Boeing's X-51A Waverider aircraft have given new blood to the scramjet concept. Aerospace-technology.com examines the technology behind the lightweight, supersonic engines and uncovers what kind of a future they could hold for passenger aircraft.


On 26 May 2010, the X-51A completed its first flight, flying more than 200 seconds and reaching speeds of up to Mach 5

Lighter, faster and far-reaching aircraft have always fought to the forefront of the aerospace industry. The Supersonic Combustion Ramjet engine, thought to be capable of speeds in excess of five-times the speed of sound and even spaceflight, has long been theorised as a potential engine for future aircraft.

"Citing the possibility of cheaper access to outer space using scramjets, Nasa were instantly interested in their feasibility."

Now the X-51A Waverider, a product of cooperation between Boeing and Pratt & Whitney Rocketdyne, is beginning to prove that the use of scramjet engines can be more than just theoretical.

The programme has once again been thrust into the limelight following further testing of the X-51, which took place on 15 August 2012. Although the US military now appears to consider it as more of a testbed for future missile technologies, any success of reaching Mach 6 is highly likely to interest companies keen on developing hypersonic commercial aircraft such as EADS.

Measuring at just 26ft long and with an empty weight of just more than 1,800kg, the X-51A Waverider fits under the wing of its B-52 Stratofortress carrier as if it were a standard, conventional engine.

Its unassuming profile and sleek, aerodynamic design hide what promises to be an immensely powerful scramjet engine, feted as a possible answer to the future of spaceflight.

Supersonic airflow

Aircraft powered by scramjet engines are capable of such speeds due to the fact they work unlike standard engines. The combustion needed to power the craft occurs in supersonic airflow, reducing the need for a craft to carry liquid oxygen onboard to create thrust, as the aircraft absorbs oxygen from the surrounding atmosphere.

"Its unassuming profile and sleek, aerodynamic design hide what promises to be an immensely powerful scramjet engine."

Scramjets rely on the vehicle to compress and decelerate incoming oxygen before combustion, with supersonic airflow taking place throughout the entire engine. Scramjets grew to notoriety as theoretical projections placed their top speed between Mach 12 and Mach 24, a vast improvement on the Mach 3.2 cruising speed of the second fastest air-breathing aircraft, the SR-71 Blackbird.

Disregarding the theoretical speeds of scramjet engines, a range of potential benefits lead to Nasa considering them to be well worth investigating.

With no rotating parts, the engines are easier to manufacture and maintain and a higher change of momentum per unit of propellant could result in vastly increased specific impulses over conventional engines.

Citing the possibility of cheaper access to outer space using scramjets, Nasa were instantly interested in their feasibility.

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During the 1990s, the Air Force Research Laboratory (AFRL) began its HyTECH programme for hypersonic propulsion.

As a result, Pratt & Whitney received a contract to develop a hydrocarbon-fueled Supersonic Combustion Ramjet, or Scramjet, engine which would later be designated the SJX61 and designed for use upon Nasa's X-43 programme.

The initial version of the craft, the X-43A, was designed specifically to operate at speeds in excess of Mach 7, representative of around 8,000km/h at an altitude of 30,000m. The single-use vehicles, designed to crash into the ocean without recovery after use, achieved mix results.

After an initial failure and crash, the subsequent two vehicles were operational before intentionally crashing into the Pacific Ocean, with the second test setting a new free-flying air-breathing speed record. The third test of the X-43A, launched on November 16 2004, beat the record set by its predecessor and achieved a new speed record of 7,456mph, equivalent to Mach 9.8.

Following the tests, Nasa Dryden engineers had expected the programme to culminate in a two-stage-to-orbit crewed vehicle, ready for launch within 20 years.

Following the cancellation of the X-43C variant, the constructed engine was later applied to AFRL's Scramjet Engine Demonstration programme in 2003.

A switch of focus

The X-51 programme, a consortium comprising Boeing and Pratt & Whitney Rocketdyne, was tasked by AFRL and the Defense Advanced Research Projects Agency (DARPA) to demonstrate a scalable, robust endothermic hydrocarbon-fueled scramjet propulsion system.


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The scramjet flight test vehicle, designated X-51 on 27 September 2005, underwent ground tests of the engine and vehicle, which were completed in 2006, followed by wind tunnel testing in July 2006, with a first captive flight conducted in December 2009.

The X-51 is carried to 50,000ft by a B-52 Stratofortress and then released over the Pacific Ocean. The craft is propelled by a solid rocket booster until it reaches Mach 4.5 before the SJY61 scramjet engine is initiated, launching the craft towards speeds up to Mach 6.

On 26 May 2010, the X-51A completed its first flight, flying more than 200 seconds and reaching speeds of up to Mach 5. Although the craft failed to reach the speeds demonstrated by its predecessor, the X-51A managed to set a new record for total scramjet flight burn time of 140 seconds. A second test, held on 13 June 2011, was ended prematurely when the engine failed to transition to the JP7 fuel, used to power the engine into the high speeds.

Looking to the future

"Much hype has surrounded the possible use of scramjet engines within the aerospace market, particularly with the possible slashing of international flight times."

After attempting to restart several times to no avail, the vehicle was ditched into the ocean as originally planned.

Although technically a failure, the X-51A team was able to collect significant data from the test and Boeing assured the immediate future of the programme with the scheduling of two future test flights. One such test flight, taking place on 14 August 2012, hoped to validate the technology further by finally reaching speeds approaching Mach 6. The US military carried out the test, with the Pentagon eager to confirm the technology for use in the development of faster missiles, over the Pacific Ocean in a carbon copy of previous test measures.

Much hype has surrounded the possible use of scramjet engines within the aerospace market, particularly with the possible slashing of international flight times.

The prospect of flying from London to New York in less than an hour would have a significant impact on business, and the Institution of Mechanical Engineers has included such aircraft in its 'Aero 2075: Flying into a Bright Future?' report.

The report cited that due to developments in aircraft design, scramjet-powered aircraft could come into construction by the end of century and even take inspiration from the animal kingdom to slash flight times further.

Copying migrating geese to fly in a 'V-shaped' formation, aircraft could utilise the airflow of the aircraft in front to reduce flight-times and increase fuel efficiency.


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Artist conception of Nasa's X-43A.
Diagram showing scramjet propulsion.
Ground testing of the SJX61-2 scramjet engine.
The X-51A Waverider attached to the B-52 Stratofortress in 2009.