Wings Magazine

Boeing’s X-51A rides the supersonic wave

Aug. 17, 2012, Chicago, Il. - Successful tests of Boeing's X-51A Waverider aircraft have given new blood to the scramjet concept. examines the technology behind the lightweight, supersonic engines and uncovers what kind of a future they could hold for passenger aircraft.

August 17, 2012  By

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.

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

B-52 Stratofortress carrier as if it were a standard, conventional


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.

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

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.

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.


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


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.


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


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.


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


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

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.


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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