hypersonics
facilities around the world all operate
under the same principle – air is
accelerated to extreme velocities around a
test article that is stationary. This is
convenient for observing the test article,
but introduces disturbances into the air,
some of which are acoustic and can have
an effect on the physics being measured.
“Other disturbances are chemical in
nature. When vehicles fly at hypersonic
velocities, the temperatures of the gas
surrounding them and the temperature of
the vehicle’s skin can become high
enough to cause the air to break down
chemically, while the outer surface of the
vehicle can ablate, melt or react with the
super-heated air.
“The chemistry of the flight
environment is very important. However,
34 MARCH 2020 \\ AEROSPACETESTINGINTERNATIONAL.COM
X-43 AND X-51
NASA awarded the contract to develop three X-43A Hyper-X aircraft
to MicroCraft in 1997. Designed to investigate air-breathing engine
technologies at hypersonic speeds, the X-43s were expected to reach
Mach 10. A combination of Pegasus booster and X-43A was carried
aloft by NB-52, the Pegasus rocket igniting after release to propel the
X-43 out to the high Mach required for its scramjet engine to function.
The first launch in June 2001 failed. Almost three years of
modifications and software work followed, before an X-43A was
accelerated to Mach 6.83 on 27 March 2004. The experiment had
proven the ability of the scramjet engine to exceed the X-43’s drag.
A third and final flight, on 16 November 2004, achieved Mach 9.6.
The scramjet proved capable of holding this speed, again proving a
mission objective.
The US Air Force returned to hypersonic flight with the X-51A
Waverider, built by Boeing. Four craft were commissioned, each
relying on a booster rocket to accelerate them to scramjet working
speed after release from a B-52H mothership. The X-51’s scramjet
was designed to run on standard jet fuel and while its investigations
into powerplant and airframe technologies were aimed at acquiring
knowledge for future space launch vehicles, it also had obvious
relevance to hypersonic missile development.
The initial X-51 launch, on 26 May 2010, achieved Mach 5 before
telemetry was lost and the aircraft was deliberately destroyed. On June
13, 2011, the second flight failed during the switch from ethylene to jet
fuel, while structural failure ended the third, 14 August 2012, mission.
The fourth and last machine was consumed during a 1 May 2013 flight,
during which it achieved and maintained Mach 5.1. Like the X-43, the
X-51 was a small, expendable crewless vehicle.
wind tunnels typically generate environments that are
either far too cold or much too hot, preventing the effects
of chemistry from being observed, or changing the air’s
composition before it reaches the test article.
“What’s exciting about performing hypersonic
testing using the two-stage light gas gun is that we are
able to environmentally control our flight range. We can
reproduce an acoustically and chemically clean flight
environment over a range of atmospheric altitudes.”
“We are continuing to develop this testing approach
to probe the physics of hypersonic flight conditions in
physically realistic flight conditions. We’re also
expanding the scientific instrumentation and
measurement techniques, so we can learn more about
the type of hypersonic flight conditions that could only
otherwise be observed in very expensive full-scale
flight tests.”
Nonetheless, the ultimate aim of hypersonic research
is to develop an understanding sufficient to enable the
reliable engineering of new-generation weapons systems
and perhaps ultimately, crewed or even passengercarrying
vehicles. SwRI’s system offers unprecedented
capability, but the limited size of its test subjects means it
will never replace full-scale flight trials. It’s a point
Mueschke happily acknowledges.
“We can use specialized imaging techniques, such as
schlieren imaging, and solo-luminescent images to
investigate boundary layer transition effects.
“We’re also using advanced diagnostics that measure
light emissions at different wavelengths to measure the
chemical species present in the environment around the
vehicle in flight.
“This data can be compared with complex
simulations that incorporate the chemical breakdown of
the air at extreme temperatures. One of the primary
uses for the facility is therefore to provide benchmark
data for computer simulations in the most physicallyrepresentative
environment possible.” \\
6 // The X-43, also known
as Hyper-X, for ‘Hypersonic
X-plane’ was a small,
single-use vehicle (Photo:
NASA/Tony Landis)
7 // Primarily a US Air Force
project, the X-51 never
achieved the outright speed
of the X-43A, but
demonstrated the ability to
run a scramjet on jet fuel
(Photo: US Air Force)
volume of hydrogen between itself and
the projectile. Once the hydrogen is
highly compressed, a rupture disk bursts
and the high-pressure hydrogen drives
the projectile down the barrel.”
For its large two-stage light gas gun
system, the distance from the breech –
where the gun powder is ignited – to the
muzzle – where the projectile leaves the
gun and begins to freely fly – is about 70ft
(21.34m). The system can launch
projectiles up to 1.5in (38mm) in diameter.
Using light gas gun systems, SwRI can
launch objects up to velocities of 7km/s
(15,700 mph, or Mach 21).
In the past the system has been used
to observe what happens when one piece
of material impacts another at extreme
velocity. But, SwRI’s largest light gas gun
has more recently been used to explore
aspects of hypersonic flight. Mueschke
says, “The numerous wind tunnel
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/AEROSPACETESTINGINTERNATIONAL.COM