- A hypersonic wind tunnel that can test aircraft at 15
times the speed of sound is being designed by the US Air Force. It will
be used to help develop planes such as NASA's X-43a Scramjet, due to have
its first test flight next week. The X-43a has a top speed of Mach
10.
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- Existing wind tunnels can only test planes at speeds
of up to Mach 7. Machines called shock tubes can reach Mach 10, but only
for a few milliseconds, explains Richard Miles, an engineer at Princeton
University in New Jersey. So Miles and his colleague Garry Brown set about
finding another way to test planes at higher speeds.
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- The chief problem with shock tubes is that the air gets
very hot. "In order to achieve the right conditions you have to heat
the air to about 3000 kelvin," says Miles. But anything exposed to
such high temperatures melts. "That's one of the reasons why you can't
run the tests for long." The heat also produces pollution from
nitrogen
oxides, which can distort test results.
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- In the hypersonic tunnel, the researchers hope to
maintain
speeds over Mach 15 (18,000 kilometres per hour) for tens of seconds.
"The
key difference is that you start with a very, very high pressure and fairly
moderate temperature," says Robert Crook, deputy director of
technology
at the US Air Force's Arnold Engineering Development Center in
Tennessee.
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- A piston initially forces air into the tunnel at high
pressure. A single piston stroke lasts for 10 seconds, but by running four
or more pistons in sequence the tunnel could run for far longer. "If
you could operate them in Gatling-gun style then you could have a
continuous
process," says Crook.
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- The air enters a thin neck at the head of the tunnel,
where a high-energy electron beam is fired into the air against the stream.
This adds energy to the air, accelerating it (see Diagram). "The speed
you can get is dependent upon the energy you've got in the gas," says
Miles. "The electron beam is a way of transferring more energy into
the gas."
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- The beam will probably have a power of about 100
megawatts,
and building such a powerful beam will be a challenge in its own right,
says Crooks. "It's a lot of energy," agrees Miles, which is why
it is aimed directly into the air stream. Electromagnetic coils guide the
beam to keep it away from the tunnel walls. "If the beam were to hit
the side of the tunnel, it would vaporise it," Miles says.
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- At this point in the tunnel the air is travelling at
Mach 1. As it leaves the neck, the widening of the tunnel accelerates it
to around Mach 12. A second electron beam ionises the air and a powerful
magnetic field accelerates the ionised airflow to Mach 15.
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- The researchers plan to carry out the first tests of
the new design at Sandia National Laboratory in Albuquerque, New Mexico,
although they will use a relatively low- energy electron beam of 1
megawatt.
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- The X-43a has already been tested in a wind tunnel up
to Mach 7, according to Larry Huebner at NASA's Langley Research Center
in Virginia. One goal, says Huebner, is to see how the X-43a performs in
these tests compared with the millisecond shock-tube tests in polluted
air.
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