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鈥檚 X-43a Scramjet, due to have its first test flight next
week. The X-43a has a top speed of Mach 10.
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.
The chief problem with shock tubes is that the air gets very hot. 鈥淚n order
to achieve the right conditions you have to heat the air to about 3000 kelvin,鈥�
says Miles. But anything exposed to such high temperatures melts. 鈥淭hat鈥檚 one of
the reasons why you can鈥檛 run the tests for long.鈥� 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. 鈥淭he key difference is that
you start with a very, very high pressure and fairly moderate temperature,鈥� says
Robert Crook, deputy director of technology at the US Air Force鈥檚 Arnold
Engineering Development Center in Tennessee.
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. 鈥淚f you could operate them in
Gatling-gun style then you could have a continuous process,鈥� says Crook.
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).
鈥淭he speed you can get is dependent upon the
energy you鈥檝e got in the gas,鈥� says Miles. 鈥淭he electron beam is a way of
transferring more energy into the gas.鈥�
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. 鈥淚t鈥檚 a lot
of energy,鈥� 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. 鈥淚f
the beam were to hit the side of the tunnel, it would vaporise it,鈥� Miles
says.FIG-mg22902901.JPG
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.
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.
The X-43a has already been tested in a wind tunnel up to Mach 7, according to
Larry Huebner at NASA鈥檚 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.