Breaking the speed limit: New Air Force Wind Tunnel
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.
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. “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.
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.
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.
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.”
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.
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’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.
Author: Duncan Graham-Rowe
New Scientist issue: 12th May 2001
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