Leak Locator: Ultrasound for finding holes in spacecraft

After the International Space Station started losing air pressure in January 2004, astronauts hunted for 2 weeks before finding the leak near a porthole. Though it worried mission controllers, the leak never endangered the people on board.

Since then, NASA’s been searching for a fast way to find such leaks. Now, NASA-funded researchers have discovered that listening for high-frequency sound waves rippling through a spacecraft’s hull can do that job.

The metal skin of the International Space Station is constantly threatened by micrometeorites and by debris from previously launched satellites and spacecraft. Zipping toward the station as fast as 15 kilometers per second, even a millimeter-size fragment can punch a hole through the hull.

The handheld, industrial-type leak detector that astronauts currently rely on detects hissing sounds in the air. When there’s a leak into the surrounding vacuum, however, most of the sound is whisked out of the spacecraft with the air.

Stephen D. Holland and his team at Iowa State University in Ames focused on the sound that gets trapped inside the aluminum skin of the spacecraft. Their new leak-detection system employs microphones attached to various points on the metal surface. When a computer combines the data from each microphone, patterns emerge that pinpoint a leak. Both the old and new detectors listen at ultrasound frequencies, which are higher than the limits of human hearing.

In each trial, Holland’s group drilled a millimeter-wide hole in an aluminum sheet similar to the sheets that make up the hull of the space station. The researchers then created a vacuum on one side of the sheet. With their system of microphones in various configurations, they located the leak in every test.

“The technique appears to work quite well,” says acoustics researcher Todd W. Murray of Boston University. “It seems that the next task will be in outfitting a spacecraft with a sensor system.” He points out that reflections of ultrasound near edges could make the processing more difficult.

Holland’s group has developed several variations on the skin-listening approach, using anywhere from 2 to 256 microphones, and the researchers are testing which arrangement is best suited for installation on the space station. Sophisticated leak-detection systems of this type could also be built into future spacecraft, Holland says.

The research appears in the April Acoustics Research Letters Online and the April 25 issue of Applied Physics Letters.

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