Finding leaks in a spacecraft
With financial support from NASA, Iowa State University (ISU) engineers have developed a sensor to quickly find leaks in a spacecraft. This sensor locates an air leak by listening to the noise generated by the air rushing out of the leak and includes an array of 64 elements that detects vibrations as they radiate along the spacecraft. Because astronauts cannot hear the noise caused by escaping air, NASA needed to design a system to help them. As one ISU researcher said, "NASA wants to be able to find these leaks. Fixing them is easy. But the question is, 'Where is the leak?'" Now that this sensor has successfully been tested on the ground, NASA is evaluating a proposal to build a prototype of the leak detection system for future missions.
You can see above a prototype of this sensor (Credit: ISU) This image was extracted from a video released on August 8, 2007 and produced by ISU's Aerospace Engineering Department undergraduate students Ricky Reusser and Steve Sulhoff. It shows "the students attaching the transducer to the simulated spacecraft skin, taking data with a computer, and processing the data in real-time to demonstrate the accuracy and sensitivity of the method." Here is a link to this movie (MPEG format, 2 minutes and 35 seconds, 38.9 MB).
This project has been led by Dale Chimenti, an Iowa State University professor of aerospace engineering (more details here) and several of his colleagues at ISU's Center for Nondestructive Evaluation (CNDE).
Here are some details about this sensor. "The different elements pick up the vibrations at different times. Those differences can be analyzed by a computer to determine the direction of the leak. Add information from multiple sensors and a leak can be found in about a minute. It can take weeks to find a leak with NASA's current handheld, ultrasonic detection devices. But it wasn't easy to improve the technology. Chimenti said a major challenge was accounting for a reinforcing grid that rises from the back of a spacecraft's shell and affects the travel of vibrations. He said the researchers solved the puzzle by targeting the frequency range measured by the sensors. They focus on lower frequencies and that reduces the effects of the reinforcing grid."
In a previous news release published on August 14, 2007, the CNDE provides more details about the technology and how it works. "The CNDE method holds significant promise over other proposed methods because it does not require listening for noise from the propagated through the air of the spacecraft. On the Space Station the walls ceiling and floor are covered with instruments, storage areas, and life-support systems. The trick is to locate the leak sufficiently well to permit astronauts to decide which instrument rack to empty in order to find the leak. Once found, most small leaks (less than 5 mm diameter) can be easily fixed with a NASA-approved patch kit. Because of all the space debris and danger of micrometeorites in low Earth orbit, NASA is concerned that a leak to the pressure vessel of a long-endurance spacecraft will occur. (Several impacts have occurred, but none yet have pierced the pressure vessel where the astronauts live and work.)"
And the CNDE adds that the method used by its researchers "unusual because it exploits a random noise signal (from the leak itself) to perform the location. To accomplish this, the random signal must first be changed to a deterministic signal, done by monitoring two channels of a 64-element acoustic array. One element acts as the reference and a cross-correlation operation between that element and each of the others in the array effectively eliminates the random nature of the noise signal. Performing a two-dimensional spatial Fourier transform on the array data and summing over a predetermined frequency range yields a bright spot in two-dimensional wavenumber space corresponding to the direction of the leak signal, and hence the leak itself."
Now that this sensor has demonstrated that it would be able to detect leaks in spacecrafts, what will happen? Chimenti will submit a joint proposal for Phase II funding from NASA's Small Business Technology Transfer Program with Invocon Inc., a company based in Conroe, Texas, and which already built several electronic sensors for NASA. If NASA funds that proposal, Iowa State and Invocon researchers will develop a prototype of the leak detection system.
Sources: Iowa State University news service, Otober 2, 2007; and various websites
You'll find related stories by following the links below.