Photonics Inspects Space Shuttle
Before starting out on a long trip, travelers examine their vehicle to make sure there will be no problems, particularly important when the mode of transportation is as complex as the space shuttle and the destination as exotic as space. For the shuttle’s return to service with the Discovery mission in July, the imperative to give everything a thorough going over carried extra weight, courtesy of the panel that was formed in response to the loss of the shuttle Columbia in February 2003.
Following the Columbia accident, NASA increased its nondestructive evaluation capabilities. To identify delaminations and other defects in the foam insulation on the space shuttle’s external fuel tank, the agency selected terahertz imaging. In this image of a test sample, purposely introduced defects are highlighted. Courtesy of Picometrix LLC.
“The Columbia Accident Investigation Board felt that it was important enough to make numerous recommendations,” said Eric Madaras, a senior research scientist at NASA’s Langley Research Center in Hampton, Va. He explained that the suggestions called for NASA to increase its nondestructive evaluation capabilities, especially with regard to the foam that coats the spacecraft’s external fuel tank and the tiles that protect the shuttle from the heat of re-entry. For that, the agency and its contractors turned to a variety of photonic-based imaging systems.
In the case of the foam, NASA wanted to peer below the surface to find delaminations, voids and other defects. After evaluating various nondestructive testing approaches, Madaras said, the agency picked two: backscatter x-ray and terahertz imaging, which employs photons with wavelengths that lie between the microwave and the infrared.
The orbiter boom sensor system features optical instruments, including a laser scanner, for the inspection of the spacecraft’s tile heat shield. Courtesy of NASA.
Picometrix LLC, an Ann Arbor, Mich.-based subsidiary of Advanced Photonix Inc., supplied the terahertz imaging systems that NASA used for the nondestructive evaluation of the foam on Discovery’s fuel tank. Given the size and shape of what had to be inspected, this presented a challenge. The foam, which can be up to 12 in. thick, was sprayed on at NASA’s Michoud Assembly Facility in New Orleans, covering a tank 153 ft long and 27 ft in diameter.
Rob Risser, president and general manager of Picometrix and chief financial officer of Advanced Photonix, noted that terahertz signals require extreme precision between the pulsed laser source and the semiconductor elements that generate and detect the radiation -- with an alignment accuracy of better than 1 µm. Because dragging the laser around the tank is not an option, Picometrix developed a 30-m-long flexible cable that transported both light and electrical signals.
As for the results, the good news, Madaras said, was that the evaluation methods developed for the foam worked. No critical voids were found, and the foam did not come off at altitudes that would cause damage. This indicated that the void problem thought to be the source of the Columbia disaster had been corrected.
Nevertheless, the foam still came off Discovery’s tank on launch for reasons that are not yet clear. Once a probable cause has been found, it will be addressed. This might include adapting the nondestructive evaluation methods.
The tile was inspected in space on the Discovery mission using instruments on the orbiter boom sensor system. Tim Fisher, lead system engineer for the instrument cluster at NASA’s Johnson Space Center in Houston, said that the choice was to go with two optical systems that had proved themselves in the past.
The first was the laser dynamic range imager from Sandia National Laboratories in Albuquerque, N.M., consisting of a broad-field illumination system and an image-intensified camera. By modulating the laser and intensifier so that the relative phase of the two signals is rotated 90° for each frame, the system creates stacked images that are used to calculate a range measurement for every pixel.
The other instrument was Neptec’s laser camera system, a name that can be misleading. Using a 1480-nm laser from Furukawa Electric Co. Inc., the system sends out a continuous beam of radiation, and the device captures the reflected signal. Iain Christie, director of research and development at Neptec in Ottawa, explained that the laser camera system is a scanner.
“It makes sequential 3-D measurements,” he said. “For the shuttle application, the measurements are generally made in a raster scan over a rectangular area.”
The inspection systems are precise but slow, with the laser dynamic range imager being the faster of the two for large areas. Both are intended for detailed inspections of such areas as the shuttle’s vital leading edge, providing measurement redundancy. Fisher said that NASA was pleased with the performance of the sensors on the Discovery mission.
As for the protruding gap filler on the spacecraft that was removed during a spacewalk, that was initially spotted on high-resolution photos taken from the International Space Station as Discovery approached. That is when the systems were brought to bear to provide precise information.
They took some 3-D measurements of it with the sensor, but it was all detected from those original photos, Fisher said.
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