T-rays may check shuttle foam

Researchers at Rensselaer Polytechnic Institute (RPI; Troy, NY) and Lockheed Martin Space Systems (New Orleans, LA) are investigating terahertz imaging as a potential nondestructive method for preflight inspection of the foam that is applied to space-shuttle fuel tanks.

Aug 1st, 2003
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Researchers at Rensselaer Polytechnic Institute (RPI; Troy, NY) and Lockheed Martin Space Systems (New Orleans, LA) are investigating terahertz imaging as a potential nondestructive method for preflight inspection of the foam that is applied to space-shuttle fuel tanks.

National Aeronautical and Space Administration (NASA) investigators believe that the Columbia space shuttle crash may have been caused by foam insulation breaking away and striking the left wing of the craft. So fuel tank manufacturer Lockheed Martin Space Systems asked X.-C. Zhang, the J. Erik Jonsson Professor of Science at RPI, to study a specially prepared sample of the foam material.

The sample was composed of material identical to the foam that is normally applied to the shuttle fuel tank, but instead of the continuous layer of foam normally applied to the tank, the sample consisted of a 2 × 2-ft block approximately 4 in. thick. An aluminum plate served as the base for two different insulating materials: A 1-in. layer of dense, corklike super-lightweight ablator (SLA) was applied on top of a 3-in. layer of closed-cell sprayed-on foam insulation (SOFI).


Researchers Xie Xu (left) and Hua Zhong (right) use a terahertz-imaging technique developed at Rensselaer Polytechnic Institute to successfully locate embedded defects in foam samples provided by Lockheed Martin Space Systems.
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A total of eight man-made defects of various sizes were scattered throughout the sample. The embedded imperfections mimicked defects that might occur in a normally produced foam application on the fuel tank. The two types of defects hidden in the sample were voids (or air bubbles), ranging from 1/4 to 1 in. in size, and debonds (separations between layers of foam or between a foam layer and the aluminum base).

Zhang's research team successfully used terahertz radiation ("T-rays," which lie in the portion of the electromagnetic spectrum between microwaves and infrared light) to detect the embedded defects (see figure). The SLA and SOFI materials making up the insulating foam sample happen to be excellent subjects for terahertz radiation, according to Zhang. "The foam has a lower attenuation, allowing the terahertz waves to penetrate to a depth of many inches," he said.

The researchers used an 800-nm femtosecond laser with a 250-ns pulse-width and a 250-kHz repetition rate to generate the terahertz signal, and a zinc telluride electro-optic crystal to detect it. They were able to locate and identify defects in the insulating foam sample by measuring the signal amplitude, temporal delay, and waveform distortion of the signal. But the sample also posed unique challenges due to reflections from different layers of the foam, according to researcher Hua Zhong.

"Because we are playing in a refractory geometry, signal collection becomes an issue, and diffusion of the terahertz beam on passing through the foam significantly lowers collection efficiency," she said. To compensate for these factors, they placed the collection components (including a pellicle, parabolic sensor, and sensor crystal) on top of the testing platform and then situated the mirrors to ensure that beams coming from different foam layers would be collected and would overlap fairly well with the probe beam on the sensor crystal.

The next major hurdle for the team is to boost the signal-to-noise ratio, which will also increase the scanning speed, added Zhong. "Right now it takes half a day to scan a 2 × 2-ft piece of foam. We could speed it up considerably, to one hour or less, by increasing the sensor crystal size to 40 × 40 cm and using a CCD (charge-coupled-device) camera."

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