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Office of Communications
Rensselaer Polytechnic Institute

CONTACT:
Joely Johnson, (518) 276-6098, johnsj2{at}rpi.edu
X.-C. Zhang, (518) 276-3079, zhangxc{at}rpi.edu

FOR IMMEDIATE RELEASE: June 11, 2003

Rensselaer Researchers Use T-Rays to Uncover Defects in Space Shuttle
Foam Sample: Could Lead to More Effective Method of Pre-flight
Quality Control of Insulating Foam

TROY, N.Y. -- Using a technique pioneered by researchers at
Rensselaer Polytechnic Institute, terahertz (THz) radiation has been
used to uncover small defects in a sample of space shuttle foam. This
nondestructive method could help National Aeronautical and Space
Administration (NASA) officials examine the insulating foam that is
applied to each shuttle's fuel tank prior to launch. 

Fuel tank manufacturer Lockheed Martin Space Systems (New Orleans)
approached X.-C. Zhang, the J. Erik Jonsson Professor of Science at
Rensselaer, and requested a study of a sample of the foam material.
Zhang and his research team have employed terahertz radiation
(T-rays) to spot defects, including air bubbles and separations,
purposely embedded in a specially prepared sample. Such defects have
proved difficult to locate using X-rays or ultrasound. Zhang's team
(including doctoral students, Hua Zhong, Xie Xu, Tao Yuan, and
Shaohong Wang) has been working closely with Lockheed Martin to study
the sample. 

The sample is composed of material identical to that which would be
applied to the shuttle fuel tank. In contrast to the continuous layer
of foam normally applied to the tank, the sample is a block measuring
two feet square and approximately four inches thick. An aluminum
plate serves as the base for two different insulating materials: A
one-inch layer of dense, cork-like Super-Lightweight Ablator (SLA) is
applied on top of a three-inch layer of closed-cell Sprayed-On Foam
Insulation (SOFI). 

A total of eight man-made defects of various sizes were scattered
throughout the sample. The embedded imperfections mimic defects that
could potentially occur in a normally produced foam application on
the fuel tank. Two types of defects were hidden in the sample: voids
(or air bubbles), ranging from one-quarter inch to one inch in size,
and debonds (separations between layers of foam or between a foam
layer and the aluminum base). 

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. 

A New Way to See

The technology behind the emitters and detectors used to produce and
sense the T-rays was developed at Rensselaer. The researchers use
electro-optic crystals and a femtosecond laser to generate and detect
the THz signal. They are able to locate and identify defects in the
insulating foam sample by measuring the signal amplitude, temporal
delay, and waveform distortion of the signal. 

"Optimal THz scan sensitivity also depends on the material being
looked at. Thickness and density of an object can affect how far the
T-rays will penetrate and how widely they will scatter. Both the SLA
and SOFI materials making up the insulating foam sample happen to be
excellent subjects for THz radiation," says Zhang. "The foam has a
lower attenuation, allowing the terahertz waves to penetrate to a
depth of many inches." 

T-rays lie within the far-infrared region of the electromagnetic
spectrum -- the large range between microwaves and visible light. The
unique properties of THz radiation make it a potentially excellent
complement to existing imaging methods such as X-rays and ultrasound.
The safety and sensitivity of T-rays may allow the technology to
someday play a part in security searches for weapons and toxins, and
could improve detection of breast and skin cancer. 

About Rensselaer

Rensselaer Polytechnic Institute, founded in 1824, is the nation's
oldest technological university. The school offers degrees in
engineering, the sciences, information technology, architecture,
management, and the humanities and social sciences. Institute
programs serve undergraduates, graduate students, and working
professionals around the world. Rensselaer faculty members are known
for pre-eminence in research conducted in a wide range of research
centers that are characterized by strong industry partnerships. The
Institute is especially well known for its success in the transfer of
technology from the laboratory to the marketplace so that new
discoveries and inventions benefit human life, protect the
environment, and strengthen economic development.

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