Tile Test System Could Make Space Shuttle Safer (Forwarded)

University Communications
University of Arizona
Tucson, Arizona

Contact Information:

Tribikram Kundu
Professor, Civil Engineering and Engineering Mechanics
tkundu @ email.arizona.edu

October 20, 2005

Tile Test System Could Make Space Shuttle Safer
By Ed Stiles

In February 2003, Space Shuttle Columbia disintegrated on re-entry,
killing all seven crewmembers. The shuttle was hit by a piece of foam that
damaged the thermal-protection tiles on its left wing, leading to failure
when superheated air surged into the wing and, possibly, a wheel
compartment.

UA Civil Engineering Professor Tribikam Kundu now is part of a team that's
designing a way to test thermal protection tiles both on the ground and in
flight to prevent similar accidents.

Kundu spent the summer at Wright-Patterson Air Force Base in Dayton, Ohio
working on ways to non-destructively test the tiles. The work was done in
labs run by the Non Destructive Evaluation Branch of the Air Force
Research Laboratory. Kundu was testing the tiles for poor adhesion or
internal cracks. The project is aimed at developing a real-time, on-line
monitoring system for the tiles on a military version of the space
shuttle.

Professor Kundu and his graduate students are continuing the project on
the UA campus by using computer models to further develop the testing
technique and to determine the number and position of sensors needed to
make it work most efficiently. The project is being conducted under the
supervision of the Air Force Materials and Manufacturing Directorate.

"When the space shuttle re-enters the atmosphere, the air friction
generates enough heat to melt any kind of metal," Kundu explained. "The
special silicon-carbide foam tiles are attached to the outer surface of
the shuttle. They protect it from the high heat generated when the shuttle
re-enters the Earth's atmosphere. The inside of the tile is like a sponge
with many air pockets that serve as shields for the outside heat."

How the Tiles Break Down

Three things can cause the tiles to break down:

* The tiles can delaminate from a space vehicle (such as the shuttle).
During re-entry partially delaminated tiles can rip away, exposing the
metal underneath.

* The tiles can develop internal cracks that provide pathways for heat to
reach the underlying metal.

* The tiles can be damaged by collisions with some of the millions of tiny
space junk particles that are leftover debris from previous missions. Even
a BB-sized particle traveling at high speed could damage a tile.

This past summer, Kundu and Air Force researchers demonstrated that
ultrasonic signals generated by piezo-electric transducers can be used to
test how well the tiles are bonded to the shuttle or if they contain
hidden cracks. The signals were generated by a transducer and sent to a
receiver through an aluminum test frame.

Elastic Waves Tell the Story

"If you have perfect bonding and no cracks, the signal energy will be low
at the receiver," Kundu said. "As soon as the energy level goes up, that's
an indication of a delamination defect."

Ultrasonic waves are what engineers call "elastic waves." We hear elastic
waves as sound waves between about 20 Hz and 20 KHz. Above 20KHz, they're
called ultrasonic waves. These waves can travel through the air like sound
waves or they can travel through solid materials, such as the shuttle
tiles. When they travel through the tiles, they generate a small amount of
stress.

"We have demonstrated this system as a proof-of-concept at the Air Force
Research laboratory," Kundu said. "Now the question is, 'How do we design
a system to make it work in the real world?' " That's now the focus of his
ongoing research.

Kundu also is working on ways to detect when the tiles are hit by space
junk so they can be inspected for damage.

"We can permanently mount a sensor on the bottom of each tile that will
send a signal when it is hit," he said. The sensors would be wireless so
engineers would not have to worry about running hundreds of feet of cable
to the tiles. "But we may not need a sensor on every tile," he explained.
"Maybe we only need a sensor every few rows." This could be a 3-by-3 area,
covering nine tiles or a 6-by-6 area covering 36 tiles.

Using DPSM to Find the Answers

Kundu is attacking these problems with a computer modeling technique
called DPSM (Distributed Point Source Method). This is a numerical
analysis technique that is simpler and faster than methods now commonly
used by engineers, such as finite element analysis.

"Although we're using DPSM to model the elastic wave propagation through a
tile structure, it can be applied to a wide variety of engineering
problems," he said.

Kundu worked on the DPSM method during summer research projects between
1998 and 2004 at the Ecole Normale Superieure in Cachan, France with
Professor Dominique Placko. They are now writing a book about DPSM.

Kundu and Professor Douglas Adams of Purdue University are working on the
thermal-protection tile project in collaboration with the research group
leader Dr. Kumar Jata of the Air Force Research Laboratory at
Wright-Patterson Air Force Base.

Related Web Sites:

* Tribikram Kundu's Homepage
http://www.u.arizona.edu/~tkundu/
* The Air Force Research Laboratory
http://www.afrl.af.mil/

IMAGE CAPTIONS:

[Image 1:
http://spaceflight.nasa.gov/gallery/...s114-s-049.jpg
(57KB)]
The sun rises on the Space Shuttle Discovery as it rests on the runway at
Edwards Air Force Base in California after a safe landing at 5:11 a.m.
(PDT) on August 9, 2005. UA Professor Tribikam Kundu is working on ways to
test thermal-protection tiles on similar space vehicles to make for safer
re-entry into the Earth's atmosphere. (NASA photo)

[Image 2:
http://spaceflight.nasa.gov/gallery/...s011e11260.jpg
(109KB)]
This underside view of the Space Shuttle Discovery was photographed as it
approached the International Space Station. It shows the shuttle's heat
shield, which is made of thermal-protection tiles. (NASA Photo)

[Image 3:
http://spaceflight.nasa.gov/gallery/...2003e61578.jpg
(92KB)]
Astronaut Andrew S. W. Thomas, STS-114 mission specialist, looks closely
at tile underneath the Space Shuttle Atlantis in the Orbiter Processing
Facility at Kennedy Space Center (KSC). (NASA Photo)

[Image 4:
http://spaceflight.nasa.gov/gallery/...2003e52340.jpg
(87KB)]
Onboard a KC-135 aircraft, astronaut Rex J. Walheim uses a tool similar to
a putty knife to remove excess material from a membrane-covered cavity on
a damaged section of thermal tiles. The aircraft flew a series of special
parabolas to afford a number of zero-g windows for rehearsing
extravehicular activity (EVA) tasks for repairing damaged Shuttle tiles.
(NASA Photo)