New TPS testing process in works

by Chris Bergin

University of Arizona Civil Engineering Professor Tribikam Kundu is part of a team that’s designing a way to test thermal protection system (TPS) tiles on a military version of the Space Shuttle.

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.
In 2003, the Space Shuttle Columbia was lost when a breach in its TPS on the leading edge of the wing allowed super-hot gasses destroy the Orbiter during re-entry.

“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.”

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.

“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.

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.

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