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Space station experiments yield unexpected results


December 1, 2011

By Karen Mowls

Earlier this year, Space Shuttle Endeavor visited the space station to retrieve a set of rods that are part of an experiment created in junction by NASA and the European Space Agency. When the samples reached Earth, they were eventually sent to Dr. Surendra Tewari of the Chemical and Biomedical Engineering Department at Cleveland State.

Tewari’s job within the experiment was to create metal rods that were to be melted down and then refrozen in space. This is the same type of metal that is used in wind turbines, and when they are created on Earth convection affects the cooling process, which makes turbines weak.

“When the liquid metal cools, it creates something called dendrites,” Tewari said. He compared the little structures to Christmas trees. They branch out and are connected. When making turbines, the goal is for these dendrites to be uniform; they should be in rows. On Earth, gravity pushes while the metal is cooling, which causes the little “tree branches” to snap. This creates what is called a spurious pattern and the metal is then weaker.

This is a problem with turbines because they are being pulled toward hot exhaust fumes of temperatures reaching up to 2,500 degrees. If the turbines are weak, they could explode— shooting chards of metal in every direction.

NASA approached Tewari and his colleague David Poirier, a professor at the University of Arizona, to create a set of metal rods to be melted down and then refrozen in space. They both have been working on NASA experiments for about 15 years.

Tewari explained that in space there is no convection and there is no gravity, so the assumed outcome of the experiment was that the rods would form non-spurious dendrites, making the metal stronger. If the experiment was a success, we could hopefully figure out a way to repeat the results on Earth.

While the rods were in space, they were each heated, and refrozen at different speeds to test how the dendrites would form in each situation. Tewari has only cut open the slow-heated rod, and there was surprisingly a section that grew the spurious dendrites.

“This was very unexpected,” Tewari said. Part of the metal rod grew spurious dendrites, and part of the crucible (the container that held the rod) was melted onto the rod, which Tewari said shouldn’t have happened.

A portion of the rod also didn’t cool in the correct shape. It indented near the top, which Tewari also said shouldn’t have happened. He is currently trying to figure out why there was a section of non-spurious dendrites in the sample.

Tewari created another sample that is waiting to be sent to space. It will take about four years to be launched because the experiment costs over $1 million to complete. This sample will be similar, but the metal pieces are shaped like actual turbine blades. This will allow Tewari to see if the metal will be stronger than if it was cooled on Earth.

Mark E. Bell is a contract engineer at NASA and said that the possibilities of what can be accomplished by these experiments are vast. He said that creating a better way to form the metal would “affect a large number of high temperature material applications.”

Bell also said that they are working on ways to make metal stronger because parts around the engine need to stay solid longer than metal. They experiment with mixing different materials, like ceramic and metal, to create something stronger. The experiments that Tewari is working on may help in this process.

Although Tewari is constantly busy with classes and NASA experiments to analyze, he said that he enjoys the excitement of it all and loves his job. “This is the only profession that allows you to do what you want to do,” he said.