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Wind Tunnel Tests Support Shuttle Safety
NASA's Glenn Research Center in Cleveland, Ohio, performed wind tunnel tests to help verify that the space shuttle can fly safely after one of the biggest aerodynamic changes in the spacecraft's history.

Photo: engineers mount hardware in tunnel ET-119, the external tank that will carry fuel for the shuttle Discovery's July 1 launch, is the first to fly without protuberance air load (PAL) ramps. These wedge-shaped layers of foam were designed to protect cable trays and pressurization lines from aerodynamic forces and vibrations during flight.

On Discovery's last mission in July of 2005, NASA's cameras captured video of PAL ramp foam falling from the external tank during liftoff. Though the orbiter wasn't damaged, NASA immediately began investigating the cause of the foam loss and ways to prevent it from happening again.

Based on mathematical models, the agency decided to remove the ramps for future flights. NASA then conducted several wind tunnel tests to verify the mathematical models.

Image right: Williamson (center), Christine Yehlik, Pete Cooper, Tom Vannuyen, and Steve Scott install scale models of the cable tray, pressurization and feed lines in the supersonic wind tunnel. Credit: NASA

The first tests took place in NASA Glenn's 8-by-6 foot supersonic wind tunnel, where Glenn engineers tested scaled down models of the cable tray, pressurization lines and liquid-oxygen feed line. The hardware models were subjected to wind speeds ranging from a half to 1.6 times the speed of sound at multiple angles.

During the tests, a team of engineers led by the Johnson Space Center collected test data in real time in the wind tunnel's control room. "About 30 people came in from Johnson, Langley, Marshall, USA and Boeing," said Glenn's Lead Wind Tunnel Test Engineer Scott Williamson. "They were looking at the test data in real time and processing it so it would be ready for analysis as soon as the tests were completed."

Photo: engineers assemble hardware for testImage left: Pete Cooper (left) and Dale Dragony (right) assemble pressurization line models. Credit: NASA

Measuring 23.5-feet long, the tunnel is NASA's only transonic propulsion wind tunnel. That means it can create speeds of airflow that vary from subsonic to supersonic at different points along the surface of the hardware, allowing it to simulate the conditions the external tank encounters in flight.

"The airflow around the tank is constantly changing," Williamson said. "As you accelerate, the density and direction of the air changes, and the shuttle components interrupt the airflow."

Data from the tests confirmed the predictions of the mathematical model -- that the shuttle hardware could withstand launch conditions without the PAL ramps.

"This test was a critical item necessary for us to return the shuttle to flight," said Jim Peters, Shuttle Program Aerospace Flight Systems Manager. "The folks at Glenn did a remarkable job in a short period of time. If it weren't for their unbelievable efforts we wouldn't be ready to launch."

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Jan Wittry (SGT, Inc.)
NASA's Glenn Research Center