F-15B gives a LIFT
As part of the Agency's return-to-flight effort, NASA engineers are acquiring data on how insulating foam debris or "divots" behave when these small pieces are shed from the Shuttle's external fuel tank during launch.
Image Right: Dryden's F-15B test fixture recently was used in the Lifting Insulating Foam Trajectory experiment as part of NASA's return-to-flight work. The LIFT experiment tested behavior of insulating foam debris when it is shed from the Shuttle's external fuel tank. NASA Photo by Jim Ross
Dryden conducted a series of flight tests of the divots as part of the return-to-flight team effort. The Lifting Insulating Foam Trajectory flight test series at Dryden used the Center's F-15B research testbed aircraft to test the divots in a real flight environment at speeds up to about Mach 2, or twice the speed of sound.
Small-scale divoting occurs when adhesive on the external tank thermal protection system, or TPS, foam fails. This occurs as a result of decreasing atmospheric pressure combined with increased heating during Shuttle ascent, which causes air trapped beneath the TPS to expand.
Image Left: Dryden engineer Kendall Mauldin pieced together these frame captures obtained using a high-speed camera during a March 16 flight. The image shows a foam divot (the round object seen in the second and third rows) ejected from the F-15B test fixture at Mach 1.97 and 47,000 feet in the lift experiment. NASA Photo
Nine divot ejection flights using BX-265 shuttle external tank foam were successsfully flown in the F-15B LIFT project. A total of 38 divots were ejected in these flights at subsonic and supersonic test points along the Shuttle's ascent trajectory, up to Mach 2 and a maximum dynamic pressure of 850 pounds per square foot. All of the divot ejections and trajectories were successfully recorded using a high-speed digital video system. The divot flight data supported the Return to Flight Shuttle Design Certification Review, one of the last engineering-review milestones prior to launch.
LIFT Project Manager Stephen Corda said objectives of the current F-15B flight tests included determining divot structural survivability and stability in flight and quantifying divot trajectories using videography. The flight data of divot trajectories also may be used for computational fluid dynamic code validation.
"We're using the unique capabilities of the supersonic F-15B aircraft and the aerodynamic flight test fixture to provide a means to eject these debris or divots from the fixture, and then photograph them with a high-speed digital video system, where we're able to video the divots in flight at up to 10,000 frames per second," Corda explained.
NASA's Space Shuttle Systems Engineering and Integration office at Johnson Space Center in Houston, Texas, funded the LIFT flight tests at Dryden. JSC aeroscience engineer Ricardo Machin said the LIFT flight tests will help validate models used for debris transport analysis.
"In particular, it's going to help us understand whether the divots break up once they come off the external tank, and secondly, whether they will trim and begin to fly or if they'll tumble. The difference between trimming and flying makes a huge difference in the amount of kinetic energy that this piece of debris can impart to the shuttle," Machin said.
The LIFT flight test required two new capabilities: an in-flight foam divot ejection system and a high-speed video system to track and record the trajectories of divots in flight. Dryden engineers developed both capabilities in just over two months.
Dryden's LIFT team designed, fabricated and ground-tested four divot ejection systems, completing 70 ground tests to determine and refine the best approach. Dryden engineers also designed and procured the very high-speed digital video equipment, including development of a system to synchronize cameras with the divot ejection system. In addition, they developed videography analysis techniques in order to quantify divot trajectories.
Staff at Marshall Space Flight Center, Huntsville, Ala., assisted Dryden researchers in the experiment and data obtained from the flights will be used in work at Ames Research Center, Moffett Field, Calif.
Marshall technicians sprayed aluminum sheets with BX-265 Space Shuttle foam for the F-15 LIFT tests. The aluminum sheets were reinforced with the foam, just as is done in preparing the Shuttle External Tank.
Ames researchers are using the information gained through the LIFT experiments to validate their three-dimensional computational fluid dynamics predictions of the divot stability and trajectories.
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