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United States Air Force
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Arnold Engineering Development Center
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Writer: Danette Duncan
Date: March 19, 1999
Release # 99-041
Photo # none
AEDC Performs Shuttle Materials Test for NASA/Lockheed Martin
ARNOLD AFB, Tenn.—Arnold Engineering Development Center is assisting the National Aeronautics Space Administration with improvements in existing Space Shuttle materials.
According to NASA, during several previous Space Shuttle flights, including the shuttle launched Nov. 29, 1998, the shuttle external tank experienced a significant loss of foam from the intertank. The material lost caused damage to the thermal protection high-temperature tiles on the lower surface of the shuttle orbiter. The loss of external tank foam material and subsequent damage to reentry tiles is a concern because it causes tile replacement costs to significantly increase, however, it is not a flight safety issue. As a result, NASA-Marshall Space Flight Center selected AEDC to perform flight hardware materials tests on the shuttle’s external tank panels in the center’s von Karman Facility Supersonic Tunnel A. The purpose was to establish the cause of failure for the tank thermal protection materials at specified simulated flight conditions. "NASA chose AEDC due to its technical expertise and historical program successes," Steve Holmes, a NASA-MSFC technical coordinator, said.
The Lockheed Martin-manufactured non-reusable external tank, the largest element of the Space Shuttle, fuels the shuttle orbiter during powered flight and is comprised of three components—a liquid oxygen tank, a liquid hydrogen tank and an intertank assembly that connects the two propellant tanks. At the full capacity of 528,600 gallons of propellant, the external tank weighs 1.6 million pounds. The tank is covered with a multi-layered, spray-on foam insulation that provides thermal insulation for the tank against the extreme internal and external temperatures generated during prelaunch, launch and flight.
Wayne Hawkins, Sverdrup project engineer, explained the foam system is exposed to multiple forces, causing difficulty in determining the actual failure of the thermal protection system. "Multiple forces act on the foam system," Hawkins said. "The environmental factors include thermal protection system cell expansion, aerodynamic loading, highly variable local flow conditions, oscillating shocks, vibration, temperature and main external tank substrate flexure."
Although NASA and other facilities have performed a number of tests in an attempt to define the underlying root cause of this foam loss, they were not successful. At one time, the center’s 4-foot and 16-foot transonic aerodynamic wind tunnels were possibilities for the test, but Tunnel A’s ability to closely duplicate flight conditions and control both ambient pressure and test sample immersion time made it the facility of choice. Tunnel A is a continuous flow-variable density wind tunnel with an automatically driven flexible-plate nozzle and a 40- by 40-inch test section and can cover the Mach number range of 1.5 to 5.5.
"The ideal success for the test is the generation of foam loss on a consistent basis with simulated flight conditions," Hawkins said.
Although the AEDC Tunnel A tests did not replicate the in-flight failures, they did provide detailed measurements to better understand the flight environment and fundamental failure mode. From these tests, NASA determined the failure is caused principally by foam cell expansion due to external heating at approximately Mach 4 combined with pressure change and aerodynamic shear. Specialized miniature shear gages and other instrumentation were installed during the test to measure these forces. The customer and sponsor were pleased with the AEDC test results. "No other facility can test with articles/models as large as AEDC with conditions that can match flight," Holmes said.