During a series of tests from Dec. 9-13 at NASA's Marshall Space Flight Center in Huntsville, Ala., engineers will apply nearly a million pounds of force to the top of an empty but pressurized rocket fuel tank. The test will eventually buckle and destroy the structure of the thin cylindrical tank wall while instruments precisely measure and record everything, millisecond by millisecond.
"What we learn will make it possible for NASA to design safe but still thinner and lighter structures for the Space Launch System and other spacecraft," said Dr. Mark Hilburger, senior research engineer in the Structural Mechanics and Concepts Branch at NASA's Langley Research Center in Hampton, Va.
In rocket science and engineering, every pound counts, and it costs to lift every pound to orbit. Rocket tanks are one of the heaviest parts of the rocket. If engineers can make tanks stronger and lighter, rockets can carry heavier payloads to space. That's the goal of the Shell Buckling and Knockdown Factor Project led by the NASA Engineering and Safety Center (NESC) in collaboration with Marshall and Langley teams.
Langley engineers are conducting their second full-scale tank test, nicknamed Can Crusher II, in Marshall's unique facility designed to test the full-size structures. Marshall engineers conducting the test have a keen interest in the results because the data will enhance the design of the heavy-lift Space Launch System (SLS), which is being developed by Marshall and will be the largest, most powerful rocket ever built.
Launch vehicles are composed of thin-walled cylindrical structures; if they are made lighter, buckling from the forces of launch and flight becomes a major concern. The project is developing a new, extremely accurate set of design standards for NASA and the aerospace industry, which has been using data that dates back to Apollo-era studies.
"In the 1960s when we went to the moon, those engineers did an amazing job with what they had," Hilburger said. "But they had to build conservative margins into their calculations because they didn't have today's materials or design, test and simulation tools. That means they built the launch vehicle heavier than it had to be, which can reduce the payload it can carry."
Since 2007, the Shell Buckling Knockdown Factors Project has been using cutting-edge test and analysis techniques to amass new data for design. The ultimate goal is to develop analyses and models that reflect the real-life test articles with extreme accuracy, so designers can use high-fidelity computer simulations and virtual tests to save time and money. "But we have to make sure that we ground those models in these carefully conducted real-world tests," Hilburger said.
In March 2011, the project team came to Marshall for what they believed to be the first test-to-failure of a full-scale, 27.5-foot-diameter, 20-foot-tall aluminum lithium test cylinder just for research purposes. It was reinforced with an orthogrid stiffener pattern, and the team squeezed it until it buckled, revealing the edges of the design margin.
The cylinder to be tested this time is External Tank-derived Test Article 2, or ETTA 2 Like ETTA 1 in 2011, it was built at Marshall from panels used for external tanks in the space shuttle program. This one is also 27.5 foot in diameter, the same diameter as SLS tanks, and 20 feet tall but will feature a different orthogrid stiffener pattern. Engineers can compare the results of this test to the first one to see if one pattern results in a stronger tank. At the top and bottom of the can are the load or pressure introduction structures made in the 1970s for the shuttle program.
"Using the heritage tank panels and Marshall’s valuable test facilities is saving millions in test dollars and time," Hilburger said.
The team prepared for next week's test to failure by running a series of sub-critical tests over the last few months. They've fitted the cylinder with more than 800 strain gauges, and 80 displacement transducers, and speckled ETTA 2 with markers used by a digital image correlation system. Cameras set up around the tank monitor the position of the dots during testing.
"We can actually track minute changes in the position of those dots and from that calculate displacements and strains on the entire test article," Hilburger said.
This week, there will be additional exercises, and then the final day will be a test to failure scenario.
"We'll pressurize the structure to simulate an internal fuel pressure," Hilburger said. "And then we'll slowly start applying a combination of compression and bending to simulate a typical rocket flight condition."
Data from the team's work is already being incorporated into designs for the core stage of the SLS.
"When the new core stage flies, our design factors will be flying with it. It's very gratifying, but it's also nerve-wracking. When you're trying to reduce excess margins, you're obviously closer to failure, and want to make sure it's being done safely and with as much knowledge as possible.”
The shell buckling project activity has also given engineers at Marshall a great opportunity to hone skills. A lot of new technicians have received on-the-job training that will translate directly into SLS testing.
"This is my first large-scale structural test," said Matt Cash, lead test engineer for ETTA 2. "It's a fantastic experience, and everything I’m learning helps me prepare for SLS structural testing." Cash earned a degree in civil engineering with an emphasis on structures from the University of Alabama in Tuscaloosa. He's been a NASA employee for three years, and worked on the 2011 full-scale shell buckling test.
Because Marshall is one of the few places in the world where this kind of testing can be done, Cash said he's thrilled to be in the right place at the right time.
"The tests will provide extremely valuable data to SLS. I couldn't be happier to get to be a part of it."
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