Think of it as high-tech can crushing. Only the can is enormous, as big as part of the largest rocket ever made.
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."
