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see:
http://blog.al.com/breaking/2011/03/nasa_engineers_say_shell-buckl.html
Photograph from a March 2011 test conducted by NASA at the Marshall Space Flight Center in Huntsville, Alabama. Two rows of buckles can be seen near the top of the huge internally stiffened cylindrical shell. The test was conducted by Mark Hilburger, Mike Roberts and others. "Pocket" buckling (local buckling between adjacent internal stiffeners occurred at about 50000 lb axial compression. "Global" buckling (overall buckling) occurred at about
770000 lb. Global buckling of this shell is predicted to occur at 786000 lb.
ARTICLE by Lee Roop, The Huntsville Times, Thursday, March 24, 2011:
HUNTSVILLE, Alabama - NASA engineers improvised in Huntsville on Wednesday when their carefully planned - and televised - "can crush" experiment didn't crush a rocket shell as expected.
But improvise they did at Huntsville's Marshall Space Flight Center, and the final results support their theory that 1960s rocket standards still in force are far higher than they need to be.
New standards coming from tests like this will mean thinner, lighter rocket shells with more room for payloads to deep space, engineers said.
"No pressure," joked principal investigator Dr. Mark Hilburger of NASA's Langley Research Center as he displayed his computer models before the test. NASA designed the test so visiting VIPs could see the results in real time alongside Hilburger's predictions.
"Let's hope this is what happens, and we'll look good," Hilburger said. "And if not, we'll learn something."
VIPs came from across the country to see what one engineer called "an historic event." Many were from commercial companies such as SpaceX and United Launch Alliance now vying to build rockets to launch cargo and crews into orbit.
"Nobody has ever done this. We want to tap into NASA's findings," said Ray Miryekta of Blue Origin, a privately funded aerospace company set up by Amazon.com founder Jeff Bezos. Miryekta and his team flew in from Kent, Wash.
The "can" being tested was a 20-foot-tall, 27-foot-wide cylinder made of segments from a space shuttle external fuel tank joined at Marshall by state-of-the-art friction stir welding. It was set between two platforms where engineers could dial up the pressure until the shell buckled.
Engineers predicted buckling at 786,000 pounds of pressure, but they passed that level with no significant change. So, the team backed the pressure off and scratched their heads.
Their conclusion was that air being pumped into the shell for structural support was providing more support than expected. The solution: Lower the air pressure enough to get results, but not enough to flatten the shell like a pancake.
It worked. High-speed photography and color stress images showed the shell buckling in pockets at 500,000 pounds of pressure, along its internal supports at 737,000 pounds and "globally" at 770,000 pounds.
The original NASA standards drafted in the 1960s predicted buckling at 348,000 pounds of pressure, Hilburger said.
"I can't make a direct comparison because we did change the test," Hilburger said afterward, but the difference is consistent with tests showing modern shells have a safety margin of 50 to 60 percent - far stronger than necessary.
"What a great test," Hilburger told engineers who applauded him afterward. "I was holding my breath the whole time waiting for the next thing to happen. I'm very positive we do have the data we were looking for."
The test was conducted at Marshall's historic engineering test laboratory, where space hardware has been tested since the 1960s. Lead Marshall test engineer Mike Roberts was enthusiastic after Wednesday's test.
"Bring on heavy-lift," Roberts grinned, referring to the deep-space rocket Marshall will lead in developing when Congress provides the money.
"We build this test article here, moved it around the center and tested it here," Roberts said. "We're ready."
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