NASA crushes rocket fuel tank for science
By David Szondy
December 18, 2013
On December 9, NASA began what is either an impressive engineering test or a classic example of world-class larking about. At the space agency’s Marshall Space Flight Center in Huntsville, Alabama, engineers are crushing an enormous can by subjecting it to almost one million pounds of force. This may seem like a party trick that’s gone out of control, but there’s a serious reason behind this … or so NASA says. The crushing is part of the project to design the fuel tanks for NASA’s Space Launch System (SLS), which will be used to launch the Orion spacecraft and deep space missions.
The problem with propellants is that you need some way to carry them. Early liquid fuel rockets had fuel tanks installed in their hulls, but in the 1950s, engineers saw this as a needless expense in weight and complexity. Their answer was to turn the fuselage of the the rocket itself into the fuel tank. By the 1960s, this had gone so far that the rockets that ran the Space Race ended up as giant, round metal envelopes that used the fuel as part of the structural integrity. Think of it as being like a plastic water bottle that can sit in a lunch bag just fine when it’s full, but crumples easily when empty.
This approach solved a lot of problems, but it added others. Not only did the hull have to cover equipment, it had to withstand pressures, control sloshing, and all sorts of things that a simple skin doesn’t have to. And it had to do this while maintaining the rocket’s structural integrity.
The tests, called the Shell Buckling Knockdown Factor Project, are taking place at Marshall’s Structural and Dynamics Engineering Test Laboratory, where the Saturn V rocket, the Space Shuttle, and components of the International Space Station underwent similar tests on the world’s largest tensile testbed. The tank is an unused Space Shuttle component. It’s 27.5 ft (8.3 m) in diameter, is made of an aluminum-lithium alloy, and NASA says that it’s similar in structure to the SLS fuel tanks.
The purpose of the tests is to subject the tank to the sort of loads expected during an SLS launch. The tank is pressurized to simulate flight conditions and to see how well the it holds up to internal pressure, and the test bed inflicts compression and bending forces on it that cause some serious squishing.
"When it buckled it was quite dramatic," says Mark Hilburger, senior research engineer in the Structural Mechanics and Concepts Branch at NASA's Langley Research Center in Hampton, Virginia. "We heard the bang, almost like the sound of thunder and could see the large buckles in the test article."
The buckling is measured using a technique called Digital Image Correlation. For this, the tank is painted with 70,000 irregular black and white polka dots. Around the tank, 22 high-speed cameras monitor the dots continuously and record any buckles, rips or strains by measuring any displacement over a wide area.
The main goal of the tests is to find a way to reduce the weight of the SLS by 20 percent. This will allow the booster to carry heavier payloads and missions farther into deep space.
"In addition to providing data for the Space Launch System design team, these tests are preparing us for upcoming full-scale tests," says Matt Cash, Marshall's lead test engineer for the shell buckling efforts and the SLS forward skirt and liquid oxygen tank structural testing. "Performing structural tests on hardware that is the same size as SLS hardware is providing tremendous benefit for our future development work for the rocket."
The video below describes the crush test.