New analysis of one factor that drives structural design in launch vehicles suggests that weight savings of as much as 20% can be achieved in some large components and gives designers a much better understanding of the level of robustness needed for safety.
The NASA Engineering and Safety Center (NESC) at Langley Research Center has spent $13 million since 2007 recalculating and experimentally validating shell-buckling “knockdown factors” that have been in use since the beginning of the space age. Derived from engineering testing that started in the 1920s, the original knockdown factors determine how much additional margin designers need to add to the predicted buckling load of a rocket body or other curved structure to create a safe design.
But excessive margin equals unnecessary weight, which cuts into the useful payload a vehicle can lift to orbit. In the past 30 years, engineers have developed a better understanding of just why a cylindrical structure buckles, and the NESC-led project is using that knowledge to develop and validate less restrictive, more robust knockdown factors.“The original design factors, or knockdown factors, were developed in an era when we didn’t have the great computing capabilities that we have now,” says Mark W. Hilburger, a senior research engineer in the Structural Mechanics and Concepts Branch at Langley and the originator of the Shell Buckling Knockdown Factor project.
Since then, engineers have learned the effects of manufacturing imperfections, different materials and other factors on structural strength, allowing Hilburger and his colleagues to begin to recalculate the knockdown factors more rigorously. Rather than the extensive empirical testing that produced the factors in the Apollo-era NASA/SP-8000 design monographs, the team is conducting controlled trials in a special facility at Marshall Space Flight Center to validate their work, and these analyses will be the basis for new knockdown factors.
To date, they have tested four 8-ft.-dia. aluminum lithium, orthogrid-stiffened cylinders to failure, demonstrating that their calculations correlate with the test data to within 5% as compared with a 30-50% discrepancy historically. That improvement in correlation will enable a reduction in conservatism and can translate into weight savings in structural design, which can be significant in large pieces like the core stage of a heavy-lift launch vehicle.
“We did a full-vehicle study on the Ares V,” says Hilburger. “. . . In the core stage there is a tremendous amount of weight-savings potential there because it’s very heavily loaded, [it has a] very large diameter and very thin walls.”
On the Ares V, with its large surface area, even a 0.1-lb. saving per square foot would add up to thousands of pounds of mass that can come out of the structure. Across the Ares V, the team found structural-mass savings of 10-20% depending on location.
In February, the shell-buckling team will test a 27.5-ft.-dia space shuttle external tank barrel at Marshall to validate the scalability of their new analysis-based design factors. In addition to the aluminum lithium cylinders NASA is fabricating for the tests, Boeing and Northrop Grumman are providing composite test articles under Space Act agreements with the agency to validate knockdown factors for composite structures, for which there are no existing design guidelines.
The commercial involvement suggests a future direction for the new knockdown factors. The original application for the recalculation was in the Constellation program’s Ares vehicles—the Ares I crew launch vehicle and the Ares V heavy lifter. Those developments have been scrapped under the new NASA approach to human space access in favor of commercial crew transport and a government-built heavy lifter, but the revised design factors can be used on those launch vehicles as well.
The NESC team already has held two workshops for NASA engineers, and they have published their findings in NASA technical memorandums. The “biggest challenge,” Hilburger says, is persuading engineers to accept the new figures his team is developing.
“The second-biggest challenge is to develop a technology-infusion strategy and a set of guidelines that will last another 20 or 30 years, and in a way that they’re applicable to who knows what vehicles get built later on,” he says.
In addition to greater accuracy in designing structures, the new knockdown factors will give engineers a better understanding of how new manufacturing techniques and build tolerances can affect the buckling of the structure. NASA spent millions on friction-stir welding tooling for the Ares vehicles, and learned how the heating associated with the process alters the geometry of the metal being welded. That knowledge has allowed much greater accuracy in predicting shell buckling, in part because the welds are so uniform compared with traditional welds and result in very repeatable quality and geometry.
“That’s why you have to start including not only your designers, but the folks who are building these parts, and get an understanding of how we can tweak these tolerances,” Hilburger says. “. . . So then you start building on your knowledge and help guide the manufacturing to make a better product.”