National Aeronautics and Space Administration

www.nasa.gov

Background

Thin-walled composite shell structures have been used in launch 

vehicles for many years. Additionally, NASA is increasingly 

considering composite structures for use in launch vehicles. 

For many such launch-vehicle shell structures, composite 

structures are chosen for structural efficiency and reasonable 

manufacturing considerations. However, it is well known that 

thin-walled shell structures can be very imperfection sensitive 

when subjected to destabilizing loads. That is, small geometric 

or loading imperfections can cause the actual buckling loads of 

the as-built shells to be significantly lower than the theoretical 

predictions, which are based on simplified linear bifurcation 

buckling analyses of geometrically perfect shells. Therefore, it is 

important to understand the structural response and imperfection 

sensitivity of such shell structures. 

To account for the imperfection sensitivity during design of a thinwalled shell, the theoretical buckling load of the perfect structure 

is typically multiplied by a buckling KDF to determine a safe 

design load level. Hence, the guidelines for determining these 

KDFs can be very important for the design of structurally efficient 

shells. The most widely used source for KDFs for cylindrical 

shells is the NASA SP-8007 [1], which has recommendations that 

were developed based on experimental buckling tests from the 

1930s-1960s. However, SP-8007 has not been updated since the 

late 1960s and few, if any, composite shells were tested in the 

development. Consequently, the SP-8007 guidelines may not be 

applicable to shells constructed from modern materials, improved 

manufacturing processes, and new structural concepts. Several 

recent NASA studies have used 0.65 as a universal buckling 

KDF for cylindrical composite shells regardless of the structural 

concept (sandwich, stiffened shell, etc.) and the design details 

(thicknesses, stiffener dimensions, layup, etc.). 

The NASA Engineering and Safety Center (NESC) is currently 

assessing the NASA SP-8007 guidelines with the intent of 

revising these guidelines for selected metallic and composite 

cylindrical structures. The purpose of this Technical Bulletin is to 

provide guidance on the KDFs that should be used for composite 

structures until the new guidelines are developed and published.

Findings and Conclusions

It is unclear from where the universal 0.65 KDF for composite 

shell structures came, or if it is always conservative. In particular, 

when applied to anisotropic composite sandwich shells, the 

SP-8007-recommended KDFs for orthotropic cylinders and 

isotropic sandwich cylinders can be significantly lower than 

0.65. Nonlinear finite-element analysis to date however suggests 

that these SP-8007 KDFs are conservative for all the sandwich 

composite cylinders considered. Because the SP-8007 KDFs for 

many composite shells are less than 0.65, 0.65 should not be 

used as a universal KDF for composite cylinders unless there is 

other appropriate rationale. 

If SP-8007 is used to calculate KDFs for composite cylinders, 

the original assumptions and limitations must be understood and 

caution should be used if the recommendations are extended 

outside their original scope. Particular caution should be used 

when applying SP-8007 recommendations to composite cylinders 

that do not consist of orthotropic layers aligned with the axial and 

circumferential directions of the cylinder (cross-ply laminates), 

those with extreme stiffness tailoring, or those with significant 

contributions from the extension-shear, extension-bend, or 

bend-twist terms. Additional consideration must be made for 

cylindrical shells with low transverse shear stiffness (such as 

some sandwich shells) because they can have significantly lower 

predicted buckling loads than similar shells with high transverse 

shear stiffness. More detailed recommendations will be made by 

the NESC when the new guidelines are published.

References

1. Anonymous, “Buckling of Thin-Walled Circular Cylinders,” 

NASA SP-8007, 1965, revised 1968.

Buckling Knockdown Factors for Composite Cylinders

Sandwich composite test article showing material failure that occurred as a 

result of global buckling.

The buckling performance of thin-walled cylindrical shells is well known to be sensitive to small geometric and loading 

imperfections. During design, this sensitivity is typically accounted for by multiplying the predicted buckling load of a 

geometrically perfect structure by an empirical design factor known as a buckling knockdown factor (KDF). The most 

widely used source of KDFs for cylindrical launch-vehicle structures is NASA SP-8007. However, general composite 

shells are outside the original scope of SP-8007, and a universal KDF of 0.65 for all composite shell designs has 

been used in several recent NASA studies, though the technical justification is unclear. If the NASA SP-8007 is used

to calculate KDFs for composite cylinders, the original assumptions and limitations should be understood and care 

must be taken. Additionally, the universal KDF of 0.65 is thought to be unconservative for certain designs and is

therefore not recommended.

NASA Engineering and Safety Center Technical Bulletin No. 16-01