Figure 2.1-8. Akron and Macon Frame Structure The multi-spring concept of supporting long, continuous column elements in triangular array is shown in Figure 2.1-10. This particular type of structure emerged during World War I, where the Germans were constructing 600-foot battle zepplins once a month, for the bombing of London. This section was also incorporated in the Hindenberg and sister ship structures from 1927 to 1929. For the Rockwell SPS structure, two structural element candidates emerged. They are shown in more detail by Figure 2.1-11, an embodiment of the Vierendeel truss in an equilateral triangular cross-section, and Figure 2.1-12, an embodiment of the multi-spring stabilization of continuous column elements in an equilateral triangular cross-section. Figure 2.1-13 is a comparison of five cross-sections normalized to have the summation of area of column cap and polygon perimeters to be equal for all five sections. The comparison is valid for dispersed material, lightweight structure in which material thickness is approximately 0.010 inch or less. The comparison appears to be a means of rapidly obtaining a structural figure-of-merit as a function of geometrical shape. With local crippling, column, and general stability structural failure modes, all occurring at equally high allowables (a requirement of lightweight structures). Included as material property parameters, the trend to triangular cross-section and multi-convolution of cap element shapes appears to be most promising for SPS structure. Further, these triangular shapes permit coaxial alignment of centroids at all assembly sequence levels. Finally, such shapes permit all column elements to obtain end-fixidity via quick-connect attachment means. Cross-bracing of columns by preloaded tensile elements, provides the lightest possible structure available for the SPS framework.