[admford]

1. You can probably design a cross structure that would hold cabin cylinders as so:

O|O --- O|O

But it creates a problem. The surface area of an A380 is about 1009 mˆ2 (45 meters long, 7.14 wide). If we use four 727 (a narrow body airliner) as models for the cabin tubes. The surface area of one tube that size is 537.2 mˆ2 (45 meters long, 3.8 wide). Multiply the result by 4 tubes, you get 2149 mˆ2. That's an increase of 213% in surface area.

Given the structural reinforcements within the host aircraft, the need of aerodynamic surfaces to make sure four interlocking tubes don't create dangerous aerodynamic forces & stresses, and the fact that just for the outer covering, the weight of the tubes increases twofold over a standard A380. A minimum reduction in weight would have to be at least by 50%, if not more in order to be competitive. The 787, with a fully carbon fiber fuselage and other weight saving methods reduces weight by 20% at most over a comparative aluminum structure (http://www.boeing.com/commercial/aeromagazine/articles/qtr_4...).

At this point, the use of separate cylinders to hold people and cargo is more or less moot since the weight reductions needed aren't currently possible (physically, or economically), even with the newest materials.