Interesting how the rocket is so skinny with a huge "hammerhead" payload fairing. The upgrades to the Merlin 1 engine series seem to have worked out so well tha

Interesting how the rocket is so skinny with a huge "hammerhead" payload fairing. The upgrades to the Merlin 1 engine series seem to have worked out so well that they have stretched Falcon 9 quite a lot. The loads in the center of the rocket must be huge. To increase diameter, they would probably need to invest so much more in either risk analysis, flight control, manufacturing or transport that it wasn't deemed necessary. (Resistance to buckling is roughly relative to rotational inertia of the cross section, which for a circle grows with the square of radius assuming a constant shell thickness, and the thickness would actually increase with increasing radius because it is a pressure vessel. Also the actual bending loads increase supralinearly with length.)