True, the space shuttle was not completely contained on its vertical axis, but I was talking about the boosters themselves. I said the lift mechanism was a vertically stacked chemical rocket, not that the entire shuttle was a uniform tower, as it obviously wasn't.

The boosters are components of the space shuttle, which is what we were talking about: simpler working components evolving into complex systems.

Simple working component: Rocket booster

Complex system: Shuttle with a crew module, fuel tanks, and multiple boosters