When I was learning Autodesk Inventor, I wanted to choose a difficult subject to model. Inspired the Merlin 1D engine, as well as the RD-180, this is a parametric modeled rocket engine.
I wanted to learn about creating objects from revolving profiles, as well as how to create complex pipes. To this effect, I think I was successful. I did not know about components at the time that I was modeling this, so if I were to re-do the model, I'd use the component system more generously.