Phantom tubes (that is, body tubes with one or more dimensions set to "0") have a long history in OR. There were needed in many situations in 15.03, and still needed (mostly with pods) in 22.02. Specifically, you must use a phantom body tube with diameter set to zero to mount fins-on-fins.

This is a hack. The user must select a body tube, and then manually set the desired dimensions to zero. The phantom tube will be invisible in the rocket view, even when selected. In the component tree, it will look just like a regular body tube, despite the fact that it is anything but.

We should do something better. We're probably going to need phantom tubes for at least the next couple of releases (or more), so they're not going away. We should be able to offer something better than a hack that requires the user to do weird stuff. I don't know the best way to do it, but this issue can serve as a collection point for ideas.