Another Open Rocket conundrum

With this design, I can't figure out how to make the 3" tube one piece. If I make it an "inner tube", then it doesn't allow me to add fins to it. The way I've done it with two tubes, it thinks there is "Discontinuity in body tube diameter". I don't know if that's affecting stability or flight sims. Any insight from an OR guru?

View attachment Pentaurus2.ork

You have to create an outer tube that is the length of the exposed section, and for the portion that is inside the larger tube, you'll need an inner tube. Personally, I make the outer (exposed) tube a PBT, and make the inner tube the full length of the actual body tube segment. I don't know how to eliminate the "Discontinuity in body tube diameter" message, nor exactly how it will change the altitude simulations.

I've got a request into the OR developers to allow adjust smaller tubes to be inserted inside larger tubes.

I would add a transition to fix the discontinuity issue. Just make the length of it extremely small. Then OR will beaware that the tube is changing size. Otherwise it won't be taken into account, hence the message.

Then add the internal part of the tube as an inner tube.