I built the Interrogator from Sirius a bit ago, which has the 18mm motor mount. Along with that rocket, Sirius also has the mid-sized Interrogator, with a 24mm motor mount, and the Interrogator G, with a 29mm engine mount.

This is the mid-sized Interrogator.

I built it almost stock. A small change I made was to swap the motor mount tube with a heavy walled 24mm motor mount tube. Mainly, it was to fit a 3.75" motor mount instead of the stock 2.75" motor mount. Since I was swapping it, I use heavy walled 24mm mount tube, since I have plenty of that stuff.

I'm not too worried about the balance, since the nose is resin cast, and on its own pulls the CoG forward of the fins.

The main impact on the build was that I had to pull a thin layer of the centering ring off to make the thicker engine mount tube fit.

edit: 18mm not 13mm

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