See Part 1: • Houdini Procedural Modeling: Rocket Part 1
Procedural modeling in Houdini Part 2:
In this video I model the fin using VEX to control X and Y scale independently. I also use VEX and attributes from the refine node to taper the fin from top to bottom. Finally, I use groups to change the width of the fin, and extrude it to control the subdivision.