At present, the calculation of leading and trailing edges of fins in FinSetCalc just works its way around the points in the Freeform Fin Set. The result is correct if the fin is on a body tube (so the y coordinate of the first and last point are the same); if it's on a transition it only takes into account points between the fin (0, 0) and the fin's highest point, and doesn't consider the intersection of the fin with the transition.

The result is that for the attached .ork file, a bunch of parameters related to the fin geometry are 0, resulting in NaN errors in the simulation.

This comes from a bug reported on the sourceforge or-bugs mailing list as:

From: Gary Aoki via Openrocket-bugs [email protected]
Subject: [Openrocket-bugs] bug with freeform fins on a transition
Date: Sun, 12 Mar 2023 04:21:06 +0000 (UTC)

1. Put freeform fins on a transition

2. CP shows N/A

3. Flight simulation reports exception, something returns Not a Number

fin on transition.zip