@neil_w comments on this thread have lead me to spending some time looking at Barrowman Equations.
I'm trying to get my head wrapped around why this rocket swing tests rock solid stable, yet Open Rocket shows it to be unstable.
If I remove the hammerhead fin, the rocket has a stability caliber of 1.97
View attachment 510834
With the hammerhead fin, the rocket has a stability caliber of -0.559
View attachment 510835
So follow me on this.. With the hammerhead in place, the large vertical stabilizer fin can be removed with almost no change in stability? (from -0.559 to -0.592)
View attachment 510839
But without the hammerhead fin, if the large vertical stabilizer fin is removed, it radically changes stability? (From 1.97 down to .878)
View attachment 510841
I've looked at the "Tools" / "Component Analysis" tab to try to wrap my head around what the equations are doing.
It seems that the way Open Rocket does the analysis the hammerhead fin so radically drives instability, that it overshadows the stability provided by the large vertical stabilizer fin.