Funny I've never seen a need for TTW fins on any Model Rocket under 3.3lbs.
If your model will be flying on BP motors and clusters Thur the wall fins is NEVER necessary, regardless of fin design or Tube thickness. I use Standard BT-80 (.021"wall) "estes style" tubing on all my Upscales with Butt mounted fin joints which are far more structurally sound then many on the board understand.
Frankly TTW fins are simply more work then required on just about any BP powered LPR and MPR models. Only real reason for using TTW fins is when you get into much higher thrust HPR motors and far heavier models.
Mark is exactly correct about not adding unnecessary mass to your lpr/mpr models, building as light as possible always improves performance without sacrificing good strength and longevity.
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