How big tube fins?

Does anyoune know how big tube fins to put on a 30mm diameter and 420mm long rocket?

I dunno...but the brand-new issue of "Sport Rocketry"...the NAR's mag...has an article on pretty much this exact question; how big (diam and/or length) should the tubes be for a given airframe size.

The norm is to use tubes that are the same diameter as the BT - 6 of them fit evenly around the main tube. The rule of thumb I'd seen is a length between 1.5 and 3 times the BT diameter. You can simulate these in RockSim and there is an article on EMRR https://www.rocketreviews.com/rocksim_library.shtml that should describe how tube fins affect stability.

The main point of the Sport Rocketry article is tube fin performance vs the overall length of the rocket. Stubby tube fin rockets perform far better than longer skinnier ones.

Summaries/teasers here:
https://rocketdungeon.blogspot.com/2008/03/sport-rocketry-marchapril-2008.html
https://rocketdungeon.blogspot.com/2008/01/tube-fin-performance.html

Dexter: Just to clarify, are you talking about tube fins attached to the side of the airframe, or concentric with it (like a FlisKits Corona)?

Tube fins can be a cool way to build a different looking rocket.

Depending on design, because of their extra drag they can reduce altitude by more than a third, when compared to straight fins. If flying in a small field, that can be desireable.

As I understood the Sport Rocketry article, the reported result was that a short stubby rocket body could greatly reduce the extra drag created by the tube fins. Interesting to note the rockets in the article had 7 slightly smaller tubes for fins, instead of the easy to size 6 tube fins the same size as the body.

A single large concentric fin is called a ring fin in RockSim. Different look, but also adds a lot of drag compared to straight fins.