Tube Finned High Power Rocket Tube Fin Length

I'm trying to simulate tube fins in Rocksim 9 and having a hard time. Is there a rule of thumb that sets the ratio of tube fin length to overall rocket length. I'm designing a 6 tube model built around a PML 3.0 inch phenolic tube

Honestly, I think most designs answer this from a practical and aesthetic basis rather than a mathematically optimized one: if, say, the donor tube was 24" to start with, you'd likely see 6 fins at 4" each. If 18", they'd be 3". I know some have done extensive research on this, and may provide a mathematical answer. I seem to recall something about seven smaller tubes being better off than six identical ones, but I'm not 100%, and that doesn't answer the tube length anyways...



Later!

--Coop

Yes 7 smaller fins is better than 5-6 . But the six fin design is the most popular. Now on the fin length . It is said 1.5-3 times the diameter or the air frame . I had to re read some reports to day with the some what not great flight of the super evil minoin. Also no video , my daughter turned the camera on then off . I was thinking I turned it back on at the pad . It didn't happen that way . It went on a I280 dm metal storm . I re did the math and had the fins at 4.5 x the diameter , it went up about 2-3 hundred feet and hit its wall and came down . Not much damage and no one was hurt . Tube fins are cool to play with .

I don't play around much with such sims but I've read that if you treat each tube as if it's three separate fins (3.14~) you can get a pretty good approximation. So a six tube fin rocket can be simulated with a 18 fins.

They're awfully stable. Even in windy conditions but they're draggy and really don't like mach. At all. Violently.

From what I've read and from my own experiences flying tube fin rockets, the best ratio seems to be about fin length equal to or less than the tube fin diameter. I usually shoot for fin length about 70-75 percent of the tube fin diameter. If this were an airplane, we'd be talking about aspect ratio where high aspect ratio wings are long and have short chord lengths while low aspect ratio wings are stubby and have long chord lengths.

View attachment 168306View attachment 168310
Picture #1 has short tube fins (high aspect ratio) while picture #2 has taller fins (low aspect ratio). Baloney sliced, too.

View attachment 168307
Picture #3 is my high powered "Super-Duper Neon 29" with very low aspect ratio tube fins. I'd estimate them length being at least 3x the diameter or more. This is what most people think of when tube fins are discussed.

View attachment 168311View attachment 168308
I have a lot of tube fin rockets. From 24mm to 54mm motor mounts.

Six tube fins are easy to do because they fit naturally. Seven fins have less drag and their coefficient of drag more closely simulates a conventional rocket but they are more difficult to construct because you must add spacers to get them to fit properly.

Google "Larry Brand tube fins" for more information and pictures.