Tube fin diameter relative to body tube diameter?

[boatgeek]

I am advising students on a project that will likely use 3” tube fins on a 5.5” body tube. A question nagging at the back of my mind is whether there’s a rule of thumb for minimum tube fin diameter like there is for flat fins. Normally you want the span of a flat fin to be at least the tube diameter to make sure it’s clear of the boundary layer. I’ve seen the problem in action on a silly rocket that had 18mm tube fins all the way around a BT-80 body tube.

Anyone have experience with tube fins about half the diameter of the body tube?

 

[cls]

Hi, make the tubes as long as their diameter. 1 caliber. So, 3" long, 3" diameter. Ok to have 4, 5, 6, 7, or 8; how many fit around your 5.5" airframe?

 

[BABAR]

. I’ve seen the problem in action on a silly rocket that had 18mm tube fins all the way around a BT-80 body tube.

hey! I resemble that remark!



https://www.rocketryforum.com/threa...r-wide-body-bt-80-horizontal-recovery.166124/

See post 14

 

[BABAR]

double post

 

[boatgeek]

Hi, make the tubes as long as their diameter. 1 caliber. So, 3" long, 3" diameter. Ok to have 4, 5, 6, 7, or 8; how many fit around your 5.5" airframe?

Looks like 8 will fit, with a little extra space between. Maybe the students will throw 4 flat fins in the gaps. It's a silly rocket anyway.

 

[JoePfeiffer]

I'm not familiar with any similar rules of thumb on that. I'll be curious to hear how it works!

 

[boatgeek]

I'm not familiar with any similar rules of thumb on that. I'll be curious to hear how it works!

I'll give a report, but it's not going to be that useful for future design/analysis. There's not only 4 flat fins in the little gaps, there's 8 more 29mm tube fins out near the tips. It's named Bride of Frankenfin for a reason.

 

[grillsjosephbus]

• Mar 14, 2023

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• #15

Chocked Full 'O Data

Drag of Tube Fins: Apogee Peak Of Flight Newsletter 347

References on Subject
1. Stability of tube fins by Seth Avecilla.
2. The Effect of Model Rocket Tube Fin Characteristics on the Location of the Center on Pressure by Andrew Polashenski 11th grade science fair project.
3. Larry Brand, “Tube Fin Rocket Aerodynamics Revisited”, Sport Rocketry magazine, January-February 2008
4. Larry Brand, “Tube Fin Rocket Aerodynamics Revisited Part 2”, Sport Rocketry magazine, March-April 2008
5. Larry Brand, “Tube Fin Rockets - Seven Beats Six Part 3”, Sport Rocketry magazine, March-April 2010
6. Larry Brands Tube Fin Construction Articles: http:// www.rocketreviews.com/larry-brand-page.html
7. United States Air Force Academy Department of Aeronautics - Laboratory Facilities Handbook

 

[Back_at_it]

I have never seen one flown with tube fins half the size. I was at a launch last summer where someone used a tube fin for their level one. It was made from LOC parts with a 4 inch body tube and 3 inch tube fins. They were about 8-10 inches in length and I'd guess the entire rocket was about 60 inched long. Flew dead straight. Don't remember the motor but the rocket stayed in sight the entire flight.