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Mid Powered tube rocket?

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Miles Goodson

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So I built a Mid powered rocket, an dit looks like it's going to fly really well. (I have yet to fly) I had some extra tube, and so I built a replica of the same rocket, but with tube fins. I hope to just be able to swap the nosecone out and fly them both on the same day. I did not know how long to make the tube fins, so I just used the same length of my fins on the other rocket. Any tips on tube fin length?
 

dhbarr

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As long as your CG didn't move too far back from the mass of the tubefins, you should be just fine. Tubefins are very forgiving.
 

Andrew_ASC

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As long as your not trying to compute tube fin flutter characteristics you should be fine. Nytrunner and I think it involves some ugly chain of differential equations to unlock. Unlike a flat plate fin which would be a standard NACA4197TN document and a few lines of algebra lol.
 

Andrew_ASC

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I wanna say there’s a high power kit called Kraken around and it can bust Mach with tube fins just fine.
 

dhbarr

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I wanna say there’s a high power kit called Kraken around and it can bust Mach with tube fins just fine.
You don't want to push Mach on a Kraken for two reasons: a) the fins don't touch each other to provide support, and b) those lovely sweeping curves concentrate flutter stress in a few areas.
 

Andrew_ASC

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Yeah weren’t there some guys shattering tube fins into a bazillion pieces?
 

Funkworks

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... Any tips on tube fin length?
Think of your rocket as a weather vane: The fins' role is to ensure the motor shoots downwards (backwards). This is achieved with fin size and fin position and depends on motor weight (entire rocket's weight distribution). Then size and shape are adjusted to minimize weight and drag.

As with any flying arrow, the center of mass (CG) has to be ahead of the weather vane center (CP).
 

boatgeek

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The general guidance I've seen is that the tube fin length shouldn't be more than 1 to 1.2 times the diameter of the tube, or air tends to go around rather than through the fins. I don't think that's a real problem for stability, it just makes the rocket draggier. Of course, you're not building a tubefin for high performance anyway. I've found that cardboard tube fins that are less than half a diameter long tend to get beat up pretty fast.
 

jqavins

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I read somewhere here on TRF that if tube fins are too long the air won't go through them and the cease to function. To the unaided eye the fins on the Kraken are obviously longer than 1.2 calibers; I don't know what the usable limit is, or if maybe what I read was wrong.
 

dhbarr

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I read somewhere here on TRF that if tube fins are too long the air won't go through them and the cease to function. To the unaided eye the fins on the Kraken are obviously longer than 1.2 calibers; I don't know what the usable limit is, or if maybe what I read was wrong.
They would only cease to function if they stopped moving the CP aft relative to the CG. So it would very much depend on their placement relative to the rocket length.
 

Alan R

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Here's all you need to know. I think this was a science fair entry by Tim Van Milligan's daughter... https://www.apogeerockets.com/education/downloads/Newsletter347.pdf

Basically... tube fins have about the same drag as regular finned rocket, you just have to calculate the surface area of your tube. Also, longer tubes create more drag.

In your particular case, if your tubes are the same length as the original flat fins ...they're too big. They can be considerably shorter and achieve the same level of stability.
 

Miles Goodson

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If my fins are too long, then could I make swooping cuts in them to decrease the surface area?
 
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