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  #1  
Old 09-26-2013, 03:46 AM
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astronot astronot is offline
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Default Tube fins Question

I'm trying to understand what benefits, or drawbacks there are related to using tubes as fins, and/or heavily incorporating tubes into a rockets overall design.
Aesthetically the tube design looks cool and unusual. I guess that why I'm drawn to learn more about it. I see videos of rockets with no fins, just tubes, flying on youtube. It may just be that I'm pretty simple minded but, it seems to me that there would an aweful lot of aerodynamic drag in a design like that. How stable is a rocket that is built like that? They don't seem to weather cock that much to be sure. Can they support heavier payloads? What exactly does the tube fin design lends itself to (do a good job at) other than it is just a cool looking design?
There is not a tremendous amount of information on the internet about this design element.
So... I was wondering if I could pick your brain. School me on the subject please.

Thanks for your help.
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  #2  
Old 09-26-2013, 07:53 AM
samb samb is offline
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Try searching for tube fin info by a guy named Larry Brand. He published some articles on tube fin design and aerodynamic analysis in the NAR's Sport Rocketry magazine. As I recall, the articles went into some depth with drag measurements and a lot of real experimental data gathered from flight tests. I personally just think they look cool !
FWIW my intuition tells me that tube fins will always be draggier and have less corrective ability than traditional shapes. The ones I've seen in person are very good windy day flyers.
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Old 09-26-2013, 08:02 AM
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gpoehlein gpoehlein is offline
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I've built and flown quite a few, going all the way back to the original Centuri Groove Tube. My current favorites are two kitbashes - my Tube Baby (a Baby Bertha with tube fins replacing the wooden fins) and my Tube Daddy (a Big Daddy built with seven BT-80 fins and a 3x24mm motor mount - it is awesome on three E9s). Tube fin rockets are very stable, but don't seem to be as susceptible to weathercocking as other designs with a similar margin of stability.

The simplest geometry is six tube fins around the central body tube with all being the same diameter. If you use different sized tube fins, you can fit more or fewer fins around the body tube, but spacers will be required (either space between the BT and fins or between the tube fins). The rule of thumb that I have heard about the length of the tube fins is that they should be no longer than about twice their diameter - much longer and fluid dynamics starts creating interference inside the tube fins. DON'T cap either the front or back of any of them - that just defeats the purpose. Likewise, power freaks are tempted to put motors inside the tube fins - again, tube fins depend on the air going THROUGH them to work, so don't do it. Oh, and more is not better - some informal research has been done on tube fins by NAR members, and seven tube fins is pretty much the best you can get - eight or more reduces the efficiency of the tube fin.

To start out, just grab a copy of your favorite kit (Baby Bertha, Alpha, Skywriter, - they all work) and a length of body tube the same size. Start with six tube fins about the same diameter as the diameter (3/4" for BT-20, 1" for BT-50, 1-1/4" for BT-55, and 1-1/2" for BT-60). With this general plan, you pretty much can't go wrong.
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Old 09-26-2013, 08:17 AM
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astronot astronot is offline
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Default Tube Fins Question

Thanks for the replies and info sharing Samb, and Greg Poehlein.
Greg Poehlein
You touched on something that made me think of another question. Does air have to flow freely around the outside of the tube as well as the inside? For instance, can you over crowd the tubes? Do they need to not touch each other? Is there a minimum gap that needs to be maintained between the tubes. If memory serves me, it seem as though I've seen designs where the tubes are touching each other all the way around the rocket and I've seen other designs where they are evenly spaced and some where they are group together in groups around the rocket.
Does a rocket design with fins and tubes work better than either fins or tubes by themselves?
Several different designs spring to mind. The flying stove pipe, and some of the Syfy designs, etc...
Thanks for all your help.

Last edited by astronot : 09-26-2013 at 08:21 AM. Reason: left off some information
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Old 09-26-2013, 09:44 AM
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gpoehlein gpoehlein is offline
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Quote:
Originally Posted by astronot
Thanks for the replies and info sharing Samb, and Greg Poehlein.
Greg Poehlein
You touched on something that made me think of another question. Does air have to flow freely around the outside of the tube as well as the inside? For instance, can you over crowd the tubes? Do they need to not touch each other? Is there a minimum gap that needs to be maintained between the tubes. If memory serves me, it seem as though I've seen designs where the tubes are touching each other all the way around the rocket and I've seen other designs where they are evenly spaced and some where they are group together in groups around the rocket.
Does a rocket design with fins and tubes work better than either fins or tubes by themselves?
Several different designs spring to mind. The flying stove pipe, and some of the Syfy designs, etc...
Thanks for all your help.


Tube designs are rather flexible and forgiving. Yes, for the best effect, air should be able to travel both through and outside the fin - This is because the geometry of the circle means that your tube will have about 3 times the surface area (and thus, aerodymanic corrective action) as a fin with the same profile as the tube. In fact, this is how we used to sim the tube fin rocket in Rocsim before the version came out that supported tube fins. If, for example, you had six BT-50 tube fins that were two inches long, you would put 18 rectangular fins around your rocket that were 2 inches along the root and a span of .976" (the diameter of a BT-50). This would effectively simulate the tube fins. Another layer of tubes around the first not only defeats the purpose, but adds a lot of weight to the rear of the rocket. And Larry Brand showed in is research that eight or more tube fins actually reduces the efficiency and you will get less altitude as a result, all other factors being equal. Seven is the optimal number (your central tube will have to be somewhat larger than the tube fins - I'd have to run some calculations to find out how much), but six is still very good, and the geometry works perfectly for those six fins (they form a hexagon around the central body tube). Having the tube fins touch each other is fine, as is having a small gap between the fins or having a gap between the ring of fins and the central body tube.

In general, unless you are trying to test the boundaries and experimenting to see what works and what doesn't, I'd say you are best off to stick to 5, 6 or 7 tube fins, equally spaced around your central body tube. If there is a gap between the tube fins, you can put a spacer (cardboard or balsa) bridging the gap or just leave it open. Obviously, using spacers will make the design a lot stronger. Also, I have found that, with larger tube fin rockets (BT-80 and higher), you want to strengthen the tube fins - a normal body tube will get deformed when it lands and your tube fins will no longer be nice and round. Several ideas proposed include putting a second layer of body tube inside the first (cut a second tube lengthwise and remove any overlap when inserted inside the tube fin - the same technique as making a tube coupler out of a body tube) or putting a ring of coupler stock inside the aftmost part of the tube fin (1/2" to 1" - does the same as a second layer but removes a lot of the weight).

Like I said in my earlier post, try a simple six tube design with the tube fins the same size as the central BT - Estes' Super Neon and Super Neon XL are fine, and can be built without the little external fins for a true tube fin experience. ASP has a couple of tube fin rockets as well, and almost anything can be converted with a little extra body tube. Mixing up the size of the tube fins and central tube is a bit more challenging, and I recommend using a draw program such as Adobe Illustrator, or using rocsim to figure out the spacing of the tubes.

Greg
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Hint #1: Do not use magician's flash paper for recovery wadding!

Hint #2: Clean your shoes after flyin' in that cow pasture - that ain't no dirt clod on the sole!
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  #6  
Old 09-26-2013, 09:59 AM
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astronot astronot is offline
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Thanks for your reply. The design sounds pretty forgiving so long as you adhere to some guidelines.

I think I'm gonna play with a Semroc Goliath for my first tube fin rocket. Do tube fine rockets like lots of speed or just just average speed. The Goliath will give me the capability of a 3 motor cluster 18mm BP, a 24mm BP motor, or a single 18mm BP motor.

Thanks for the Google search tip Samb. I will be looking into that pretty soon as well.
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  #7  
Old 09-26-2013, 10:24 AM
samb samb is offline
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NP astro, good luck. Larry's kinda famous in tube fin circles and you won't do much better than studying all the knowledge that Greg dropped here.
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