New to tube fins

[UPscaler]

Hey All,

I know I already have plenty of projects, but I am just in the designing stage for this one. Dad and I want to have significantly more rockets to launch in spring. Plus, he wants to have a few different rockets he can choose from for his L2.

So, if I had a three inch diameter rocket, and want four tube fins, how do I calculate the diameter the tube fins must be so they all sit against each other?



Thanks guys!
-Braden

 

[SCE to AUX]

Rocksim would be the best way to play around with different configurations, and determine tube diameters, etc. The latest version supports tubefins, but the altitude predictions seem quite optimistic in my experience.

If your dad is thinking about an L2 size tubefin, I flew one for my L2 a couple months ago. Build thread here:

https://www.rocketryforum.com/showthread.php?t=14088

 

[KennB]

The geometry is pretty simple. The radii of the four tube fins will form a square that is the diameter of the tube fin on each side. The diagonal from corner-to-corner of that square will be two radii plus your three inch body tube.

D = the side of the square = the diameter of the fin tube

D+3 = the diagonal

You can then use the cosine of 45 degrees to calculate a value for D.

Of course, I say this late on a Sunday night so you'd better check my idea and your work twice and even break out the graph paper to check your results (CAD would help if you have access to it).

 

[UPscaler]

Okay, we have changed it to six 3 inch fins, just so it will all be the same sized tubing. If the main airframe is six feet long with no nosecone, will six one foot long tube fins suffice?


-Braden

 

[o1d_dude]

You should do some research on this. High aspect ratio fins vs low aspect ratio. Stubby bodies vs long bodies. Six fins vs seven. Phenolic fins reinforced with fiberglass. That sort of thing.

Look up "Larry Brand" on EMRR. Specifically, you might want to look up his "Golden Acorn" as well. It's a fairly uncomplicated L2 ship that will fly on F50s up to whatever your wallet can bear (54mm motor tube) as long as you keep it under 600 mph. Larry flew his on a J210 to an altitude of 4600'.

 

[UPscaler]

SCE to AUX,
Your rocket "purple people eater" is essentially the same rocket as the one we are planning. The fins are almost identical to what I have drawn up on cad... Great minds think alike!

The only difference is our airframe will be six feet long.

We are getting together with our friend Justyn Palmer, who has a rocket similar to yours, that I just saw fly yesterday on a 54mm L, no problems. He will probably have some tips and tricks for us. He fiberglassed it all, but we are going to use blue tube, because we want the finished product to be beautiful, not to mention, I have some 54mm blue tube sitting here :wink:

-Braden

 

[El Cheapo]

As a rule of thumb, I have heard tube fin height is equal to one inch of body tube diameter.

 

[AHansom]

I did a clone of the King Kraken from Pemtech using BlueTube and also put in a bigger MM from 38 to 54mm. The Kraken uses a 3" body and stands 42" tall. The tube fins are also made from 3" tubing so they dont touch each other but thats why I like the design. First flight was on a CTI J-760 to 6000'. and the BlueTube took it with no problem.

 

[UPscaler]

As a rule of thumb, I have heard tube fin height is equal to one inch of body tube diameter.

Wouldn't all tube fins then be one inch? Because no matter how big the body tube gets, one inch is one inch

maybe I'm not reading this right...


-Braden

 

[El Cheapo]

If the tube fin is same diam as the bt, the tube fin height would be 1" x 3". Your tube fin height would be 3". That's just what I heard.

 

[o1d_dude]

I usually make my fins slightly smaller in height than the diameter.

The comparison between fin diameter and length can be called aspect ratio.

One inch of length for one inch of tube diameter is "unity", ie. one to one.

I make my 3" diameter tube fins 2-3/4" tall/long so the ratio of height to diameter is less than one to one.

 

[gpoehlein]

I seem to remember that the rule of thumb was that the tube fins should not be more than about twice their diameter in length or they will suffer enough turbulance to reduce their effectiveness. Of course, the classic tube fin design, Centuri's Groove Tube, has a tube fin that is more like three times its diameter, and it flies just fine.

 

[o1d_dude]

Usually the longer tube fins are cut that way because the single thickness fin wall is not all that sturdy. Tube fin flutter is a Bad Thing.

Make the tube fins shorter but reinforce them to avoid flutter.

Gotta admit that SCE to Aux's tuber is quite striking, tho.

 

[powderburner]

Just in case you somehow didn't know, using six tube fins with a main airframe tube of the same diameter is sort of the "standard" way to go about this. That config makes it easier to align an assemble the first two tube fins (on a flat table) and then the next two (on top), and makes for super-quick assembly.

Okay, we have changed it to six 3 inch fins, just so it will.....

We recently had someone post a nice graphic here on TRF that showed various core and fin tube size combinations. Does anyone still have that link handy? (I couldn't find it)

 

[gpoehlein]

When I'm building a 6 tube fin model, I usually glue together the tube fins in three pairs. I can then easily glue the pairs around the main body tube.

 

[o1d_dude]

I've always done mine the same way PowderBurner describes. Even my Super Neon XL in spite of what the directions said.

 

[boomtube]

Here’s one of my takes on the “TubeFin” concept I call “Turbo-Tube”.

Six BT-50s surrounding a BT-55 main body. The gimmick is cutting and reattaching the ends of the tube-fins so they are at an angle. These act in the same manner as fin-tabs or canted fins in that they direct the airflow out the back in order to spin the rocket.

Truthfully they do that so well the rocket spins so fast it seriously degrades the performance. I can’t even fly it with anything larger than a C-motor or it spins so fast it becomes unstable.

 

[Pem Tech]

I did a clone of the King Kraken from Pemtech using BlueTube and also put in a bigger MM from 38 to 54mm. The Kraken uses a 3" body and stands 42" tall. The tube fins are also made from 3" tubing so they dont touch each other but thats why I like the design. First flight was on a CTI J-760 to 6000'. and the BlueTube took it with no problem.

Whazooooo

I am going to have to try a 54mm....

Great job.

 

[JAL3]

I did a clone of the King Kraken from Pemtech using BlueTube and also put in a bigger MM from 38 to 54mm. The Kraken uses a 3" body and stands 42" tall. The tube fins are also made from 3" tubing so they dont touch each other but thats why I like the design. First flight was on a CTI J-760 to 6000'. and the BlueTube took it with no problem.

This one looks like a good candidate for the Kraken gallery. May I copy it over there?

 

[AHansom]

Thanks John that would be great to put it in the gallery

Andy

 

[MarkII]

I like the idea of using just 3 tubes in tube-fin rockets. I haven't seen any degradation of stability by using half the normal number of tube in the rockets pictured below. They all go poker straight all the way to apogee, even in a breeze. The Viking I (on the far left), for instance, goes wicked high on a B6-6. I'd love to see what any of the first three would do on Quest D5s (or an F32 for the fourth one), but there's a very good chance that I would never see them again.

From left to right: Viking I, Viking II, Viking III, Viking IV.

 

[bradycros]

From left to right: Viking I, Viking II, Viking III, Viking IV.

Normally very few tube rockets catch my eye. However the Viking II is interesting. Do you have a plan for it you could share?

 

[MarkII]

Normally very few tube rockets catch my eye. However the Viking II is interesting. Do you have a plan for it you could share?

It probably helps that it's the only rocket in that picture than isn't still in primer. (That photo was take 1-1/2 years ago, but it could have been taken yesterday. The other 3 rockets are still in primer.)

The plans are right here. Click on the image to download and open the PDF file.

https://www.oldrocketplans.com/fsi/fsi1010/fsi1010.htm

PM me for parts and supplementary instructions.

 

[Scotty Dog]

So, if ya have two dif size tubes stacked like this, on a dif size main BT with smaller tubes with fins and NCs in the outer tubes , that must really throw a wrench in the equation. :gavel: I flew it first time on a B6-4 this weekend and it was PERFECT!