Tube fin query

[Barracuda]

I've read quite a lot in the archives here about tube fins.The 3 fin per tube rule and the same dia.tube fin as the BT.It's hard to put it in OR also,my question I guess is if it passes the swing test it's good to go?I've figured out how to put them in OR as an inside tube and adjust the radial but OR doesnt take any of that into consideration for stability...is there an easier method?Got BT-80 and BT-60 tubes and such "on the way"I wanted the airframe BT-80 and the tube fins BT-60 around 45" tall.....I'm POSITIVE someone will offer suggestions

[powderburner]

What's the "three fins per tube" rule?

You can use tube fins that have a smaller diameter than the main body tube, just realize that when they get really small, they will likely be buried in the airflow turbulence layer from the front end of the rocket and will not be as aerodynamically effective as you might expect. (IOW, they will contribute little stability benefit.)

The main reason you see so many tube-fins-same-size-as-main-BT is that they stack up nicely in a clean geometric pattern. This makes them easy to align, easy to assemble, and (in the opinion of some) a visually pleasant configuration. You can go bigger, you can go smaller.

What you can't do is make the tube fins too long themselves. Airflow through the inside of the tube fins will have its own boundary layer build-up, and when this gets too thick it will effectively choke the flow through the tube fin. You should be fine at length/diam ratios around two to five, might want to do some private flight testing if you go much beyond that.

[Rex R]

in open rocket, tube fins are not (yet) supported. so to guestimate stability one uses 3 fins of approx the same surface area per tube fin to get a close simulation.
rex

[[POW]Eagle159]

Heres some of my tube fin/ ring fin rockets... They are all are scratch built.

I juts look where the Cg is and estimate with my educated guess if its stable. I usually put some nose weight just to be sure.

They are all Bt-55 tubes... I think a Bt-60 and -80 tube fin rocket would me really cool!

[[POW]Eagle159]

The ones in these pictures are Bt-55 tubes, they all have flown well and multiple times.

[o1d_dude]

I like tube fin rockets and build them almost exclusively.

Don't bother with sim programs because most of them "can't handle the truth!" Yes, you can approximate using three fins for each tube although some say you should use 19 fins. I've also heard there are other considerations beside just the fin area that come into play but 18 or 19 fins is probably close enough government work.

I usually figure that the CP of a tube fin rocket is down inside the fin can...maybe 2/3 up from the bottom. My tube fins are usually shorter than the tube fin diameter by about 10-15 percent. By that I mean a 4" diameter tubefin is only 3-1/2" tall. Aspect ratio, if you will. I have two rockets where I have built the tube fins even shorter than that. These are the two rockets in the foreground of the above photo #1. If you examine the length of the tube fins in the above photo #2, you can see the fins of the foreground rocket are shorter than the rocket behind it. Both tube fins are the same diameter.

As to the number tube fins, six tubes of the same diameter as the central body tube will give you an exact fit. To use five larger diameter fins or seven smaller diameter fins will require some experimentation and/or the use of spacer shims to make everything come out right.

Larry Brand has done several year's worth of experimentation with tube fin rockets and generally states that while six tube rockets are the easiest to build there are not as efficient as the seven tube variety. There is much less drag and such rockets have a fairly consistent coefficient of drag across the range of velocities that can be encountered with different motor choices.

An aerodynamicist (is there such a word?) can probably explain this better but I am only a retired telcom guy who builds a lot of tube fin rockets.

EDIT: Forgot to mention that tube fin rockets are extremely stable with respect to wind. Weathercocking and tube fin rockets are things that you almost never hear in the same sentence.

[MarkII]

What's the "three fins per tube" rule?

Well, I wouldn't call it a rule, but...


...nothing wrong with using three instead of the usual six.


Just make like you're putting on six tubes (Groove Tube-style), but only glue on three. Use a fourth tube as a spacer between them to correctly line them up.

I have yet to see a tube-fin rocket that was not stable.

[mcderek]

Here is the one I'm working on now:



Bt60 tubes. Tube fins doubled with 1/64th ply. 29mm motor mount. Streamer goes in the Vagabond nose cone. 114 grams at the moment. I call it the "See Ya!" D-F powered

[Barracuda]

Well this is what I came up with going to call it..Vesuvius.BT-80 airframe 34"tall,4 BT-60 tube fins 7.5",24mm eng.mount.I'm liking it so far.Paint scheme hmm..don't want it to be a kraken look a like.I put a couple of stringers on the mm thinking it would distribute the thrust better,had another similar rocket that aquired a small crease in the BT just above the fins thought it was from the thrust(E18-4)because it landed in a grassy field.Anyone else done a "different"kind of mtr mnt,I've used the shear panel and gusset ideas.

[BABAR]

What you can't do is make the tube fins too long themselves. Airflow through the inside of the tube fins will have its own boundary layer build-up, and when this gets too thick it will effectively choke the flow through the tube fin. You should be fine at length/diam ratios around two to five, might want to do some private flight testing if you go much beyond that.

I didn't know this. I did an upscale of Apogee SR-72 Darkbird. I left off the inlet cones because I didn't know how to make them upscales and because I figured if I just left the tube open it would function as a tube fin and add stability. Not sure of exact dimensions now, but length was definitely more than 2.5 times diameter (if I understand your "length/diameter ratios around two to five" correctly.)

Puppy flew great, despite the drag I had unknowingly built into it.

Plans for the SR-73 Raven are currently dancing in my head. I will keep the above in mind.

THANX!

Tom

[Pem Tech]

What's the "three fins per tube" rule?
.

It was introduced to me as the Levinson Method and it is how we simulate the sculpted tube fins on the Kraken family of rockets. Essentially, you use three flat fins that have the profile of the tube fin, for each tube. Results have been very positive.
Here is an example...
http://pembertontechnologies.com/mod...structions.pdf

[Pem Tech]

Well this is what I came up with going to call it..Vesuvius.BT-80 airframe 34"tall,4 BT-60 tube fins 7.5",24mm eng.mount.I'm liking it so far.Paint scheme hmm..don't want it to be a kraken look a like.I put a couple of stringers on the mm thinking it would distribute the thrust better,had another similar rocket that aquired a small crease in the BT just above the fins thought it was from the thrust(E18-4)because it landed in a grassy field.Anyone else done a "different"kind of mtr mnt,I've used the shear panel and gusset ideas.

Looks great!
I'm looking forward to the flight report.

[Barracuda]

Here's a better pic,built a ejec.baffle on the upper half,might try the short version first I think it would do better on a D12..it's the same rocket just haven't glue the BT baffle section in yet.I was thinking(yea I know)maybe a splash of red and orange up top and the rest black

[UncleVanya]

It was introduced to me as the Levinson Method and it is how we simulate the sculpted tube fins on the Kraken family of rockets. Essentially, you use three flat fins that have the profile of the tube fin, for each tube. Results have been very positive.
Here is an example...

Layne - the original method used by Levinson was to use 3 fins per tube fin. However he later revised his findings for a more conservative result. The newer method he advocated in this article: http://www.apogeerockets.com/educati...sletter119.pdf suggests that only 6 fins are needed to simulate the CP of 6 tube fins.

I (like another on the thread) like shorter tube fins - mine are usually what I call square tubes (length = diameter) which is a borrowed from an old engine marketing bit that I remember (stroke vs. bore).

In any case, my experience is that tubes are very stable particularly with respect to changing wind conditions. However they do have their limits... I built a BT20 C6 powered tube fin that was higly unstable using dimensions that were very close to those used in a highly stable 3" design. I later traced it to the weight distribution - the balsa nose was too light in proportion and I had not thought about that adequately and misestimated the location of the CP.

[o1d_dude]

I like mine "oversquare". That is when the tube fin length is just a bit shorter than the diameter. To use
Brad's method, length<diameter. My 3" diameter tube fins are 2.75" long to put it in practical terms. I suppose one could call this type of measurement an aspect ratio of sorts.

[Barracuda]

Well this is what I came up with on a color scheme,hasn't flown yet.FF D12-3 maybe..then E28-4 reload.I'm thinking I'll still have to get me one of those Krakens

[o1d_dude]

Great paint work!

Be sure to snag some launch pics and share them.

[FastCargo]

...suggests that only 6 fins are needed to simulate the CP of 6 tube fins.

That makes more intuitive sense to me. In side profile (like if you did the old style 'silhouette' method of determining CP), a tube fin wouldn't look any different than a regular fin. Now, I know that's not completely true...I suspect the tube fin creates more drag (more surface area), but it should be close enough...

FC

[Barracuda]

Well my "Vesuvius" had its' maiden voyage last Saturday, very nice flight on a E18-4w reload.It had some spin at take off but shortly after it was the straightest flight I've seen.The video was almost as good as the pics.My daughter was down and has never seen my "research" she almost jumped when it ignited(love the sound of white lighting motors).
I'll get the vid up later,haven't uploaded to youtube yet

[Barracuda]

Here's a short clip of "Vesuvius".The music is the first one I picked off of the youtube editor..thought it kind of fit..chaotic.I think the initial spin was getting caught on the igniter wires

[Barracuda]

I'm trying to keep all these in the same thread..here's the second flight of "Vesuvius" better video and flight.There was almost no wind but it did arc over just a little to soon.Maybe try a F motor next time,I've not flown above an E class because I thought I didn't have the room.Still trying to break a 1000' with this rocket don't think I can do it with an E..really like these tube fins!

[accooper]

I've read quite a lot in the archives here about tube fins.The 3 fin per tube rule and the same dia.tube fin as the BT.It's hard to put it in OR also,my question I guess is if it passes the swing test it's good to go?I've figured out how to put them in OR as an inside tube and adjust the radial but OR doesnt take any of that into consideration for stability...is there an easier method?Got BT-80 and BT-60 tubes and such "on the way"I wanted the airframe BT-80 and the tube fins BT-60 around 45" tall.....I'm POSITIVE someone will offer suggestions

But it takes a real rocket man to make a tube fin rocket with seven or eight or nine tubes equally spaced around the main tube. My best so far is eight. Send me your email address and I will send you the equations to do this with.

Andrew

[o1d_dude]

But it takes a real rocket man to make a tube fin rocket with seven or eight or nine tubes equally spaced around the main tube. My best so far is eight. Send me your email address and I will send you the equations to do this with.

Andrew

You didn't mention whether your 8 tuber was LPR or bigger.

Larry Brand, the tube fin guru, shared his less than stellar experiences with his Octopod H E R E.

Then again it was a speed/power dependent thing and the H180 reload revealed the instability for certain.

[accooper]

You didn't mention whether your 8 tuber was LPR or bigger.

Larry Brand, the tube fin guru, shared his less than stellar experiences with his Octopod H E R E.

Then again it was a speed/power dependent thing and the H180 reload revealed the instability for certain.

Mine are all mid power go to my sites gallery to see them. Although I guess I could shove a G138 into my great white to make it high power. It has flown on a Road Runner F60, Aero-tech G38, G53, and a G64. My Ground Hog 2-R has flown on everything from a D24 to an F39.

Andrew

[CarVac]

I've always wondered: Why haven't I seen a tube fin rocket with two rows of tube fins: for example, an 18-tube-fin'd rocket. You could do cool things with the last row, like have them alternate lengths, or extend way back behind the rocket, or something like that.

[o1d_dude]

I've always wondered: Why haven't I seen a tube fin rocket with two rows of tube fins: for example, an 18-tube-fin'd rocket. You could do cool things with the last row, like have them alternate lengths, or extend way back behind the rocket, or something like that.

No reason why this couldn't be done.

It would have a LOT of drag, tho. OTOH, if you enjoy seeing the whole flight, that might just be your buttercup.