Tube Fin Two Stage

[accooper]

OK, John has got my interest up. I have built at least 10 rockets using tubes for fins. Now I would like to build a two stage with BOTH stages having tubes for fins.

I would like the center tube to be a BT-20 and the tube fins to be BT-55. Both stages would us 18mm engines. The stages would be the old standard engine to engine staging. The sustainer length would be 10.5 inches in length. and the booster 2.75 inches.

Here is the question. How long would the tubes need to be on the booster and how long on the sustainer?

Any ideas where to start?

Andrew From Texas

 

[accooper]

I've started the gathering of parts to make my two stage tubber. Here are some pictures. I will try and get some more done on it tomorrow so I will upload more pictures as it goes together.

Basic parts include:

1 Semroc balsa nose cone BNC 20G3

3 2.5 inch pieces of BT-50 cut at a 45 degree angle

3 3.5 inch pieces of BT-50 cut at a 45 degree angle

1 9.25 inch piece of BT-20 for the sustainer

1 3 inch piece of BT-20 for the booster

2 BT-20 size engine blocks

24 inch piece of Kevlar thread to connect the shock cord to

18 inch piece of 1/4 inch elastic

2.5 inch 1/8 inch launch lug

Wood glue to glue it together with.

All straight forward construction techniques are used nothing fancy.

 

[JAL3]

Andrew,

One of the things you need to know about tube fins is that for any given tube size, you can exactly fit six tube fins around it if they are the same size. That means if your main tube is a BT20 you can evenly space 6 BT20 tube fins around it and not have any space left over.

You can use other sizes but the math doesn't work out for even spacing.

As to length, they can be pretty short...to the point where you would call them ring fins.

My unsanctioned "made it up myself" definition of a ring fin os one where the length is less than the diamter.

 

[BobCox]

The generic rule of thumb for tube fin rockets is to make the length be 1/6th of the main body length. There is not a lot of science in that rule. It is done mainly because it is easy to get two tubes the same length, cut one into 6 equal pieces, and they will fit exactly around the main tube.

In general, the tube fins start to become less effective the longer they are. Once their length gets much beyond 2x their diameter, the air no longer flows through the tube but gets bunched up inside. This decreases the stabilizing effect of the tube fin. On a 2-stage rocket, you need to make sure that the combined length of the two stages' fins do not get too long.

The tube fins in your photos look like they will probably be fine. Adding those bevels should help improve its ability to work at non-zero angles of attack.

You should do a swing test with both motors loaded before flying it, but it will probably be fine.

 

[MarkII]

These rather longish tube fins work quite well, but then, there are only three of them on the rocket, not six.

MarkII

 

[accooper]

The first thing I have done is to marked both the first and second stages for three fins and "tacked" them on with thick CA.

Andrew

 

[accooper]

After "tacking" on the tubes, and letting them dry, I mounted the engine block of the first stage flush with the back end of the BT-20 body tube. I used a small amount of wood glue for this.

Andrew

 

[accooper]

If there is any tricky items in building this rocket, installing the second stage engine block is it.

First I took the engine block, cut a small grove into it, not deep, then tied the kevlar string with it lying into the grove. This keeps it from distorting the tube. A small amount of wood glue on the knot doesn't hurt anything.

THEN, I taped two engine casings end to end. (see picture 1)

Then inserting the two taped engines into the first stage and making sure they butted up against the engine block of stage one (see picture 2).

I then fished the Kevlar string through the long stage 2 body tube, smearing a little wood glue into the end of stage 2, I inserted the engine casing pushing the engine block into the stage 2 tube. Then immediately removing the engine casing.

That's As Far As I've gotten. More later!

Andrew From Texas

 

[MarkII]

It looks like a good start, Andrew. Your use of oversized (1.8x) tubes for the fins is quite interesting; I have not seen that done very often.

John, I'm not 100% sure, but I think that your description of ring vs. tube fins is missing the key distinction between them. I have seen several examples of tube fins that are shorter than they are wide, In fact, there was an article in Sport Rocketry by Larry Brand that summarized quite a bit of research that he had done, in which he concluded that short tube fins are more effective and less draggy than long ones (using the typical 6-tube, or in his case, 7-tube, configuration).

The way I understand it, a ring fin consists of a stabilizing cylinder that contains the rocket's principal vertical axis within its diameter. Typically, a ring fin is centered on the rocket's vertical axis. In other words, the rocket's airframe and the ring fin are concentric, and their vertical axes are identical. Think of the Astron Sprite, the FlisKits Tumble Weed or the Starlight Hyperon.

I would define a tube fin as being a stabilizing cylinder whose entire diameter is laterally displaced away from the rocket's vertical axis. It has a vertical axis that is parallel to the airframe's vertical axis, but is not concentric with it. Although the FlisKits Borealis is usually described as having ring-fin stabilization, I think that it's large outboard rings are technically tube fins.

A rocket can have multiple ring fins, all concentric. The Estes Hyperion was one such design. A ring fin can also be longer than it is wide. The Estes World Federation Star Probe had a long, tube-like ring fin.

A rocket can be stabilized by just one tube fin if it is combined with other stabilizing structures. The Estes EAC Firecat, for instance, uses two wing-like fins plus a tube fin for stability. Larry Brand has built a number of rockets using fins made out of tubes that are shorter than they are wide. Here's one example. Here's another. And another. Larry Brand appears to be the man to talk to about tube-finned rockets.

MarkII

 

[JAL3]

It looks like a good start, Andrew. Your use of oversized (1.8x) tubes for the fins is quite interesting; I have not seen that done very often.

John, I'm not 100% sure, but I think that your description of ring vs. tube fins is missing the key distinction between them. I have seen several examples of tube fins that are shorter than they are wide, In fact, there was an article in Sport Rocketry by Larry Brand that summarized quite a bit of research that he had done, in which he concluded that short tube fins are more effective and less draggy than long ones (using the typical 6-tube, or in his case, 7-tube, configuration).

The way I understand it, a ring fin consists of a stabilizing cylinder that contains the rocket's principal vertical axis within its diameter. Typically, a ring fin is centered on the rocket's vertical axis. In other words, the rocket's airframe and the ring fin are concentric, and their vertical axes are identical. Think of the Astron Sprite, the FlisKits Tumble Weed or the Starlight Hyperon.

I would define a tube fin as being a stabilizing cylinder whose entire diameter is laterally displaced away from the rocket's vertical axis. It has a vertical axis that is parallel to the airframe's vertical axis, but is not concentric with it. Although the FlisKits Borealis is usually described as having ring-fin stabilization, I think that it's large outboard rings are technically tube fins.

A rocket can have multiple ring fins, all concentric. The Estes Hyperion was one such design. A ring fin can also be longer than it is wide. The Estes World Federation Star Probe had a long, tube-like ring fin.

A rocket can be stabilized by just one tube fin if it is combined with other stabilizing structures. The Estes EAC Firecat, for instance, uses two wing-like fins plus a tube fin for stability. Larry Brand has built a number of rockets using fins made out of tubes that are shorter than they are wide. Here's one example. Here's another. And another. Larry Brand appears to be the man to talk to about tube-finned rockets.

MarkII

MARK:

Boy am I glad I qualified my response!

In reading through yours, I think you are mostly right. I would have answered something along your line a year ago, still vague but same general idea. Concentric circles form the basis of my original understanding of tube fins.

I suppose my understanding of the term shifted when I built a Fliskits Dead Ringer ( https://www.fliskits.com/products/01prod_fs.htm ). Those ring fins are not concentric but do fit my criterion of shorter along the axis than diameter. Without ever really thinking about it then or now, my use of the term shifted.

Visually, I was surprised when I read the descriptions of Mr. Brand's rockets you provided links to. Only on the third one, the Aquafina, do the tube fins appear to be shorter than their diameter. When looking at the first two, they seemed (to my eye at least) to be a bit longer and about the same respectively. I even have a piece of paper with tick marks on it on my desk where I tried to gauge them. I was surprised when I read in the construction descriptions and he actually gave dimensions and they are shorter. So much for trusting my eyes!

There may well be formal definitions of both ring and tube fins out there; I have not bothered to look. What I provided was my conceptual understanding and it looks like I will have to modify it yet again.

As for the Dead Ringer, I don't think that I could ever bring myself to call those fins tubes, whatever the actual definition is. They just look too much like rings.

Thanks for the clarification.

ANDREW: stop wasting your time reading our drivel and get back to building!:grinch:

 

[accooper]

Right now I am on my way to Seguin to pick up my son for Christmas break. I'll right some more latterz.

Andrew

 

[dragon_rider10]

That looks like an interesting concept. I hope to see more soon!

 

[MarkII]

The following is completely beside the point, Andrew, so just skip over it.

OK, John:

If you read the articles that I linked to, you will see that Larry Brand mentioned the lengths that he made the tube fins. In all three, the tubes were shorter than their diameters. For example, in one he used 3" diameter tubes that were 2.75" in height. His Aquafina was just the most dramatic example of this that I could find in a quick survey of his many articles about tube-finned rockets on EMRR.

What I was trying to do in my response was to provide definitions of "ring fins" and "tube fins" that were free of any mention of height. The reason was that I don't think that the cylinder height has anything with whether a circular stabilizer on a rocket is a ring fin or a tube fin. I realize that a discussion of what to call them is really pedantic, because it doesn't affect in any way how we use them. But like any profession, academic field or even hobby, we have our own language and vocabulary, and so we should recognize that our terms have precise meanings and that we all understand and agree about what our terms mean.

Tube fins do not have to be taller than their diameter, as I mentioned above. And ring fins do not have to be shorter than their diameter, either. Did you take a look at the Estes World Federation Star Probe? Most people would call the cylinder at the aft end of the model a ring fin, but as you can see, it is considerably longer than its diameter. In fact, the part that was used was BT-80S, which measured 2.6" in diameter and 4.5" tall. Since tube fins can be shorter and ring fins taller than their diameter, then the length (or height) of the component doesn't really have anything to do with whether we should call it one type or the other.

In my last post, I rashly (even foolishly) went out on a limb and attempted to describe the one characteristic that I saw as distinguishing a ring fin from a tube fin. When we see something that is called a ring fin, it is just about always positioned so that the center of the ring's diameter is in the same spot as the center of the airframe's diameter; in other words, they are concentric. Whenever we see something that we call a tube fin, it is always positioned along or immediately adjacent to the airframe's perimeter. The center of the tube fin's diameter is never concentric with the airframe's diameter; in fact, it is always positioned completely outside of the airframe's perimeter.

When I went on to define a ring fin, I wanted at first to include the requirement that it be concentric with the airframe. That is almost always what we see, after all. But then I realized that a rocket could have one or more ring fins that were "off center", so that the the center of the ring's diameter was not in the same place as the airframe diameter's center. But if it was far enough off to the side that all or nearly all of the ring fin was outside of the airframe's diameter, it would meet my definition of a tube fin. So I came up with the requirement that the center of the airframe's diameter had to fall within the perimeter of the ring in order for the attachment to be called a ring fin.

An analogy would be with the game of horseshoes; in order for the thrown shoe to be a "ringer," it just needs to land in such a way that the post is somewhere within the incomplete circle made by the arms of the shoe. If the "post" (the center of the airframe's diameter) is within the circle described by a cylindrical stabilizer, that stabilizer is a "ringer."

If the center of the airframe's diameter falls outside of the circle described by a cylindrical stabilizer that is attached to it, the stabilizer is called a tube fin. Note that according to this definition, the entire circle described by the tube fin does not have to be outside of the airframe. That is almost always how they are positioned, though.

The FlisKits Borealis and the Dead Ringer both utilize very large, thin sections of tubing (rings) for stabilization. But because the center of the fuselage or airframe diameter is outside of all of these rings in both cases, the rings on these two models are, in fact, tube fins.

In no way did I post this to disparage your earlier comments. This is simply a discussion about which term to apply to these structures when they are placed on a rocket. Your comments, and those of anyone else who wants to wade into this swamp, are welcome.

Now back to our regularly schedule program, which is already in progress.

MarkII

 

[JAL3]

The following is completely beside the point, Andrew, so just skip over it.

OK, John:

If you read the articles that I linked to, you will see that Larry Brand mentioned the lengths that he made the tube fins. In all three, the tubes were shorter than their diameters. For example, in one he used 3" diameter tubes that were 2.75" in height. His Aquafina was just the most dramatic example of this that I could find in a quick survey of his many articles about tube-finned rockets on EMRR.

What I was trying to do in my response was to provide definitions of "ring fins" and "tube fins" that were free of any mention of height. The reason was that I don't think that the cylinder height has anything with whether a circular stabilizer on a rocket is a ring fin or a tube fin. I realize that a discussion of what to call them is really pedantic, because it doesn't affect in any way how we use them. But like any profession, academic field or even hobby, we have our own language and vocabulary, and so we should recognize that our terms have precise meanings and that we all understand and agree about what our terms mean.

Tube fins do not have to be taller than their diameter, as I mentioned above. And ring fins do not have to be shorter than their diameter, either. Did you take a look at the Estes World Federation Star Probe? Most people would call the cylinder at the aft end of the model a ring fin, but as you can see, it is considerably longer than its diameter. In fact, the part that was used was BT-80S, which measured 2.6" in diameter and 4.5" tall. Since tube fins can be shorter and ring fins taller than their diameter, then the length (or height) of the component doesn't really have anything to do with whether we should call it one type or the other.

In my last post, I rashly (even foolishly) went out on a limb and attempted to describe the one characteristic that I saw as distinguishing a ring fin from a tube fin. When we see something that is called a ring fin, it is just about always positioned so that the center of the ring's diameter is in the same spot as the center of the airframe's diameter; in other words, they are concentric. Whenever we see something that we call a tube fin, it is always positioned along or immediately adjacent to the airframe's perimeter. The center of the tube fin's diameter is never concentric with the airframe's diameter; in fact, it is always positioned completely outside of the airframe's perimeter.

When I went on to define a ring fin, I wanted at first to include the requirement that it be concentric with the airframe. That is almost always what we see, after all. But then I realized that a rocket could have one or more ring fins that were "off center", so that the the center of the ring's diameter was not in the same place as the airframe diameter's center. But if it was far enough off to the side that all or nearly all of the ring fin was outside of the airframe's diameter, it would meet my definition of a tube fin. So I came up with the requirement that the center of the airframe's diameter had to fall within the perimeter of the ring in order for the attachment to be called a ring fin.

An analogy would be with the game of horseshoes; in order for the thrown shoe to be a "ringer," it just needs to land in such a way that the post is somewhere within the incomplete circle made by the arms of the shoe. If the "post" (the center of the airframe's diameter) is within the circle described by a cylindrical stabilizer, that stabilizer is a "ringer."

If the center of the airframe's diameter falls outside of the circle described by a cylindrical stabilizer that is attached to it, the stabilizer is called a tube fin. Note that according to this definition, the entire circle described by the tube fin does not have to be outside of the airframe. That is almost always how they are positioned, though.

The FlisKits Borealis and the Dead Ringer both utilize very large, thin sections of tubing (rings) for stabilization. But because the center of the fuselage or airframe diameter is outside of all of these rings in both cases, the rings on these two models are, in fact, tube fins.

In no way did I post this to disparage your earlier comments. This is simply a discussion about which term to apply to these structures when they are placed on a rocket. Your comments, and those of anyone else who wants to wade into this swamp, are welcome.

Now back to our regularly schedule program, which is already in progress.

MarkII

Relax.

I didn;t take it as disparagement. And I did read where he gave the lengths of the tubes on his review. You are absolutely right. All I was saying is that my perception in looking at them was that they were longer than the diameter, except the Aquafina. That DID look shorter.

As I said, it was all my perception, modified over time by the Dead Ringer. There was nothing rigorous at all in my treatment, just perception.

I'll even go so far to agree that with the right definition, the rings on the Dead Ringer and Borealis are really tubes. To me though, they still look like rings.

I took no umbrage in anything you said.

:santa-smile:

 

[MarkII]

I took no umbrage in anything you said.

:santa-smile:

I didn't think that you would, but I was concerned about what other forum members might think if they somehow blundered into this exchange in what is otherwise an interesting thread.

The stabilizers in the Borealis and Dead Ringer ARE rings (what else could you call them?); rings that, in those designs, are used as tube fins. The stabilizer on the WFSP is obviously a tube that just happens to be used as a ring fin.

:hohoho: , indeed, and :merry-christmas: , my friend.


OK, Andrew, you can have your thread back now.

MarkII

 

[accooper]

Sorry For The Delay. had to go get my son from TLU for the Christmas Holidays.

Where was I?

Oh Yea, The Next thing is after the tubes are good and dry, cured overnight is best I used Tite Bond Molding and Trim Glue to fillet the tubes. After letting it cure over night I repeated this on all tubes.

Andrew

 

[accooper]

The next item to be installed was the launch lug. 2.75 inches from the bottom of the sustainer body tube. I tack mine on with thick CA, let it cure over night, then fillet it as I did the tubes using Tite Bond Molding and Trim glue. Let that cure over night and then do it again.

Andrew

 

[accooper]

Since this is a pretty straight forward construction, I really didn't have any unforeseen problems, until I went to "swing" test it. I found that the CG was a little too far back, so I drilled out the nose cone, inserted 1/8th ounce weight as far into the nose cone as I could. I then plugged the hole using a 1/4 inch round dowel. Gluing it in using wood glue and then trimmed it off with my Dremel tool. After it cured I drilled a 1/16th inch hole for the screw eye.

I have since "swing" tested it with satisfactory results.

More to come as the weather here clears up.

Andrew
Dark Lord Of The Scratch Builders

 

[BobCox]

Did you try the swing test in both single-stage and two-stage configuration?
For the two-stage configuration, did you have the tube fins aligned or interleaved?

 

[accooper]

I tried them both ways. It was a little more stable interleaved, but not enough for my liking.

With the nose weight it is very stable.

Andrew

 

[accooper]

Here is a picture with everything together. She is ready for painting and installing the recovery system.

Andrew
Dark Lord Of Scratch Builders

 

[accooper]

here she is all ready to fly next weekend. On my experimental rockets I like to keep the paint scheme simple and bright. Makes it easier to find.


Andrew
Dark Lord Of The Scratch Builders

 

[Bone Daddy]

Beautiful.

Can't wait to see launch report.

I love multistage rockets. There's just something too cool about watching that booster tumble to the ground and then too aggravating to then realize you have no idea where the rest of the beast went.

I would love to see an A or even a 1/2 A booster.

 

[accooper]

Well, in all my years of building and flying model rockets I have only ever built one 2 stage rocket, and that was back in 7th grade. I built an Astron Apogee II for the science fair. I was trying to prove that you could seed clouds with model rockets. Lost every one I flew.

Back in those days there was no "D" or "E" Estes engines. To get that Apogee to 2000 feet with a 1/2 ounce payload of Sodium Chloride , I had to use a B14-0 and a C6-7. I don't care what anyone else says, the B14-0 was a more powerful booster engine. I just couldn't reach 2000 feet with a C6-0. I miss the B14 series of engines. I also miss the A8-0 and 1/2A6-0 boosters.

BTW I hope to fly the 2 For Tubes on Saturday when out group has their launch. John Lee takes some mighty nice pictures. Always gets my good side!

Andrew
Dark Lord Of The Scratch Builders