Help with desgn of 3" tube fin scratch build

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

kjkcolorado

Well-Known Member
TRF Supporter
Joined
Jan 7, 2013
Messages
352
Reaction score
3
Location
Denver, CO
I have a few questions for the skilled rocket craftsmen and engineers of TRF. Here are my intentions:

3" tube fin design with 54mm motor mount - I want the flexibility of flying any CTI motor from 38mm (assuming safe thrust-weight and velocity off the rail) to a 6-grain, 54mm (possibly 6XL, but don't own that case....yet)

Initially I was thinking about Blue Tube for the airframe and tube fins. However, I am really intrigued with what I have read/seen on TRF regarding the MAC Performance canvas phenolic tubing and would really like to try it. The biggest downside is that it is about 2x the cost of Blue Tube per inch, and I will have to pay shipping (Apogee is close enough that I can drive to buy Blue Tube there). But, if it matters for my questions below, let's assume MAC Performance's canvas phenolic tubing.

Nose Cone TBD - I'm up for suggestions here. The cones that MAC Performance sells are 5:1. For what I have in mind aesthetically I want a shorter rocket with a shorter looking nose cone (maybe 3:1).

Dual Deploy using nose cone volume (and short payload section if necessary) to pack main chute

GPS transmitter (TBD) housed in av-bay aft of nosecone

Tube fins cut at angles both foreward and aft (not a 'square' 90 degree cut) - think of a similar profile as the Madcow DX3 fins except with tube fins

Aeropoxy ES6209 for all bonding

Possible name: 'UNFINITY' is the only one I keep coming back to

OK. Here are the things I need help with:

1) I plan on borrowing a friends 12" chop saw capable of angled cuts (but not compound angles/mitres) to cut the tube fins. Any suggestions on how to keep track of the long axis of the tube fin when I turn it around to make the second angled cut with the chop saw?

2) I want to keep the rocket as light as possible. What is the shortest length you would go for the 54mm motor mount? My Aeropac 54-38 adapter is just over 7" long, so I need it at least long enough to fit it. Remember I want to be able to fly 6 grain 54s (possibly 6XL).

3) Again, for weight considerations, 2 or 3 centering rings for the motor mount? If it makes a difference in your answer, I will most likely order the centering rings from MAC Performance and they make an aft ring that is stepped, so it also functions similar to a thrust plate.

4) Any thoughts on how the angled tube fins will react at higher velocities? For sake of discussion, right now my design sketches have the 'root' edge of the tube fin at 3 calibers (9"). I am anticipating the outer edge of the tube fin tapering to one caliber (3") in length. For strength considerations I don't think I should go any 'thinner' than that, but I have no objective information to justify that thinking.
 
One piece I'd give is to make sure the first tube fin is absolutely straight when you glue it on, otherwise they'll all come out crooked and won't line up when you get to the last one. I did this by placing both the BT and the fin flat (parallel to the long axis) on a piece of glass until the glue started to set.
 
Zeroignite, good call. I've already been thinking about that. Definitely going to follow your advice and the method of bonding the first two tube fins (on opposite sides) with everything flat. I need to take my level out and check the possible work surfaces in my house. Wondering if a piece of glass still might sag a bit if the work surface isn't completely flat/level to start...
 
I use 2 pieces of 1 inch angle aluminum placed along the rocket tubes to ensure that it's straight. Hold with rubber bands. Cheap, easy, and fool proof. As a bonus, it doubles to be able to draw straight lines on larger body tubes as well.
 
Last edited:
Contact Mike at MAC to see what he would recommend as a compatible 3:1 nose cone.

For your other questions:

1 - Mark a line along the length of the tube before you cut. Use this as a reference against the fence or some other convenient feature on the saw to keep the tube properly aligned.

2 - I use 14" for motor tubes, but you can go shorter. If you're really trying to save weight, just make it long enough to hold the adapter, say 8".

3 - 2 centering rings is sufficient, especially with a short motor tube. Make decent fillets.

4 - That's a good question. I know some of the King Kraken rockets have shredded their fins, but they are unsupported, unlike a 6 tube fin rocket. Bond well between the fins and add some fillets and I think you'll be fine.

Check out Iter's Bluefin Tuba threads (38, 3" and 4") for some great build techniques.
 
One piece I'd give is to make sure the first tube fin is absolutely straight when you glue it on, otherwise they'll all come out crooked and won't line up when you get to the last one. I did this by placing both the BT and the fin flat (parallel to the long axis) on a piece of glass until the glue started to set.

I've built a lot of tube rockets (including a 3" diameter tube fin for my failed L1 cert - nose separated from body - wasn't pretty!). The easiest way to make sure tube fins are straight is to glue all the fins into pairs. THOSE are laid flat on table while they set up, then each pair is glued to the body tube. Makes building tube fin models a snap!
 
Contact Mike at MAC to see what he would recommend as a compatible 3:1 nose cone.

For your other questions:

1 - Mark a line along the length of the tube before you cut. Use this as a reference against the fence or some other convenient feature on the saw to keep the tube properly aligned.

2 - I use 14" for motor tubes, but you can go shorter. If you're really trying to save weight, just make it long enough to hold the adapter, say 8".

3 - 2 centering rings is sufficient, especially with a short motor tube. Make decent fillets.

4 - That's a good question. I know some of the King Kraken rockets have shredded their fins, but they are unsupported, unlike a 6 tube fin rocket. Bond well between the fins and add some fillets and I think you'll be fine.

Check out Iter's Bluefin Tuba threads (38, 3" and 4") for some great build techniques.


Thanks for the advice.
1. I already contacted Mike at MAC Performance Rocketry. He can cut my tapered tube fins for me keeping the long and short axis 180 degrees from each other, so that problem is solved.
2 & 3. I do want to save as much weight as possible, particularly in the rear. So, I am leaning towards a shorter motor tube with only 2 centering rings. I'll pay special attention to good bonding techniques and good filets on both rings.

I've read through all the threads of Ari's 'Blue Fin Tubas' (and a bunch more on tube fin rockets). Awesome stuff....that's what has me all excited to build my own tube fin rocket.
 
Last edited:
I've built a lot of tube rockets (including a 3" diameter tube fin for my failed L1 cert - nose separated from body - wasn't pretty!). The easiest way to make sure tube fins are straight is to glue all the fins into pairs. THOSE are laid flat on table while they set up, then each pair is glued to the body tube. Makes building tube fin models a snap!

I've seen the preferred technique of bonding the first 2 tube fins on either side of the body tube while on a perfectly flat surface, and pairing up the remaining 4 tube fins. One of the challenges with this design will be alternating 3 tapered fins with 3 straight cut fins. I need to make sure the long 'axis' of the tapered fins lines up to the body tube. So, I should be able to do the first 2 fins on the flat surface to the body tube as normal. However, I think it will be best to carefully do the remaining 4 one at a time to get the tapered tube fins aligned correctly. Not sure I am making sense. Once I get to this build I'll try to post some pics so it's easier to see what I am trying to describe.

Thanks to all for the input/ideas.
 
I've seen the preferred technique of bonding the first 2 tube fins on either side of the body tube while on a perfectly flat surface, and pairing up the remaining 4 tube fins. One of the challenges with this design will be alternating 3 tapered fins with 3 straight cut fins. I need to make sure the long 'axis' of the tapered fins lines up to the body tube. So, I should be able to do the first 2 fins on the flat surface to the body tube as normal. However, I think it will be best to carefully do the remaining 4 one at a time to get the tapered tube fins aligned correctly. Not sure I am making sense. Once I get to this build I'll try to post some pics so it's easier to see what I am trying to describe.

Thanks to all for the input/ideas.

Not as difficult as you might think - if you are using six tube fins the same size as the BT at least. All you need is a tube marking guide that allows for six fins (the Estes guides can be made to work, or you can use an end marking guide with six fins). Just mark each fin with three marks 60 degrees apart - the center mark is the line where each fin will glue to the body tube. Glue the fins into pairs with one of your slanted fins and one flat, making sure to use opposite outer marks. Besides - if you are using white or yellow glue and double glue joints, you need those three lines on each fin to show where the glue goes! ;)
 
Not as difficult as you might think - if you are using six tube fins the same size as the BT at least. All you need is a tube marking guide that allows for six fins (the Estes guides can be made to work, or you can use an end marking guide with six fins). Just mark each fin with three marks 60 degrees apart - the center mark is the line where each fin will glue to the body tube. Glue the fins into pairs with one of your slanted fins and one flat, making sure to use opposite outer marks. Besides - if you are using white or yellow glue and double glue joints, you need those three lines on each fin to show where the glue goes! ;)

OK. I got it. Your explanation of marking the fins with 3 marks, 60 degrees apart makes sense. Thanks.
 
Just a quick follow up for those who checked in to this thread. As I noted above, I exchanged a few emails with Mike at MAC Performance Rocketry regarding the specs for the tapered tube fins and placed an order. He put together a custom 3" 'kit' for me with all the components for this dual deploy tube fin build. Everything looks awesome and fits great! Can't wait to build it, but I promised myself I would finish two other projects before starting this one.
 
Did you ever build this? I'm particularly interested because it's the only tubefin I'm aware of using canvas phenolic, and also because the design is extremely similar to one I'm still sketching on :)
 
Back
Top