Through-the-wall fin gluing techniques

[4regt4]

I am attempting my first through the wall fins. Not sure how to go about gluing something that I can neither see nor reach.

Estes basically says "apply glue to edge, insert into slot". Some suggest a double dip, insert once as per instructions, remove, reapply glue. I question how well the glue gets applied this way. In particular, it would seem a filet would be desirable. But difficult. Also, it seems that being the surface (MMT) is curved, the glue would tend to flow away from the joint.

I've seen people slit the tube all the way out the back. Then assemble a fin canister and slide it in. But others have argued that weakens the tube structure.

FWIW, I'm making a LPR, 24mm motor tube.

Or maybe I'm over thinking this.

Hans.

 

[rharshberger]

Double dip glue method... just like Estes says, it is a LPR. OR Double Dip method HPR style, coat fin root with a heavy bead of glue, insert, press down, remove fin, recoat with a second heavy bead of glue, re-insert and place in fin jig or immobilize fin by some method until glue has dried/cured.

 

[Hobie1dog]

"over analysis leads to paralysis". I don't double dip and have never even heard of it , a single layer of the glue on the edge and then stuck down through the slot is good enough for low power rocketry. You're going to have external fillets in addition to this. Remember the glue is a lot stronger than the materials that you're using in low power.

 

[K'Tesh]

My preference is to leave the rear CR off (if possible) then after gluing the fin in place and doing the internal fillets, gluing it in place. I also have a habit of protecting the motor tube by inserting a pencil balloon and inflating it to prevent epoxy from dripping inside. Also, I now wrap the motor tube's back end with electrical tape to prevent the epoxy from building up and preventing the installation of the rear CR.

 

[jimzcatz]

I don't do through the wall, that's so 80s. I glue my fins directly to the motor tube making a fin can. I am very hesitant to ever say anything is "best", but I firmly believe this method is the best way to build a High power rocket. It gives you strong internal fillets without having to leave off the rear ring or having to inject (yeeeccchhh) epoxy. If your BT has precut slots, just extend them all the way aft. Install the fin, MT assembly as a unit and add external fillets. Internal BT fillets are a waste of time and epoxy.

 

[OzHybrid]

Hmm... A glue thread.

Check the fin tabs are long enough to reach the motor tube. A common issue is they don't get to the tube so no glue CAN stick. Ideally you want to have the fin vertical and you chosen method of alignment readily available. As others have said leaving the end centering ring off until the fins are in place gives you access to glue stuff. I use bamboo satay skewers and shave the ends flat. This gives you plenty of reach and lets you do small fillets in places that would otherwise be impossible.
It's only a 24mm motor so don't over glue it as this is just wasted glue if the strength of the glue is heaps more than the balsa? fins.
Good luck
Norm

 

[jimzcatz]

Bob Smith

 

[lakeroadster]

I am attempting my first through the wall fins. Not sure how to go about gluing something that I can neither see nor reach.

Estes basically says "apply glue to edge, insert into slot". Some suggest a double dip, insert once as per instructions, remove, reapply glue. I question how well the glue gets applied this way. In particular, it would seem a filet would be desirable. But difficult. Also, it seems that being the surface (MMT) is curved, the glue would tend to flow away from the joint.

I've seen people slit the tube all the way out the back. Then assemble a fin canister and slide it in. But others have argued that weakens the tube structure.

FWIW, I'm making a LPR, 24mm motor tube.

Or maybe I'm over thinking this.

Hans.

Cutting the slot so the fin fits tight into it makes the process much easier. The slot then holds the fin in place.

Follow the Estes instructions.... but then before inserting the fin into the slot I also use an oral syringe to apply a line of wood glue, through the slot, onto the motor mount tube. Then once the fin is inserted through the slot, rotate the rocket so the nose is vertical pointing up, that keeps the glue on the joint.. Then after a couple minutes, invert the rocket so the nose is pointing down, then the glue will want to run back towards the nose. I typically do this for about 5 minutes. This will give you a glue fillet between the MM tube and the fin. Super strong glue joint.

Unless your building a competition rocket, where extra weight is an issue, a bit to much glue is better than not enough.

On my ring fin booster I actually drilled holes in the body tube, so I could "inject" wood glue into the TTW cavity to glue the fins. The booster is a tumble recovery... rough service.

Try out the oral syringes... they make precise gluing so much easier.

 

[neil_w]

My process for LPR TTW:
1) apply a small amount of yellow glue to fin root.
2) Insert into slot, and then remove
3) Look in (use flashlight if necessary) to check if glue was left on the entire root area. If not, the tab might be too short, and some sanding might be necessary.*
4) If it looks good, then apply a bit more glue to fin root and insert. Let dry.
5) Apply external fillets

This is more than good enough for LPR.

*When scratch building, I usually cut my tabs a bit too long, so I can sand them down to precise fit. That's much easier than trying to lengthen them.

 

[BigMacDaddy]

When you insert your fin through the tube be sure that you do not distort the tube - easy to create a hollow where the fin pushes the body tube down along the slot. I use a small metal ruler to check that the body tube is not distorted. If you see this happening, may need to sand the slot a bit so that fin fits in the slot snug but not too tight.

 

[prfesser]

My preference is to leave the rear CR off (if possible) then after gluing the fin in place and doing the internal fillets, gluing it in place. I also have a habit of protecting the motor tube by inserting a pencil balloon and inflating it to prevent epoxy from dripping inside. Also, I now wrap the motor tube's back end with electrical tape to prevent the epoxy from building up and preventing the installation of the rear CR.

For those who are wondering "How do I keep the MMT centered in the body tube when gluing it into the BT?", here's an example with my Super Big Bertha scratchbuild.

1. Gather materials. Notice that the MMT here has three centering rings, for other reasons. This method works just fine for rockets with only two rings.
Sand thoroughly those areas on the MMT where the fin tabs will touch. Wipe down the sanded surface with rubbing alcohol (91% IPA) or a microfiber cloth.
(The holes drilled in the fin tabs have a purpose. Read on.)


2, If your rear CR doesn't have T-nuts or similar object that can be gripped, take two long strips of scotch tape or packing tape, and wrap each around the CR as shown below. The objective here is to make "handles" so that the CR can be removed after it's been pushed into the tube.


3. Apply glue inside the airframe to the location of the forward centering ring(s). Be careful to NOT allow glue to drip onto the location of the aft centering ring! Slide the aft centering ring onto the MMT, then insert this assembly into the body tube.


4. Now you should have a MMT that is perfectly centered in the BT, with the aft CR being removable. Allow the glue to dry/cure COMPLETELY, then grab the screws or the two pieces of tape and pull the aft CR out.



5. Apply glue to the root edge of a fin tab, and to the area on the MMT where that root edge will land. Slide the fin into its slot and against the MMT. As @BigMacDaddy notes above, DON'T push the fin against the MMT so hard that it distorts the BT. Tape or hold the fin in the proper position until the glue dries/cures. Be sure to wipe off any glue that runs onto the MMT where the other fin tabs will land. Also wipe off any glue that may prevent the aft CR from landing on the back edges of the fin tabs.
Repeat with the other fins, one at a time. Then add MMT-fin tab fillets, using a straw or similar to apply the glue. Don't worry about looks, those fillets will never be seen by anyone but you.

6. The holes drilled in the fin tabs (see the first image) permit expanding foam to get a better grip on the tab. I mixed up a little foam and poured it into the spaces between the fin tabs. Any foam that went where it wasn't supposed to was easily removed with a Dremel. Then the aft centering ring was glued in place, and external fillets applied as usual.

Best -- Terry

 

[bjphoenix]

My preference is to leave the rear CR off (if possible) then after gluing the fin in place and doing the internal fillets, gluing it in place.

This is what I do. I put the rear ring in place, unglued, when gluing the forward ring. Then take out the rear ring, do the fin fillets, and then glue the rear ring back in place.

 

[4regt4]

Glad to see great minds think alike. I've started the build, and have only temporarily set the rear CR in place....

Hans.

 

[dhbarr]

It's 24mm. Double dip wood glue, fillet trim glue, still overkill.

 

[beeblebrox]

I have started making alignment tools for holding the motor mount straight while initial gluing of TTW fins. You don't need a lot of glue to tack the fins in place. Remove the tool when glue is set, then apply your internal and external fillets per your favorite method. I like to use pieces of those bamboo kabob skewers on the inside of the body tube next to the fins. light weight and you don't need much glue. Then glue in the rear centering ring.

This one happens to be for standard LOC/AeroTech 2.6" tube and 29mm motor tube. STL files for three and four fin versions in the zip file.
Print settings not critical. Since every printer is different, print a small slice first. Lower the part in you slicer so that only about 2-3 layers of the base prints. Either set the slicer to stop after about 2-4 mm or cancel the print after part of the three or four top parts print. Then test it to fit your tubes. If too loose or too tight scale the model a tiny bit. These printed perfectly on my Ender 3 Pro. 0.6mm nozzle, 0.2mm layer height. Filament - eSun PLA+

 

[bjphoenix]

I should have said before but for MPR I typically add small t-nuts in the rear centering ring to use for my motor retention system, so that gives me something to easily pull the rear centering ring out when I'm ready to do the internal fillets.

 

[4regt4]

Follow up question:

If one (well, say maybe me..) is papering the fins. Given the strength increase, would it be a good idea to paper the entire fin, including the internal tab?

Edit: I'd probably leave a bit of "naked" wood for the glue joint on the motor tube.

Hans.

 

[BABAR]

Follow up question:

If one (well, say maybe me..) is papering the fins. Given the strength increase, would it be a good idea to paper the entire fin, including the internal tab?

Edit: I'd probably leave a bit of "naked" wood for the glue joint on the motor tube.

Hans.

Good question. I am thinking no, as I think you will need a bare wood area on the fin for your OUTSIDE body tube fillets (you want a joint wood to body tube, not PAPER to body tube.)

as people have said however, you are dealing with low power, so it may not matter than much.

Regarding strengthening the tab segment, I am curious for those who routinely do thru-the-wall fins (papered or not) how frequently the TAB section has been a failure point (I.e., papering will strengthen that segment, but if the fin rarely breaks there, papering may simply add weight and as above compromise the outer tube fillet.)

 

[4regt4]

Good question. I am thinking no, as I think you will need a bare wood area on the fin for your OUTSIDE body tube fillets (you want a joint wood to body tube, not PAPER to body tube.)

as people have said however, you are dealing with low power, so it may not matter than much.

Regarding strengthening the tab segment, I am curious for those who routinely do thru-the-wall fins (papered or not) how frequently the TAB section has been a failure point (I.e., papering will strengthen that segment, but if the fin rarely breaks there, papering may simply add weight and as above compromise the outer tube fillet.)

I would think, however (yes, I shouldn't do that), that the filet would still be a strong bond, as there would be a wood-glue-paper-glue bond. The paper would be sandwiched between 2 layers of glue and would likely be saturated with it.

As you alluded to, I am thinking that the most likely failure point would be at the surface of the tube.

Or I could be totally wrong...

Hans.

 

[kcobbva]

I have started making alignment tools for holding the motor mount straight while initial gluing of TTW fins. You don't need a lot of glue to tack the fins in place. Remove the tool when glue is set, then apply your internal and external fillets per your favorite method. I like to use pieces of those bamboo kabob skewers on the inside of the body tube next to the fins. light weight and you don't need much glue. Then glue in the rear centering ring.

This one happens to be for standard LOC/AeroTech 2.6" tube and 29mm motor tube. STL files for three and four fin versions in the zip file.
Print settings not critical. Since every printer is different, print a small slice first. Lower the part in you slicer so that only about 2-3 layers of the base prints. Either set the slicer to stop after about 2-4 mm or cancel the print after part of the three or four top parts print. Then test it to fit your tubes. If too loose or too tight scale the model a tiny bit. These printed perfectly on my Ender 3 Pro. 0.6mm nozzle, 0.2mm layer height. Filament - eSun PLA+

View attachment 515432

Thank you for sharing the STLs. I will play around with this as well as it's a method that I wouldn't have to put screw holes in my lower CR any longer. Thanks!

 

[neil_w]

I don't paper my tabs. It wouldn't hurt, but it doesn't really help either (the strength of the tab is not generally a limiting factor in anything), and it's much harder to trim around the tabs. Oh, and it means the slots need to be a bit wider (relevant if you're using pre-slotted tube.

E.g.:

 

[BABAR]

there would be a wood-glue-paper-glue bond. The paper would be sandwiched between 2 layers of glue and would likely be saturated with it.
.

For STANDARD (not thru the wall) fin mounting, a relatively common failure is tube delamination, the whole fin and fillet rip off the tube, with varying thicknesses of the body tube paper.

So IMO papering is a great technique for both strengthening fins and cosmetic appearance, but just like you shouldn’t glue a piece onto a painted surface, you shouldn’t try to form a bond ACROSS a papered surface. Paper strengthens the fin, but traction force to the PAPER joint risks delamination of the paper off the FIN.

Again however, for low power WITH through the wall fins, you would probably be OKAY, but I strongly suspect you would not be BETTER off (stronger unit) by extending the paper into the tab.

OTOH where I might do this is if, for whatever reason (bad QA in a kit or you cut them yourself and overestimated the width, etc.), your slots are too big. Adding one or more layers of paper to make it snug and straight may be a GOOD solution.

 

[bjphoenix]

When I paper fins I run the paper all the way edge to edge, then put fillets over it. If you paper to the glue fillet but not under it, you have a weak line right there where the paper ends. The paper will still strengthen the fins in other ways but won't strengthen the fins with respect to breakage at the glue fillets.

A balsa fin with paper on it does create a surface where the paper can delaminate. When I paper fins I use a minimum of glue because moisture in the glue can cause the paper to wrinkle. Maybe using a different glue and more of it would make the paper bond better.

I still think papering all the way to the ends of the tabs would be worthwhile. If there is a significant concern for delaminating I would suggest using different glues and/or something other than paper. And speaking of which I would like to know what readily available papers might be stronger than common printer paper. I've read suggestions to use velum. I've wondered about using tyvek but in my experiments it was hard to glue to and later I learned that it doesn't resist heat very well.

 

[rharshberger]

For STANDARD (not thru the wall) fin mounting, a relatively common failure is tube delamination, the whole fin and fillet rip off the tube, with varying thicknesses of the body tube paper.

So IMO papering is a great technique for both strengthening fins and cosmetic appearance, but just like you shouldn’t glue a piece onto a painted surface, you shouldn’t try to form a bond ACROSS a papered surface. Paper strengthens the fin, but traction force to the PAPER joint risks delamination of the paper off the FIN.

Again however, for low power WITH through the wall fins, you would probably be OKAY, but I strongly suspect you would not be BETTER off (stronger unit) by extending the paper into the tab.

OTOH where I might do this is if, for whatever reason (bad QA in a kit or you cut them yourself and overestimated the width, etc.), your slots are too big. Adding one or more layers of paper to make it snug and straight may be a GOOD solution.

I have a SBR Lil' Fusion with TTW fins and a 24mm motor mount, it has been flown a number of times on several unobtanium motors a AT G110T, AT G55W, and CTI F240 V-max, havent managed to rip the fins off yet and it is just built with the double dip fin root method, and Titebond Quick and Thick fillets. No leaving off the rear CR to fillet internals or anything, if I had a 24mm H I would fly it on that with no fears. Its an LPR built to LPR standards being flown on MPR motors. This weekend I watched 3" School Rockets being flown on G's and H's with raw balsa fins, and some not so pretty build technique, they all survived. We as a group way overbuild LPR and MPR rockets.

 

[lakeroadster]

Follow up question:

If one (well, say maybe me..) is papering the fins. Given the strength increase, would it be a good idea to paper the entire fin, including the internal tab?

Edit: I'd probably leave a bit of "naked" wood for the glue joint on the motor tube.

Hans.

Another vote for no papering on TTW tabs. I cut the paper so it also isn't involved in the fin fillets.

 

[4regt4]

When I paper fins I run the paper all the way edge to edge, then put fillets over it. If you paper to the glue fillet but not under it, you have a weak line right there where the paper ends. The paper will still strengthen the fins in other ways but won't strengthen the fins with respect to breakage at the glue fillets.

A balsa fin with paper on it does create a surface where the paper can delaminate. When I paper fins I use a minimum of glue because moisture in the glue can cause the paper to wrinkle. Maybe using a different glue and more of it would make the paper bond better.

I still think papering all the way to the ends of the tabs would be worthwhile. If there is a significant concern for delaminating I would suggest using different glues and/or something other than paper. And speaking of which I would like to know what readily available papers might be stronger than common printer paper. I've read suggestions to use velum. I've wondered about using tyvek but in my experiments it was hard to glue to and later I learned that it doesn't resist heat very well.

I'm using the thick, slow cure CA to paper the fins and am very pleased with it. If you tap it against something, it sounds like plastic or a plywood fin. Completely different "feel" to it versus a wood glue paper finish. It has been a bit of a learning curve, though, as there are definitely some tricks to getting it right. What I'm struggling a little with currently is shaping the edges, as the CA doesn't sand well when it dries thick, which occasionally happens with sloppy technique. But I have another idea to try that should make things easier

Others have commented that it's not necessary, as they haven't had fins rip off with monster motors. But I'm doing it to hopefully reduce chance of breakage on landing. Yes, the CA might make them more brittle. However I think the extra strength will offset that. If successful, I might use somewhat smaller parachutes to avoid drift - I'm flying on a small field.

Hans.

 

[4regt4]

Another vote for no papering on TTW tabs. I cut the paper so it also isn't involved in the fin fillets.

View attachment 515788

If I were doing short tabs like in your example, I'd probably do the same. My current build is a BT-80 tube with 24mm motor. So the tabs are longer and I like the idea of strengthening them.

Hans.

 

[lakeroadster]

If I were doing short tabs like in your example, I'd probably do the same. My current build is a BT-80 tube with 24mm motor. So the tabs are longer and I like the idea of strengthening them.

Hans.

What's your end goal? Why are the fins being papered?

To prevent chipping or damaging the wooden fin at ground contact? Papering the fin where it penetrates the body tube doesn't help in this case.​

​

Fin flutter reduction.​

The TTW fins strength is incredibly strong at the body tube due to the cantilever support of the fin attachment to the MMT.​

Where the advantage of papering fins comes into play is the added rigidity the fin gets as it gets out away from the body tube.​

​

 

[Back_at_it]

If the rocket is a small diameter meaning that I don't have room to leave the rear centering ring off and access the inside of the fins then I do the double dip method but I use Thick and quick as it stays put and doesn't run everywhere. Thick and quick on the root edge where it inserts into the body tube. TBII everywhere else.

If you can leave the rear ring off, the best option is to do so and do internal fillets. then install the rear ring.