Anybody use "Fin Pockets"

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rharshberger

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I have been conversing with my L3CC and he recommended a method of fin attachment referred to as Fin Pockets, basically it sounds like a slot between the CR's created by "dams" on both sides of where the fin will attach. The slots for the fins can be cut and the assembled motor mount unit can be epoxied into place. Epoxy is then put into the fin pockets and the fin inserted with no further need for internal fillets. I guess my real question is are the fin pockets larger than the fin and and extra epoxy is put in the pocket to self fillet ,example: fin is .25" thick do I make pocket .5" wide so a .125 thick "fillet is made on each side internally OR just wide enough to fit the fin and press out a small amount of epoxy. If anyone has used this technique for L3 rockets and has pictures that would be great if you would be willing to share.
 
I have considered doing this for a decade, but I always back down as I think "there's no way I'm going to be able to get those things (i.e., two tubes, three slots, six dowels) to all line up I'm gonna end up with one fin way outta whack." But that's just me...
 
Im not planning to dowel the fin just epoxy it into the pocket, the fin line up is easy since I slot my CR's so the fins self jig. The fins will be constructed of Baltic Birch ply, end grain balsa cores, and two layers of 5oz carbon each side. There is thread here somewhere of another Cherokee-D upscale with nearly identical fin construction. Actually IIRC its your fin design I am using, did it work out ok for you?
 
Interesting concept and one that I have not thought of. I am wondering what the advantages/disadvantages are of using this method, assuming TTW fin-mount, and using the more common method of root glued to MMT with internal fillets? I realize "internal fillets" is something that can be implemented differently. In my case I run internal fillets both on the fin/MMT and the fin/body tube, then my external fillets, fin/body tube.

One potential advantage I can see is that the epoxy used on root would not be subjected to heat-soak, which could compromise your epoxy depending on what is used.
 
Interesting concept and one that I have not thought of. I am wondering what the advantages/disadvantages are of using this method, assuming TTW fin-mount, and using the more common method of root glued to MMT with internal fillets? I realize "internal fillets" is something that can be implemented differently. In my case I run internal fillets both on the fin/MMT and the fin/body tube, then my external fillets, fin/body tube.

One potential advantage I can see is that the epoxy used on root would not be subjected to heat-soak, which could compromise your epoxy depending on what is used.

The fin root is still attached to the MMT as normal, its just there are "dams" on each side of the fin to act kind of like fillets I guess. The fin as it is slotted into the pocket (if the CR is slotted as I plan to) and a 1/16" gap is on each side of the fin the excess epoxy from the fin root would then squeeze up through that narrow gap insuring full epoxy coverage on the fin root and where fillets would normally contact the fin, mmt, and airframe. I didn't mention that the TTW tabs will only be about 3/4" long due to the airframe being Loc 5.38" and a 98mm MMT, I will also be using 3 CRs and an AeroPack 98mm Retainer. I will definitely doing my usual style buid thread once I get the build underway.
 
The fin root is still attached to the MMT as normal, its just there are "dams" on each side of the fin to act kind of like fillets I guess. The fin as it is slotted into the pocket (if the CR is slotted as I plan to) and a 1/16" gap is on each side of the fin the excess epoxy from the fin root would then squeeze up through that narrow gap insuring full epoxy coverage on the fin root and where fillets would normally contact the fin, mmt, and airframe. I didn't mention that the TTW tabs will only be about 3/4" long due to the airframe being Loc 5.38" and a 98mm MMT, I will also be using 3 CRs and an AeroPack 98mm Retainer. I will definitely doing my usual style buid thread once I get the build underway.

Okay I see what you're doing, looking forward to seeing the build thread. Are you taking the "dam" all the way down to the MMT, like a coffer dam, if not will you still do internal fillets on whatever side of the dam that is not touching. Finally what do you use to adhere your root to the MMT, epoxy, JB Weld, etc?
 
Okay I see what you're doing, looking forward to seeing the build thread. Are you taking the "dam" all the way down to the MMT, like a coffer dam, if not will you still do internal fillets on whatever side of the dam that is not touching. Finally what do you use to adhere your root to the MMT, epoxy, JB Weld, etc?

The dams will be Baltic Birch ply and epoxy I am tempted to use here is System 3's T88 in the mixing tip cartridge that fits a regular caulk gun from Aircraft Spruce. This rocket while it has a 98mm mount will be flown on 75mm AT M1297W for the cert flight , and a 54mm AT L1000 DMS. So there are no 98s in the near future but I may go ahead and use JB Weld for inside the pocket for its higher heat properties.
 
Closest I've got to this is having dado cuts placed in the CRs so the fins interlock. Nat from Upscale CNC did a fantastic job of making my fins and CR's for my L3 project...


Later!

--Coop
 
Closest I've got to this is having dado cuts placed in the CRs so the fins interlock. Nat from Upscale CNC did a fantastic job of making my fins and CR's for my L3 project...


Later!

--Coop

Thats how i do my CRs as well, then the fins self jig nice and straight.
 
Rich, I did this on my titan missile, in that case the fins are lexan and removable. I also did it on my titan dynasoar. I dont see an advantage in making them loose, fillet the dams then they take bending loads, epoxy is there on the fin root to hold it to the mmt and keep it attached to the dam or fin box. I also had Nat make mine and had the dams interlock to the centering rings. Alternatively you could tab/slot the centering rings and fin tab.
 
Would love to see some pictures to "paint a picture". Being new to all the real HPR concepts, I love learning from you guys
 
I did something similar on one my builds. It works but may not be needed. You can get enough strength without the dams

Then again, the dams will be easier to hold in place as the epoxy dries. Fins can move a little.
 
Would love to see some pictures to "paint a picture". Being new to all the real HPR concepts, I love learning from you guys

Check out the link in the first few posts, on page 8 of the L3 package is a picture on the lower left of the fin pockets"
 
I did something similar on one my builds. It works but may not be needed. You can get enough strength without the dams

Then again, the dams will be easier to hold in place as the epoxy dries. Fins can move a little.

I usually use the next size thicker CR than needed and slot them heavily for the leading and trailing edges of the TTW tab to slot into in this case the CRs will be 1/2" baltic birch ply with 1/4" deep slots, I still use a payload bay fin guide to make sure everything stays in perfect alignment but it really is not necessary because the slots do their job. The dams are really there I believe to add some reinforcement in the nature of internal fillets withhout have to actually try and fillet inside a narrow area. This rocket will have approximatly 3/4" between the MMT and the inner airframe. Here is a picture of my slotting technique for my 3" Frenzy XL clone with the fins dry fitted into the slots. I built a jig for my router to slot centering rings. Not sure how much additional strength it slotting like this adds, I know the rings and fins are harder than heck to break apart without fillets because of the fillets, and non-slotted CRs seem to allow the fins to break away easier( all non-scientifically tested of course, just redneck stress testing).


L2project9-2.jpgThese pictures are not representative of fin pockets. They just demonstrate the slotted Centering rings for self jigging fins.
 

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That is an interesting idea and looks like a good bet when space is tight between the MMT and the body tube. However, it does mean that all epoxy joints are in tension, whereas epoxy is stronger in compression (typically 3X stronger). So I'd be reluctant the skip the internal fillet between the fin and the body tube for high-stress rockets.

Of course that's not to say that L3 cert rockets are, or should be, high-stress rockets.
 
That is an interesting idea and looks like a good bet when space is tight between the MMT and the body tube. However, it does mean that all epoxy joints are in tension, whereas epoxy is stronger in compression (typically 3X stronger). So I'd be reluctant the skip the internal fillet between the fin and the body tube for high-stress rockets.

Of course that's not to say that L3 cert rockets are, or should be, high-stress rockets.

For some reason I like to put the largest MMT in a rocket I can without being MD, so far this one sims to .98 Mach on a AT M1297W. I probably will never fly with a larger than M motor in this rocket or any rocket anytime soon simply because they are out of my price range. I can see epoxy being stonger in compression because its a solid, but in tension the strength of the bonded material will most likely be the weak point. The fins for this rocket will be a 1/4" Baltic Birch Ply core skeletonized and filled with either endgrain balsa (most likely) or possibly a honeycomb material or foam with 2 layers of 5oz CF both sides. Main reason for endgrain balsa over honeycomb is cost. As for the "damns I will laminate them similar to tip to tip style, and will bevel the top down to center so epoxy can be forced between the tube and the top of the dam up to where the one edge meets the airframe which would give a small internal fillet. Thanks for your contributions to the hobby John your website and Thrustcurve.org are great tools.
 
This is from NAR website. Page 8 has some pics & a little description.

https://www.nar.org/wp-content/uploads/2014/08/Sample-L3-Package.pdf

That's superfluous as is the all-thread. Adds weight with overkill on the strength. I prefer to use a longer motor tube with two forward centering rings at least. Epoxy the mount in with the trailing ring abuting the leading fin edge root. Epoxy the fins in and then use a syringe and tubing to lay internal fin fillets. When done, epoxy the aft ring in place.

I see the project was from 2008 that had the totally stupid "safety switches" on every ematch circuit which are totally useless. I was glad when that rule was dropped.

Trying to get all the "fin pockets" to align up would be a problem in my book and is not necessary.
Kurt Savegnago
 
The only allthread will be a single piece with an eyenut on each end through AvBay. Woodworking is my other hobby so "complicated" lineup is a matter of perspective. Believe it or not the Frenzy XL fin can a few posts previous took some extra work but but the mmt slid right in and went together without fin guides.
 
The dams will be Baltic Birch ply and epoxy

Apologies if this is a stupid suggestion but I just wondered whether balsa would be a better dam material than birch ply? My thinking is the balsa would be lighter, and would soak up some of the epoxy as it was applied. I read about fin pockets some time back and was thinking about using balsa for the dams on a 38mm MMT / 2.26" airframe combination I have planned.
 
Balsa would work as well, and be lighter, however it would add little strength if placed too close to the fin. I plan on using the fin pockets since fillets between a 4" and 5.5" tube doesn't leave much room.
 
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Apologies if this is a stupid suggestion but I just wondered whether balsa would be a better dam material than birch ply? My thinking is the balsa would be lighter, and would soak up some of the epoxy as it was applied. I read about fin pockets some time back and was thinking about using balsa for the dams on a 38mm MMT / 2.26" airframe combination I have planned.

Balsa would work as well, and be lighter, however it would add little strength if placed too close to the fin. I plan on using the fin pockets since fillets between a 4" and 5.5" tube doesn't leave much room. A 38mm MMT in a 54mm airframe leaves a small gap where I believe the fin pockets are an advantage. By putting the dams approximately a 1/8" to 3/8" from the fin root you basically are creating a self filleting situation, yes its a little more work but not much especially if you use balsa which is very easy to cut.
 
Balsa would work as well, and be lighter, however it would add little strength if placed too close to the fin. I plan on using the fin pockets since fillets between a 4" and 5.5" tube doesn't leave much room.

Yes - I was thinking of leaving a small gap between dam and fin tab. I was also thinking of making small channels/ holes in the balsa dams with an awl or drill bit to allow the epoxy to fully wick in / form 'pins'.

A 38mm MMT in a 54mm airframe leaves a small gap where I believe the fin pockets are an advantage.

I have some 7mm / 4/16" balsa which should match the depth of the CRs that form the fin can with minimum sanding. I'm still gathering together parts for my project, but once I'm underway I'll start a build thread with pictures.
 
to me it seems needlessly complicated, especially for a cert rocket The purpose of the rocket is to prove that you can make a rocket that stands up to the flight and follows the correct profile. If you're interested in fin pockets they could be done on a non cert rocket. I'm sure you've read multiple other posts on other cert rockets that say "keep it simple" and "build them the way you always do" or "don't do your first DD on your cert rocket"
It's like telling you to cert on the O8000. (but that would be cool)
Now if you always did that, then cool, it's how you build them. Many ways to get to the same place.
 
to me it seems needlessly complicated, especially for a cert rocket The purpose of the rocket is to prove that you can make a rocket that stands up to the flight and follows the correct profile. If you're interested in fin pockets they could be done on a non cert rocket. I'm sure you've read multiple other posts on other cert rockets that say "keep it simple" and "build them the way you always do" or "don't do your first DD on your cert rocket"
It's like telling you to cert on the O8000. (but that would be cool)
Now if you always did that, then cool, it's how you build them. Many ways to get to the same place.

I get told not to do all kinds of things. Some people seem to think fin pockets are complicated, they really aren't. As for slotting CR's that takes me about 5 extra minutes per ring and makes assembly so much easier, especially since I built a jig for my router, I just drop the ring in lock it down, make a pass with the router, turn ring to next position and do it again, etc.

The fin pockets take about 5 mins per fin. I cut a strips of wood the width of the gap between the mmt and airframe and the length of the distance between the centering rings. The CR's are already attached to the mmt but not filletted. I place a fin in the slots (mmt still just tube and rings). Place a spacer next to the fin root and tack the dams in place with CA one on each side of fin root. Once all the dams are attached, I fillet the CR's on both sides. Once the mmt is solid I epoxy it into the airframe, fill the fin pockets with thickend or structural epoxy and insert the fin into the slots, making sure a small amount of epoxy is squeezed out of the pocket around the fin airframe joint, wipe off the excess epoxy, check my fin alignment and set it aside to cure. No further messing with the fin beyond external fillets. Advantage is I don't have to fillet inside a .75" gap between airframe and mmt 16" deep, and I know I get solid fin attachment.

This is also a L3 cert rocket not a L1.
 
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