Anybody use "Fin Pockets"

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Thats how i do my CRs as well, then the fins self jig nice and straight.

How does everyone align the MMT with the body tube, both lengthwise and radially? I worry I'll be off a few degrees or a few 32nd's fore/aft.

Lots of great info in this thread, and the links too. Thanks all! Total noob and trying to soak it all up before I start asking too many questions.
 
How does everyone align the MMT with the body tube, both lengthwise and radially? I worry I'll be off a few degrees or a few 32nd's fore/aft.

Lots of great info in this thread, and the links too. Thanks all! Total noob and trying to soak it all up before I start asking too many questions.

I usually draw a line along the airframe where the launch lug/rail buttons will be , then a corresponding mark on the CR. As the mmt (complete with CR's) is inserted into the airframe I can eyeball the line on the CR to the one on the airframe aligning the mmt radially, optionally since I have already cut the fin slots the mmt can be aligned via the slots in the centering rings to the fin slots, and the lengthwise alignment is accomplished by the same method (front and rear CR slots line up with the beginning and end of airframe fin slots). The fin slots and launch lug/rail guide lines are all marked using a paper wrap fin location guide printed out from OpenRocket, and all the lines extend the full length of the airframe.

Here is a 3D representation of a fin pocket the two dams are on each side of the fin, and the area between the dams and the fin should be completely filled with epoxy (epoxy not shown). Second picture has rear centering ring removed for clarity. Representation is really close, the dams would actually be parallel to the fins, not angled like they appear, OpenRocket has limitations for this type of drawing (gap between fin and dams would be the same top to bottom, not wedge shaped).
FinPocket3D.JPG FinPocket2.JPG
 
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Good info thx! So with the fin pockets being so wide (as they are in the pic) what keeps the fins perpendicular to the body? Seems to me they can 'tip' left or right several degrees if you're not careful. Alignment jig of sorts?
 
Good info thx! So with the fin pockets being so wide (as they are in the pic) what keeps the fins perpendicular to the body? Seems to me they can 'tip' left or right several degrees if you're not careful. Alignment jig of sorts?

I use slots in my centering rings, or I use the fin guides from Payload bay, just print them out and glue to foam board cut out the shape and it works. The fins are held straight by the fin slots, and tipping is handled by the fin guides, just like the ones in the photo. I do cut the corners off the guides where the fin and airframe meet creating small triangles so epoxy/glue doesn't stick to the guides, I also label them with airframe size and fin thickness so they are re-usable.

https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES

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I gotta say, this method just makes too much sense to me. I'm surprised it's not the defacto way to do it. I've watched John videos on his AGM-33 Pike and I cannot imagine what a messy pita it was to fillet those long fins. With my upcoming 1/2 Scale Phoenix build I'm trying to learn every handy construction tip I can. And I'm learning a ton from Rich and John!

Here's how I'm going to make my fin pockets, at least for my 4" Madcow Phoenix. Simple 1/2" foam cut and hot glued to the MMT. Once the MMT is inserted in the body (sans fins) and affixed, I'll pour or inject some epoxy through the fin slot in the body. It'll be captured by my foam fin pockets and I'll lower the fin in place and wait for it to dry. Zero support is provided by the foam, but it doesn't need to provide any. The strength of the bond will come from where the epoxy contacts the MMT and the fin, just like a regular fillet does but thicker, and heavier now that I think about it.

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I gotta say, this method just makes too much sense to me. I'm surprised it's not the defacto way to do it. I've watched John videos on his AGM-33 Pike and I cannot imagine what a messy pita it was to fillet those long fins. With my upcoming 1/2 Scale Phoenix build I'm trying to learn every handy construction tip I can. And I'm learning a ton from Rich and John!

Here's how I'm going to make my fin pockets, at least for my 4" Madcow Phoenix. Simple 1/2" foam cut and hot glued to the MMT. Once the MMT is inserted in the body (sans fins) and affixed, I'll pour or inject some epoxy through the fin slot in the body. It'll be captured by my foam fin pockets and I'll lower the fin in place and wait for it to dry. Zero support is provided by the foam, but it doesn't need to provide any. The strength of the bond will come from where the epoxy contacts the MMT and the fin, just like a regular fillet does but thicker, and heavier now that I think about it.

View attachment 280330

View attachment 280331

Different, but I don't see why it wouldn't work. Definitely a nice job of thinking outside the box.
 
Looking nice and solid. I take it that's birch ply as you mentioned earlier ?

The centering rings are 12mm Baltic Birch ply, the dams/edges of the pockets are actually balsa since their only purpose is to keep the epoxy in the pocket so it can form a fillet on its own as the fin is pushed in. The slots in the CR are to help the fin "self jig" into a straight and perpendicular position to the motor tube. I could easily have made the dams out of Baltic Birch ply but its not necessary.
 
I'm thinking that I'll be using fin pockets for my "Stubby" or "Stubbier" design. Since I'll be using Plexiglass fins, I want them to be easy to replace, and that means pockets that the fins lock into.
 
The centering rings are 12mm Baltic Birch ply, the dams/edges of the pockets are actually balsa since their only purpose is to keep the epoxy in the pocket so it can form a fillet on its own as the fin is pushed in. The slots in the CR are to help the fin "self jig" into a straight and perpendicular position to the motor tube. I could easily have made the dams out of Baltic Birch ply but its not necessary.

Ah - ok - yes I used balsa for the dams also. Slotting the CR is a good idea - ensuring equal thickness of epoxy filling the respective tab/dam gap on each side. The fin setting on my current build is now complete (didn't get round to posting pictures yet) and pleased with the results so far.
 
Ah - ok - yes I used balsa for the dams also. Slotting the CR is a good idea - ensuring equal thickness of epoxy filling the respective tab/dam gap on each side. The fin setting on my current build is now complete (didn't get round to posting pictures yet) and pleased with the results so far.

The slots in the CRs when done right actually allow me to put the fins in perfectly straight.
 
Since this thread was recently brought up again, I figured I'd add the method I used for fin dams, which was 3D-printing. Here's the structure for a 54mm MMT in a 3" airframe (RW Osprey 75). The CR's are 3D-printed since they didn't need to be structural, all of the strength comes from the epoxy between/surrounding the MMT and fins. The 3D-printed rings also made it easy to do the channels for the airstart conduit (this particular rocket was the sustainer in a 2-stage Osprey 75) and the notches for the MMT-attached shock cord loop. The Osprey 75 has long fin roots (~13"), longer than any single piece I can make flat on my printer's bed, so I printed each 'dam' as 2 parts that locked together in the middle, and it only took a bit of epoxy at each joint to keep things from moving around. I kept things as thin as I could while still being stiff, so the weight of all of the printed parts was minimal. The CRs have notches for the fins as well as for the dams, to ensure everything ended up in just the right spot.

P9130002.jpgP9130001.jpgP9130003.jpg

The one thing I would do different the next time (I have a 75mm MMT in a 4" airframe, customized Madcow X-celerator build coming up where I plan to do this), in the above design none of the CR's were epoxied to the airframe. Instead I had poured a layer of JB-Weld at the top to help 'insulate' the top CR (which is just PLA) from the burning BP, though since then I've used PLA for 3D-printed charge wells and they're holding up just fine, so I think I was unnecessarily concerned in this regard. But my focus now is on ensuring a good seal between the payload section and the outside, so having one point where the CR is filleted to the MMT (the JB-Weld pour did that) as well as filleted to the airframe (which wasn't done by my JB-Weld since it was cured outside the rocket) to seal things off. What I did above seems to hold pressure fine, but it's just because things are a fairly tight fit, not because there's an actual seal anywhere, so next time I want to focus on creating a seal rather than just protecting the PLA.

As built above, my sustainer actually came out slightly lighter than my original booster which didn't use this design but instead an attempt at injected fillets. I say "attempt" because the epoxy (30-minute Z-poxy) didn't flow well, so it really just created 3 "blobs" running MMT-to-airframe around the injection sites, nothing like a proper fillet. Of course there are lots of other possible variables affecting the weight (especially since the booster and sustainer were ordered/built over a year apart, so the tube weight and such could have changed, and a bit of the sustainer's fin root was trimmed for the ISC mate with the sustainer), but the point is it didn't add significant weight to the design. I used Proline 4500 w/milled fiberglass for the epoxy in the dams.
 
I've done this on 3 models for clear plastic fins. On my Gemini-Titan model this fins were epoxied in. On 2 different size Atlas Mercury models the fins were slid in from the back and attached to a tab by way of a single screw for each fin.
 
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