Foaming Fin Cans

[KenECoyote]

Kramer Rocket-Pockets
54 mm body tube - 38 mm motor 4 fin .125 thick fins ( I also have 38mm to 29mm 3 fin pockets, and am going to do a 54-38 three fin version)

(3) files

1. STL for lower fin pocket (orange)
2. STL for upper fin pocket (brown)
3. STL for drill jig. The jig corresponds to the (4) holes in the lower fin pocket. The holes can be used for alignment of the bodytube to the slots. there are (4) additional holes that are at the base of the fin slots, I use these to make sure the fin slots are in the correct position.

• Notes
  • There are (4) holes on the bottom that can be tapped for a 'clip retainer' if you want. I use aluminum snap ring retainers.
  • be careful with the lower it is kind of fragile until bonded.
  • this design does not use any centering rings - the 3d printed parts act as the centering rings.
  • there are (4) radial holes that line up with the drill jig holes, they can be used for;
    • aligning the holder into the rocket
    • attaching a lower rail button
    • using small screws to attach the motor tube / fin pocket to the body tube instead of bonding it.
    • Hook for hanging a bottle opener - handy on the range
  • Prep the OD of the motor tube and ID of the body tube, bond the motor retainer to the motor tube
  • Bond the lower (orange) and upper (Brown) fin can pocket to the motor tube with 5 minute Epoxy or CA (the fin fillet will add A LOT of bond from the inside to the outside.
  • Prefit the fins, put a piece of tape where the fin hits the body tube (for cleanup), put tape on the side of the fin slots.
  • fill the pocket full with epoxy, make sure there are no bubbles, wipe the root of the fin with epoxy (to make sure you dont trap any bubbles and for a better bond)
  • slowly push the fin all the way in until it bottoms out, wipe off all the adhesive that came out, allow to cure

YMMV

If you use it as is, great, if you improve on it, please send me the file.

View attachment 622829

View attachment 622821

Out of curiosity, do you know how much extra weight is added (just the 3D print and the also with epoxy as a finished unit)?

 

[kramer714]

Out of curiosity, do you know how much extra weight is added (just the 3D print and the also with epoxy as a finished unit)?

Total for 3d printed parts and adhesive

Adhesive (epoxy)- Total for (4) adhesive fills (red) = 17.7 cc @sg 1.2= 21 grams (just the adhesive for the fin)
Upper Guide (PETG) - total for (1) piece (brown) = 7.3 cc @ SG 1.27 = 9.3 grams
Lower Pocket (PETG) - total for (1) piece (orange) = 35.7 cc @sg 1.27 = 44. 4 grams

Total - 74 grams

Eliminates (2) centering rings (Fiberglass) total volume 5.8 cc @sg 10.5 grams + plus adhesive to bond them in place

The weight gain isn't as bad as is shown when you figure all the fillet weights and centering ring adhesive. The design of the lower and upper pocket were based on easy to print designs, with 100% fill.

They both could be lower in weight by just making the 'cylinders' shorter. Once the adhesive is cured, the fins will hold the motor tube, arguably all the upper pocket does is seal ejection charge gas. The rings on the outside are there to make sure there is a 'gas seal' against the body tube. The length of the cylinder on the bottom of the lower pocket (orange) was sized based on slotting the tube, With the bottom flush to the body tube with the fins in place. I could have removed some of the material between the fins on the 'inside' of the fin can. would remove a few grams.

The lower (orange) can be made lighter by forming the pocket seal like a blade seal not like a 'comma seal'. (see picture below showing cross section. I wanted the edges of the adhesive to form 'molded fillets' at the body tube and the motor tube, not square fillets. Making the lower pocket like a 'comma' makes it much easier to print. but carries a little extra weight. The web between the commas is 1.1 mm thick, again sized for printing, any thinner and the quality drops on a part oriented this way.

 

[KenECoyote]

Total for 3d printed parts and adhesive

Adhesive (epoxy)- Total for (4) adhesive fills (red) = 17.7 cc @sg 1.2= 21 grams (just the adhesive for the fin)
Upper Guide (PETG) - total for (1) piece (brown) = 7.3 cc @ SG 1.27 = 9.3 grams
Lower Pocket (PETG) - total for (1) piece (orange) = 35.7 cc @sg 1.27 = 44. 4 grams

Total - 74 grams

Eliminates (2) centering rings (Fiberglass) total volume 5.8 cc @sg 10.5 grams + plus adhesive to bond them in place

The weight gain isn't as bad as is shown when you figure all the fillet weights and centering ring adhesive. The design of the lower and upper pocket were based on easy to print designs, with 100% fill.

They both could be lower in weight by just making the 'cylinders' shorter. Once the adhesive is cured, the fins will hold the motor tube, arguably all the upper pocket does is seal ejection charge gas. The rings on the outside are there to make sure there is a 'gas seal' against the body tube. The length of the cylinder on the bottom of the lower pocket (orange) was sized based on slotting the tube, With the bottom flush to the body tube with the fins in place. I could have removed some of the material between the fins on the 'inside' of the fin can. would remove a few grams.

The lower (orange) can be made lighter by forming the pocket seal like a blade seal not like a 'comma seal'. (see picture below showing cross section. I wanted the edges of the adhesive to form 'molded fillets' at the body tube and the motor tube, not square fillets. Making the lower pocket like a 'comma' makes it much easier to print. but carries a little extra weight. The web between the commas is 1.1 mm thick, again sized for printing, any thinner and the quality drops on a part oriented this way.



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Thanks! That info is very much appreciated. I'm always looking for new methods and thought that this would be a great one to try. I had actually mentioned epoxy dams at one point above but have never tried it myself. Very nice!

 

[Back_at_it]

I don't have a ton of experience foaming fins but I have done it once. When building my LOC Hawk, I didn't have enough room to get in between the motor tube and body tube to do proper fillets. I did have a can of spray foam with a long thin straw. After taping the motor tube closed and placing tape around the outside of the body tube, I stuck the straw in as far as I could and filled it up.

I honestly didn't feel any additional rigidity in the fins themselves. The process added a touch over 1oz in the exact wrong place. Not sure what my traditional fillets would have weighted but I feel like the foam was heavier.

Overall, I don't see the benefit from it and would probably never do it again. With that said, I do use spray foam in nose cones to keep nose weight in place all the time.

 

[MClark]

I don't have a ton of experience foaming fins but I have done it once. When building my LOC Hawk, I didn't have enough room to get in between the motor tube and body tube to do proper fillets. I did have a can of spray foam with a long thin straw. After taping the motor tube closed and placing tape around the outside of the body tube, I stuck the straw in as far as I could and filled it up.

I honestly didn't feel any additional rigidity in the fins themselves. The process added a touch over 1oz in the exact wrong place. Not sure what my traditional fillets would have weighted but I feel like the foam was heavier.

Overall, I don't see the benefit from it and would probably never do it again. With that said, I do use spray foam in nose cones to keep nose weight in place all the time.

No offense but…..
it didn’t add any strength. You used the wrong foam. Spray can foam doesn’t cure properly in large cross sections and is not for reinforcing.
The two part foam will cure in any thickness and comes in several densities to suit use.
I use in large diameter cardboard rockets with a big gap between tubes and to fill in between motor tubes in clusters

 

[Back_at_it]

No offense but…..
it didn’t add any strength. You used the wrong foam. Spray can foam doesn’t cure properly in large cross sections and is not for reinforcing.
The two part foam will cure in any thickness and comes in several densities to suit use.
I use in large diameter cardboard rockets with a big gap between tubes and to fill in between motor tubes in clusters

No offense taken. You are absolutely correct. I had heard of people, foaming fin cans, and I happen to have a can of Spray foam on the shelf so I figured I would give it a shot.

It was a learning experience