Foaming Fin Cans

[jqavins]

I get the general idea that filling the entire fin can with expanding foam will lock everything together and make it sturdier. I get that it's overkill for LPR and probably for most if not all MPR.

Does anyone have any real data on just how much it helps? On when it's really worth doing? Good rules of thumb? Or is it just one of those things that people do because other people have done it and it "seems like a good idea"; I'm not trying to imply that it is that, only that I don't know.

I don't have anything coming up that is at all likely to need it; the question has just been nagging at me now and then.

 

[Rschub]

I think it might help for rockets that use plastic nosecones as tail cone faring's to help support the plastic from cracking or separating, since it is notoriously difficult to bond to.

The rest of the time it's just extra weight in the wrong place IMO.

 

[tfish]

It may have helped some..with cardboard rockets back in the day.
How many rockets have you seen that the fin attachment failed..and that you feel..foaming the fin can would have prevented..the failure?

Tony

 

[jqavins]

How many rockets have you seen that the fin attachment failed..and that you feel..foaming the fin can would have prevented..the failure?

How many have I seen? None. But that's why I'm asking.

 

[jderimig]

Many colorful discussions on this subject. In my opinion it is never worth doing. A good decent TTW fin construction is the strongest part of the rocket and the area between the airframe and MMT is crazy strong and stiff.

Foaming the fin can just hits your wallet, adds weight where you dont want it and adds strength where you don't need it. It also makes fin repair a nightmare if you break a fin.

But that is my personal opinion.

 

[jadebox]

It also makes a terrible mess if you don't do it right.

 

[KenECoyote]

John Coker had a video on foaming fin cans and he did it for his 5.5" fg AGM-33 Pike, which he flew on an M685.

 

[KenECoyote]

Personally I like using it for nose cones and tapered fin cans (like V2), where getting to internal fillets is difficult - plus the taper means less extra weight at the further end. Also it works well for a quick build and/or when there are more than 4 fins in the back.

I think that my rockets are built pretty stout and the foamed fin can feels strong enough for my standards; however, I don't have enough flights on them to make a hard call yet.

 

[KenECoyote]

It also makes a terrible mess if you don't do it right.

Yes! However, to me it's the same with CA or Epoxy until you learn to use it or read up enough about it to prepare for the "worst".

 

[dhbarr]

I've noticed some well-filleted card-birds that get tears at the top of the TTW slot. I would think that foam would have helped in these cases, since they would have stuck to the total inside surface.

 

[jqavins]

Also it works well for a quick build and/or when there are more than 4 fins in the back.

Is that because more than four fins can make it hard to do internal fillets? If there is room for good internal fillets, is there some other special reason to foam?

 

[KenECoyote]

Is that because more than four fins can make it hard to do internal fillets? If there is room for good internal fillets, is there some other special reason to foam?

Just my opinion and I was thinking more of internal fillets when they're already glued into the fin can vs. assembled outside beforehand. Usually those "split fin" rockets have less space in the rear to work with and sometimes you don't want to cut a huge slot in the rear for inserting a pre-assembled 8 fin can since to fill in the slot between the fins afterwards and make them look good takes some work (more than you'd think imho). You can also make glue dams, but that's extra work too.

 

[rvanler65]

I've noticed some well-filleted card-birds that get tears at the top of the TTW slot. I would think that foam would have helped in these cases, since they would have stuck to the total inside surface.

I foamed my Wildman 4" Eagle Claw as there was no good way to get internal filets and I am planning on putting some big motors in it

 

[jderimig]

Is that because more than four fins can make it hard to do internal fillets? If there is room for good internal fillets, is there some other special reason to foam?

Internal fillets are also over-rated. The fillet at the inside of the airframe is the least bending stressed element in the structure. Fillet the outer fin attachment and the fin to MMT are you are good to go. Especially if using a FG airframe.

 

[jqavins]

Just my opinion and I was thinking more of internal fillets when they're already glued into the fin can vs. assembled outside beforehand. Usually those "split fin" rockets...

Oh (the light dawns) split fins. I thought you were talking about 5 or 6 regular fins, i.e. 72 or 60 degrees apart.

 

[jqavins]

Internal fillets are also over-rated. The fillet at the inside of the airframe is the least bending stressed element in the structure. Fillet the outer fin attachment and the fin to MMT are you are good to go. Especially if using a FG airframe.

How is the fin to MMT filet not "internal"?

 

[KenECoyote]

Oh (the light dawns) split fins. I thought you were talking about 5 or 6 regular fins, i.e. 72 or 60 degrees apart.

Well, even those since they become more work to do internal fillets on both sides of each fin.

 

[jderimig]

I define internal as a fillet on the inside of the tube. The fillet to the mmt is on the outside of the tube. My definition may be the not same as others...

 

[BigMacDaddy]

I was thinking about this on a model I am 3D printing -- JB-3 Tiamat -- it has very complex geometry and I want to keep weight down as much as I can. However, I do worry that the think wall print could take some damage if it lands wrong on something hard. Thinking about foaming inside the rear fins but could maybe do the whole booster (worry it might swell up).

 

[KenECoyote]

When I think of internal fillets, I think of both to the bt and mmt.

Here is an example of what I usually do with epoxy:

 

[KenECoyote]

I was thinking about this on a model I am 3D printing -- JB-3 Tiamat -- it has very complex geometry and I want to keep weight down as much as I can. However, I do worry that the think wall print could take some damage if it lands wrong on something hard. Thinking about foaming inside the rear fins but could maybe do the whole booster (worry it might swell up).

View attachment 622723

I don't know if foam is good in this case given what you said about added weight and worry about it swelling the 3D printed can. Of course you can try some testing.

 

[kramer714]

I have been using 3d printed fin pockets for through the wall on smaller stuff. Truly attaches the bodytube to the motor tube, fins 'potted in' without adding too much weight. Better solution than foaming a fin can.

• Prep the OD of the motor tube and ID of the bodytube, 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

 

[kramer714]

oh, i also mold in a slot to bond in the shock cord

 

[Hobie1dog]

IMHO foaming is a waste of time. A fin will snap off most likely outside the airframe.

 

[beeblebrox]

When I think of internal fillets, I think of both to the bt and mmt.

Here is an example of what I usually do with epoxy:
View attachment 622714

Yep, Exactly! Foam is a good insulator, and good for where pipes and wires go thru a hole in a floor/ceiling. It has almost no structural strength at all. It will rip easily. The one use I can see though is if you want to add a nose weight inside one of those plastic cones that almost nothing sticks too, you could put in a bit of lead shot for the mass you want to add, then foam the nose to lock that in place. (I need to do that on my T-Loc. Love that rocket but they are marginally stable...)

 

[beeblebrox]

I have been using 3d printed fin pockets for through the wall on smaller stuff. Truly attaches the bodytube to the motor tube, fins 'potted in' without adding too much weight. Better solution than foaming a fin can.

• Prep the OD of the motor tube and ID of the bodytube, 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

View attachment 622724View attachment 622726

I like that! I have printed full fin cans that have performed well... Got Files?

 

[cwbullet]

I have been using 3d printed fin pockets for through the wall on smaller stuff. Truly attaches the bodytube to the motor tube, fins 'potted in' without adding too much weight. Better solution than foaming a fin can.

• Prep the OD of the motor tube and ID of the bodytube, 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

View attachment 622724View attachment 622726

I like doing this both through the full fin can or just slots a portion of the length. They help get those fins straight.

I would be interested in seeing your files also. I often improve my designs once I see a better way to do it.

 

[kramer714]

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.

 

[kramer714]

Yep, Exactly! Foam is a good insulator, and good for where pipes and wires go thru a hole in a floor/ceiling. It has almost no structural strength at all. It will rip easily. The one use I can see though is if you want to add a nose weight inside one of those plastic cones that almost nothing sticks too, you could put in a bit of lead shot for the mass you want to add, then foam the nose to lock that in place. (I need to do that on my T-Loc. Love that rocket but they are marginally stable...)

um, sort of....

Little background, I owned a company where we made structural foam cores for aircraft composite parts. very structural.
So here is the difference, these cores were molded under pressure in heated molds. The pressure makes the foam uniform and makes it fill the edges evenly. The temperature lowers the viscosity initially, and then causes the foam to (almost) fully react in a controlled exotherm. This keeps the reaction from continuing after the foam is taken from the molds. The heat also increases the HDT of the foam. For aircraft EVERYTHING needs to be able to handle -65 to +165F in service.

Most poured in place foam will want to shrink over time, causing the foam to be preloaded in tension (not a good thing) causing cracks and voids. The molds were carefully engineered to allow air out but keep foam in. Anything at all structural was molded at about 3 lbs density. Free rise foam 'blanks' are don in large 'loafs' with the edges cut away and discarded.

 

[AtlPhilip]

My $0.02, take what you will from it.
The fin can pictured below was in a 6in booster (Kosdon L1860) that suffered an ejection failure and came in ballistic from about 1200ft. It has not been modified since the crash except for removing the tubing so I could inspect the inside. It's hard to see, but there is a single layer of 4oz glass from fin root to fin root. You can see the 1/4 wood cleats used to align and hold the G10 fins during glue up. I rebuilt the airframe around this fin can and have since shot it on a K2050 and an K1100.