Today I learned how not to foam a fin can

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Idunno

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I was trying to build a Wildman mini kit according to these excellent instructions:
https://www.rocketryforum.com/threads/wildman-wild-child-darkstar-minis-all-glass-are-here.16370/
Sadly I didn't read over them very well, because I missed the part about refrigerating part A and B before starting work. Of course after mixing parts A and B, the expansion started immediately, and I scrambled to fill my can. I should have just aborted but I didn't, and now I have learned the hard way how not to foam a fincan. I think I've also learned that I do not like foam, but I might give it a second chance with the refrigerator trick. There are 3 fins, and 1/3 of the fincan is good, 1/3 is okay and 1/3 is not good, err uh I mean optimized to reduce weight.

1. The 'weight optimized' segment has a void taking up about 2/3 of the space of where foam should be. It is sealed with foam on top centering ring and out the back, but there is a huge gap in the middle, I can see it through the fiberglass wall. I guess I have to drill a small hole to fill the void? Out of curiosity how much weaker would the rocket be without filling that void? Not that I plan on leaving it that way.

2. The second chamber is about 95% full. There is one bubble. Anything to worry about?

3. Any tips on getting the foam off the outside of the BT without scratching the fiberglass? I had a few dribbles down the side.


Also, I noticed the foam (PML) has an expiration date of 2/21. That's less than a year from now. Did I get an old batch or does it normally have a short shelf life?
 
I was trying to build a Wildman mini kit according to these excellent instructions:
https://www.rocketryforum.com/threads/wildman-wild-child-darkstar-minis-all-glass-are-here.16370/
Sadly I didn't read over them very well, because I missed the part about refrigerating part A and B before starting work. Of course after mixing parts A and B, the expansion started immediately, and I scrambled to fill my can. I should have just aborted but I didn't, and now I have learned the hard way how not to foam a fincan. I think I've also learned that I do not like foam, but I might give it a second chance with the refrigerator trick. There are 3 fins, and 1/3 of the fincan is good, 1/3 is okay and 1/3 is not good, err uh I mean optimized to reduce weight.

1. The 'weight optimized' segment has a void taking up about 2/3 of the space of where foam should be. It is sealed with foam on top centering ring and out the back, but there is a huge gap in the middle, I can see it through the fiberglass wall. I guess I have to drill a small hole to fill the void? Out of curiosity how much weaker would the rocket be without filling that void? Not that I plan on leaving it that way.

2. The second chamber is about 95% full. There is one bubble. Anything to worry about?

3. Any tips on getting the foam off the outside of the BT without scratching the fiberglass? I had a few dribbles down the side.


Also, I noticed the foam (PML) has an expiration date of 2/21. That's less than a year from now. Did I get an old batch or does it normally have a short shelf life?

2 part foam has a "relatively" short shelf life once exposed to moisture, I just tossed some that was about 10 or so months old and one component had crystallized badly, unlike epoxy I have had no success undoing the crystallization process. Following CJ's directions carefully is advised.

To be honest my .02 cents is that there is nothing to be gained from foaming most fincans under 4" and a 1.6" like the mini's is a p.i.t.a. to get the foam into.
 
Oh, Lordy, can I relate! Being in hurry, and lazy, and impatient, and generally preoccupied wth other aspects of the build, I foamed a 2.5” can to save weight and secure the fins without the mess of internal fillers. Not only did I not chill anything, the foam was from an LDRS maybe 2-3 years ago. Not crystallizd, but darkened. I ran a test batch anyway. It still worked! Scrambled to get three batches measured out so I could go quickly once I was ready. Masked the tube and fins (I am naturally messy) and mixed first batch. Stirred until it was milky as normal. Too long! Foamed right up and over the cup, securely fixing it to the table. Lucky I had paper down ‘cus I KNOW I am messy. Made another batch. Tried again. Better, but too little for each bay/fin section. Drat! Mixed another batch. Poured again. Better, but still not enough. Damn! Mixed ANOTHER batch ( you’d think I learned by now, but I’m old, stubborn and will argue with a door knob). Poured again. Frack! Too much! Naturally. It was UGLY! I trimmed it up and put the bulkhead on to hide my shame and ineptitude. It’s perfect now. Of course, if the fins fall off, it was my math at Mach speeds. Yeah, that’s it, my Mach speed math! Good luck mate, you got this. I have actually drilled holes and used a ‘meat flavour injector’ install foam, but that takes real finesse and some practice. Epoxy and sand the hole on f/g birds prior to finishing. On c/f, pound to fit and paint to match. Straight smoke and good chutes.
 
For that little rocket I would not worry about it. You can just put some good fillets on the outside and fly it. Unless you're looking to eliminate outside fillets I really see no benefit to foaming fincans for smaller rockets like that one.
 
I finished my Wildman Intimiditor DD A49B047B-5321-44AE-9AE9-FFE35AC27A9E.jpegmini this past winter. I had plans to foam the can and chickened out. I’ll make my first attempt on something with slightly more volume.

I double wiped the root edges and sorta put internal filets in. Put small external filets on and been flying it on G motors for now. Seems plenty strong.
 
Nothing to gain by foaming that. You did it just right.
I finished my Wildman Intimiditor DD View attachment 413763mini this past winter. I had plans to foam the can and chickened out. I’ll make my first attempt on something with slightly more volume.

I double wiped the root edges and sorta put internal filets in. Put small external filets on and been flying it on G motors for now. Seems plenty strong.
 
I finished my Wildman Intimiditor DD View attachment 413763mini this past winter. I had plans to foam the can and chickened out. I’ll make my first attempt on something with slightly more volume.

I double wiped the root edges and sorta put internal filets in. Put small external filets on and been flying it on G motors for now. Seems plenty strong.

Looks great! Does that kit have the nosecone coupler built in or does the nose cone accept a coupler?
 
Idunno,
What did you decide? I don’t have experience with foam so take my musings for what they are worth. Looking at the space in the Mini and your described issue I’d drill two holes on opposite sides of the voids and try shooting the foam with a syringe in one side and give it a place to vent and expand to the opposite hole. Or drill next to the fin and run some 30min along the root edge.

Did you epoxy the root edge prior to foaming? If you did, I’d bet applying external fillets would be plenty of strength.
 
The whole idea behind foaming fincan on mini's was simply the time factor. We were building and flying same day in many cases at many fields. No waiting for epoxy to cure. Note that the instructions came with both foaming AND typical epoxy methods. Choice is left to builder. We would do group builds of 4-6 at a time night before launch. And there was always someone with past experience to help mitigate any pit falls...lol When done correctly it is a fast simple way to build.

The biggest downfalls for beginners are not working fast enough and trying to clean up any foam that leaks while still raw. If ya wait till cure it just scrapes off with knife and sands smooth. Definitely not for everyone...but definitely has it's place.:cool:
 
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