foaming fin cans

Ok, here's a tech post for us all.
I've foamed mild Hi-Power rocket's fin cans with spray can based expanding
insulation foam before , I don't know why.
If I want to foam a fin can that I've reinforced on the inside with dowels
and 'glass cloth (I reinforce on insides not outsides for some reason, looks
better)
what would you recommend for KLM type loads ? 5.5" tubes to 98mm motor
mount fin can, dowels and 'glass cloth reinforced.
Art
Reply to
Art Upton
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Dont use the can stuff - it will expand after a while - and will ruin the fin can.
Reply to
Stephen Corban
Do you really need to foam the fincan? I haven't convinced myself that it is necessary.
I have one 4" rocket in which I filled the fincan with 2 part foam. I believe the foam has saved that fincan through 2 sever lawn-dart experiences. The whole rest of the rocket has been replaced twice. On the other hand, I believe it added excess weight in the wrong part of the rocket. It really wouldn't have been much harder to start all over again after those crashes even if they had destroyed the fincan. There is no way to know if the foam made a difference or not.
I have never foamed a fincan since that one. I am currently flying a 7.5" rocket on AMW 54mm K motors and it is showing no problems. This rockets' fincan was built by someone else and survived a hard landing due to a deployment problem. I cut off the damaged forward end of the airframe and repaired one G10 fin. My rebuild added more than 4 feet to the forward end of the rocket and a good bit of weight overall. The last time it flew on a K975 WW, it weighed about 30 lbs on the pad. It seems fine without foam and I think it would be fine on L motors too, if it had a big enough motor mount tube.
Foam it if you want, but I don't see a need for it. My L3 project won't get foamed. If you use TTW fins with a centering ring at the forward and aft ends of the fin tabs and really good filleting, I think the foam is useless weight. It also makes it much more difficult to replace a fin.
Just one opinion.
-- Eric Benner TRA # 8975 L2 NAR # 79398
Reply to
Eric Benner
5.5-7.5" airframes, 98mm host mount. Wood glue construction with epoxy and glass reinforcing at all fin/body and ring/body joints.
Figger a easily done method and I'll put those instructions in a kit.
Jerry
Reply to
Jerry Irvine
I agree. It seems to me that if you do a good job with filleting your fins to the motor tube and to the inside and outside of the body tube, foam would not be necessary. I think that it is extra weight and expense.
-- Joe Michel NAR 82797 L1
Reply to
J.A. Michel
I'll do that on the inside with Phen fins using epoxy and glass cloth. the wood won't hold on the Phen fins. it does do a great job on plywodd however when I've used it !
Now, I've got many a 5.5 rocket with no reinforcments on the outside. Ala, I-roc
what's everyones thoughts on that ? inside dowles and 'glass only.
Reply to
Art Upton
...and here is proof that your opinions are true...
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Doc
-- Drake "Doc" Damerau
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NEPRA President NAR Section 614 NAR 79986 L3
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Remove "My Shorts" to reply
Reply to
Doc
The spray can based stuff should not be used for rocketry.
There is no doubt that the two part stuff adds strenght to a fincan, but is it worth the weight penalty in the aft end of the rocket? I dont think so. A well planned glassing job will add considerably more strengtht.
Where two part expanding foam shines is in adding electronics to a nose cone. Check this out:
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RDH8 NAR L3
Reply to
Robert DeHate
Point.
Non user of foam.
Jerry
Reply to
Jerry Irvine
Ahhhhhhh . . . . dataaaaaaaa . . . . .
Jerry
Reply to
Jerry Irvine
On Sun, 5 Oct 2003 18:53:02 -0400, "Art Upton" is alleged to have written:
As many others have pointed out, foaming fin cans is overkill, and adds weight where you don't need it. Doc has data to prove it, too: see
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If it makes you feel better to foam your fincans, I'm not going to say not to. But, it's really not necessary except for very unusual applications.
I personally have 2-part foamed an entire airframe, and it was necessary in that application. I built a rocket out of my old fiberglass leg cast, and the foam held my motor mount tube in place, since there was no way I was going to be able to cut "centering" rings to hold a motor mount in *that* odd-shaped airframe. Also, since the cast had been cut off my leg, it was split in two pieces. The foam helped to hold the rocket together.
Where two-part foam is valuable in ordinary rockets, however, is in nosecones. If you need to add nose weight, a very good method to use is to mix lead shot, BBs, or ball bearings with thin (laminating) epoxy, such as System Three, and pour it into the tip of the nose cone. Once it hardens, back it up with a layer of 2-part urethane foam, to keep it from "breaking loose" under acceleration.
Note: Do not use "spray can" foam. It relies on atmospheric moisture to catalyze its expansion and cure, and can keep expanding for *months* after you apply it. Also, while it may seem solid at the surface, it can remain semi-liquid (uncured) in the middle for *years* if used to fill an enclosed air-tight space, such as a plastic nosecone or a fin can.
Also, I mentioned using laminating epoxy to wet out the lead shot used for nose weight. This is *not* 5-minute epoxy, nor is it the epoxy sold in syringes in a hardware store. Most hobby stores sell decent epoxy for fiberglassing. Much, much better are the epoxies used for boad building and aircraft construction.
If you haven't done so yet, go to
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and order the $10 (postpaid!) epoxy trial kit. You'll get resin and hardener sufficient to make a pint and a half of mixed epoxy, an assortment of fillers, a few mixing cups and other assorted stuff, and "The Epoxy Book". This book is *well* worth the entire price of the kit, as it will explain everything you ever wanted to know about epoxies. Even if you later decide you like another brand better (I have tried a few, and still prefer System Three, myself), you will not go wrong with this kit, or the book.
While you're there, look at the "Success Stories" page....
- Rick "Crayon" Dickinson
Reply to
Rick Dickinson
Thanks Rick.
all good advice. I've decided, "no foam for you" , er I mean no foam for me.
art
Reply to
Art Upton
I concur. I have used the 2 part foam in 2 fin can assemblies. Both of them have blown out seams in various places when they heated up. The most recent one had been foamed for 2.5 years.
Stephen Corban wrote:
Reply to
Andrew MacMillen

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