Foam the fin can or not??

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Well-Known Member
May 1, 2002
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Central Iowa
I'm building a Binder Design Jaguar and am starting to wonder if I should foam the fin can or not.

Build is with 30 min epoxy with milled fibergalss. Fins will have fillets to the motor tube, to the inside of the body tube, and to the outside of the body tube so basically six fillets to each of the three fins.

I will be pushing this rocket with a J570 and it should be breaking mach and over 50g's according to my sims. (wrasp)

So will foaming the fin can really do anything for me in this application..??

If so are there any good foams to use that can be picked up at Lowes or Home Depot.. I really don't want to delay the build anymore waiting on foam by mail order if i don't have to..

Thanks for any advice....
I'd say you're probably OK without foaming the fincan since you've got fillets everywhere (especially with the milled fiber in the epoxy!) you can put one, but here's a little food for thought in case you decide to pursue it further:

1) The Jaguar is so long that the additional weight from foaming the fin can isn't going to make it anywhere close to unstable. A plus for considering to foam the fin can.

2) However (to the best of my knowledge), all of the expanding foams found in home improvement/hardware stores tend to be the kinds that need heat and/or air to cure. In other words, the foam can continue to expand looooong after you might want it to! :eek: PML's expanding foam doesn't have these requirements not to mention it cures much faster!

That's my $0.02... HTH,
Hi Firemanup,

I have become a believer in foaming the fin can. Here is a shot of my rocket that might have survived if I had read someone's advice on EMRR. It used to be a PML Little Lunar Express.

Now, this rocket had HUGE fins so it was a good ecample in my case of lack of experience to NOT foam it. I even had the stuff on my shelf.:rolleyes:

If the weight increase is minimal, I will do it when I am pushing a rocket with 500+N average thrust.

Len Bryan
I just used PML foam for the first time on a Vaughn Bros. Javelin. It's being rebuilt from a prang several weeks ago...all that was left were the fin unit and nose cone. One of the fins was a bit loose so it seemed better to use foam instead of cutting everything up and starting over.

I drilled three 1/4" holes in the aft CR and just poured foam in a little bit at a time, until it finally started expanding thru the holes.
Before that, it was easy to judge how far up the foam was expanding by feeling the heat thru the body tube. Once it dried the excess was cut away, and the mess was covered up with a fresh centering ring (which I needed for PVC cap motor retention anyway).

Granted, this was not the optimal way to use foam but the fins feel rock-solid now. I plan to give it a shake-down flight on a G64 this weekend, if it holds up it'll fly again on an H210R.
I don't think you really need it, but it certainly wouldn't hurt to do it. The other possibility (if you want to save a little bit of weight) is to 'glass the fins to the body tube on the outside (or inside, or both!! lol)
Foam adds alot of strength to the fin can area. I have heard that some guys tack the the fins in place with CA then foam, no epoxy or filets and have flown on M motors. I say, let someone else test this method on their own rockets

Generally, I dont beleive that foam is required, but I now do it to all of my rockets. It's just fun to play with and I am not going for record altitudes.

In a hurry? Check your local hobby shop. My local shop sells what appears to be the same stuff as PML. It is used as flotation for R/C boats. It also will work in confined spaces and cures completely.

If you are worried about the weight shift, foam the nosecone as well. This is something I recomend for Mach + flights anyhow. It also, for some reason, makes paint stick to the nosecones better.
I am on both sides of the issue. The purpose of the rocket define how I will build it.

For my L2 I wanted a solid rocket, it was a BSD 38 Special I foamed the can and it is solid as a rock. Got my L2 with it but generally I do not fly it much because of the weight. However two launches ago we had high winds and gusts not the best day for flying. I think you were at that lauch Jason? I popped a J330 in her and let it rip the rocket came down on a reefed drouge, the main got stuck, so it hit hard on the fin can but not even a ding.

For rockets that I want to fly high and light I will not use it. For rockets that I want tough and durable I will. So for me at least it comes down to 'intended use'.

Foaming does strengthen the structure substantially. My first experiance with this was three years ago when my MiniBBX/Terrier suffered a cato during sustainer ignition. The interstage coupler of the Terrier was blown to bits (including the rather solid cast transition) and the quantum tubing of the sustainer stressed so much the the paint, lugs, and fin fillets were all blown off. Yet the fins were still on rock solid. I am sure if it wasn't foamed the motor tube would have completely ruptured (intead of a couple of cracks) and blown the whole aft end of the airframe apart.
Foam also serves another function, at the beginning of this season I landed my PML Eclipse (glassed phenolic version) in a river. Earlier in the day someone elses Eclipse made the same landing and sank in moments. Mine floated like a champ due to the floatation provided by the foam.
Use only the two part expanding foams, do not use any of the spray can stuff, it will never cure.
A better source for foam than PML, though, is U.S. Composites:

Much cheaper. Check out their epoxy and fiberglass too. Great prices all around!
Not only is the US Composites foam cheaper, but also available in different densities. I use both the 4lb and 8lb density stuff, much stronger than the PML type foam.
After using something that I haven't played with much until today...I also want to propose a middle ground for foaming the fin can:

Use polyurethane glue.

Polyurethane glues (like Gorilla Glue) are pretty much the exact same product as the PML 2-part foam except that there's nothing to mix & it doesn't foam quite as much, especially since it is used in smaller quantities... All you need to do is get the surface damp with water (not soaked) & smear a light amount of glue. Unlike the PML foam, which goes into a rapid chain reaction within 60 seconds of mixing the 2-parts & fully cures in about 20 minutes, poly glue instead takes about 15 minutes to begin to foam & 4 hours to cure. This stuff would be excellent for internal fillets. (I only use it in the future tense here because I am *definitely* going to be using it in the future but have only used it sparingly in the past...) Oh, you can find it at your local home improvement store & a 4oz bottle of Gorilla Glue is ~$5.

I almost forgot to mention that another difference is that it doesn't generate heat (exothermic reaction) during curing like the PML foam. It's worth mentioning because poly glue will stick to pretty much anything!

Give it a shot... HTH,