To foam or not to foam...

Maybe someone could point me in the right direction. I've been searching, but I'm not coming up with any results. I'm looking for more information on the two part foam systems for the fin can. I've watched videos and I understand how it works. I'm just trying to find out when people decide to use it compared to just having internal fillets. Is it personal preference?

I would say let's have a foam party!!!

Seriously though I think foaming is easier... fillet seems a lot of work. Plus foaming helps insulate the rocket (not sure if its needed) and provides structural rigidity.

I think you'll find it is more of a personal preference

Pros - adds strength and helps grab the fin to hold it

Cons - Adds weight where you don't want it, and if you do break a fin can make repairs more difficult. Also can make a real mess if you use too much

Well, if I am foaming I'll make sure I use fiberglass board for the fin, it's very hard to break. If it does break chances are the rest of the rocket is ruined.

I have used foam an small fiberglass rockets, short and 2-3" diameter.

I used foam on my 4" Nike with a 54mm motor. I also added (small) internal fillets. Those fins aren't going anywhere.

Consider foan as an internal fillet but it bonds to the entire fin tab in the fin can versus just a small portion that typical fillets would do. Weight would probably be less than traditional internal fillets if used instead of. I have built both ways and I think it just comes down to personal preferance.

Foam is way easier, especially for small diameters. As for mass, well, obviously, the mass of the foam is just the weight of the two liquid parts you mixed together. Compare that to the mass of epoxy you mix up for internal fillets. I have done this and found that for 3 inch and smaller diameters, foam is probably less weight. Above that, modest epoxy internal fillets probably weigh less.

Generally I would say HPR it is OK to do internal fillets and skip foam but not the other way around.

I do both, I use Aeropoxy ES6279 structural epoxy to bond the root of the fin to the MMT then I use a syringe to lay down fillets on the fins against the MMT as well as internal fillets on the fins against the inside of the BT. I then use PML 2 part foam at full strength. Externally I also use fillets but it's more for aerodynamics and looks after all the internal stuff.

Total overkill, maybe. However I usually load up the biggest motors my builds can take and have never left a fin on the pad or ripped one off on the way up. I have also had a 5.5 lb rocket fall a mile on a 12" drogue, land in hard dirt and besides some scrapes there was no damage.

I would argue why not...it makes for a very sturdy rocket and then you can just concentrate of having some fun.

mpitfield said:

I do both...Total overkill, maybe.

Click to expand...

Totally. HPR birds with foam and no internal fillets work fine for me.

In fact, the instructions for my L2 Madcow Frenzy (cardboard and plywood version) made no mention of foam nor internal fillets at all. Just a heavy bead of glue on the fin root is probably good enough.

+1^

You don't need either. The fin can is the strongest part of the rocket. A good joint at the the fin root and external fillets is way strong enough. I have never seen a fin being "pulled" out of a rocket, if anyone has please send a picture for evidence, I'd like to see it.

If you do break a fin, a foamed fin can makes the repair almost unrepairable.

Personally I consider the insulation properties of the foam a minus. It would contribute to the heat soak around the motor tube.

Gerald

Skip it. It doesn't add more than it subtracts from the rocket, in the end.

I foamed my L1 rocket, but years down the road wish I had not. The extra weight it did not need and a no-main landing on the dry lake bed had the fin can come apart around the foam--complete loss. I suspect it may have turned out for the better, perhaps repairable, if not for the foam. As John said, the fin can is already plenty strong already.

I build wildman minis with foam, but otherwise it's relegated to nose cones, where I use it with bbs for weight.

I like putting a tube into the nosecone to form a forward bay, and foaming around it. It makes for a very solid nosecone and puts a little extra weight where it is often most needed.

Gerald

G_T said:

I like putting a tube into the nosecone to form a forward bay, and foaming around it. It makes for a very solid nosecone and puts a little extra weight where it is often most needed.

Gerald

Click to expand...

Thats actually a really cool idea. I was thinking of using John Coker's nose cone modification from his how to videos. Maybe I could modify it a little with your technique as well.

I always use both foam and fillets. Its really not much more work and the the foam really doesn't add that much weight. It gives me a little peace of mind that I won't pop fillets on a hard landing.

jderimig said:

I have never seen a fin being "pulled" out of a rocket, if anyone has please send a picture for evidence, I'd like to see it.

Click to expand...

Unfortunately, I don't have the pics you want to see, but this type of failure is really not uncommon. I've had it happen to two rockets (and have seen it in others'). One was carbon with FG fins, the other all FG. Both had fin roots "double butter" epoxied to the motor tube. Both had good ProLine external fillets. In both cases the rocket apparently landed on the outer/bottom corner of a fin, which cracked the fillets, and while the fin did not pull out of the rocket completely, both times they pulled out about 1/2".

Since you are probably wondering - I fully rough sanded and cleaned all bonding surfaces before glue-ups, and even went as far as deeply scoring/notching the gluing surfaces as well on one of them. Also, my launch sites have pretty hard surface landings, so this is of course a factor.

I've never had this happen on rockets where I've either foamed the fin can, or did internal fillets.

Fins CAN and DO "pull out" in rough-landing scenarios.

s6

stealth6 said:

Fins CAN and DO "pull out" in rough-landing scenarios.

s6

Click to expand...

Easy repair though right? Easier than a broken fin in a foamed fin can.

Yep, pretty simple repair.

s6

Buckeye said:

probably good enough.

Click to expand...

Yeah I have never really subscribed to "good enough", but I have no issues if you want to.

To borrow a pic and phrase from o1d_dude:

View attachment 247548

The difference between as strong as possible and as strong as necessary is a problem area for me too.

mpitfield said:

Yeah I have never really subscribed to "good enough", but I have no issues if you want to.

To borrow a pic and phrase from o1d_dude:

View attachment 247548

The difference between as strong as possible and as strong as necessary is a problem area for me too.

Click to expand...

"Good enough" doesn't refer to me. It refers to the kit instructions supplied by the manufacturer. If Madcow says no fillets needed, I believe him. But, I wimped out and put small internal fillets anyway - only on the MMT, not the BT.

To me, aerodynamics is all about efficiency and the challenge of working on the performance edge. Overbuilding rockets defeats this purpose. Your goals may be different. That's cool.