Suggestions on "filling the void"

[rbelknap]

My first attempt at filling a fin-can with 2 part foam was marginally successful:



As you can see in picture 2, there is a small void along the fin root. I'm not too worried about it, but the other side (picture 3) is a bigger problem. At this point I'm wondering if I should drill some holes in the side of the airframe to inject more foam, or try to dig out enough of the foam in the rear to clear a channel through which to inject the foam.

Holes through the airframe would probably be easier, but I know I'm never going to get them filled and smoothed out enough to hide them when the rocket eventually gets it's final coat of paint. I'm also concerned about getting the holes placed correctly to give the foam room to escape/expand. I don't want to blow out the side of the G12 tube.

I'm looking for advice on the best way to get that large void filled.

 

[Eat, sleep, and Fly]

Honestly, If that is the darkstar mini, as I think it is, I Wouldn't worry about it. Put on an external fillet, and you could drop that thing from heaven without a probelm.

 

[rbelknap]

It is a Darkstar Mini, and I am planning to do external fillets with epoxy clay. I'm somewhat concerned with the weight of the rocket being off-center, but perhaps I'm just being paranoid? With such a small volume, perhaps it doesn't really matter. Truth be told, I'm probably going to strap an external camera to this thing on most flights, wich would affect the stability more than some foam voids.

A related question, is there an easy-ish way to clean the foam drips? (A mild solvent of some sort?) or am I in for a bit of careful cutting and sanding to clean the outside of the airframe?

 

[hornet driver]

I'd say cut it off and give it a lite sanding---You might try acetone or high end laquer thinner but you might end up with a sticky mess and have to sand anyway--not to mention the fumes !

 

[jsargevt]

I think that you're being paranoid I suspect that with some external fillets you'll be rock-solid. Worst case is that if the fin ever does crack loose you can jb weld it back in, and I am pretty sure that you won't have to bother. I like E,S&F's heaven image.

If you want to "balance" the weight then inject a little epoxy via a hole. I think that adding more foam might be just asking for trouble - I don't know want to find out what a G12 foam explosion looks like. As far as filling in a drilled hole - a little bondo spot putty will make quick work of that - I doubt that you'd notice there was a hole there once you filled it in and sanded it down.

I will be interested to hear what you think about epoxy clay. Personally, I find it to be better in places you can't see since I have had trouble making it look good (although I didn't have Bondo at the time so that might help a bit).

 

[rbelknap]

Thanks for the advice so far. I've also gotten a couple of responses on the actual build thread. I'm still thinking about how I'm going to proceed, but I'm leaning towards cleaning up, moving on, and relying the external fillets for strength.

I will be interested to hear what you think about epoxy clay. Personally, I find it to be better in places you can't see since I have had trouble making it look good (although I didn't have Bondo at the time so that might help a bit).

I got it to work pretty well on the fillets for my King Kraken canards, granted that was a relatively small job compared to fillets on 6 fins:



Not the best photo, sorry.

I've been following the "Level 2 Rocket" series of videos on the Apogee Components website, and he used CWF to smooth out and feather the epoxy clay fillets. Not something I would have thought of using on a fiberglass rocket.

 

[AZ_Ron]

Once the foam cures fully, it sands off like a dream.
On the next build, do your fillets first. That will keep any foam
from seeping out from any small gaps.
On a lot of my builds, I don't bother with fillets at all.
Just foam the whole fin can, when it's cured, sand, prime and
paint.

Ron

 

[JDcluster]

Why are you foaming in the first place; water landings?

With all the extra weight in the tail you might need nose weight.


JD

 

[rbelknap]

Why are you foaming in the first place; water landings?

With all the extra weight in the tail you might need nose weight.

I'm using the "foam and fly" construction method outlined in the kit's assembly instructions. I'm doing it because its a new skill for me. I like trying new things

 

[Pem Tech]

Once the foam cures fully, it sands off like a dream.
On the next build, do your fillets first. That will keep any foam
from seeping out from any small gaps.
On a lot of my builds, I don't bother with fillets at all.
Just foam the whole fin can, when it's cured, sand, prime and
paint.

Ron

Ron is right, once cured two part foam is easily sanded and worked. Much easier than urethane glue.
Wish I could help with the void issue....

 

[SAC of MMMSClub]

My first attempt at filling a fin-can with 2 part foam was marginally successful:

View attachment 115406 View attachment 115405 View attachment 115407

As you can see in picture 2, there is a small void along the fin root. I'm not too worried about it, but the other side (picture 3) is a bigger problem. At this point I'm wondering if I should drill some holes in the side of the airframe to inject more foam, or try to dig out enough of the foam in the rear to clear a channel through which to inject the foam.

Holes through the airframe would probably be easier, but I know I'm never going to get them filled and smoothed out enough to hide them when the rocket eventually gets it's final coat of paint. I'm also concerned about getting the holes placed correctly to give the foam room to escape/expand. I don't want to blow out the side of the G12 tube.

I'm looking for advice on the best way to get that large void filled.

That is strange, if you poured the A+B in to the void and it ran all the way to the forward CR it should have grown uniformly up and around everything?? I do not know what Acetone will do to the FG but it will eat gone nada zip the foam and you could try again. Good luck

 

[blackjack2564]

what Acetone will do to the FG but it will eat gone nada zip the foam and you could try again. Good luck

Not this stuff. I've tried everything.... acetone, gas, MEK, lacquer thinner, alcohol,and some really strong reducers you wouldn't even want to be caught dead around. It's there to stay. Just some scraping & sanding makes it come off quite easily.

I looked at the rest of the internals. There is enough there between the other 2 sides, I would not worry, just move on to the externals......you will be fine. Everyone wants to overbuild stuff this is a tiny rocket, don't get sucked into the "overthink---overbuild" thing.

 

[SAC of MMMSClub]

Not this stuff. I've tried everything.... acetone, gas, MEK, lacquer thinner, alcohol,and some really strong reducers you wouldn't even want to be caught dead around. It's there to stay. Just some scraping & sanding makes it come off quite easily.

I looked at the rest of the internals. There is enough there between the other 2 sides, I would not worry, just move on to the externals......you will be fine. Everyone wants to overbuild stuff this is a tiny rocket, don't get sucked into the "overthink---overbuild" thing.

Jim, wow! what did you use for foam? But I do agree with you, move on and make it look good on the outside. Spin and stability are important but, not to get nuts about.

 

[rbelknap]

Thanks for the input. In the end I did a bit of experimenting and got the void filled in nicely:



Yes, I know I probably didn't need to, but It's the principle of the thing. I should be smarter than the rocket.

I'm still very much the novice builder, and I wanted to figure out if I could fix it. It gave me a chance to learn more about how to handle these materials.

There's a quick write up of what I did here.