PML's Adjustable Density Expanding Foam

In my first HPR build (PML's Spitfire) Public Missile sells "Adjustable Density Expanding Foam". Is this a better alternative to just straight epoxy when attaching rocket fins?? Or would you use it in addition to epoxy?? The field I fly at has a 2500' restriction, so I really won't be pushing anything higher then a H size engine. (FYI)

Geoff

I've never used it alone. I've epoxied the fins then if I felt the need, added the expanding epoxy

With the power you are talking about straight epoxy will be sufficient

The foam is intended to be used in addition to the epoxy. I would agree with les, not really necessary in this application. I seriously overbuild, so I use foam on just about everything I build. One should be cautious with shorter rockets however. The foam is light but not weightless. It adds some tail weight that you may have to deal with in balancing the rocket.

Jim

Echoing Jim ... if you can handle the weight of the foam, then a foam-filled fin can is very strong. It is much more difficult to loosen a fin with foam.

A better application of foam is filling hollow nosecones to add nose weight.

BTW, all two-part foams are "variable density" by adding water, AFAIK.

And a cheap 2 part foam is probably available at your local Lowes or Home Depot, Sika Post-Fix is a 2-3lb/cu. ft. foam sold for use with fence posts in place of concrete. I did some tests with it last year and used it in the nose cone of my Nuclear Sledgehammer.

Bat-mite said:

Echoing Jim ... if you can handle the weight of the foam, then a foam-filled fin can is very strong. It is much more difficult to loosen a fin with foam.

AIK.

Click to expand...

i drill some holes through the fin in the area that will be inside the BT. allows the foam to flow through and lock em in a bit more.

gcanroc said:

Is there any difference of what Public Missiles is offering (Adjustable Density Expanding Foam) and the window & door sealer that you buy in a aerosol can that you spray into spaces so as to block out drafts and add to the insulation?? Just curious.

Click to expand...

Absolutely. The canned foam requires open air space to evaporate and cure. If you inject or spray it into a nose or fin area, only the top skin will cure and you're stuck with a soupy gunk mess.

2-part foam cures by itself just like epoxy.

DO NOT USE CRAP IN A CAN!!!
It is for windows and doors...not rockets. Heat will make it countinously cure for months/years .every time rocket is in sun/hot day the foam will expand more.
It will expand inside your airframe and bulge or burst out the fin slots. It cannot fully cure inside a sealed airframe correctly.
The difference is one is 2-part glue the other is crap in a can. [1 part moisture cure]

I must also differ in opinion from others. It can be used without any other epoxy or glue.
I built a Gizmo with wire ties and foam only [actually bit of CA to hold fins in place] I defy anyone to remove those fins! Have built several rockets with foam only.

Wildman mini kits have instructions for building with foam.

...nuff said about "crap in a can". Thx for all the great input, there's nothing better then learning from "experienced" advice. Back to rocket building.

Geoff

Alternate sources, several threads about this stuff here on TRF

https://www.homedepot.com/p/Sika-33-fl-oz-Fence-Post-Mix-7116170/300934597

https://www.lowes.com/pd/Sika-Polyurethane-Fence-Post-Mix-Actual-Net-Contents-33-fl-oz/999977070

I would avoid building a rocket with the idea that it will only fly on a field with a very limited ceiling.

At some point in the future you may move, your club may change fields, or you may visit a launch at a field where you can fly bigger motors.

Foaming a fin can with PML foam can greatly strengthen it while adding little weight. Of course if your find are really solidly attached they may break on a hard landing instead of pop off, making repair much more difficult.

Here's the link to a write up about the Sika Post-Fix foam I did a while back.

https://www.rocketryforum.com/threads/sika-post-mix-expanding-foam.150303/