Foamed Fin Cans & Stability

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Dec 24, 2003
Reaction score
I am building an AT Sumo for a L1 attempt and decided to go a little overboard as I want to reall push this rocket to the edge. That said I foamed the inside of the fin can with a 2 part polyester modeling foam.

That is the windup, here is the curveball.

I can't seem to figure out using Rocksim (v7) to determine if I have made a substantial change in CG. Other than do a swing test or a static balance to see if CG has shifted, does anybody know if the 2 part foams weigh enough to make a serious difference in CG?

For the record, I am planning on an H165R for the launch.


I would love to know that too. I intend to fill some fins on a Estes v2. How much does expanding foam weigh?
The foam is not terribly heavy, but could certainly make a rocket unstable. I would weigh the model and find the CG (with motor, chute, etc). I'd then plug these into the Rsim's mass override and see what the sim says.
I'm with rstaff3. The Sumo is a pretty stubby rocket, so I'd definitely take the extra step to be sure.

I have just foam filled a new rocket fin can with 2-part PML foam. The space filled is 9.5" long using a 2.6" bodytube and TTW fins that take up a little space. The foam weighed 1.9oz (58g) and moved my CG back around 1". With your motor you may have to add some noseweight. The weight foam will add could make your rocket unstable.

Be sure to weigh your rocket before and after adding the foam to get the correct foam weight. Plug it into RocSim, then, recheck your CG location and see if it has moved aft.

I have RocSim 5 but and I put the weight, mass override, over the center of the motor tube.
an easy fix would be to foam the nosecone, this would balance the rocket and not add too much weight. Also, since you want to push this rocket to the edge as you say, it will help keep the nosecone from getting destroyed (okay maybe this is way overboard but better to overstrengthen)
You could look at the effect of the foam on the CG of the rocket in RockSim if you know the density of the cured foam.

To calcualte the desity of the foam, cast or cut several 1 cm on edge squares of it. The weight of the cubes in grams will be the density of the foam in g/cm3. Or you might be able to find the density of the cured product in the information provided by the manufacturer.

You will then need to create a new material in the materials database for the expanding foam. Open RockSim from the main screen under Rocket, select materials. Add the foam as a material in the database and make it available for use in body tubes. Simulate the foamed in area as an inside tube of the appropriate inside and outside diameter and length.

I always check the CG of the completed rocket using a loop in a piece of string (static balance test) to see if I got everything entered into RockSim correctly. I have caught myself more than once mistakenly selecting things like solid poystyrene nose cones!
If the RockSim CG agrees with the balance point found with the loop of string, then you know you have simulated the foam correctly.

Bruce S. Levison, NAR #69055
teflon, I have a question. In the newsletter, you to get the mass numbers for the foam from a MSDS spec. I was wondering, are the values shown in the sample screen (fig2) real values from such a sheet. If not, you you have some typical numbers?

The actual values I used were for 2 part Urethane foams from the Us Composites web site at:

I used the 2 lb and 4 lb densities since I didn't think anyone would use the 8lb and 16 lb density materials routinely for rockets.

Bruce S. Levison, NAR #69055