Foam Fincan Retrofit

In this thread I will be displaying the process I use as I add 2 part foam to the fincan of a preassembled rocket. In this case it is a Fiberglass Madcow Little John (4") with a 54mm motor mount. Since the internal fillets were done poorly, and this rocket will now be used for a level 2 certification attempt (already flown level 1 motors with it), I think it would be a good idea to make this modification.

Materials:
Rocket (preassembled)
Drill (1/4" bit) ~ the size of holes you tap depends on the amount of space you have between the motor mount tube and the airframe. (Bigger is better)
Deburring tool
Funnel or dispenser bottle with a small tip.
Masking tape and stuff to make sure you don't get foam on your rocket (it is VERY sticky).

The foam I am using is from TAP Plastics; X-30 polyurethane (2 parts).

https://www.tapplastics.com/product...ation_materials/tap_x_30_polyurethane_foam/79

I have done that before. You either have to really have a feel for how much to use....or use very small batches. Put in too much and you can have more problems than just a mess (oh, yeah, problems PLUS a mess).

I have the same kit that I used for my L1 earlier this year. Although I don't plan on using it for my L2 cert, I do anticipate using such motors down the road. Rather than foam the fin can, I went with an SCP thrust plate. It was a perfect fit and the Aeropack retainer bolted right up to it.

Thrust plates look nice, but really are overkill for anything that isn't at least a L3 rocket. Not to mention expensive. They are good for transferring the forces to the airframe, but don't really help reinforce the fincan itself.

Here is the aft end, with 4 1/4" holes in the centering ring.

The bottles that will be used to inject foam. I was able to get these at a local supermarket for $1.50 each. Fortunately, the nozzle fits perfectly into the holes, insuring that spillage will be minimized.

That is what I was thinking. I already calculated about how much volume I need to fill, but will add a bit less in order to assure it doesn't overflow. Small increments sound like the best option.

Calculate volume to fill:

V1 (Volume of fincan from ID of airframe):

pi*(49.5mm)^2 = 7697.681mm2
(*length = 254mm)

V1 = 1955210.97mm3

V2(Volume of motor mount tube from OD):

pi*(29mm)^2 = 2642.0772mm2
(*length = 254mm)

V2 = 671087.61mm3

Now subtract V2 from V1 to get the volume that needs to be filled:

V1 - V2 = 1955210.97mm3 - 671087.61mm3 = Vfinal =1284123.36mm3

Convert Vfinal to mL Vfinal = 1284.123mL

Now, divide by the expansion factor of the specific foam. In my case, this is 30X

so Vmaterial = 42.80mL
(meaning 21.4mL of each part)

It should be noted that I didn't take into account the volume which is taken up by the fin tabs. Another reason to add LESS than the calculated amount, to prevent it from overflowing.

I wouldn't worry about the volume of the fin tabs, as you'll never get all the foam out of the bottles.

tellinz said:

Thrust plates look nice, but really are overkill for anything that isn't at least a L3 rocket. Not to mention expensive. They are good for transferring the forces to the airframe, but don't really help reinforce the fincan itself.

Click to expand...

After flying my L3 rocket both with and without thrust plate, I am convinced they are overkill there too.

However, there are reasons to use them other than pure strength. They look great, and provide excellent attachment for retainers. In my case it helped me out of a jam after I broke a glue on retainer and had to perform surgery on the motor mount tube.

You are absolutely correct that thrust plates do not help with fin strength from outside forces but I think the point being made was that foam or injected fillets help with transferring motor forces to airframe.

I was thinking more of how to deal with the increased thrust and not so much the integrity of the fins. Looking back at your original post, I now realize that's your main concern.

Tonimus said:

I wouldn't worry about the volume of the fin tabs, as you'll never get all the foam out of the bottles.

Click to expand...

Very good point. Another thing to consider.

My Gypsy said:

I was thinking more of how to deal with the increased thrust and not so much the integrity of the fins. Looking back at your original post, I now realize that's your main concern.

Click to expand...

Understood!

The material arrived today:

Got everything taped off, don't want to get any of this stuff on the air frame.

Success. I was able to get almost exactly the right amount on the first attempt. As you can see, a little bit spilled through the holes. The reaction is exothermic, so the aft end of the rocket got quite warm after a few minutes. One thing I will say about this stuff is that you MUST work fast, and have everything ready to go before hand.

Cool, glad it worked out.

After about 15 minutes, the material is no longer tacky, and excess can easily be shaved off. An exacto knife is all I needed to clean up the aft centering ring. Soon I will plug these holes with some epoxy clay. Once painted, you won't be able to see the difference.

Lookin' good!