2-Part Foam Fincan
One way to make a rocket fincan more solid is to fill the spaces between the body tube, motor mount tube and fins with polyurethane foam. Not only does this fill the cavity, adding mechanical strength, but also creates an additional bond between all components. This short video shows how to use 2-part expanding foam (running time 7m).
Calculating the volume to fill isn't hard, but it helps to know the basic formulæ. The area of a circle is πr², which means the half the diameter, times itself, times 3.14. The volume is just the area times the length. Our situation is a bit more complex because we aren't filling the motor mount tube, so we need to subtract that volume.
In addition the foam expands, so we need to calculate the pre-expansion volume. Generally, the foam will expand a bit less than what's stated on the can. The X-30 foam says it expands 30 times, but I never get that much expansion. I calculate to fill the cavity, then do a second pour if necessary.
Or use this little worksheet to calculate the amount of foam to mix:
The foam I'm using is "X-30" polyurethane foam from Tap Plastics. Note that you must use 2-part foam, not the stuff that comes in the tubes for caulking guns, because you need foam that will cure without the presence of air.
The rocket featured in this video is my Madcow AGM-33 Pike.