Polystyrene Fins vs. ABS Fins vs. Balsa Fins vs. Balsa Core Fins
I have always disliked mounting balsa fins on my rockets. They are too delicate for extended use and can be a pain to finish properly. If they are not finished properly you can see the grain patterns on the fins which seems out of place on a "rocket." Even though balsas lightweight characteristics cannot be beat, there is something that makes me cringe about mounting wooden fins on a rocket.--Even if the heart of that rockets airframe is an oversize toilet paper tube!
On a recent kitbash of a Big Bertha rocket, I decided to upgrade the motor mount to a 24mm short (C & D only) and also cut the stock 4 fin design down to a through-the-wall (TTW) 3 fin using 2.5mm polystyrene. Polystyrenes biggest drawback for fin construction is its flexibility. Anything less than 2.5mm will just flap in the wind creating a very unstable rocket. Polystyrene will take rougher landings that would regulate the balsa finned rocket to the spare parts bin.
The best solution by far, is to use a poly/ABS fin with the inner portion of the fin cut out and replaced with balsa, thus getting the durability of a manmade material where it is needed AND the lightness and stiffness of balsa. This is all extra work on initial construction but, it seems to be worth it due to less time spent repairing the rocket over its service life. I used this construction on my latest series of airframes. I shaved 30 grams off of my TL-96 payload rocket fins using this method. I saved 10 grams per fin and they are MUCH more durable than balsa all by its lonesome.
I used 2.5 mm ABS cored with 1/8" balsa. It wasnt that tricky cutting out the core with a Dremel multi purpose bit, but my fins were around 16 sq. in.--a decent size pallete to work with, as your fin size decreases this method would get trickier. I sanded the fins flat after gluing them up. The resultant fin was noticebly lighter and stiffer than its pure ABS sibling. I saved approximately 28% of the total weight of EACH fin.--Definitely worth the extra effort in fin construction. The build of TL-96, the first in a series of payload rockets I am designing, will have its own article posted soon.