Adding fins to bottle rockets contributes to the stability of the rocket in flight. A rocket works by conserving momentum as water (in the case of the bottle rocket) or burning gases escape from the rocket’s nozzle, the rocket will feel a push in the opposite direction. Unless the escaping material is exquisitely aimed, then the rocket may not get pushed exactly in the direction you want it to go.The fins counteract sideways motion of the rocket. Air flows smoothly past them if the rocket is traveling along its axis. If there is any sideways motion, then the air striking the fins pushes the rocket back towards straight motion.

The fins can be made out of cardboard and taped to the bottle. Their shape probably isn’t too important (the fins on real rockets take a wide variety of shapes), although their area is. It probably isn’t good to make them too heavy. If you make them with pointed ends it will be easier for them to bend out of shape when you set the rocket down. The plane of the fins should contain the axis of the rocket don’t bend them or they will cause drag on the rocket in flight and may cause it to spin. Spin is used to help stabilize rifle bullets, but in the case of a water bottle rocket it probably won’t make much difference because the bottle may spin but the water inside would not.

i couldnt find much information but i researched for like an hour. i found that this high school teacher did this using Newton's 3 Laws, and she said that it really helped her. she also said that if you use firm fins, it will work so then the fins aren't flapping around or else they would be useless. the size of the fins matters, like long and narrow. it also says on this website to stick a ball of clay in the nosecone

what website?

oh go to http://catalog.pitsco.com/content/item.aspx?ap=1&art=22 if you need help on the Center of Gravity or the Center of Pressure!