Adding fins to bottle rockets, or any rocket, for that matter,
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. Even a small deviation from the right
direction will make the rocket take a curved path, and maybe even go in
circles or tumble randomly. A balloon with its lip untied, with air
escaping, is an example of an uncontrolled rocket.
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. It's not perfect -- the rocket may
still go around in circles, but the idea is to make the path straighter
than it otherwise would be.
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. That would
rob energy from the flight anyhow, so it sounds like not such a good
(republished on 07/30/06)