Water Rocket - Fins

Fins

Fins are extremely important on a water rocket. They ensure stability, and it is very likely that your water rocket flies higher with fins than without. Fins increase drag, but the stability achieved makes a much larger difference to the height your rocket will fly. A second thing that is very important is where you position the fins. It is best if they are placed near the back of the bottle where the center of mass is found. A water proof, stable, light material to make the fins would be "Coroplast". This is a cardboard like material that is almost indestructible. The only negative it has is that it is harder to glue, but with the right glue it is possible.

As the propellant level in the rocket goes down, it can be shown that the centre of mass initially moves downwards before finally moving upwards again as the propellant is depleted. This initial movement reduces stability and can cause water rockets to start tumbling end over end, greatly decreasing the maximum speed and thus the length of glide (time that the rocket is flying under its own momentum). To lower the centre of pressure and add stability, fins can be added which bring the centre of drag further back, well behind the centre of mass at all times, ensuring stability.

In the case of custom-made rockets, where the rocket nozzle is not perfectly positioned, the bent nozzle can cause the rocket to veer off the vertical axis. The rocket can be made to spin by angling the fins, which reduces off course veering.

Another simple and effective stabilizer is a straight cylindrical section from another plastic bottle. This section is placed behind the rocket nozzle with some wooden dowels or plastic tubing. The water exiting the nozzle will still be able to pass through the section, but the rocket will be stabilized.

The fins on a rocket are important for stability during flight. They should be placed near the rear of the rocket.

Aerodynamic drag acts on the fins as well as on the rocket body. Fins add to the frontal surface area on which the drag force acts (and therefore should be designed not to add too much drag). The drag forces on all frontal surfaces of the rocket can be resolved into one force acting at the center of pressure Center of pressure (fluid mechanics). This acts to oppose the forward motion, but if the rocket nose is not pointed in the direction of its motion at a given time (perhaps due to wobbling or instability), then there will be a torque, due to the resolved drag force, acting around the center of gravity. This torque will stabilize the rocket by returning its nose to the direction of travel.

Since the torque is the cross-product of the drag force magnitude and the moment arm, torque can be maximized without increasing drag force by increasing the moment arm. The larger the distance between the center of gravity and the center of pressure, the greater the moment arm on the restoring torque. Therefore, it is desirable to have the center of pressure, and therefore the fins, as far back as possible on the rocket body.

The lift force acts to push the back end of the rocket so that the nose will face the flight direction, and the drag force does the same, even though it is pointing orthogonally to the lift force.