<P> The size of the nozzle affects the thrust produced by the rocket . Larger diameter nozzles provide faster acceleration with a shorter thrust phase, while smaller nozzles provide lower acceleration with a longer thrust phase . </P> <P> As the propellant level in the rocket goes down, the center 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). </P> <P> To lower the center of pressure and add stability, fins or other stabilizers can be added which bring the center of drag further back, well behind the center of mass at all times . Stabilizers of any sort are normally placed near the back of the bottle where the center of mass is found . The increase in stability which well - designed fins give is worth the extra drag, and helps to maximize the height to which the rocket will fly . </P> <P> Stabilizing fins cause the rocket to fly nose - first which will give significantly higher speed, but they will also cause it to fall with a significantly higher velocity than it would if it tumbled to the ground, and this may damage the rocket or whomever or whatever it strikes upon landing . </P>

How does a parachute work on a bottle rocket