You can make many modifications to your water bottle rocket to make it more powerful, aerodynamic and efficient, maximizing the height it reaches and the amount of time it stays in the air. You can add or remove fins, use nose cones of varying shapes, adjust the contents of the water bottle and even adjust the conditions of the launch. The right combination of adjustments will make your rocket fly higher, farther and straighter.
Make a nose cone out of modeling clay and attach it to the nose of your rocket. A nose cone decreases the drag force on the rocket, allowing it to fly higher and stay in the air longer. It also moves the rocket's center of mass closer to the nose, which increases the rocket's stability in flight. The best shape for a nose cone is a parabola ("U" shape).
Make two or three fins out of plastic or Styrofoam and attach them as close as possible to the water bottle's mouth. The fins increase the surface area of the rocket, which increases its stability by moving its center of pressure closer to the mouth.
Vary the nozzle size. A wider nozzle will often increase the thrust power of your rocket because the mass of the rocket decreases at a faster rate.
Vary the ratio of water to air in your water rocket. Increasing the amount of water usually results in a greater thrust and a higher flight.
Use an air pump to increase the air pressure inside the water rocket. 100 pounds per square inch is a typical pressure for launching normal-sized water bottle rockets.
The lighter you make your water bottle rocket, the higher it will fly. However, lighter rockets are less stable in flight.
Always launch water bottle rockets in an open area, a safe distance from any buildings, cars or people. Be sure to point the rockets away from people or animals.