Body Materials

• There is very little difference in the quality of 2-liter soda bottles. They're all made from a very thin and brittle plastic. But that thinness and flexibility is what allows the fuel chamber to store even more compressed air.
  
  Fusing the body of two soda bottles together will increase the size of the fuel chamber and allow it to shoot higher. Use a strong glue like Gorilla Glue or an epoxy. These adhesives will keep the bottles together but still be flexible enough to allow the body to expand during the air compression. However, don't bend or crush the body rocket when fused together, as any damage to the frame may cause it to lose the air seal.
  
  Think outside the soda bottle paradigm by using other materials such as PVC pipe, plastic casing, and, if you're feeling daring, a lightweight metal body. These bigger chambers are sturdier and will allow for more water and air to enter the chamber. But there's more designing to do in fashioning a nozzle and top. As a point of inspiration, the "X12", world record holder for highest water rocket launch, was made from multi-chambered metal casing.
  
  Retrofitting a wider nozzle to the rocket will allow for more propellant to escape and resulting a more powerful launch.
  
  

Fins

• Fins are necessary if you want to keep the rocket ascending in straight trajectory and not sputter out just after launch. Either three or four fins work, depending on the kind of rocket you have. Test out the different options to see which works best.
  
  Foam board is a perfect material, as it is sturdy and easy to work with. If your rocket has a metal casing, then a light and thin metal fin is possible. Use metals such as aluminum, tin or zinc.
  
  

Cones, Parachutes, etc

• Adding a cone to the top of the rocket helps streamline the body and creates a more aerodynamic rocket. A sturdy poster board that fits snugly over the top is a good start.
  
  A parachute is a vital addition to your rocket. It will keep the rocket from crashing to earth and damaging the body, as well as giving it a dramatic descent. Attach the parachute to the top of the rocket and fold it correctly under the cone to ensure a consistent deployment.
  
  The ratio of water to air in the fuel chamber makes a large difference in the height of the launch. Every rocket is different, so each ratio will vary based on a number of factors. Test different ratios to discover which is best for your design. There are also a number of computer simulators that can estimate what ratios will be optimal.