This instructable is based on my knowledge of water rockets, that I have learn in the past few years.
I am not responsible for any damage you infict on your self or others from launching or building water rockets
Have fun building and flying!
Step 1: Lets get Started
If you took a standard two litre fizzy soft drink bottle and pressurised it to120 psi the rocket would reach about 100 or so feet.
But then if you took 2, two litres bottles and pressurised it to 120 psi again the rocket would go about 150 feet or so because the rocket has more air in it therefore creating more thrust. The rocket will only go 50 feet more because of the added mass. You can stop this by making a two stage rocket. A two stage rocket will work better because it would not have to carry the full payload on all of its flight.
Step 2: How to build a fins and nose cones
This step is to get to grips on what a rocket needs.
Step 1: nose cone
cut the top and bottom off a 1.25 litre bottle. Then cut the neck off the top.
Step 3: Step 2: Nose cone
Cut a ping pong ball in half with a craft knife,
Then glue half of the ping pong ball into the top of the 1.25 litre bottle 'top'.
Use a Plastic adhesive glue, Or if apperance doesn't matter you could just tape it on!
Step 4: Step 3: Nosecone
Adding wieght to the nose cone may help, It will move the centre of gravity higher, thus being more stable.