The Water Rocket is a plastic-bottle based rocket which uses pressurized water to propel it upwards and then a parachute to keep it's payload, here, a tennis ball, in the air for a longer period of time. The model in this Instructable is based on a 2-liter bottle, and should, if properly built, stay in the air for over 25 seconds.

Step 1: Step Zero: Gather Materials

To properly build the rocket, have these materials on hand:

" Two 2-L bottles
" Cardboard
" Scissors
" String (30 meters should be more than enough)
" Painters Dropcloth (Can be found at most hardware stores)
" Tape (Not scotch, preferably)
" Tennis Ball
" Baby/Talcum Powder [optional]

For Launch:

" Water
" Launcher
" Pressure Pump
" Additional Cardboard

Step 2: Step One: Prepare the Bottles

First, remove the labels from all bottles, keeping one bottle fully intact afterwards.

Step 3: Step One: Prepare the Bottles

Next, cut the bottom and the rim off of the 2nd bottle, leaving only a chunk of the top with a hole in it. Then, tape over top of bottle until the top of the cone is as close to round as possible. For comparison, our cone is pictured above.

Step 4: Step Two: Prepare the Fins

Cut three fins in an approximation of the shape above, making sure that the flat part of the fin is shorter than the length of the rocket. Bend the fins in half. Tape the crease to the rocket, spacing the three fins apart evenly (if possible, measure 120 degrees of spacing on the main body of the rocket). Make sure the tips of the fins are pointed downward. After attachment, gently curve the bottom tips of each fin in the same direction.

Step 5: Step Three: Prepare Parachute

Cut the plastic sheeting into a 4-diameter circle. Poke eight holes, evenly spaced apart, on the rim of the circle of sheeting. Tie a 3m length of string into each hole.

Step 6: Step Four: Assembly

Tie the ends of the strings not attached to the parachute together. Tape those ends to the tennis ball.

Step 7: Step Five: Loading

Coat the inside of the cap with baby powder, if available, to promote release. Load the tennis ball-parachute combination into the cap, and place the cap on top of the bottom end of the main, finned rocket. Fold the parachute in carefully to ensure that it does not expand inside the cap.

Step 8: The Launch

1. Attach the assembled rocket to the launcher.
2. Pressurize the water, using the pump, to about 60 psi.
3. Release, with launch.

The water, pressurized, will be pushed by the air out of the rocket and into the ground. The force the ground exerts on the water will push the rocket upwards until there is no more water in the rocket. The rocket's acceleration will then become negative, with the upward velocity at first decreasing, ceasing movement, and then falling back to Earth. At the point of equilibrium, the cap of the rocket should separate from the main body, at which point the tennis ball, with the parachute attached, will come out of the cap. The tennis ball, being heavier than the parachute, will, if not already below it, pass it fairly quickly. This will cause air to expand the bottom of the parachute. Now expanded, the force of air resistance will act against gravity and cause the tennis ball to fall more slowly.