Sooner or later, your water rocket is going to come down hard on top of a rock or patch of concrete, and you'll need a new rocket. If you've just got into this hobby, that's not much of a problem, since most beginners start with little more than an empty 2-liter soda bottle. But if you want to move on to some fancy fins and maybe a payload or second stage, a softer re-entry would be nice. Parachutes are tricky, and don't always deploy, but experimentation is half the fun of water rocketry.
Mark a slightly scalloped line around an empty 2-liter soda bottle approximately 5 inches from the top. Cut along the line to remove the top of the bottle, creating a nosecone. Make sure it fits loosely onto the top of your soda bottle water rocket. Trim back the scalloped edge of the nosecone if necessary to make sure it will slip off easily. A tight nosecone won't allow the parachute to deploy properly.
Dangle a rubber band through the hole in the top of the nosecone. Arrange the rubber band so that most of it is inside the nosecone. Hold the other end of the rubber band tightly as you screw on the bottle cap. Tie a 2-foot piece of nylon string to the long end of the rubber band.
Cut a 2-foot diameter circle out of a plastic bag. Cut eight, 2-foot pieces of nylon thread. Use a dab of epoxy glue to attach one end of each thread to the edge of the circle at equal intervals. Place a small piece of tape over each glued end for extra strength. Gather the other ends of the threads and slip them through the eye of a fishing snap swivel. Knot the ends to the swivel securely.
Glue a small screw eye to the top of the water rocket with epoxy. Be sure the eye remains upright and open until the epoxy sets. Thread a rubber band through the eye, then insert the rubber band back through itself to attach it to the eye.
Tie the string attached to the nose cone to the eye on top of the rocket. Use the snap of the fishing swivel to attach the parachute shroud lines to the eye.
Fold the parachute carefully and wrap it loosely in the shroud lines. Place the parachute into the nosecone, and the nosecone on the rocket. Launch the rocket to test the parachute deployment. The parachute should deploy just after the rocket reaches apogee, as the change in direction and air pressure dislodges the nosecone.
Water rocket enthusiasts are constantly searching for dependable parachute deployment systems. If this design doesn't work for your rocket, experiment with one of the many others, or design your own.
Water rockets can be dangerous if used improperly. Never launch a rocket at anyone or anything.