Water rockets are loads of fun to experiment with.
The following instructable shows the construction and use of a zip tie based release mechanism.
Length of fluorescent tube protector (source TAP plastics)
Zip ties, enough for the circumference of the launcher
The photos should make the construction steps clear.
This is my first instructable. All work creating this instructable was done on my Android device.
Step 1: Water Rocket Launcher, Rocket,