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,
Step 2: Rocket Detail
Step 3: Sheath, Release Mechanism
Step 4: Zip Tie Based Release Mechanism Detail
Step 5: Zip Tie Mechanism With Sheath
Step 6: Zip Tie Mechanism Detail
Step 7: Rocket and Launcher
Step 8: Rocket and Launcher Detail
Step 9: Rocket View From Nose
Step 10: Rocket Loaded Detail
Step 11: Rocket Loaded Detail
Step 12: Close Up Rocket Loaded Before Securing
Step 13: Close Up Rocket Secure
Step 14: Video of Launch