Water Rocket Recovery
Deploying a parachute at the rocket's apogee is a very popular way of water rocket recovery. It is relatively easy to design and the only challenge is to meet the moment of minimal vertical speed. Within a certain time slot around apogee the recovery system will still work but the forces on rocket and parachute will increase. If the parachute deploys far too early (or too late) damage is very likely.
It is very common among water rocketeers to use a timer to "tell" the rocket when it is time to deploy the parachute. Usually the time until apogee is known from simulations and it can be set before the launch. When the rocket leaves the launcher the timer starts counting and after the programmed time is over the deployment mechanism is triggered. How close to apogee this happens depends on the accuracy of the timer and the simulation.
The timer controlled recovery is usually very reliable. This is probably the reason why it is so widespread in the water rocket world.
Among the timers there are three different kinds which can be found in water rockets:
timers are probably the most common ones.
However, timer controlled deployment has a drawback if something goes wrong during launch preparation.
The simulation results could be wrong, the timer could have been set to the wrong time, too much (or less) water could have been filled into the rocket or the pressure gauge could be defective.
There are even more scenarios which can lead to malfunction of the timer recovery system. This should be considered when deciding which recovery system shall be used in the next water rocket.