When a water rocket takes off, it goes through two stages of flight. In the first, the water being forced out the rear accelerates the rocket up. When the water runs out, the rocket coasts to its apex and then begins to fall to the ground. In this lesson, you will follow a water rocket as it shoots up into the sky. Different experiments use different amounts of water and different starting pressures of the air in the rocket. The water in a water rocket is its reaction mass. More reaction mass means more mass to push the rocket and also more mass that has to be pushed. However, the more water in the rocket, the less air there is to push it out.