Once the bottle is empty of water, its flight starts to slow. If the rocket doesn't fly at all, check two things: Is water escaping from the bottle? Look around the cork to check if it's sealing ...

Once the bottle is empty of water, its flight starts to slow. If the rocket doesn't fly at all, check two things: Is water escaping from the bottle? Look around the cork to check if it's sealing ...

The problem with a lot of these water bottle rocket projects is that they leak ... 100 PSI at which point a yank on the string lets it fly. Most of the time we look on these as casual projects.