My design for a very simple water-rocket

Despite the rain, yesterday's test launch in the park was an impressive success. The only modifications to a 2-litres plastic bottle were a tennis ball taped to the front and four small, stiff fins attached with Gaffer (Duct) tape. Adding weight to the front makes a huge improvement to stability in flight. The fins must be stiff and 'swept-back' or they will be ripped off during the launch. The bottle accelerates to a measured 100 miles per hour - 160 kilometres per hour - in a small fraction of a second. (I once calculated the velocity by analysing video frames.)
No special 'nozzle' is needed for this design, if a suitably tight-fitting bung can be found for the tube from the air pump. It means that children can build and launch their own rockets.
Notice the important safety feature: the rocket is launched at a child's head-height. It must NOT be possible for it to hit anyone on it's way up...