According to [NASA](https://www.grc.nasa.gov/www/k-12/rocket/rktbot.html):

>Usually the rocket is made from a 2-liter soda pop bottle. Before launch, the bottle is filled with some amount of water, which acts as the "propellant" for the launch. Since water is about 100 times heavier than air, the expelled water produces more thrust than compressed air alone. The base of the bottle is only slightly larger than the launch tube. When the rocket is placed on the launch tube, the body tube becomes a closed pressure vessel. The pressure inside the body tube equals the pressure produced by the air pump. Fins are attached to the bottom of the body tube to provide stability during the flight. 

Always be safe when testing model rockets and avoid a rapid unscheduled disassembly!
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