Water rockets are made by filling a vessel to the brim with water and sealing it so that it is airtight. The vessel can be made from any used soft drink or water bottle, preferably made from plastic to avoid injury during the launch or landing. The wide availability of material out of which to make the body of this kind of rocket is a large contributor to its widespread popularity. The two liter soda pop bottle is currently the most popular vessel for constructing water rockets.
The filled and sealed vessel is placed upside down on top of a launcher, which pressurizes the vessel with compressed air or gas. A simple launcher can be made from a bicycle pump, a rubber stopper, and a basic framework to hold the bottle in place while it is pumped full of air. The bottle can also be pressurized with an air compressor or even a cylinder filled with nitrogen or natural gas.
As the compressed air or gas is added to the water-filled vessel, a bubble is created inside the rocket that, being lighter than the water, rises to the top of the bottle. As this bubble expands it creates a great volume of pressure inside the bottle.
The compressed gas or air stores energy as it fills the bottle, and the water amplifies its force and increases the mass fraction. The reaction can be modified by including various additives in the water. Salt, for instance, adds to the density of the water, which increases its efficiency as a fuel and gives the rocket more momentum even when less water is used. Soap, on the other hand, takes away from the density of the water but causes the rocket to expel less of it as it rises so that the fuel lasts longer.
When the rocket is ready to be launched, the seal at its mouth – usually a rubber stopper or nozzle – is quickly released. This is generally done by means of a pull cord to ensure the safety of the hobbyist by increasing the distance between him and his rocket. When the seal is ripped off, the vessel is released from the pump and, due to the great deal of pressure inside, water is expelled from the neck of the bottle very rapidly. This reaction causes the bottle to shoot into the air at a very high speed. It will continue to rise until all the water has been used up and the pressure inside the bottle has returned to normal.
The duration of the flight as well as its altitude depend in large part on how big a bottle is used to form the body of the rocket. The size of the bottle determines how much water and pressurized air can be used, which is directly connected to the rocket’s performance. Sometimes multiple bottles are joined together to increase the volume of water that can be used. These must be sealed very carefully and tightly to prevent pressure from escaping at the time of launch. Other factors that affect the flight duration and altitude include how much air is initially pumped into the bottle, the type and size of the rocket’s nozzle, and the weight and shape of the rocket before it is filled with water.