Water rockets are like their model rocket cousins, except that these are powered by a combination of water and air pressure. The pressure vessel, the engine of the rocket, is usually a used soft drink bottle.
How they work
The bottle is half filled with water, then inverted so the nozzle points towards the ground. The bottle is then pressurized (usually by means of a bicycle pump) and then released. The water will be pushed out by the compressed air, and because of Newton's third law the rocket will fly upwards.
As important as the design of the rockets themselves is the design and construction of the launcher. This is the part that holds the rocket in place while it is being pressurised and can let the rocket go at the right time. Launchers come in many different forms, from very small rubber stoppers with a bicycle valve in it to immense complicated contraptions of metal and plastic.
Fins are not essential on a rocket (the rocket in the picture has no fins and flies very well). However, as the pressurized bottle loses its thrust, it has the tendency to start spinning end over end. This will decrease the ballistic flight time of the rocket (time that the rocket is flying under its own momentum). But fins add yet another problem. Because the rocket is now going straight up and down, it will hit the ground nose-down very hard and possibly break parts. Some people have put tennis balls on the top as a sort of bumper. Of course if a parachute is going to be added, this will not be a problem.
The fins on a rocket should ideally be as far to the rear of the rocket as possible. They should be stiff (flexible fins would not have any effect), and attached to the rocket firmly, because the forces involved are immense.
A simple but effective stabilizer is a straight cylindrical section from another plastic bottle. This section is placed behind the rocket nozzle with some wooden dowels or plastic tubing. The water exiting the nozzle will still be able to pass through the section but the rocket will be stabilized.
Parachutes can be made out of dustbin liners (trash bag s). The hard part is getting the rocket to release the parachute at the right moment. Many ingenious ideas have been thought of. The simplest, but not the most reliable, is adding a heavy nosecone. The theory is that the nosecone will at the apex fall faster than the rocket body which is being slowed down by drag, and the two should separate.
Another method is by means of a flap which goes along the rocket body. While the rocket is in flight the flap is pushed down by the air rushing past it. As it reaches the apex, the flap is no longer being pushed down and can swing upwards, releasing the nose cone. See the external links below for more details.
Increasing volume and multiple stages
There are ways of connecting several bottles to increase the total volume of your rocket. It will also get heavier, though. Two 2 liter pop bottles can be joined together. Cut the top off of one, and the bottom off the other, and join the large pieces together by sliding one over the top of the other. Using glue between the two layers of plastic and wrapping duct tape around the top of the join can create a fairly secure seal.
It is also possible to have a multistage rocket, but this is quite complicated.
Please check the external links below for more details.
Other sources of pressure
Using a bicycle pump can get tiring after only a few launches. Other possible means of pressurizing a rocket include:
- A gas compressor, like those used in workshops to power pneumatic equipment and tools.
- Compressed gasses in bottles, like carbon dioxide (CO2) and nitrogen gas (N2). CO2 bottles are also used as pressure sources in paintball, for example. Scuba tanks are another possibility. Please be careful with bottled gasses: as the compressed gas expands, it will cool down (see gas laws) and components will cool as well. Some materials such as PVC and ABS can become brittle and weak when severely cooled. Keep a safe distance by using a long airhose. Use a pressure gauge (known as a manometer) and a safety valve on your launcher installation to avoid over-pressurising your rocket and having it explode. Pressure test your rocket as described in Safety section before actual launching. Know the pressure limit of the bottles you are using.
Compressed air has a lot of potential energy, which can be very dangerous if the plastic bottle ruptures. Plastic shards will go flying everywhere at high speed. In other words: experimenting with water rockets is not without risk!
When you build your rocket, before launching it, please pressure test it first by filling the rocket completely with water, and then pressurising it to higher than you plan to go for the actual launching of the rocket. If the bottle ruptures, the amount of compressed air inside it (and thus the potential energy) will be very small and the bottle will not explode in spectacular fashion. Water is hardly compressible, and thus will not store any appreciable amount of energy, as air does.
While pressurising and launching your rocket, please keep bystanders at a distance. Have a mechanism for releasing your rocket at a distance (with a piece of string, for example) so that if it veers off in an unexpected direction, it will not hit you.
Only launch bottle rockets in large grassy fields, preferably empty of large numbers of people. This will prevent damage to property and harm to others.
Never ever fire a water rocket at any person or animal. They can break bones.
The people at U.S. Water Rockets have the current world record for height achieved by a water propelled rocket. Their design flew to an amazing 490 meters (1606 feet) (2 flight average). They flew an onboard video camera as payload and used compressed air as a pressure source. They used a carbon fiber reinforced fluorescent lamp cover (FTC), and a special low friction large nozzle shape to lift the heavy payload.
- U.S. Water Rockets – Current altitude record holder – Aerial photography using water rockets.
- Water Rocket Discussion Group
- Water Rocket Index
- Water Rocket Annex
- Another Water Rocket Page
- Anti-Gravity Research - A commercial company making water bottle rocket parts.
- Bigfoot Water Rocket Launcher Systems - Another commercial company selling parts.
- Rokit Water Rockets - Another commercial company, based in England.