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.
Launchers
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.
Modifications
The same sort of modifications can be done on a water bottle rocket as on model rockets. Fins, parachutes, cameras, etc. It's all possible.
Fins
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
Parachutes can be made out of dustbin liners (trash
bags). 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.
Safety
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.
Records
The people at U.S. Water Rockets (http://home.earthlink.net/~waterrocketnews/world_storyd.htm) 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.
External links
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