Water rocket

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Water Rocket Launch

A water rocket is a type of model rocket powered by stored compressed air instead of a chemical propellant, and using water as its reaction mass. The pressure vessel, the engine of the rocket, is usually a used plastic soft drink bottle. They are also called bottle rockets—which can be confusing as this term traditionally referred to a firework.

1 How they work
2 Safety concerns
3 Water rocket competitions
4 Altitude Record
5 See also
6 References
7 External links

[edit] How they work

The bottle is partially filled with water (typically a third full), and then inverted so the nozzle points towards the ground. The bottle is then pressurized with a gas (usually compressed air) and then released. An excess of water will cause the maximum altitude of the rocket to decrease.

Water and gas are used in combination, with the gas providing a means to store potential energy, as it is easily compressed, and the water improves the mass fraction and provides greater momentum when ejected from the rocket's nozzle.

Typically a pressure of at least 80 psi (550 kPa) is required, although over 100 psi (700 kPa) is usually used. If the pressure is too low, then the rocket lacks energy, and will reach a reduced altitude.

[edit] Multi-bottle rockets and multi-stage rockets

Multi-bottle rockets and multi-stage rockets are considered to be "proper" water rockets. They are not as reliable as single water bottles, but they generally have improved performance, and the risks are considered to be worth the increase in both the rockets' range and entertainment value.

Two multi-bottle rockets with a cat for scale.

A larger multi bottle rocket with cylindrical fins.

Multi-bottle rockets are created by the joining of two or more bottles in any of several different ways; bottles can be connected via their nozzles or by cutting them apart and sliding the sections over each other to increase volume. The increased volume will lead to increased weight and duration of the thrust of the rocket.

Multi-stage rockets are much more complicated. They involve two or more rockets stacked on top of each other, designed to launch while in the air, much like the multi-stage rockets that are used to send payloads into space. Methods to time the launches in correct order and at the right time vary, but the crushing-sleeve method is quite popular. Multi-bottle rockets are unreliable, as any failure in sealing the rocket can cause the different sections to separate. To make sure the launch goes well, pressure tests are performed beforehand, as safety is a concern.

[edit] Sources of gas

Several possible methods of pressurizing a rocket include:

A standard bicycle/car tyre pump, capable of reaching at least 80 psi (550 kPa).
An air compressor, like those used in workshops to power pneumatic equipment and tools.
Compressed gases 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. Care must be taken with bottled gases: 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. Long airhoses are used to maintain a safe distance, and pressure gauges (known as manometers) and safety valves are typically utilized on launcher installations to avoid over-pressurising rockets and having them explode before they can be launched. It is recommended by all responsible organisations, that highly pressurised gases such as those in in diving cylinders or vessels from industrial gas suppliers should only be used by trained operatives, and must be delivered to the rocket via a regulator device (e.g. a SCUBA first-stage).

Ignition of a hydrogen-oxygen mixture in the bottle, the explosion creates the pressure to launch the rocket into the air.

[edit] Fins

As the propellant level in the rocket goes down, the center of mass goes backwards, towards the center of drag of the rocket. This greatly reduces stability and tends to causes water rockets to start tumbling end over end. This will greatly decrease the maximum speed and thus decrease the length of its glide (time that the rocket is flying under its own momentum). To lower the center of pressure and add stability, fins can be added which bring the center of drag further back, helping ensure stability.

However, stabilizing fins will cause the rocket to return with a significantly higher velocity, possibly damaging the rocket upon landing. This should be taken into account when designing rockets. Crumple zones or parachutes can be utilized to minimize this.

In the case of custom-made rockets, where the rocket nozzle is not perfectly positioned, the bent nozzle can cause the rocket to veer off the vertical axis. The rocket can be made to spin by angling the fins, which reduces off course veering.

Another simple and 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.

Another possible recovery system involves using the rocket's fins to slow its descent. By increasing fin size, more drag is generated. If the center of mass is placed forward of the fins, the rocket will nose dive. In the case of super-roc or backgliding rockets, the centre of gravity and the centre of pressure are as close as possible, so it spins around, which slows down its descent.

[edit] Safety concerns

Water rockets employ considerable amounts of energy and can be dangerous if handled improperly or in cases of faulty construction or material failure. Certain safety procedures are observed by experienced water rocket enthusiasts:

When a rocket is built, it is pressure tested. This is done by filling the rocket completely with water, and then pressurizing it to higher than anticipated pressures. 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.
While pressurizing and launching the rocket, bystanders are kept at a safe distance. Typically, mechanisms for releasing the rocket at a distance (with a piece of string, for example) are used. This ensures that if the rocket veers off in an unexpected direction, it is less likely to hit the operator or bystanders.
Water rockets should only be launched in large open areas, away from structures or other people, in order to prevent damage to property and people.
As water rockets are capable of breaking bones upon impact, they should never be fired at people or animals.
Safety goggles or a face shield are typically used.
A typical two-liter soda bottle can generally reach the pressure of 100 psi safely, but preparations are made for the eventuality that the bottle ruptures.

[edit] Water rocket competitions

The Oscar Swigelhoffer Trophy is an Aquajet (Water Rocket) competition held at the Annual International Rocket Week in Largs, Scotland and organised by STAAR Research through John Bonsor. The competition goes back to the mid-1980s, organised by the Paisley Rocketeers who have been active in amateur rocketry since the 1930s. The trophy is named after the late founder of ASTRA, Oscar Swiglehoffer, who was also a personal friend and student of Hermann Oberth, one of the founding fathers of rocketry.

The competition involves team distance flying of water rockets under an agreed pressure and angle of flight. Each team consists of six rockets, which are flown in two flights. The greater distance for each rocket over the two flights is recorded, and the final team distances are collated, with the winning team having the greatest distance. The winner in 2007 was ASTRA. The competition has been regularly dominated over the last 20 years by the Paisley Rocketeers.

The United Kingdom's largest water rocket competition is currently the National Physical Laboratory's annual Water Rocket Challenge. The competition was first opened to the public in 2001 and is limited to around 60 teams. It has schools and open categories, and is attended by a variety of "works" and private teams, some traveling from abroad. The rules and goals of the competition vary from year to year.

The Water Rocket Achievement World Record Association 1000 Foot Challenge. Teams compete to be the first to fly a water rocket over 1000 feet (305 meters),

The most popular water rocket competition in Germany is the Freestyle-Physics Water Rocket Competition. The competition is one part of a larger part of a student physics competition, where students are tasked to construct various machines and enter them in competative contests.

(Placeholder - Several well-organized, well-attended, long-standing competitions have been taking place in Thailand and other parts of the Far East for several years - A More detailed contribution is requested)

[edit] Altitude Record

Apogee photograph taken by the onboard video camera from U.S. Water Rockets' record breaking X-12 Water Rocket at an altitude of 1918 feet (585 meters).

The current record for greatest height achieved by a water and air propelled rocket is 1918 feet, (585 meters), held by U.S. Water Rockets on May 8, 2006

[edit] See also

[edit] References

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[edit] External links

U.S. Water Rockets – Current altitude record holder – Aerial photography using water rockets.
Team Parental Advisory – Little site with team updates and photos.
Air Command Water Rockets - An Australian based water rocket website with construction details and lots of photos.
Includes in-flight video.
The Water Rocket Achievement World Record Association
The International Water Rocket Association-
Damos Water Bottle Rockets – Another Australian based site with lots of cool Onboard movies!
Association in Scotland to Research into Astronautics - An Astronomy and Spaceflight Society in Scotland with an interest in water rockets since the 60's! Winner of the 2005 International Rocket Week Aquajet (Water Rockets) Competition.
International Rocket Week