Water rocket

Water Rocket Launch (click on image for larger version and description)

Water rockets, also called Bottle rockets, are like their model rocket cousins, except that these are powered by a combination of water and air pressure instead of a chemical propellant. The pressure vessel, the engine of the rocket, is usually a used plastic soft drink bottle.

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 air and then released.

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

Typically an air pressure of at least 80 (PSI) is required, although over 100 is usually used. If the pressure is too low, the water is not forced out fast enough, causing the rocket to use a lot of its energy to propel not only the rocket but the water propellent as well. This severely reduced achieved height.

Parachutes

Parachutes for water rockets can be made from many things, including simple dustbin liners (trash bags). The difficulty lies in getting the rocket to release the parachute at the right moment. There are many methods for accomplishing this. The simplest is adding mass to the nose cone. The theory is that the nose cone will at the apex fall faster than the rocket body, which will be slowed down by drag, separating the two.

Another method is an airspeed flap, which goes along the rocket's 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.

As most bottle rockets launch at over 100mph, it is very likely that loose parts will be ripped off by wind shear. This can be dangerous to observers or opperators, so securing everything properly is an important safety consideration. Parachutes are particularly likely to be ripped off if even slightly misplaced.

Multi-bottle rockets and multi-stage rockets

Multi-bottle rockets and multi-stage rockets are considered to be "proper" water rockets. Although they generally improve performance, they are not as reliable as single water bottles, but the risks are consieered 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 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 thrust in 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.

Nose cone

Hemispherical nose cones are considered to be better than conical or parabolic nose cones as they generate less drag.

If fins are not used on a rocket, the cone should be made heavy so as to raise the centre of gravity (for stable flight, the centre of gravity should be above the centre of pressure by 1 or 2 rocket diameters).

Sources of air pressure

Several possible methods of pressurizing a rocket include:

• A standard bicycle/car tyre pump, capable of reaching at least 80 PSI.
• An air 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. 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.
• A combustion reaction between hydrogen and oxygen inside the rocket, initiated by a spark.

Fins

As the rocket loses its thrust, it has the tendency to start tumble end over end. This will 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. 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.

Aerial photography

File:Sept42005 Flight 1.jpg

Aerial Photograph taken by U.S Water Rockets' X-14 water rocket

Cameras and video cameras can be launched along with water rockets to take photographs in-flight. These aerial photographs can be taken in many ways and with many different types of cameras. Mechanised timers can be used to take photographs. Passive methods are also employed, such as strings that are pulled by flaps that respond to wind resistance. However, the rocket's speed and motion can lead to blurry photographs, and quickly changing lighting conditions as the rocket points from ground to sky can have an impact on video quality. As parachute systems tend to fail, cameras also need to be protected from impact with the ground.

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.

Competitions

The sport of Water Rocketry has been growing at a fast pace in recent years, which has led to an escalating competetive spirit. The Water Rocket Achievement World Record Association (WRA2) has established the first officially sanctioned set of rules for competitions. WRA2 Class ‘A’ Water Rocket World Record Rules set forth the basic guidelines for the initial class of competition, based on the types of rockets presently holding the top positions in the record books. This class of rockets, powered by compressed air and water, are completely fabricated from ordinary materials, such as soda bottles and balsa wood.

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 1996 was ASTRA after regaining it from the Sheffield Rocketry Association. 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 travelling from abroad. The rules and goals of the competition vary from year to year.

Records

The overall record holders are:

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Apogee photograph taken by the required onboard video camera from U.S. Water Rockets' record breaking X-12 Water Rocket at an altitude of 1803 feet (550 meters).

The people at U.S. Water Rockets have the current overall world record for height achieved by a water and air propelled rocket. Their design flew to an amazing 554 meters (1818 feet) (2 flight average as required by the WRA2 altitude record rules). They flew an onboard video camera as payload and used compressed air as a pressure source for the water reaction mass. They used a carbon fiber reinforced fluorescent lamp cover (FTC), and a special low friction large nozzle shape to lift the heavy payload.

The world record runners up are:

The current second highest altitude is Antigravity Research company. On 17 July, 2003, the Antigravity Research company, reaching 378.5 meters (1242 feet). Their rocket was made using an ordinary 2 liter soda bottle with carbon for reinforcement, allowing a pressure of 1150 psi to be achieved using compressed Nitrogen. They also used soapy water, creating a foam reaction mass that more evenly distributed the weight in the rocket and allowing the use of a nozzle to use the energy stored in the rocket more efficiently.

The current third highest altitude holder is Sam Mulock with his Red Arrow III, a 12-liter water rocket constructed from standard soft drink bottles reinforced with kevlar fabric. His 1230 foot (375 meter) flight on November 7, 2004 was achieved using compressed air with a water reaction mass. Mulock has flown a camera to altitudes of 1131 feet (345 meters) during his photo flights.

The current fourth highest altitude is Robert Youens with Insane Air, an ultra lightweight design made from a fluorescent lamp cover (FTC) which achieved an altitude of 1105 feet (336.8 meters) using only compressed CO₂ at a pressure of 130 psi, and no water or other reaction mass at all, on the 22nd of June 2002. Specifications, flight data, construction details and other explanations can be seen on his website.

The current fifth highest altitude is Bruce Berggren with his two-stage Millennium VIII rocket reaching 1060 feet (323 meters) on 3 July, 1998. His former world record holding rocket was constructed combining both a fluorescent lamp tube cover and ordinary soft-drink bottles. He used compressed air and water, and all parts used to build the rocket were standard, off the shelf components available to anyone with access to a good hardware store. This rocket was the first to break the 1000 foot (314 m) altitude. The 8th rocket in the millennium series (the name indicates the altitude goal of 1000 feet) was build with the following constraints:

• a one-cubic ft tank of 150 psi air would supply all the power.
• No pyrotechnic or electrical power would be used for chute deployment
• The rocket should weigh less than a pound (dry)
• Everything built from inexpensive parts in a garage workshop.

See also

• Model rocket
• Pneumatics
• Gas laws
• Hot Water Rocket

External links

• U.S. Water Rockets – Current altitude record holder – Aerial photography using water rockets.
• 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
• Water Rocket Discussion Group
• Water Rocket Index
• Water Rocket Annex
• Another Water Rocket Page
• Kevin's H2O Rockets
• Nick's Water Rockets
• 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.
• Water rocket - A French water rocket website.
• Water rocket equations - The physics behind the rocket, including a simulator.
• Basic rocket and launcher designs for beginners.