Water rocket

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Image:WaterRocketLaunch.jpg

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. The pressure vessel, the engine of the rocket, is usually a used plastic soft drink bottle.

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How they work

The bottle is filled 30% to 40% with water (usually about a third), 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.

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Parachutes

Parachutes for water rockets can be made out of 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, but not the most reliable, is adding a heavy 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, and the two should separate.

Another method is by means of 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.To attach the nose cone you can shape it as a cone and then tape it around the bottom of the bottle...

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Multi-bottle rockets and multi-stage rockets

Multi-bottle rockets and multi-stage rockets are the 'proper' water rockets, per se. Although they generally improve performance, they are not as reliable as single water bottles, but the risks are worth the huge increase in fun and range.

Image:Two Rockets.JPG Image:Ethereal.JPG

Multi-Bottle rockets are created by the joining of either 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 that launch while in the air, much like the multi-stage rockets 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 not very reliable, as any failure in sealing the rocket can cause the different sections to separate. This is usually not dangerous but it could be. To make sure the launch goes well, pressure test beforehand (see safety section).

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Nose cone

The best nose cones for water rockets are hemispherical ones. Conical or parabolic nose cones do not enhance performance as they generate a lot of drag.

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

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Other sources of pressure

Using a bicycle pump can get tiring after only a few launches. Other possible means of pressurizing a rocket include:

  • 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.
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Fins

As the rocket loses its thrust, it has the tendency to start spinning end over end. This will decrease the length of its glide (time that the rocket is flying under its own momentum). To lower the centre of pressure, fins are added, this makes the rocket more stable. However, stabilizing fins add another problem, because the rocket will now go straight up and nose-down very hard, possibly becoming damaged in the process. This should be taken into account when designing rockets. Crumple zones should be implemented, or parachutes should be used.

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'. By angling the fins by a few degrees, you can make the rockets spin, which reduces this off course veering.

The further back the fins are, the more effective they are when generating spin. To lower the centre of pressure, it is essential that the fins be at the back. They should be stiff (as 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.

Another possible recovery system involves using the rocket's fins to slow its descent. By increasing fin size, you will be creating more drag for the rocket as it is falling. If you place the center of mass 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.

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Aerial photography

Image:Sept42005 Flight 1.jpg As mentioned before, camera 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, as well as passive methods such as flaps pushed by wind resistance pulling strings to take the photographs as the rocket descends. Problems encountered can be blurriness in the photo caused by the speed of the rocket or its motion as it spins, as well as quickly changing lighting conditions as the rocket points from ground to sky while taking a video.






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Warning

  • When a rocket is built, it should be pressure tested first by filling the rocket completely with water, and then pressurising it to higher than is planned to use 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 an appreciable amount of energy, as air does.
  • While pressurising and launching the rocket, bystanders should be kept at a 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 will not hit the operator or bystanders.
  • Water rockets should only be launched in large grassy fields, preferably empty of large numbers of people. This will prevent damage to property and harm to others.
  • Water rockets should NEVER be fired at people or animals, as they are powerful enough to break bones upon impact.
  • Anyone close to the rocket should wear safety goggles, and have a shield, such as a board to hide behind.
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Records

The overall record holders are:

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 523 meters (1715 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.

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See also

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External links

fr:Fusée à eau it:Razzo ad acqua nl:Waterraket pt:Foguete à água ja:ペットボトルロケット

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