Water rocket

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Water Rocket Launch

A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel—the engine of the rocket—is usually a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air.

The term "aquajet" has been used in parts of Europe in place of the more common "water rocket" and in some places they are also referred to as "bottle rockets" (which can be confusing as this term traditionally refers to a firework in other places).

Water rocket engines are most commonly used to drive model rockets, but have also been used in model boats, cars, and rocket-assisted gliders[1].


[edit] Operation

Simplified animation of how a water rocket works. 1) compressed air is added which creates a bubble which floats up through the water and then pressurizes the air volume in the top of the bottle. 2) The bottle is released from the pump. 3) The water is pushed out the nozzle by the compressed air. 4) The bottle moves away from the water because it follows Newton's Third Law.

The bottle is mostly filled with water and sealed. The bottle is then pressurized with a gas, usually air compressed from a bicycle pump, air compressor, or cylinder up to 125 psi, but sometimes CO2 or nitrogen from a cylinder.

A student who tests a water rocket launch.

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 increasing the mass fraction and providing greater momentum when ejected from the rocket's nozzle. Sometimes additives are combined with the water to enhance performance in different ways. For example: salt can be added to increase the density of the reacton mass resulting in a higher specific impulse. Soap is also sometimes used to create a dense foam in the rocket which lowers the density of the expelled reaction mass but increases the duration of thrust. It is speculated that foam acts as a compressible liquid and enhances the thrust when used with De Laval nozzles.

The seal on the nozzle of the rocket is then released and rapid expulsion of water occurs at high speeds until the propellant has been used up and the air pressure inside the rocket drops to atmospheric pressure. There is a net force created on the rocket in accordance with Newton's third law. The expulsion of the water thus can cause the rocket to leap a considerable distance into the air.

In addition to aerodynamic considerations, altitude and flight duration are dependent upon the volume of water, the initial pressure, the rocket nozzle's size, and the unloaded weight of the rocket. The relationship between these factors is complex and several simulators have been written by enthusiasts to explore these and other factors.[2][3][4]

Often the pressure vessel is built from one or more used plastic soft drink bottles, but polycarbonate fluorescent tube covers, plastic pipes, and other light-weight pressure-resistant cylindrical vessels have also been used.

Typically launch pressures vary from 75 to 150 psi (500 to 1000 kPa). The higher the pressure, the larger the stored energy.

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

Two multi-bottle rockets with a cat for scale.
A larger multi bottle rocket with cylindrical fins.

Multi-bottle rockets are created by joining two or more bottles in any of several different ways; bottles can be connected via their nozzles, by cutting them apart and sliding the sections over each other, or by connecting them opening to bottom, making a chain to increase volume. Increased volume leads to increased weight, but this should be offset by a commensurate increase in the duration of the thrust of the rocket. Multi-bottle rockets can be 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. These are very good if you want to make the rocket go high however they are not very accurate and may veer of course.

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.

[edit] Sources of gas

Several methods for pressurizing a rocket are used including:

  • A standard bicycle/car tire pump, capable of reaching at least 75 psi (500 kPa).
  • An air compressor, like those used in workshops to power pneumatic equipment and tools. Modifying a high pressure (greater than 15 bar / 1500 kPa / 200 psi) compressor to work as a water rocket power source can be dangerous, as can using high-pressure gases in from cylinders.
  • Compressed gases in bottles, like carbon dioxide (CO2), air, and nitrogen gas (N2). Examples include CO2 in paintball cylinders and air in industrial and SCUBA cylinders. Care must be taken with bottled gases: as the compressed gas expands, it cools (see gas laws) and rocket components cool as well. Some materials, such as PVC and ABS, can become brittle and weak when severely cooled. Long air hoses 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-pressurizing rockets and having them explode before they can be launched. Highly pressurized gases such as those in diving cylinders or vessels from industrial gas suppliers should only be used by trained operators, and the gas should be delivered to the rocket via a regulator device (e.g. a SCUBA first-stage). All compressed gas containers are subject to local, state and national laws in most countries and must be safety tested periodically by a certified test center.
  • Ignition of a mixture of explosive gases above the water in the bottle; the explosion creates the pressure to launch the rocket into the air.[5]

[edit] Fins

As the propellant level in the rocket goes down, the center of mass may move backwards. This reduces stability and tends to cause water rockets to start tumbling end over end, greatly decreasing the maximum speed and thus the length of 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 cause the rocket to fall with a significantly higher velocity, possibly damaging the rocket or whatever it strikes upon landing. This is noteworthy if the rocket has no parachute or other recovery system or it has one which malfunctions. This should be taken into account when designing rockets. Rubber bumpers, Crumple zones, and safe launch practices can be utilized to minimize damage or injury caused by a falling rocket.

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 rocket is designed such that the relationship between center of gravity and the center of pressure of the empty rocket causes the fin-induced tendency of the rocket to tip nose down to be counteracted by the air resistance of the long body which would cause it to fall tail down, and resulting in the rocket falling sideways, slowly. The article cited above is a detailed exploration of the phenomenon.

[edit] Nozzles

Water rocket nozzles differ from conventional combustion rocket nozzles in that they do not have a divergent section such as in a De Laval nozzle. Because water is essentially incompressible the divergent section does not contribute to efficiency and actually can make performance worse.

There are two main classes of water rocket nozzles:

  • Open also sometimes referred to as "standard" or "full-bore" having an inside diameter of ~22mm which is the standard soda bottle neck opening.
  • Restricted which is anything smaller than the "standard". A popular restricted nozzle has an inside diameter of 9mm and is known as a "Gardena nozzle" named after a common garden hose quick connector used to make them.

Nozzles larger than the "standard" nozzle are sometimes used but are more rare.

The size of the nozzle affects the thrust produced by the rocket. Larger diameter nozzles provide faster acceleration with a shorter thrust phase, while smaller nozzles provide lower acceleration with a longer thrust phase.

Different nozzle types generally require different launcher arrangements.

[edit] Launch tubes

Some water rocket launchers use launch tubes. A launch tube fits inside the nozzle of the rocket and extends upward toward the nose. The launch tube is anchored to the ground. As the rocket begins accelerating upward, the launch tube blocks the nozzle, and very little water is ejected until the rocket leaves the launch tube. This allows almost perfectly efficient conversion of the potential energy in the compressed air to kinetic energy and gravitational potential energy of the rocket and water. The high efficiency during the initial phase of the launch is important, because rocket engines are least efficient at low speeds. A launch tube therefore significantly increases the speed and height attained by the rocket. Launch tubes are most effective when used with long rockets, which can accommodate long launch tubes.

[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 at least 50% 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.
  • Using metal parts on the pressurized portion of the rocket is strongly discouraged because in the event of a rupture, they can become harmful projectiles. Metal parts can also short out power lines.
  • 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.
  • The water jet from a water rocket is sufficiently fast that it can break fingers, thus hands should not be near the rocket upon launch.
  • As water rockets are capable of breaking bones upon impact, they should never be fired at people, property, 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 (689 kPa) safely, but preparations must be made for the eventuality that the bottle unexpectedly ruptures.
  • Glue used to put together parts of water rockets must be suitable to use on plastics, or else the glue will chemically "eat" away the bottle, which may then fail catastrophically and can harm bystanders when the rocket is launched.

[edit] Water rocket competitions

The Oscar Swigelhoffer Trophy is an Aquajet (Water Rocket) competition held at the Annual International Rocket Week[6] in Largs, Scotland and organized by STAAR Research[7] through John Bonsor. The competition goes back to the mid-1980s, organized by the Paisley Rocketeers who have been active in amateur rocketry since the 1930s. The trophy is named after the late founder of ASTRA[8], 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[9]. 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[10]. Teams compete to be the first to fly a water rocket over 1000 feet (305 meters),

The oldest and most popular water rocket competition in Germany is the Freestyle-Physics Water Rocket Competition[11]. 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 competitive contests.

Science Olympiad also has had a Water Rocket event in past years.

[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 2068 feet (630 meters).

The current record for greatest height achieved by a water and air propelled rocket is 2044 feet, (623 meters), held by U.S. Water Rockets[12] on June 14, 2007. This altitude was calculated by averaging two flights. The first flight achieved 2068 feet, (630 meters) and the second 2020 feet, (615.7 meters). The rocket also carried an onboard video camera on both flights.

[edit] Hot water rockets

A hot water rocket (or steam pressure rocket) is a water rocket which uses hot water as its propellant. Water is kept in the rocket under pressure, at below its boiling point at that pressure. As it exits through a rocket nozzle, the pressure drops and the water instantly boils and expands against the nozzle and this greatly increases the exhaust speed and thrust.

The idea of such rockets was conceived by Germany before the Second World War, with the suggested use of an alternative rocket engine for launching fighter jets.

[edit] References

[edit] Bibliography

D. Kagan, L. Buchholtz, L. Klein, Soda-bottle water rockets, The Physics Teacher 33, 150-157 (1995).

[edit] External links

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