Operation
Simplified animation of how a water rocket works. 1) compressed air is added which creates a bubble that 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 partly 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.
Students test a water rocket.
Water and gas are used in combination, with the gas providing a means to store potential energy, as it is compressible, 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 reaction 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.[1][2][3]
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 off 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 75psi (520kPa).
· 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.[4]
[edit] Fins
As the propellant level in the rocket goes down, it can be shown that the center of mass initially moves backwards before finally moving forwards again as the propellant is depleted. However this initial movement 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, well behind the center of mass at all times, ensuring 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 back-gliding 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.
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.
It can be shown that the equation for the instantaneous thrust of a nozzle is simply:[5]
F = 2PAt
where F is the thrust, P is the pressure and At is area of the nozzle.
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 100psi (690kPa) 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.
ALL ABOUT WATER ROCKETSWater (or Bottle) Rockets
Bottle rockets or water rockets, what are they?
When someone mentions bottle rockets, do you envision placing a firecracker attached to a stick into a glass bottle and launching it?
Water rockets have been a source of entertainment and education for many years. They are usually made with an empty two-liter plastic soda bottle by adding water and pressurizing it with air for launching (like the image to the right).
Soda companies began using plastic bottles in 1970. The Polyethylene Terephthalate (PET) material used in most plastic soda bottles today was introduced in 1973.
Water rockets are used in schools to help students understand the principles of aeronautics. The Science Olympiads provide challenges of bottle rocket design and flight, including altitudes and distances reached. Many interesting designs and additional information on bottle rockets can be found with a simple Web search.
Teachers and students provide the following feedback to the Secondary Science Education Department at the University of Nebraska:
"Two-Liter Pop Bottle Rockets may well be the GREATEST PHYSICAL SCIENCE TEACHING TOOL EVER CREATED!!" Middle grades students can manipulate and control variables, see their hypotheses verified or refuted, and graph their findings. High school students experience the nature of science at its best. They can document their abilities with the following concepts: inertia, gravity, air resistance, Newton's laws of motion, acceleration, relationships between work and energy or impulse and momentum, projectile motion, freefall calculations, internal and external ballistics, and the practice of true engineering.
How could something that sounds so simple be so complex? Open your mind to the science and mathematics behind this educational "toy." Below are links to a brief history timeline of rocketry, a comparison between water rockets and a NASA rocket, and additional information on the parts of a water rocket
A complete bottle will be the fuel tank that will also hold the pressurised gas.
Bottle Preparation
1. Get a 1.25L bottle and wash it out with dishwashing detergent to get the sticky residue out. The shape of the bottle can play a role in the aerodynamics, water flow within it and its center of gravity. For this reason a bottle with straight walls, no ornate protrusions and a smoothly tapered neck is a good choice. (Shaped Coke bottles are an example of an unsuitable bottle)2. Remove the label.
3. If the label leaves a sticky residue you can easily remove it by using a little mineral turpentine on a cloth. You should then wash the turpentine off with a soap and water.
4. Inspect the bottle for any kinks or scratches. The bottle may burst at these places when pressurised to a higher pressures.
5. Measure the capacity of the bottle, don’t necessarily believe the label. Knowing the capacity will help you determine how much water should be put in.
That’s the end of the bottle preparation.
Storing the contents of the bottle in a plugged up sink with a note “will drink later” is probably less than ideal. Make sure you buy bottles with contents you will drink. Because a bottle looks aerodynamic in the store does not mean you will want to drink some cheap imitation lemonade. While making rockets you will need plenty of bottles to make different components. The best way get bottles is from your friends, that way they feel they have contributed to the race for the lower atmosphere.
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