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 fireqork 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.
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.
A very important technique for building bigger and better rockets is splicing. It is generally quite difficult to find bottles larger than 2L (although there are a few 3L bottles around) and so for volumes larger than that you must use a method to join bottles together. Besides, larger volume bottles get wider rather than taller (I would guess this is to ensure that they can still fit inside a fridge), and since drag is a function of the cross-sectional area of the rocket, which in turn is a function of the square of the radius, larger bottles generate much more air resistance. For example, a bottle with diameter double that of another bottle will generate four times as much drag.
Before I talk about splicing, I should probably mention the Robinson Coupling (Dave Johnson's website), another method used to join bottles. This joint consists of a threaded lamp rod, two rubber gaskets, two washers and two nuts. You can join bottles neck-to-bottom, neck-to-neck or bottom-to-bottom using this method. You drill holes through the two ends you want to join, insert the threaded lamp rod through the holes and then tighten the nuts onto the lamp rod (with the gaskets and washers of course). This method is easier to get right than splicing, but in the long run it's definitely worth perfecting the splicing technique for a number of reasons. First of all, a Robinson coupling is heavier than a splice because of the metal parts involved - splicing only requires some glue which is relatively light. Secondly, the Robinson coupling only provides a very narrow 'throat' between the joined sections, meaning that it takes much longer for the air to pass through the coupling all the way down to the nozzle. This means less thrust (although the 'burn' time is longer) and so the rocket won't go as high as it would if it were spliced instead. For more information about this method, click on the link above.
Nose cones, along with fins, are one of the most critical aerodynamic components of a rocket. A simple paper cone taped on top of the rocket is enough to significantly reduce the rocket's coefficient of friction, however more complex shapes may be used to further improve a rocket's performance (and some are surprisingly easy to make). There are three common shapes used for nose cones: conical, ogive and parabolic, as shown in the diagram below.
A common misconception is that the most aerodynamis is the conical shaped nose cone. This probably comes from the fact that frequently space-going vehicles have nose cones this shape (for example the space shuttle's solid rocket boosters). However this shape is only suitable for supersonic flights (above the speed of sound). For water rockets, which only achieve a speed of about 1/4 to 1/3 the speed of sound, a parabolic shape turns out to be the most efficient. Similarly, you often see model rockets (the pyrotechnic type) with parabolic nose cones as well. Several methods exist to construct nose cones of this shape. The simplest and quickest is to use the top of another bottle cut off and taped or glued (or attached some other way) to the top of the rocket. Another technique that is used is guppying. This involves heating the bottom of a pressurised bottle so that it expands into a rounded shape. While this method does give very good results, it takes a lot of practice to get right.
Water rocket fins can be made from a variety of materials. I personally tend to use either cardboard (the thin type e.g. cereal boxes) or a thin plastic sheet, similar to corrugated cardboard. I believe it's called correx, however I've heard it called by other names. It can be somewhat difficult to locate unfortunately, which is why I sometimes use cardboard. Correx comes in several thicknesses - I first managed to get my hands on some 4mm thickness sheet, which worked very well; it's extremely stiff and so isn't susceptible to vibrations or bending during flight. 4mm is fairly thick however, resulting in increased drag. Recently I found some 2mm sheet which is much better aerodynamically but obviously also much weaker. As long as you make sure you cut out the fins so that the 'corrugated' parts will be perpendicular to the body of the rocket the material is still strong enough. This also applies to fins made out of balsa wood - make sure that the grain of the wood runs outwards! Another thing that is useful when making fins out of cardboard is to glue two layers of cardboard together. For example, I cut out 8 identical fins and then glue them together using rubber cement (a fairly flexible glue) so that I have 4 double thickness fins (greatly improving the strength).
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.
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
The Oscar Swigelhoffer Trophy is an Aquajet (Water Rocket) competition held at the Annual International Rocket Week in Largs, Scotland and organized by STAAR Research 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, 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 oldest and 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 competitive contests.
Science Olympiad also has had a Water Rocket event in past years.
See more about it here:
http://www.youtube.com/watch?v=yd39dtOp1zQ&feature=related
been used in model boats, cars, and rocket-assisted gliders.
been used in model boats, cars, and rocket-assisted gliders.
