Water rocket

Not to be confused with bottle rockets powered by black powder.
Water rocket launch

A water rocket is a type of model rocket using water as its reaction mass. Such a rocket is typically made from a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air. Like all rocket engines, it operates on the principle of Newton's third law of motion.

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.

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 force 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 fluid and enhances the thrust when used with De Laval nozzles.

Launching a water rocket. The rocket is in its peak with no water inside it.

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 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.

Predicting peak height

If aerodynamic drag and transient changes in pressure are neglected, a closed-form approximation for the peak height of a rocket fired vertically can be expressed as follows:

h=\left({M_i \over M_R}\right)^2 \left({P_i \over \rho g}\right)[4]

(h = peak height reached, M_i = Initial mass of water only, M_R = Rocket mass with water, P_i = Initial gauge pressure inside rocket, \rho = density of water, g = acceleration due to gravity) Assumptions for the above equation: (1) water is incompressible, (2) flow through the nozzle is uniform, (3) velocities are rectilinear, (4) density of water is much greater than density of air, (5) no viscosity effects, (6) steady flow, (7) velocity of the free surface of water is very small compared to the velocity of the nozzle, (8) air pressure remains constant until water runs out, (9) nozzle velocity remains constant until water runs out, and (10) there are no viscous-friction effects from the nozzle (see Moody chart).

An independent variable that influences peak height is weight/mass. Depending on the thrust of the rocket propulsion system, a rocket requires a minimum mass to overcome the deleterious effects of drag. For example, the greater the thrust/the less the original weight of the rocket, the more weight or mass must be added to the rocket to insure maximum apogee. The mass is generally referred to as ballast. This principle is demonstrated by having a student throw a straw with and without a piece of clay attached to the 'nose' of the straw. The straw with the greater mass will travel further, provided that there is sufficient thrust to overcome the ballast or extra mass.

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 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.

Sources of gas

Several methods for pressurizing a rocket are used including:

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:

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:[6]

F = 2 P A_t

where F is the thrust, P is the pressure and A_t is area of the nozzle.

Fins

As the propellant level in the rocket goes down, it can be shown that the centre of mass initially moves downwards before finally moving upwards again as the propellant is depleted. This initial movement reduces stability and can 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 centre of pressure and add stability, fins can be added which bring the centre of drag further back, well behind the centre of mass at all times, ensuring stability.

Thus, fins are extremely important on a water rocket. By ensuring stability, they are very likely to increase its launch height. Fins increase drag, but the stability achieved makes a much larger difference to the height the rocket will fly. A second thing that is very important is the position of the fins. It is best if they are placed near the back of the bottle where the center of mass is found. A waterproof, stable, light material to make the fins would be "Coroplast". This is a cardboard like material that is durable in use. The only negative it has is that it is harder to glue, but with the right glue it is possible.

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.

Aerodynamic drag acts on the fins as well as on the rocket body. Fins add to the frontal surface area on which the drag force acts (and therefore should be designed not to add too much drag). The drag forces on all frontal surfaces of the rocket can be resolved into one force acting at the center of pressure Center of pressure (fluid mechanics). This acts to oppose the forward motion, but if the rocket nose is not pointed in the direction of its motion at a given time (perhaps due to wobbling or instability), then there will be a torque, due to the resolved drag force, acting around the center of gravity. This torque will stabilize the rocket by returning its nose to the direction of travel.

Since the torque is the cross-product of the drag force magnitude and the moment arm, torque can be maximized without increasing drag force by increasing the moment arm. The larger the distance between the center of gravity and the center of pressure, the greater the moment arm on the restoring torque. Therefore, it is desirable to have the center of pressure, and therefore the fins, as far back as possible on the rocket body.

The lift force acts to push the back end of the rocket so that the nose will face the flight direction, and the drag force does the same, even though it is pointing orthogonally to the lift force.[7]

Landing systems

Stabilizing fins cause the rocket to fly nose-first which will give significantly higher speed, but they will also cause it to fall with a significantly higher velocity than it would if it tumbled to the ground, and this may damage the rocket or whomever or whatever it strikes upon landing.

Some water rockets have parachute or other recovery system to help prevent problems. However these systems can suffer from malfunctions. This is often 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.

Another possible recovery system involves simply using the rocket's fins to slow its descent and is sometimes called backward sliding. By increasing fin size, more drag is generated. If the centre 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 centre of gravity and the centre 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.

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.

Safety

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:

Water rocket competitions

The Oscar Swigelhoffer Trophy is an Aquajet (Water Rocket) competition held at the Annual International Rocket Week[8] in Largs, Scotland and organized by STAAR Research[9] 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,[10] 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.[11][12] 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.

The Water Rocket Achievement World Record Association[13] is a worldwide association which administrates competitions for altitude records involving single-stage and multiple-stage water rockets, a flight duration competition, and speed or distance competitions for water rocket–powered cars.

The oldest and most popular water rocket competition in Germany is the Freestyle-Physics Water Rocket Competition.[14] ([15]) 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 a Water Rocket event.

In Pakistan Water Rocket Competition[16] is held every year in World Space Week by Suparco Institute Of Technical Training (SITT). In which different schools from all over the Pakistan take part.

In Ukraine Water Rocket Competition[17] is held every year in Center for Innovative Technology in Education[18] (CITE). In which different schools from all over the Ukraine take part. The design of the standard rocket (download).[19][20] The Competition promotes the selective collection of solid dry waste in schools.[21]

In Russia Water Rocket.[22]

World record

Apogee photograph taken by the onboard video camera from U.S. Water Rockets' record breaking X-12 Water Rocket at an altitude of 2,068 feet (630 m).
Past most water rockets launched simultaneously, by Gotta Launch

The Guinness World Record of launching most water rockets is in hands of Kung Yik She Secondary School[23] when on 7 December 2013, they launched 1056 of them at the same time, together with primary school students in Tin Shui Wai, Hong Kong.[24]

The current record for greatest altitude achieved by a water and air propelled rocket[25] is 2723 feet (830 meters), held by the University of Cape Town[26] which achieved the feat on 26 August 2015, beating the previous record of 2044 feet (623 meters) held by US Water Rockets.[27] The rocket also carried a video camera as payload as part of the verification required by the competition rules.

Steam rockets

Main article: Steam rocket

A steam rocket (or hot water rocket) is a rocket which uses steam as its propellant. Before launch, water in the sealed rocket is heated. As the rocket remains sealed, the pressure increases. This pressure is sufficient to keep the water as superheated water, rather than boiling into steam, as the boiling temperature of water increases with pressure. On launch, the pressure vessel is vented through a nozzle. The released water drops in pressure as it passes through the nozzle, allowing it to boil or 'flash' instantly into steam. The high velocity of the steam, and its expansion through the nozzle, gives rise to the usual reaction force for a rocket.

The idea of such rockets was conceived in Germany before the Second World War, with the suggested use of an alternative rocket engine for jet-assisted takeoff fighter jets. Some of the few practical steam rockets constructed have been used for drag racing and for Evel Knievel's Skycycle X-2 canyon jump.

Bibliography

References

  1. ↑ Water Rocket Computer Model from NASA
  2. ↑ Sim Water Rocket from Dean's Benchtop
  3. ↑ Water Rocket Simulation from Clifford Heath's website
  4. ↑ Schultz, William W. "ME 495 Winter 2012 Lecture." University of Michigan, Ann Arbor. Mar.-Apr. 2012. Lecture.
  5. ↑ Dean's benchtop: hydrogen powered water rocket
  6. ↑ Hydroflite: Rocket Science
  7. ↑
  8. ↑ Annual International Rocket Week
  9. ↑ STAAR Research
  10. ↑ ASTRA
  11. ↑ National Physical Laboratory's annual Water Rocket Challenge
  12. ↑ Playlist
  13. ↑ Water Rocket Achievement World Record Association
  14. ↑ Freestyle-Physics Water Rocket Competition
  15. ↑ Rangliste Wasserraketen
  16. ↑
  17. ↑ Oleksandr, Zahoruiko. "Фестиваль "Еко-техно-Патріо" | ueeu.in.ua/CITE". ueeu.in.ua.
  18. ↑ Александр, Загоруйко. "Центр інноваційних технологій освіти | ueeu.in.ua/CITE". ueeu.in.ua.
  19. ↑ Олександр, Загоруйко. "Робимо ракети, озброєного робота та ліс Еко-дерев | ueeu.in.ua/CITE". ueeu.in.ua.
  20. ↑ Design
  21. ↑ Олександр, Загоруйко. "Програма "ТЕРИТОРІЯ ІННОВАЦІЙ" | ueeu.in.ua/CITE". ueeu.in.ua.
  22. ↑ Пневмогидравлическая ракета
  23. ↑ http://www.guinnessworldrecords.com/world-records/7000/most-water-rockets-launched-simultaneously
  24. ↑ http://www.sphrc.edu.hk/birthday.htm
  25. ↑ Single stage water rocket altitude record competition rules
  26. ↑
  27. ↑ U.S. Water Rockets

External links

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