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 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 . It is an example of ewton!s third law of motion.

Contents

• " #peration

• $ %redicting peak height

• & 'ulti(bottle rockets and multi(stage rockets

• ) *ources of gas

• + ozzles

• -ins

• /anding systems

• 0 /aunch tubes

• 1 *afety

• "2 3ater rocket competitions

• "" 3orld record

• "$ *team rockets

• "& 4ibliography

• ") 5eferences

• "+ 6xternal links

Operation7edit8

*implified animation of how a water rocket works. "9 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. $9 The bottle is

released from the pump. &9 The water is pushed out the nozzle by the compressed air. )9 The bottle moves

away from the water because it follows ewton!s Third /aw.

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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 "$+ psi, but sometimes

:#$ or  nitrogen from a cylinder.

3ater 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 e;ected from the rocket!s nozzle. *ometimes additives are combined with the water to

enhance performance in different ways. -or example< salt can be added to increase the density of 

the reaction mass resulting in a higher specific impulse. *oap is also sometimes used to create adense 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 =e /aval 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 ewton!s

third law. The expulsion of the water thus can cause the rocket to leap a considerable distance

into the air.

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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.7"87$87&8

#ften 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 + to "+2 psi >+22 to "222 k%a9. The higher the pressure,

the larger the stored energy.

Predicting peak height7edit8

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<

7)8

> ? peak height reached, ? Initial mass of water only, ? 5ocket mass with

water, ? Initial gauge pressure inside rocket, ? density of water, ? acceleration due to

gravity9 Assumptions for the above e@uation< >"9 water is incompressible, >$9 flow through the

nozzle is uniform, >&9 velocities are rectilinear, >)9 density of water is much greater than

density of air, >+9 no viscosity effects, >9 steady flow, >9 velocity of the free surface of water

is very small compared to the velocity of the nozzle, >09 air pressure remains constant until

water runs out, >19 nozzle velocity remains constant until water runs out, and >"29 there are

no viscous(friction effects from the nozzle >see 'oody chart9.

 An independent variable that influences peak height is weightmass. =epending on the thrust

of the rocket propulsion system, a rocket re@uires a minimum mass to overcome the

deleterious effects of drag. -or example, the greater the thrustthe 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 rockets7edit8

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Two multi(bottle rockets with acat for scale.

 A larger multi bottle rocket with cylindrical fins.

'ulti(bottle rockets are created by ;oining two or more bottles in any of several different waysB

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. 'ulti(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.

'ulti(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. 'ethods to time the launches in correct order and at

the right time vary, but the crushing(sleeve method is @uite popular.

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Sources of gas7edit8

*everal methods for pressurizing a rocket are used including<

•  A standard bicyclecar tire pump, capable of reaching at least + psi >+$2 k%a9.

• 3ater pressure forcing all the air in an empty water hose into the rocket. %ressure is the

same as the water main.

•  An air compressor, like those used in workshops to power pneumatic e@uipment and

tools. 'odifying a high pressure >greater than "+ bar "+22 k%a $22 psi9 compressor to

work as a water rocket power source can be dangerous, as can using high(pressure

gases from cylinders.

• :ompressed gases in bottles, like carbon dioxide >:#$9, air, and nitrogen gas >$9.

6xamples include :#$ in paintball cylinders and air in industrial and *:C4A cylinders.:are must be taken with bottled gases< as the compressed gas expands, it cools

>see gas laws9 and rocket components cool as well. *ome materials, such

as %D: and A4*, can become brittle and weak when severely cooled. /ong air hoses

are used to maintain a safe distance, and pressure gauges >known as manometers9

and safety valves are typically utilized on launcher installations to avoid over(pressurizing

rockets and having them explode before they can be launched. Eighly 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 *:C4A first(stage9. All compressed gas containers are sub;ect to local,

state and national laws in most countries and must be safety tested periodically by a

certified test centre.

• Ignition of a mixture of explosive gases above the water in the bottleB the explosion

creates the pressure to launch the rocket into the air. 7+8

Nozzles7edit8

3ater rocket nozzles differ from conventional combustion rocket nozzles in that they do not

have a divergent section such as in a =e /aval nozzle. 4ecause 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 FstandardF or Ffull(boreF having an inside diameter of

G$$mm which is the standard soda bottle neck opening.

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•   Restricted which is anything smaller than the FstandardF. A popular restricted nozzle has

an inside diameter of 1mm and is known as a FHardena nozzleF named after a common

garden hose @uick connector used to make them.

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

acceleration with a longer thrust phase.

It can be shown that the e@uation for the instantaneous thrust of a nozzle is simply< 78

where is the thrust, is the pressure and is area of the nozzle.

Fins7edit8

 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 momentum9. 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. 4y ensuring stability, they are very

likely to increase its launch height. -ins increase drag, but the stability achieved makes amuch 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 F:oroplastF. 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. -ins add to the frontal

surface area on which the drag force acts >and therefore should be designed not to add

too much drag9. The drag forces on all frontal surfaces of the rocket can be resolved into

one force acting at the center of pressure :enter of pressure >fluid mechanics9. 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 instability9, then there will be a tor@ue,

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due to the resolved drag force, acting around the center of gravity. This tor@ue will

stabilize the rocket by returning its nose to the direction of travel.

*ince the tor@ue is the cross(product of the drag force magnitude and the moment arm,

tor@ue 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 tor@ue. 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. 78

Landing systems7edit8

*tabilizing 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.

*ome water rockets have parachute or other recovery system to help prevent problems.

Eowever these systems can suffer from malfunctions. This is often taken into account

when designing rockets. 5ubber bumpers, :rumple zones, and safe launch practices can

be utilized to minimize damage or in;ury 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 . 4y 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 tubes7edit8

*ome 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 e;ected 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

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the rocket. /aunch tubes are most effective when used with long rockets, which can

accommodate long launch tubes.

Safety 7edit8

3ater rockets employ considerable amounts of energy and can be dangerous if handled

improperly or in cases of faulty construction or material failure. :ertain safety procedures

are observed by experienced water rocket enthusiasts<

• 3hen 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 +2 greater than

anticipated pressures. If the bottle ruptures, the amount of compressed air inside it

>and thus the potential energy9 will be very small, and the bottle will not explode.

• Csing metal parts on the pressurized portion of the rocket is strongly discouraged

because in the event of a rupture, they can become harmful pro;ectiles. 'etal parts

can also short out power lines.

• 3hile 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 example9 are used. This ensures that if the rocket veers off in an unexpected

direction, it is less likely to hit the operator or bystanders.

• 3ater rockets should only be launched in large open areas, away from structures or

other people, in order to prevent damage to property and people.

•  As water rockets are capable of breaking bones upon impact, they should never  be

fired at people, property, or animals.

• *afety goggles or a face shield are typically used.

•  A typical two(litre soda bottle can generally reach the pressure of "22 psi >12 k%a9

safely, but preparations must be made for the eventuality that the bottle unexpectedly

ruptures.

• Hlue used to put together parts of water rockets must be suitable to use on plastics,

or else the glue will chemically FeatF away the bottle, which may then fail

catastrophically and can harm bystanders when the rocket is launched.