How a Water Rocket Works

7/28/2019 How a Water Rocket Works

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How a Water Rocket Works

A water rocket works using the same principles as other rockets. There are three

main forces in action: thrust , drag (air friction) and weight (w=mg).

The water, which is expelled out by the difference between the internal and

atmospheric pressure, is a reaction mass that provides the thrust.

All rockets have a reaction mass. For the space shuttle, it is the hot gasses that

are expelled when a fuel is burnt.

In our case,the water bottle rocket, the reaction mass is water that is forced out

The air molecules moving along the side of the rocket as it is moving create

friction.

This is the drag or air friction.

This drag or air friction acts in the opposite direction as the velocity of the

rocket.

Therefore it slows down the rocket.

We learnt about weight in our Form 4 Physics.

Weight is simply the mass of the rocket multiplied by gravity.

This force works in the opposite direction as the thrust of the rocket. This

weight that acts in the downward direction, brings the rocket back down to

Earth


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Water and air make up the fuel of a water rocket.

Water,an incompressible fluid, is poured into the rocket before it is placed on

the launcher and acts as the reaction mass.

Air stores much more energy than the water because it is compressible .

Air is then pumped in and pressurised, the greater the pressure, the greater the

energy stored.

When a water rocket is launched, the difference between internal and

atmospheric pressures forces the rocket off the pressure seal, followed by the

expulsion of water and air out of the nozzle until the internal and atmospheric

pressures are equalised.

This action creates a downward force.

By applying Newtons Third Law of Motion and the law of conservation ofmomentum in order for the total momentum of the system to remain constant


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and equal to zero, there must be an equal but opposite force upon the rocket

pushing it upwards.

This is the thrust, which causes the acceleration of the rocket according to the

equation F=ma.

As the air stores the bulk of the energy inside the rocket, a correct balance

between the volumes of water and air must be found.

If there is too much water, compared to the air volume, too little energy will be

stored since water does not store energy inside the rocket while only the air

does.

If there is too little water, the reaction mass will be insufficient to provide much

acceleration, since air is far lighter than water. In this case the performance of

the rocket will be less than optimum.

Experimentally, a water-to-air ratio of 1:2 has been found to generally be the

most efficient.

That is one third of the volume is water.

Newton's First Law of Motion:

Every object in a state of uniform motion tends to remain in that state of

motion unless an external force is applied to it.

Newton's Second Law of Motion:

The relationship between an object's mass m, its acceleration a, and the

applied force Fis F = ma. Acceleration and force are vectors.

In this law the direction of the force vector is the same as the direction of

the acceleration vector.

Newton's Third Law of Motion:

For every action there is an equal and opposite reaction.


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Stability of the water bottle rocket

A rocket's stability is important for achieving better heights.

An unstable rocket will go up a few feet, then flutter back down to the ground.

There are two ways to change the stability:

1. change the surface area of the fins, or

2. add ballast to the nose.

Increasing weight to the nose increase the overall weight of the rocket so

increasing the surface area of the fins is usually the preferred method.

But increasing the surface area of the fins also increases the drag or friction.

So a stabil rocket is generally a compromise between drag and weight.

According to Newton's First Law of Motion, an external force exerted upon a

body will cause that body to accelerate in the same direction as the force

applied.

Any body in space will tend to rotate about its center of gravity (CG) - the

point where all of the mass of the body is balanced.

If the rocket has no means of stabilisation whatsoever, the external forces will

cause the rocket to go entirely off course.

For this reason fins are used; they create a restoring force that helps to keep the

rocket going as straight as possible.

It is important however to note that the restoring force created by the fins

causes the rocket to rotate around its center of pressure (CP) rather than its CG.

Flight of a water bottle rocket


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DURING ASCENT


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The weight of the bottle rocket is constantly changing during the ascent because

the water is leaving the rocket.

As the water leaves the rocket, the volume occupied by the pressurized air

increases.

The increasing air volume decreases the pressure of the air, which decreases the

mass flow rate of water through the nozzle, and decreases the amount ofthrust

being produced.

Weight and thrust are constantly changing during the powered portion of

the flight.

When all of the water has been expelled, there may be a difference in pressure

between the air inside the bottle and the external, free stream pressure. The

difference in pressure produces an additional small amount of thrust as the

pressure inside the bottle decreases to ambient pressure. When the pressures

equalize, there is no longer any thrust produced by the rocket, and the rocket

begins a coasting ascent

DURING COASTING


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The water bottle then slows down under the action of the weight and drag and

eventually reaches some maximum altitude.

The rocket then begins to fall back to earth under the power of gravity.

The parachute is a recovery system sothat we can fly the bottle rocket again.

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How a Water Rocket Works