Bottle Rockets

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Bottle Rockets - a future mode of transport?

Big Ideas:
• Bottle rockets can be easily made, and are capable of traveling at very high speeds

What is a water rocket?
It is a model rocket that uses a plastic 2L pop bottle as the engine and water as the propulsion fuel. Water is forced out of the rocket by compressed air.

How does it work?
The 2L pop bottle is partially filled with water and turned upside-down. Air is then pumped into the bottle using a bicycle pump or an air compressor, up to 100psi. Once pumped, the pipe nozzle is removed from the bottle and the compressed air expands, forcing the water out at high speeds.

So what happens?
Newton’s third law comes in here: the compressed air expands exerting a force on the water, pushing the water down and out of the bottle. The water in turn exerts an opposite force, pushing up on the air. This force propels the bottle upward, and the water gets ejected from the bottle. The compressed air expands when the pump is removed, rapidly expelling the water out of the bottle creating a large upward thrust. The bottle will accelerating up until all the water has been expelled and the pressure in the rocket drops down to atmospheric pressure.

You can see a video of the initial take off here:

Bottle_Rocket_Take_Off.mp4 (right-click and choose "Save Link As..." to download to your computer)

Why use air and water?
The air and water together provide the rocket with a source of energy and a propellant; the air is highly compressible and thus acts as a means to store potential energy. It is ideal in this case because for a very small weight it can store a lot of energy . The water is the fuel for the rocket – because it is very dense, it provides a large inertia that the rocket pushes against as it expels the water

So can bottle rockets be used as a mode of transport?
Some scientists have carried out tests to investigate the speed of bottle rockets. It was found that the rockets can travel up to 4m in just 0.2s, and can accelerate to about 100g’s – 100 times the acceleration due to gravity! This acceleration is too high for the bottle to be deemed an appropriate mode of transport for humans. 1 Look at what happens when some people try to make a man fly using bottle rockets:

You can see how fast it actually travels, in the video below. Notice how quickly it takes off and disappears briefly from the camera view - it travels too fast for the camera person to follow!

Bottle_Rocket_Full_Flight.mp4 (right-click and choose "Save Link As..." to download to your computer)

Things to try at home
Do you think changing the air/water ratio will make a difference to how high the rocket goes? Try changing the ratio and see what happens. Also, try a rocket with just air, and one with mostly water. What do you notice? Some have said that changing the density of the water, e.g. by adding salt results in a higher change in momentum per litre of fuel, making the rocket travel faster. Is this true? Try timing the rocket with salty water and see how much longer it goes for. Others also say that using soapy water creates a dense foam which lowers the density of the expelled mass, but increases the thrust duration. Try using soapy water, does the rocket travel for a longer period of time? 2

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C21: Physics Teaching for the 21st Century
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