Water Rockets for beginners

Hey everyone,

our goal with Project Poseidon is to develop an advanced waterrocket. Before we start with such an ambitious project, it is very important to actually understand the key physical principles behind waterrockets in general. We want to achieve that with the next three blog articles: This one will cover the basics. The second article will go into detail about our hypothesis regarding thrust and mass. The last one will be an analysis of the test data collected with FAR and put it into comparison to our hypothesis.

The basics of thrust

Everybody heard about Newton’s third law of motion at least once in their life. But what does it actually mean? Let’s not start with the boring physical explanation. Instead imagine that you are sitting in a boat and want to move from one point to another. Yeah, you could paddle to accelerate yourself, but that’s not cool enough. Instead, you could throw things away from you and you will start moving in the opposite direction. That is Newton’s third law. If you don’t have enough things to throw, a fire extinguisher could do that job for you with all the foam it spits out.

A rocket does the exact same thing.

The thrust (accelerating force) of a rocket is dependent on how fast and how much mass can be propelled out of the rocket. Water Rockets, as in the name, use water as the mass, and the pressure of compressed air to accelerate this water. It propels the water out backwards, so the rocket accelerates forward because of Newton’s third law of motion, as explained above.

If you want to learn more about he math behind the motion of water rockets, check out the upcoming second part of this series.

Structure of water rockets

A simple water rocket basically just consists of one tank (e.g. a PET bottle), where the water and compressed air are stored.

The tank has an opening at the bottom, which is also called a nozzle. This is where all the water exits the rocket. The nozzle is closed off (e.g. with a plug/cork) after filling the rocket to about one third with water. Next, the tank is pressurized with compressed air. You could do this with a bike valve inserted into the plug and a pump.

A diagram of a simple water rocket

As soon as the plug is removed from the nozzle, the pressurized air propels the water out and the rocket lifts off. Why pushing water out of the back accelerates the rocket forward should sound familiar to you.

That’s it for today. Currently we are working hard on our physical understanding of water rockets and on collecting the necessary data to check our hypothesis. We hope to publish the second part of this trilogy soon, until then: Thanks and bye, bye!

Aaron & Finn

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