Water Rocket Basics

What Makes a Water Bottle Rocket Work?

Water bottle rockets are one of the popular projects we see in Science fairs. Aside from the fact that it’s easy to build and use, the way it works is fun and interesting, as well. But what exactly makes a water bottle rocket fly?

Based on some explanations, the combination of water and air pressure makes it work. To further explain this, let’s look into Newton’s three laws of motion.

Newton’s 3 Laws of Motion

  • First Law: An object will stay at rest unless there’s a force that acts upon it.
  • Second Law: Force is equaled by mass times acceleration.
  • Third Law: For every action, there’s an equal but opposite reaction.

What is the connection of Newton’s laws of motion with how a water bottle rocket works? Well, based on the first law, an object will not move unless there’s a force that acts upon it. When a rocket is filled with pressure and all the air is compressed in the bottle, the force of the launch pad latch will equal the force of the compressed air. In this case, the water bottle rocket is at rest and will not launch.

The second law tells how much force will come out of the rocket’s nozzle. In this case, the accelerant will be the air, and the mass will be the water. For example, you have a 20-ounce water bottle. You have to figure out what ratio of compressed air and water would produce the most force. It’s good to have more air for acceleration and more water for mass.

When you pull the launch cord, it will force the water out the bottom of the bottle at a great speed. The water and air pressure which are going downwards will push the bottle upwards. This is an example of the equal but opposite reaction stated in Newton’s third law of motion.

How Does a Water Bottle Rocket Work?

Newton’s laws of motion explained that air pressure and water pressure are the two components which make a water bottle rocket fly. To better understand how it works, here is some further explanation.

First, the bottle is partly filled with water and is sealed. Then, it’s pressurized with gas using a bicycle pump or air compressor. The pressurized air provides a means to store energy, while water, on the other hand, provides the ejection mass to create the force needed to be able to launch the water bottle rocket at a high speed.

Once the water bottle rocket launches, the pressurized air starts to push the water out of the rocket’s opening at the bottom. While this occurs, a push force is applied back to the rocket in the opposite direction. As the water is released from the rocket in a downward direction, the push force it experiences is also moving downwards. The rocket experiences the same force, but in an opposite direction which is upward. This force is what makes a water bottle rocket fly straight up at a high speed into the air.

A plastic bottle is usually used in creating a water bottle rocket. It is pressurized with air and water until the pressure becomes high enough to make the water bottle rocket be released and fly high up the air.

How do you make a water bottle rocket fly higher? Well, the secret is how high the pressure is. A higher air pressure can make the water shoot out faster, therefore, it will give a greater push to the rocket. As a result, the water bottle rocket will launch faster to the air and of course, much higher.  

What are the fins for? When building a water bottle rocket, cardboard is usually used to make fins. Fins are attached to the bottom of the water bottle rocket to stabilize it and keep its flight straight. Fins are an important part of the rocket because, without them, the rocket might turn end over end in the air because of the forces caused by air resistance. The fins are responsible for catching the wind which may ruin the straight flight of the water bottle rocket. Also, without the fins, the rocket will not travel very far nor high enough.

The record for the greatest altitude reached by a water bottle rocket is held by the University of Cape Town. They were able to make a water rocket fly for 2,723 feet or 830 meters high. That is extraordinary!