Water Rocket Basics

A Guide to Water Rocket Launchers

Water rocket launchers have been a source of entertainment and education for many years. Water rockets use water as reaction mass or mass against which the system operates to produce acceleration. Water rockets are made for Science projects, but others also enjoy building them as hobby. They are used in schools to teach students the principle of aeronautics. Plastic soda bottles are popularly used in creating a water rocket launcher, but other materials can be used as well, such as, polycarbonate tube covers, plastic pipes, and other lightweight and pressure resistant cylinder-shaped containers.

The Parts of a Water Rocket

Water rockets consist of five basic parts which are, nose cone, payload section, body, nozzle, and fins.

  • Nose Cone: This is the extension of the bottle and it can come in different shapes. It is responsible for improving the aerodynamics of the water rocket.
  • Payload Section: This is an optional part of a water rocket. It can hold a parachute or a payload. This may also help the water rocket to land safely preventing damage.
  • Body: The body serves as the engine of the rocket. It is made of a two-liter soda or pop bottle.
  • Nozzle: This is the part that goes into the bottle’s opening. It affects the thrust made by the rocket and provides the mounting point for launchers. It has two main classes:
  • Open – An open nozzle is also known as the standard nozzle because it has a diameter of 22mm which is the typical size of soda bottle’s neck opening.
  • Restricted – A restricted nozzle is smaller than the standard which diameter is usually about 9mm.

The larger the diameter of the nozzle, the faster its acceleration will be with a shorter thrust phase. Meaning, smaller nozzles provide slower acceleration with a longer thrust phase.

  • Fins: This part helps in stabilizing the water rocket to avoid tumbling end over end which causes a decrease in speed.

Aside from these parts, some water rockets have launch tubes as well. A launch tube helps increase the speed and height achieved by the rocket.

After learning the parts of water rocket launchers, let’s further understand how they function.

How Does It Work?

Rocket engines use high pressure in forcing fluid through a restricted opening at a high velocity. This creates a force that pushes the launch vehicle in the opposite direction from the exhausting fluid. The same concept goes with water rockets. Its operation is based on Newton’s third law of motion also known as the action-reaction law, which is similar with other rocket engines. Based on Newton’s law, every action has an equal and opposite reaction. Meaning, the mutual actions of two bodies on each other are permanently equal, and directed to opposing parts.

A water rocket launcher works by filling the bottle with water then sealing it. The bottle is then pressurized with gas, usually compressed air, using bicycle pumps or air compressors. In the combination of water and gas, gas provides means to store energy because it is compressible. Water on the other hand, increases the propellant mass fraction, or the ratio between the propellant mass and the initial mass of the water rocket. Some additives are also mixed with water which helps enhance the water rocket’s performance in different ways. Salt, for instance, is added to achieve a higher specific impulse. Another example is adding soap to water which creates a thick foam resulting to a lower density of expelled reaction, but an increase in the length of thrust.

After the bottle is pressurized, the seal on the nozzle is then released. This results to a rapid expulsion of water at high speeds until the propellant is used up and the air pressure in the water rocket drops to atmospheric pressure.

Also, the altitude and flight duration of the water rocket depends on the volume of water, the initial pressure, the rocket’s nozzle size, and the unloaded weight of the rocket. So, how much water is ideal to put in the rocket?

Well, the amount of water depends on how high you want the rocket to go. If you put too little water, the reaction mass wouldn’t be enough to boost the rocket efficiently. Too much water, on the other hand, reduces the maximum height the rocket will reach. Therefore, the ideal amount of water depends on the empty weight of the rocket, and if a launch tube will be used. The heavier the rocket weighs, the more water it needs, and the longer the launch rod is, the less water should be put into the rocket, to be able to reach the maximum height.

Types of Water Rocket Launcher

There are simple requirements for water rocket launchers. They should have a way to pressurize the bottle, a seal to keep water and pressure in the bottle until it’s launched, and a way to hold the rocket from launching until there is enough pressure in it. Water rocket launchers come in variety of designs and they are divided into different types.

  • Hand Held: This launcher is held by hand. It is sealed and is usually an O-ring. The pressure on the water rocket is increased by using an air pump. To launch the rocket, the user simply let’s go of the bottle.
  • Friction Plug: This is the simplest form of a launcher. It is composed of a cork with a tube through it. A tire pump is attached to the tube which pressurizes the bottle, and at some point, the pressure in the bottle overcomes the friction fit. The plug is then blown out of the bottle which causes the bottle to launch. The downside of this type is, the pressure at which the bottle will launch is unpredictable. A guide rod may also be helpful for this type to help the water rocket launch straight, because there’s a tendency that the rocket will fly to a different direction.
  • Latched: This type of launcher beats the unpredictability of the friction plug. It holds the bottle securely until it’s ready to launch. Once the desired pressure is reached, the bottle will be unlatched causing it to launch. Same as the friction plug, a guide rod may also be used with this type of launcher.

Launch Tubes: It is a tube which fits inside the nozzle of the rocket and extends toward the nose. It is anchored on the ground, therefore, as the rocket accelerates upward, it will block the nozzle and very little water will be ejected until the rocket leaves the launch tube. In this type, the water rocket is launched when it’s desired. Also, since runs up inside the bottle, there is no need for a guide rod.

If you’re interested to learning to build your own water rocket there are many DIY yourself options.  They are easy to build, and the materials needed can be mostly found in your household. But if you’re not a DIY type person, you can also purchase an assembled one in stores and online as well.