Water Rocket
| Polish version is here |
The following article was originally published in the journal Młody Technik (eng. Young Technician) (6/2014):
What Is a Jet Engine?
A jet engine is a device that operates by ejecting a high-speed stream of fluid (gas or liquid), which creates what's known as thrust. The existence of this force is explained by Newton’s Third Law of Motion. This law states that for every action (force) in nature there is an equal and opposite reaction.
Most jet engines in use today work by expelling hot gases produced by fuel combustion. The expanding combustion products are accelerated backward, pushing the engine — and the vehicle — forward.
In the early 20th century, scientists experimented with rocket-type jet engines powered by… water and compressed air. Today, we’ll build a model of just such a water-powered rocket engine.
Blaise Pascal (1623–1662) was a French mathematician, physicist, and philosopher. From a young age, he showed a keen interest in science, which was supported by his father. In his work, he advocated for the scientific method as a means to understand the world. His achievements include the construction of the first mechanical calculators and foundational contributions to fluid mechanics. Building on the work of Torricelli, he also clearly defined the concepts of pressure and vacuum.
How It’s Built
The construction of this engine is fairly simple, as we can use a 1.5-liter (about 50.7 fl oz) PET soda bottle. The trickiest part is compressing enough air into it — this requires generating a fairly high pressure. So, we’ll need to create a simple but functional apparatus. Here's what you'll need:
- rubber stopper — sized to tightly and securely seal the bottle opening
- a short piece of flexible plastic tubing
- copper or brass tube — slightly longer than the stopper's length
- bicycle Schrader or Dunlop valve
- cyanoacrylate glue (super glue)
The construction of the valve-stopper assembly is shown in Fig. 1.
Making the Stopper-Valve
First, drill a hole through the center of the stopper along its main axis. Push the copper tube snugly into this hole. On the bottle-facing end, solder in the bike valve. The valve must be positioned so that air can be pumped into the bottle. Then, glue the plastic tubing onto the outside end of the copper tube. The other end of this tubing connects to a standard air pump (the type used for car tires). The finished assembly should look like Photo 1 below.
The valve’s purpose is to allow air to be pumped into the bottle while preventing any gas from escaping.
Ready for Launch!
Once everything’s ready, fill the bottle about two-thirds full with water (approx. 1 liter or 33.8 fl oz), then seal it tightly with the prepared stopper. Turn the bottle upside down, or tilt it so that the base points upward.
Note: Make sure the bottle is securely positioned to ensure the launch direction is safe and predictable.
Now start pumping air into the bottle — but don’t overdo it! Too much pressure could cause the bottle to burst, which can be dangerous. Once the internal pressure builds up enough, the stopper will be forced out. The expanding air will rapidly eject the water backward, launching the bottle forward at high speed. Photo 2 shows the sequential stages of the rocket’s flight, with time stamps counted from the moment the stopper is ejected. The rocket traveled over 100 meters (about 328 feet), which is pretty impressive for such a simple build.
How It Works
Pascal's law (also Pascal's principle or the principle of transmission of fluid-pressure) is a principle in fluid mechanics that states that a pressure change at any point in a confined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere. That means both the water and air inside the bottle are under the same pressure. The compressed air naturally tries to expand, which lowers its pressure — this makes it push the stopper out and force the water behind it out of the bottle.
Water, being relatively dense, has large inertia. When it's forced out backward, the resulting thrust is powerful enough to launch the bottle rocket high into the air.
Enjoy the experiment!
References:
- Wróblewski A. K., Historia fizyki - od czasów najdawniejszych do współczesności, Wydawnictwo Naukowe PWN, Warszawa 2006
- Davidson H., Blaise Pascal, Twayne Publishers, Boston 1983
- Gaj J., Laboratorium fizyczne w domu, Wydawnictwa Naukowo-Techniczne, 1982
All photographs and illustrations were created by the author.
Addendum
The effect of this experiment can be seen in the following video:
Marek Ples