Water rocket with automatic parachute deployment using recycling materials

Published in

Reverse Engineering

7 min readJul 12, 2020

Back in 2012, I and a couple of colleagues built a water-powered rocket for the final project in the fluid mechanics course of the Computer Engineering program at UFRN. This rocket was made of recycled PET bottles and some other easy-to-find materials like duct tape and garbage bags. Since this project was meant to teach us about mechanics and propulsion, I’ll include some of the math involved in the design of this prototype. I promised this won’t be a boring post, and it’ll be suitable for all ages.

Rocket design

The rocket frame was made by combining two used plastic bottles. The nose cone carried the parachute inside which was made of a garbage bag and sewing thread put together with duck tape.

List of materials:

2 PET bottles of 2L
Styrofoam
Duct tape
An air pump
A ball pump with needle and tube
Some PVC water pipes
A 50L garbage bag
Wine cork
Rubber bands, paper clips, and sewing thread

Before deep diving into our rocket design, it’s important to know a few basic concepts about the rocket's parts. Below we can see an image from the NASA website which describes the basics parts of a rocket.

If you take a look at the bottom of the rocket, you’ll see the fins. These fins were made of styrofoam, and they were designed to be lightweight and air dynamic.

The nose cone is attached to the frame of the rocket with an automatically activated system made of rubber bands and lockers. Inside this nose, we can find the parachute which will be explained later on.

On the bottom of the rocket, we can find the nozzle. This is one of the fundamental parts of the propulsion system.

below, we used a wine cork combined with a ball needle and tube to launch the rocket.

All the materials and the steps we need to follow in order to replicate this design is described in the section down below.

Launch pad

Let’s begin with is perhaps the easiest part: the launch pad. You will need to put together some water pipes in the following structure.

Parachute

Now is time to build the parachute. In order to do so, you have to use the garbage bag and the sewing thread to create the following design.

Nose lock system

Using the rubber bands and the paper clips, create two devices as shown below. You can use a piece of a PET bottle to create the frame.

Launch system

There are a few ways to do this part. We went with a simple solution. You can use a wine cork and parts of your ball pump to create a device like the one shown below.

Rocket frame

Finally, is time to build the rocket itself. For this part, you’ll need to combine two plastic bottles, and then create the fins and attache it to the frame using duck tape.

The final project with the parachute

After attaching the parachute into the rocket frame, you should have a rocket similar to the one shown on the left.

The parachute will open as soon as the pressure inside the fuel tank is equal to the outside pressure. This will be the time where the lock device will detach from the frame of the rocket. This should happen when the velocity of the rocket is zero.

When it’s released from the nose, the air will fill the parachute, causing the expansion and opening of it. The parachute will decrease the overall speed as the rocket descent so it can land without any damage.

Center of mass

The center of mass of a given object is the average point of all the parts of the system, weighted according to their masses. In other words, it’s the position where the gravitational forces reach the equilibrium.

This position can be obtained by summing all forces multiplied by the mass divided by the sum of all masses.

We then applied this formula to all the parts, and came up with the following values:

Now, we can calculate the center of mass of the entire rocket.

This value is:

Center of pressure

The center of pressure is the point where the total sum of a pressure field acts on the frame of the rocket. This value is obtained by using the formula below.

Since we have the values and dimensions of the rocked already, we only need to apply the formula and calculate the area of each part.

This value is:

Maximum speed and height

The reason we calculated the center of mass and pressure was to be able to estimate the maximum speed an height of our rocket. Now, my friend, we are about to do some serious math. I will make life easy and throw only the formula and values for you. If you want to know more about this topic, I do recommend reading the book Fluid mechanics of Frank White.

When we model the dynamics of our rocket, we can see where each principle is applied.

To obtain the velocity u of the rocket, we need to apply the Bernoulli equation:

Before doing so, we need to calculate the total pressure inside the rocket fuel tank.

For our rocket, we have y=1.4. When it comes to the total volume, consider the volume as defined below.

Now, we can apply the formula to find the pressure P.

Finally, the velocity u is calculated below.

Since we found the maximum speed above, now it’s time to calculate the total time in which the rocket reaches speed zero (stalls and begins to descend). This time is determined by applying the formula below.

Then, we came up with t = 2.273 s. The maximum height is then calculated.

Launch day

Finally, is time to validate the efficacy of our rocket. We did launch it several times and got results similar to the ones calculated, especially for the maximum height.

We made a video on the final launch day, and you can check it out below.

Conclusion

This post showed a water rocket made of material everyone has access to. It helped us to understand how engineers calculate and estimate the hundreds, if not thousands, of variables when creating an actual rocket. When people refer to the term rocket science as something really hard to comprehend, they aren’t joking about it.

I hope you liked this content, and if so, leave a comment and let me know.

Thanks for reading it!