MAE 250
Rachita Shah, Ashby Scruggs
Project: Water bottle rocket
For this project, we decided to make a water bottle rocket. At first, we planned to make a standard rocket using the materials provided. We would use two 2 liter bottles and the poster board to make the body of the rocket. This would also include the fins and the nose cone. The egg would fit within the nose cone and be attached to a plastic shopping bag that would function as the parachute. Upon apogee, the nose cone would fall off and the egg would begin its descent to the ground. Upon further analysis, we decided to make a few changes to the design to make it more efficient while staying within the constraints of the materials that could be used.
Fig 1: Preliminary design
This preliminary design would not be successful as there would not be enough poster board to create it. The surface area of the wings in addition to the surface area of the cone would be greater than 100 in2. Making the nose cone out of the poster board would reduce the amount of poster board left to make the fins of our rocket. So, we looked for alternative solutions to the pointed nose cone. We decided that it would be better to make the tip more rounded rather than narrow. Research has shown that in subsonic speeds, rockets with nose cones in the shape of a rounded curve reduce air resistance better than a sharp curve. In supersonic speeds, sharper and more pointed nose cones function better. So, we decided to scrap the nose cone and keep the egg in one of the bottles as the end of the bottle was rounded in shape. This, in turn saved poster board which enabled us to make larger fins.
We also had an idea to protect the egg upon its descent. As there was no limit on strings, we decided to weave a basket out of strings that would protect the egg inside the rocket and also when it was making its way down. Another advantage of this basket was that it was lightweight so it would not affect the center of mass.
Fig 2: Basket for the egg
The fins of a rocket contribute to the stability of the flight. They prevent the sideways motion of the rocket. For our rocket, we decided that the fins should not be too heavy and pointed. If they were so, they might bend at the end and affect the motion of the rocket. We decided on four fins made from poster board and attached them 6 cm above the levels of the bottle’s opening to ensure that the rocket fit on the launcher.
For the parachute, we used a large shopping bag to increase surface area thereby increasing the drag. This would decrease the speed at which the egg fell to the ground and increase the probability of the egg to not be cracked. After doing some research, we discovered that a good parachute has shroud lines that are at least as long as the diameter of the canopy. Our parachute was in the form of an ellipse. We attached a lot of strings to the ends to make sure our parachute opened.
Our final design consisted of two 2 liter bottles, one cut in half, stacked on top of one another.
The bottle on top can be removed, but is attached to the main body by on piece of tape. This will allow the parachute to be released at apogee. There are four equally spaced wings that are 15 cm long and 10 cm wide. These wings are approximately triangular in shape, but curved at the tips to make them less bendable. The parachute, egg, and basket rest inside the top bottle upon the rocket’s ascent. At apogee, the top bottle will fall off, releasing the egg, parachute, and basket.
The parachute will slow the descent of the egg to an acceptable final velocity to ensure that the egg does not break upon impact.
Fig 3: Image of final design
Fig 4: Dimensioned drawing of solution made using Solidworks
Performance Estimates of the Rocket:
To check the performance of the rocket, we used a computer simulation for a water rocket found on the National Physics Laboratory website. We entered values of certain parameters such as the volume of the bottle, initial pressure, nozzle diameter, mass of the empty rocket and so on to get the trajectory of the rocket. As the launch angle was set to 90 degrees, the range turned out to be zero meters. We also had to assume a drag factor for our rocket and decided on 60. For
comparison, a tennis ball has a drag factor of about 10 and a football has a drag factor of about 100. From this simulation we were able to find the data we needed.
Volume of bottle (L)
Initial Pressure (atm)
Nozzle Diameter (mm)
Mass of Empty
Rocket (g) Flight Duration (s) Apogee (m)
2.032 4.08 25 300 3.81 17.3
Burnout (s)
Initial Velocity (m/s)
Maximum Velocity (m/s)
Kinetic Energy (@ max V)
Potential Energy
(@ max V)
0.062 0 26.3 69.4 1.9
Fig 5: Filling out the parameters to estimate results of the simulation
Fig 6: Velocity vs. time graphs to find the max. Speed
The horizontal velocity is shown in red and vertical velocity in blue. Vertical lines have been drawn to mark events in the rocket's flight. A green line shows the time at which the last of the water was ejected. A cyan line shows the time at which the 'Air Blast' finished. An orange line shows the time at which the parachute (if present) was deployed.
From the simulation results, we understood that the flight duration will approximately be 3.81 seconds, the maximum height would be 17.3 m (56.78 ft.) and fall exactly where it was
launched. From the velocity time graphs, we inferred that the maximum speed would be 30 m/s.
However; these numbers will not be achieved due to uncalculated restraints on the rocket such as wind, air resistance, and other factors. Additionally, the rocket we have designed includes a parachute, which was not accounted for in these calculations. The ascent of the rocket will be similar to the one described by the results; however, the descent will differ greatly. The
parachute will decrease the speed of the egg to an acceptable speed for landing, causing the total flight time to be longer than expected.
