Emma Borg

May 2017

Many people use water rockets for entertainment, and they are often used as demonstrations in science classes, but not everyone understands the physics that cause water rockets to work. Water rockets are rockets that are launched with water and pressurized air, “A water rocket is a chamber, usually a 2-liter soft drink bottle, partially filled with water.  Air is forced inside with a pump.  When the rocket is released, the pressurized air forces water out the nozzle…” (Dunbar). Some volumes of water cause the rocket to stay in the air longer, allowing the rocket to travel higher. There is an adiabatic expansion inside of the closed off rocket (Shaviv). This means that the pressure decreases, but the volume stays constant along with no heat flow (Giancoli 411).  This adiabatic expansion and the increase of pressure that goes along with it is what causes the rocket to be propelled through the air. There are several different forces that act on the rocket: thrust, gravity, and drag (Kian). Newton’s First Law can be used to explain water rockets. The law states that,  “An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force” (Water Rocket Physics…). All of the forces acting upon the rocket are balanced, when it is sitting on the base, but as soon as the pressurized air is pumped in, the forces become unbalanced, causing the rocket to be launched into the air (Water Rocket Physics…). Newton’s Third Law also applies, stating that for every action there is an equal and opposite reaction. The water escapes from the bottle, pushing down, so there is an upward reaction force (Water Rocket Physics…).  Overall, the pressurized air that is pumped into the rocket causes it to be propelled through the air. After enough pressurized air has been pumped into the rocket, a trigger is pulled, releasing the rocket, allowing it to jet upwards as the water streams out of the bottom of the rocket (Podesta). In other words, the water is forced down and this pushes the rocket up.

The water to air ratio inside of the rocket is crucial to the time that the rocket will spend in the air. If there is more water in the rocket, there is less room for pressurized air. In this case, there is lots of fuel (water), but very little energy (pressurized air). If there is a lot of air in the rocket and little water, the rocket will not have enough fuel to expel to keep itself in the air for a sustained period of time. The ideal water to air ratio will be the amount of water that allows the rocket to stay in the air for the longest amount of time.

Statement of the Problem .:. Top

The purpose of this investigation is to determine the ideal amount of water to put in a water rocket to make it stay in the air for the longest amount of time. What is the effect of the amount of water in a rocket on its time in the air?

The time the water rocket spends in the air will increase until it hits 400 mL then it will decrease because if there is too little water, the rocket’s fuel, the water, will run out quickly, but if it’s filled too full it will become heavy, making it harder to launch. In this situation, the independent variable is the amount of water in the rocket, which is measured in mL, and the dependent variable is the time the rocket spends in the air, which is measured in seconds.

In this experiment, the independent variable is the amount of water. The dependent variable is the time in the air and velocity. The experiment also had several control variables: the rocket, the timer used, the person timing, and the pressure.

1 L water rocket

Cardboard fins (3)

Cardstock nose

Soda bottle

Bike pump

Launch tube

Timer

Water

500 mL beaker

Release pin

Pressure gage

To begin the lab, gather all materials needed to complete the lab. It will be best to have at least two people to successfully carry out the lab. After that find an open area without any tree branches or plants that could potentially interfere with the water rocket’s path. After all of the equipment is gathered, and a good location has been found, use the beaker to fill the rocket with 50 mL of water. This will be done five times for each amount of water. The amount of water will start at 50 mL and increase in increments of 50 mL up to 500 mL. After the rocket has been filled with the appropriate amount of water, secure the rocket to the launch tube with the release pin. After the rocket has been secured, pump the bicycle pump five times. Immediately after the bicycle pump has been pumped five times, pull the release pin. Have the other person start the timer as soon as the rocket takes off. Make sure to record the data. After all the data has been gathered and recorded, clean up all materials, making sure to leave nothing behind. Since this lab is done outside, it is crucial to make sure that the test location is cleaned up and left how it was before the experiment.

 Amount of Water (mL) Trial 1 (s) ± 0.387 s Trial 2 (s) ± 0.387 s Trial 3 (s) ± 0.387 s Trial 4 (s) ± 0.387 s Trial 5 (s) ± 0.387 s Average (s) ± 0.387 s Uncertainty ± (s) 50 3.72 3.58 2.79 3.56 3.43 3.42 .465 100 2.38 2.64 2.79 2.88 3.11 2.76 .365 150 3.91 3.07 2.97 3.13 2.91 3.20 .500 200 3.35 3.13 3.37 2.98 3.25 3.22 .195 250 4.07 3.58 3.58 3.13 3.14 3.50 .470 300 3.31 3.52 3.25 3.14 3.34 3.31 .190 350 3.38 3.15 3.15 3.37 3.17 3.24 .115 400 3.17 3.13 2.93 3.26 3.12 3.12 .165 450 3.17 3.12 3.19 3.11 3.29 3.18 .090 500 2.99 2.73 2.98 2.96 3.31 3.00 .290

Data: Excel  .:. Text

In the beginning the times were not steadily increasing, but as they got closer to 250 mL, they started to increase more. At 250 mL was the maximum time in the air. After 250 mL, the time decreased. From this data, it can be inferred that the optimal volume of water in a 1 L rocket is 250 mL. This is a quarter of the total volume of the rocket. The water changes the time in the air because the more water there is, the less air there is, so there will be less pressure, but if there is not enough water, then the rocket will not be able to propel itself. 250 mL of water is the optimal volume because it spent the longest amount of time in the air.

The first data point at 50 mL was an outlier. It did not follow the trend that the rest of the data points followed. The cause of the outlier is uncertain. It could have been that there was a lot of room for pressure in the rocket, but it also could have been caused partly by human error. Barring the 50 mL data point, they all seemed to increase until they reached 250 mL, which resulted in the maximum airtime, then they generally decreased until the last trial of 500 mL.

In general, the times increased from 50-250 mL. The maximum average time occurred at 250 mL, and the average times decreased from then on. The hypothesis was incorrect. The airtime increased until the volume reached 250 mL. At 400 mL, the airtime was decreasing. While every measure was taken to ensure accurate results, there were some errors. One factor that impacted the results was the wind. The rocket was also made with cardboard fins and a cardstock top, so it got tbanged up. If this experiment was to be repeated, making the rocket fins and nose out of something more durable like plastic would improve accuracy, but for this experiment those materials were not available. Human error was another source of error. Human error is inevitable regardless of the precautions taken to ensure accuracy. To minimize the effect of human error, the person doing the timing took a reaction time test. This is where the uncertainty of ± 0.387s came from. The rest of the uncertainties were calculated based on the times in the air for each amount of water. The formula used to calculate the uncertainty was the maximum value minus the minimum value divided by two.

In conclusion, the hypothesis was incorrect in stating that the time in the air would increase until the volume was 400 mL. In reality, 250 mL was the best volume for a bottle that could hold a liter of water. In the future, it would be interesting to carry out an experiment to see if filling a bottle one quarter of the way full always yielded the maximum airtime.

This website explains the history of rockets and water rockets. It provides helpful tips on how to build a water rocket, and it compares a water rocket to an actual rocket.

This website provides very detailed instructions on how to build a good water rocket. It goes in depth about the optimal design for a rocket and why it is important to have this design.

This website gives an in depth explanation into the physics and calculations of the water rocket. It breaks down and solves equations that are important to understand how a water rocket works.

This website breaks down how a water rocket functions. It goes in depth, describing the function of each part of the water rocket and its significance to the overall function of the water rocket.

This website is helpful if you plan to build a water rocket. It provides detailed instructions as well as a video. It also addresses possible errors and how to fix them.

Dunbar, Brian. "Materials for Rocket Construction - II." Astronautics 6.33 (1936): 14-15. NASA. NASA, 2011. Web. 16 Nov. 2016.

Giancoli, Douglas C. "The Laws of Thermodynamics." Physics: Principles with Applications. 6th ed. Upper Saddle River, NJ: Pearson/Prentice Hall, 2009. 410-11. Print.

Kian, Jong Tze. "Learn Physics with a Water-Propelled Rocket." Learn Physics with a Water-Propelled Rocket (n.d.): n. pag. Web. 16 Nov. 2016.

Podesta, Michael De. "A Guide to Building and Understanding the Physics of Water Rockets." Water Rocket Booklet (n.d.): n. pag. National Physical Laboratory. National Physical Laboratory,

June 2007. Web. 16 Nov. 2016.

Shaviv, Nir. "Water Propelled Rocket." Science Bits. Nir Shaviv, 2016. Web. 17 Nov. 2016.

"Water Rocket Physics Principles Forces and Motion."  Thrust Continues until the Water Is

Gone. F (n.d.): n. pag. Water Rocket Physics Principles Forces and Motion. Web. 16 Nov. 2016.