This semester I’m taking AME 309, Dynamics of Fluids. I know what you’re thinking, “ew,” it was my same thought when I saw that I needed to take it this year. The class is challenging, that is no question, but all of the learning that we have done so far has culminated with one really fun project: building a bottle rocket!

If you’re interested in Engineering I’m sure at some point you were excited by the thought of a rocket lifting off. Well, bottle rockets aren’t quite as massive as the ones NASA, ULA, and the EU launch, but for 20 oz. they have some serious power!

This project was interesting for two reasons in particular.

1^{st}: we got to apply our experience with Bernoulli’s Equation, and the Reynold’s Transport Theorem to a real world object. I’m not a huge fan of theory, but seeing theoretical concepts in action is very satisfying and informative to me. Doing the theoretical calculations and then adjusting variables to see how the theoretical and actual values lined up gave me a better understanding of what’s really going on.

2^{nd}: I really like hands-on projects. Between working on a race car in USC Racing, and printing real objects in 3D4E I was more than in my comfort zone putting together a new object. In fact, thanks to my involvement in 3D4E I was able to 3D print the “nosecone” of our rocket!

You did not explain how a “fluid-powered” bottle rocket is meant to work. Fluids not being compressible as gases are where does the thrust come from? In the last analysis you must have something that expands (or pumps the fluid) to get a Newtonian “actio = reactio” effect?

My apologies Maureen, I totally didn’t explain how the bottle rockets work! The term ‘fluid’ refers to anything that flows; therefore the term is comprised of both liquids and gases which are more aptly described as ‘states’ of matter. Going back to your question: liquids are not compressible, but gases are. The mechanism that pumps out the liquid, ‘fuel,’ is the compressed, pressurized, gas inside the rocket. For our purposes the liquid was water and the gas was air.

With that in mind, the objective of our project was to determine the optimal mix of water and air to achieve the maximum altitude for the rocket. Using the equations I mentioned in the post, and kinematic equations, we were able to set up a Matlab script that output data predicting the maximum height of the rocket. This was simplified by the restriction of the maximum pressure and volume of the bottle; with those constraints in place the height the rocket could achieve was solely based upon the initial volume of water inside the rocket.

Once our code was working we were able to run it over and over with different volumes of water until we reached an optimal volume. On test day we measured out that volume of water and launched the rocket. We measured the altitude we reached through spotters who used a device that measured the angle the rocket was above the ground. Since we knew their distance away from the launch location we could determine the altitude through trig. Assuming our calculations were correct (my groups were!) we reached the height we predicted in the code.

Did that automatically mean that our height was the highest altitude achieved? Sadly, no. Despite the restriction on the internal volume of the rocket, some groups changed the exterior of the rocket – adding weight with other exterior features – and got higher maximum altitudes.