Tuesday, January 17, 2017

Rockets: An Introduction

by Ethan Fahnestock

An important ability of any rocket is the ability to get off of the ground. To do this, the rocket has to overcome the opposing force of gravity. Different types of rockets do this in different ways. The type showcased in this blog post - essentially an overpowered stomp rocket - uses air pressure to throw the rocket skyward.

An easy way to represent the forces acting on the rocket is using something called a free body diagram, or FBD. As visible in the picture on the right, the “rocket force” is larger than the gravitational force, resulting in a net upwards force. Physics students will remember Newton’s second law, depicted to the right as well. Dividing the net force by the mass of the rocket gives you the acceleration the rocket experiences upward.

Physics Sidenote: As a rocket expends its fuel, its mass decreases and the same force packs a bigger punch. You can see this effect on SpaceX’s livestreams.

An Example

Alright, enough physics! Over the past semester I have been messing around with different rocket designs I would like to share with you. I encourage you to play around with some of these ideas if they intrigue you.

Now most large scale rockets use some form of combustion to propel them upward. Unfortunately introducing combustion to your homemade rocket can be difficult to do right off the bat. As mentioned earlier, this rocket uses pressure to propel it. This means that the rocket doesn’t require an on-board tank to hold propellant, making it easy to build.

Making your own propellant isn’t impossible - I plan to do it soon - but it requires more precautions. Starting a forest fire is not a fun thing to do. If you are interested in making your own propellant, check out this video.  

The Launcher

Here is a video of the launcher. It uses a bicycle pump, PVC, and a sprinkler valve. After everything is put together, the launcher can get up to an astonishing 120 PSI! If something gave out, plastic shrapnel would go everywhere, so be careful. After pumping it up, a button is clicked to trigger the sprinkler valve and the rocket flies. Check it out in the video. If you want to make your own, check out the guide this one was based off of.

The Rockets

The rockets shown above were all grouped into the “Mk1” series. Most of the rockets were made of paper or tin foil combined with tape and a 3D printed nose cone. Besides occasionally blowing up, these rockets did quite well. Using rough trigonometric estimations, we measured the maximum height to rest around 55 meters! They made a satisfying thud when they crashed back into the ground. The “Mk2” series was defined by their rigid build qualities and housing of a data logger shown to the left below. The rocket on the right was completely 3D printed! The larger rocket - shown in the video above - launched to a height of 80 meters!

The data logger was made using a cheap pressure sensor and an Arduino Micro. Check out this website for more information on the circuitry.

Alright, that brings this to a wrap! I hope you learned something about DIY rocketry. If you are looking for an interesting weekend project I couldn’t recommend this highly enough.

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