For this project, I will be designing the fins for a rocketry project in Technology Through Engineering and Design II, taught at NC State University. While previous classes have modified the nose cone, or the rocket as a whole, I will only be changing the design of the fins in order to show how little changes can greatly impact flight. Using this project, I will also show you how to create your own personal model rocket.
First, you will need a number of materials to begin:
- A laptop with Solidworks, or access to an on-campus computer lab with Solidworks installed (if your a student).
- A basic rocket package, obtained through your local craft or hobby store.
- A set of rocket engines, preferably with an A8, B6, and C6 engine.
- Material to construct your fins. Balsa wood is provided in the package, however you may choose to use whatever material you desire. Keep in mind the number of variables, including surface texture, malleability, density, and more, that can affect the location of your center of mass and time required to produce.
- For this instructable, I will be using fiberglass constructed in our workshop.
- Glue, both Elmer's and superglue/epoxy/etc.
- A craft or Exacto knife.
You will also need access to a large, clear launch location, along with a door frame needed for rocket construction. (Seen in Rocket Assembly Instructions)
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Step 1: Research Your Design
For the first part of this project, it is suggested that you research possibilities for your model rocket's fins. There are many possibilities, and virtually no restrictions.
Begin by laying out what you know, and start with the basics. Do you know what direction your rocket will travel? For ours, we will be launching vertically. In what direction is drag in relation to launch direction? Drag is always opposite of direction traveled, so for us: down. What other factors influence flight? Lift? Do you even want any?
Add to your existing knowledge by using your favorite search engine to research how documented changes have affected others' flight times and paths.
Some specific questions to ask include:
- Where do I want my fins?
- How many fins should I have?
- What shape of fin works well?
- Do surface textures and shapes of leading and trailing edges have an effect on drag?
Websites from rocketry clubs, organizations involving flight construction, and other schools' classes are all good sources to reference.
It is important to keep in mind that your rocket will remain under Mach 1. Flight physics change after the sound barrier is broken, however our rockets will not reach speeds this high.
Step 2: Choosing a Fin Design
Once you have researched possibilities for your fin construction, you must decide on a way to engineer your fins. Sketch your fins on paper and make predictions for sizes along each variable dimension. While there are very few restriction on your fins, it is important to run your idea past others to avoid any drastic miscalculations.
For my design, I have chosen to use a parallelogram-type style. Using a parallelogram allows a large amount of surface area to remain further from the fuselage. The tips of the fin, being wider, help stabilize the rocket. I will also curve the leading edge toward the nose (Nakka's Rocketry Site). This is to help reduce drag as air transitions from surrounding the body to passing over the fins. I have also decided to shape my fins similar to a plane wing. With a rounded leading edge and tapered trailing edge, I am attempting to create small amounts of lift (perpendicular to the fin)in hopes of causing the rocket to spin. In order to do this, I will have to orient all lift in the same direction. In other words, all lift must be facing either clockwise, or counterclockwise about the rocket's fuselage. I intend on spinning the rocket due to research from "Apogee Rockets" that has claimed an increase in stabilized flight.
After it has been approved and you feel comfortable moving forward, it is time for the next step: Mathematical Modeling.
Step 3: Mathematically Modeling Your Rocket
Your next step is to determine the mathematically optimum characteristics of your fins to help achieve your desired goal.
- Do you want it to go the highest?
- The straightest?
- Do you want it to spin?
Using the Barrowman Equations sheet provided, decide the sizes for your fins in all areas of modification. An Excel sheet can help with this, and allow you to create graphs and charts for analyzing changes in dimensions.
If you need a copy of the Barrowman Equations sheet, I have provided you with one above.
Important characteristics to keep in mind for this portion are:
- Where is your center of mass?
- Seen above, my center of mass is located approximately 250mm from the tip of the nose.
- Where is your center of pressure?
- Based on the design of your fins, the location of your center of mass will only change a little. Approximate this point to be placed centrally compared to the fins. You will more closely locate the the center of pressure once you graphically model the rocket.
- How is drag changed as I alter my fins?
- Drag significantly increases as the thickness of the fins increases. As more air is displaced from
While the center of mass can be found in the Solidworks program, an estimate of the center of pressure can be found using your Barrowman equations. You want to keep your center of pressure (CoP) below your center of mass (CoM). You can research how far below is optimum. While I read that it is best to keep the CoP between 1x and 2x your rocket diameter below, your may find more or less to be beneficial.
Step 4: Graphical Model
After mathematically modeling your rocket, you will be required to create a graphical model with your designed fins. This is where our computer with Solidworks will come in handy.
First, download the rocket parts provided. These will contain every part needed to create a testable rocket, including pre-made fins.
Access the "balsa_fin" filename from the parts provided. You will change this to match your designed fin.
*NOTE: It is suggested that you orient the fin in your drawing with the profile on the frontal plane, and the attachment side (The side that attaches to the rocket) on the left. Observe the picture provided if you are confused on the orientation. While this is not required, it is suggested to help with a later step.
Next, when saving, overwrite the previous "balsa_fin" file with your new fin. Open the "rocket_assembly" file and click 'rebuild'. You will receive 2 errors involving mates.
To solve this you must delete these relationships, along with the circular pattern, from the "Mates" list under the feature tree.
*Do Not try to change the existing mates. Problems have arisen with attempting to do this.*
Once you have attached your fin in the appropriate location, you can create a circular pattern using as many fins as you desire. For mine, I used three fins due to research claiming proportionally higher drag values.
Step 5: Flow Simulation
This is one of the coolest parts of this project. You will now run a flow simulation on your graphical model. Under tools, you will find "Add-Ins" toward the bottom in Solidworks. This is where you will find the "flow simulator" ability; Check this box to open this section of the program.
Follow the steps in the flow simulation wizard to construct simulated air travel. After you have set all conditions and boundaries, run your animation.
Once you have animated the flow simulation, alter the scale on the left side by removing values that cannot be seen. The maximum and minimum will get closer together until you can see changes along the body of your rocket. Viewing the model, the color changes indicate changes in the pressure of the air as it passes over the rocket. Ignoring the changes of pressure around the fins, the pressure change located towards the bottom of your rocket is the center or pressure. As seen in the picture, my center of pressure is located below my center of mass. If these are switched you need to alter your rocket.
Step 6: Constructing Your Fins
Once you have completed the flow simulation, ask yourself if you feel comfortable building the rocket and fins now. If so, acquire whatever material you chose to make your fins. For this project, I have chosen to use fiberglass to construct mine. If you chose this material as well, here is how you can create your fins:
- Cut a 2 foot section of fiberglass mat the total length of the roll. Cut this section into at least 4 equal parts. *Note: I suggest using at least 6. I used four and, while it worked, it was hard to accurately sand the fins to achieve my intended goal.
- Spray a heavy coat of mold release on a flat sheet of plastic or other object that can be discarded. You want to make sure that every spot of this plastic is covered with mold release. Allow this to dry for 10-15 minutes.
- Place the plastic sheet down on a flat surface with the mold release side facing up. Uniformity and flatness assist greatly in the creation.
- Place one sheet of fiberglass on the plastic.
- Cover with epoxy and hardener mixture. Make sure the coat is thick and even.
- Allow to dry for 3-5 minutes.
- Place another sheet of fiberglass on top of the previous with the grain oriented 45 degrees to one direction.
- Cover with epoxy and hardener mixture. Make sure the coat is thick and even. Also ensure the fiberglass sheets are flat and without creases.
- Repeat steps 7 and 8 for the remaining layers.
- You can choose to place another plastic sheet, covered in mold release, on top of the fin, along with a heavy flat object. I did not do this, and one side turned out with a slightly rough surface that required sanding.
- Allow the fin to dry for at least 48 hours.
After you have completed the fabrication of your fiberglass, sand the whole sheet keeping in mind the orientation of your fins. This is to keep each fin uniform with the others. Afterwards, outline your fins and cut them out with a scroll saw. (You can choose to use a band saw, however the scroll saw provides a cleaner cut requiring less sanding and less frayed edges.)
*Optional*: You can coat your fiberglass fins in another coat of the epoxy and hardener mixture after cutting to give your fins an extremely smooth surface. This is not required, though.
Step 7: Building Your Rocket
Caution: This is the part you've been waiting for. It Is Here.
Yes, it is time to build your rocket. Follow the paper instructions included in your rocket kit carefully to construct your rocket. Remember that the cleaner it is, the less drag your rocket will have. You want to have lines and ratios perfect. Attach your fins using whatever glue you desire. I used a gel-based superglue purchased from Home Depot. It provided a strong hold and allowed me to smooth the bead into a rounded edge. This is not required, though; you can stick with classic Elmer's glue should you choose to.
I also chose to coat my rocket in paint to help smooth out any rough edges. This is also not required, however, I suggest it to help with any small imperfections and help seal the fins to the fuselage.
After your rocket is constructed and dry, make sure to weigh and record the mass of your rocket.
Step 8: Launching Your Rocket
The time has come.
On a date with little wind, and average conditions, set up your launch area. Pay close attention to any instructions provided for how to pack your streamer and load your engine. We completed 3 runs (or more if possible), each with a different engine. If you run into any launch issues, make sure the wires of your ignition don't cross. This will cause the circuit will short and your engine will not ignite.
While launching the rocket, we were able to use two students with tools made for finding altitude. The recordings gathered are found on the following page.
Step 9: Record Your Flights
Make sure to obtain your flight records from whoever is keeping track. You can compare these with the trajectory equations used in your mathematical model. You will find that the A motor is the easiest to predict. With the power increasing as you go from A to C, the motors get less predictable. This is due to a 10% possible fluctuation in fuel capacity. (in both directions)
Overall, we achieved lower maximum heights as a class due to, what we believe to be, high wind conditions. However, I achieved a much higher first flight compared to my initial calculations. I suspect this was due to the lighter weight of my fins. I believe that the mass calculated was higher, along with the thickness also ending up thinner than planned. The combination of these characteristics drastically reduced weight which helped my thrust to weight ratio.
For my flights, you can see in the photos that my A-engine launch was much higher. This was due to a drastic turn my rocket took mid-air. Both height readers lost the rocket mid-flight and estimated height achieved.
So, what do all of these findings mean for education?
This project has shown me the value of testing. While you can plan, model, and calculate to consider almost every variable, there is no definitive way of knowing exactly what will happen. Many of my classmates' models and concepts worked theoretically, while veering in random directions and somersaulting through the air. If you are an educator, consider that the most applicable form of education is the one that you actually make yourself.
Step 10: Cite Sources and Produce Documentation
This project is heavily graded on documentation. For this reason, it is important that you note all steps taken throughout your process. Show results to your equations, explain your process for building, and why you chose the design you went with. Ensure that other sites or publications used are cited for this documentation; It is important to give credit where credit is due.
Last but not least. Make your presentation fun. Remember your audience and show how you have enjoyed each step.
Websites used for research:
Milligan, T. V. (2010, February 10). Why do spinning rockets fly straighter. Retrieved from Apogee Rockets website: http://www.apogeerockets.com/downloads/Newsletter...
Nakka, R. (2001, August 26). Fins for rocket stability. Retrieved from Richard Nakka's Experimental Rocketry Site website: http://www.nakka-rocketry.net/fins.html
(2011, November 30). Rocket aerodynamics. Retrieved from Science Learning website: http://www.sciencelearn.org.nz/Contexts/Rockets/S...