Introduction: Custom-Made Model Rocket Fin
With this Instructable, we will go through the design and creation process to create your very own rocket fin. For this tutorial, we used a 20mm diameter rocket with an ESTES A8-3 engine. This tutorial can apply to most any model rocket.
These steps are designed to take you through the Engineering Design Process (EDP) and are organized as such:
(To learn more about the EDP, go to Teach Engineering)
So along with a sweet custom fin, you'll also get a lesson on the EDP; two for one!
Step 1: Ask
The EDP is used to create a solution to a problem. The first part of of the EDP is to find the problem. Along with the problem, we need to identify the constraints. Constraints are boundaries within the problem. Two example constraints are: being allowed to only use a certain material or needing to maximize certain criteria about our design to have the best solution.
A problem is a need that requires a solution. Designing a custom fin usually means that you don't think the ones included will work the best. This is our problem: the fins provided are not up to our standards. Next, we need to research our problem to know how to create an effective solution. This great webpage on a New Zealand Science Hub site explains that rocket fins alter center of mass (where all the weight/gravity forces meet, i.e. where the rocket balances) and center of pressure (where all the air pressure forces meet). Fins create stability by separating the center of mass from the center of pressure by lowering (in height) the center of pressure closer to the tail of our rocket.
This means that bigger fins (fins with a larger surface area) have better stability, but bigger fins also create more drag. Therefore with larger surface area, the fins cause the rocket to take up more space in the air and thus have more air to "fight against" as it ascends.
So the problem is coming up with better fins than provided and the constraint is that we have a trade-off to deal with between drag and stability when we try to design the best fin for our rocket.
Step 2: Imagine
Here's where we come up with designs for the fin. Simple sketches will do. We will attempt to brainstorm simple shapes so we can see how our constraint works with our designs. Figure 1 contains some of the sketches I came up with. The ones on the left are designed to reduce drag by having a low profile while the ones on the right expand on that and add more surface area to increase stability. I went with something of a mix of the two on the right. I like the retro-futuristic look (Buck Rodgers style for example) and the cut away at the bottom keeps it from being melted by the engine. The curved edge also streamlines the fin for less drag and provides more surface area for more stability (more on that in the next step).
Other than working with our constraint, you can really sketch any design you want, so go crazy! Make YOUR custom fin.
Step 3: Plan
Here's where we test if our fin is a good answer to our problem or not. Draw up a detailed diagram (I used SolidWorks because I'm in college and its free and I can do all sorts of math to it, but stuff like Google SketchUp works too) and start applying the Barrowman Equations. What are the Barrowman Equations, you ask? They are a set of close approximation equations that will help us calculate where the center of pressure is on our rocket to see if our fins will do well. Take the dimensions of your fin (in mm) and plug them into the equations to get the final distance from the tip of the nose to the center of pressure, simple plug-n-chug. After doing your equations, you can go back to your fin design and tweak it to get better numbers on the equations for better flight. If you want to do further equations to see how high your rocket will go and how long it will take to get there, you can go here.
If you have Solidworks (or other fancy simulation software), you can do a flow simulation to find the center of pressure as well as your center of mass. SolidWorks will also find your drag (as shown in the pictures), but this is overkill and drag can be "eyeballed". You can do this seeing how much it weighs, how much horizontal surface area it has (the sides facing the top of the rocket), and how streamlined it is. Remember our constraint has us wanting to minimize all of that. Your center of mass can be found when you build the rocket. Tie a string around it and adjust it until the rocket can balance on the string by itself. That is your center of mass.
Step 4: Create
Here's the fun part!
To create your fins, print a template (side view of your detail sketch 1:1 scale) of them and cut them out of balsa wood or any other desired material (I 3D printed mine, go nuts!). You also need to make sure you make the right amount of fins (mine had 4 because I thought the drag sacrifice was worth the extra stability) based on your equations and maybe an extra or two in case you lose on on launch day.one of mine fell off, as you can see. Good thing I had some extra glue.
To build the rocket, just follow the instructions that come with it, but instead of using the included fins, use your custom made ones! I suggest building or purchasing a fin attaching rig that will help you glue your fins in place because doing by hand might not give you the accuracy you need.
Now you get to launch the rocket. Set up the launch pad and follow the instructions to launch your rocket! Here's the extra part: to see how off or how well your calculations did, you'll need a friend or two. Measure, from a known distance (I used 300ft), the angle from the launch pad to the peak of the rocket's flight. Take that angle and do some simple trigonometry by using this equation Y = Xtan(A), with X being the distance from the launchpad and A being the angle to calculate the height of your rocket Y.
Step 5: Improve
To see how well you did, you can calculate percent error. Percent error is how far off you were from all of your math that you did versus the actual height of the rocket that you measured in the last step. To do that, let's walk through mine and you can just plug in your numbers. For a calculated height, I got 246.5ft and actually got a height of 210ft. Subtract the smaller number from the bigger number (in the actual equation you subtract the theoretical number from the actual value then take the absolute value, but this way is easier to explain). Then, divide by the theoretical value (the one we calculated) and multiply by 100. So 210 - 246.5 = (-)36.5 then 36.5 / 246.5 = .1481 then .1481 x 100 = 14.81% error. This is not bad, but the smaller the percent, the better.
We can also take this time too look back on things you could change to your fins to perhaps achieve a higher flight. For mine, I think I should have gone with three fins instead of four. The rocket did fly pretty straight and I think it would have flown just as straight with 3 fins, so less weight and drag with three fins might have added more height. I also might try a smaller fin. The look of my fin was nostalgic of the fat curves of "futuristic" rocket designs of the 1950's and I liked the aesthetic so I kept it, adding more weight and drag.
We also can look back on what we have learned through this project. I like projects like this because there is a simple, real-world goal and product to go with the education we got. We learned some basic aerodynamics, physics, and trigonometry all because we wanted better, custom fins for our hobby rocket.
That's it! I hope you enjoyed this tutorial and had a great time creating some sweet custom fins for your rocket!