Rocketry-fin design

[doreen1020]

My project is How does fin design affect altitude performance. I am designing 3 different fins based on shape. I'm in 8th grade & my goal is to advance to county. I'm looking for ways to advance this idea & so I need expert guidance! Can you offer suggestions/ideas & guide me to what else I should include regarding aerodynamics? Please help! Thank you

[norman40]

Hi doreen1020,

Welcome to the forum.

I think it's great that you want to follow up your project from last year with some additional study! To help with defining what to do next you might consider these questions about your work from last year.

What would you have done differently if you repeated your project?
What was surprising about your results and how would you investigate any unexpected results?
Was there anything from last year's work you'd like to improve?
What other information about last year's project would you like to learn?

If you could post a summary of the procedures and results from last year's project, we might be able to help with some more specific guidance.

I hope this helps. Good luck with your project!

A. Norman

[doreen1020]

Thanks so much for responding! I don't believe my project for this year is a continuation of my previous project on rockets. This year, I am designing 3 different fin designs to test how the 3 different shapes affect altitude performance. What I am looking for is suggestions of how to make this idea more advanced & other than calculating the rocket's altitude performance on each fin shape, which i figure out based on the rocket's descent rate, what else should I calculate or consider?

[norman40]

Hi doreen1020,

You're most welcome.

I'm certainly not an expert in rocketry so maybe other experts on the forum can add some more specific advice for you.

It seems that you already have a method in mind for measuring altitude performance based on descent rate. One idea would be to try other techniques for this measurement and compare them for accuracy or repeatability.

Another idea is to calculate the center of gravity for rockets equipped with each of you fin designs. You might try changing the center of gravity with different fins and fin locations. Then test for the center of gravity effects on stability and altitude.

I hope this helps. Please let us know if you have more questions.

A. Norman

[audreyln]

I definitely this rocket design has lots of different variables to test. Investigating the center of gravity of each rocket is a great idea, you could vary this by changing the weight of the fins. Additionally you could look at how straight the trajectory of the rocket was (although it will be difficult to control for wind speed/direction).

Testing more rockets and looking at the repeatability of each fin design is a good way to add more complexity to your project.

Finally, you could do some additional background research on rocket design and aerodynamics for inclusion in your presentation.

Good luck!

Audrey

[theborg]

Doreen1020,

A really neat project. A couple of things to keep in mind and perhaps use as a metric for comparison. 1) center of gravity (COG). This is the point where half the total mass of the rocket is on one side and half is on the other. Although it's already been discussed, keep in mind that this is the point through which thrust will act. 2) center of pressure (COP). This is the point where half the total surface area of the vehicle is on one side and half is on the other. It is the point through which drag forces will act. For stable rocket designs COG needs to be above, closer to the noze, the COP, which should be closer to the tail of the rocket. The closer they are together or if they are reversed, the rocket will not fly straight. Since you are testing fin designs, try to hold COG, overall weight of the rocket and thrust constant and measure the difference in the location of the COP with different fin sizes. What you should find is that larger fin size will produce a more stable rocket, but increase aerodynamic drag. How this translates into altitude difference will be a good result.

[theborg]

Here are a couple of links that provide a good explanation of the concepts of COG and COP.

http://exploration.grc.nasa.gov/educati ... tstab.html

And

http://exploration.grc.nasa.gov/educati ... rktcp.html

Basically, you want most of the mass of the rocket near the noze and most of the surface area (helped through the use of fins) near the tail.

[bradleyshanrock-solberg]

Speaking with my other hat on as a Science Fair judge, the one thing you can do to help your chances the most is to run enough trials that your tests have some statistical meaning.

If you test 3 fin types, but do only one test of each, we don't really know if the fin caused the variation in performance, or if the variation was caused by something else. This is especially true on something like a rocket where the same configuration may well have significant differences in how it behaves without changing anything.

If you do 10 trials each (30 total) and each fin type behaves very similar to itself (all ten trials have close results) and each different fin type shows significant performance differences, you can be pretty sure the fin changes are the reason, and not something else.

At a minimum you want three trials for each configuration, with everything else as exactly the same as you can manage. The difference between one trial and 3 is huge from a statistical standpoint (you'll have to trust me on this, the math is not something you'll be taught until late high school or college, but you need at least 2 trials to know anything about whether your experimental procedure for a given fin type is consistent at all, and each trial after that makes it more and more likely that your results are "real" as long as all trials for a given fin type are more like each other in results than a different fin type). Consistent results within a trial is fairly good proof that your entire experimental procedure is sound, and you're controlling all the other variables well.

[doreen1020]

Thank you SO much for all of the feedback! I designed &constructed my 3 fin designs(rectangular, swept, delta), completed 4 trials of each design & results were consistent! I have a few questions I was hoping to get some help on:
1. I did the "swing test" to ensure stability & then post flight, calculated distance from the launch to its landing(learned that a more stable rocket will land closer to its takeoff/launch point) Is there a better way to test stability that could be graphed?
2. Also, to take this a step further, after getting my results, I made a larger size fin with the swept design (which had the highest altitude performance & greatest stability out of the 3 designs)to test how the fin size of that rocket design would change altitude performance/stability. Is this too much to include in this project in addition to the 3 designs? or is it good to add this( changing fin size on that 1 design)?
3. What would my control group be?
Thank you in advance for helping!!!!!

[bradleyshanrock-solberg]

First, very good job. It sounds like you're well on your way.

1. For a rocket, the true measure of stability in my mind would be whether it flies in a straight line (assuming no winds, etc), so you're on the right track with your attempt to measure distance from launch point. The trouble with this approach is a cross-wind could affect the outcome, and the parachute drop also could make a difference (a big parachute might drift further than one that lets the rocket drop faster, given even very mild wind conditions.

The most obvious way to improve the accuracy of this result would be to aim the rocket at a target, close enough that it flies on a more or less flat trajectory. Please don't go and do that, as the safety risks are pretty obvious - that's a fire hazard, might damage whatever you hit (or injure somebody), not to mention the high odds of damaging the rocket. What usually looks good in a science fair project is outlining how you would do better in the measurement if you had perfect facilities - eg, if you COULD do your test in a building with no wind and very high ceiling you could then aim the rocket at a target in the ceiling and see how it drifted. The judge will understand that such a test is beyond your resources, but will give points for thinking it through (or at least I would, if I was judging your project).

2. Nobody is going to penalize you for doing more work than you originally intended. The way to present such results is to describe how the first trials gave you the idea to make the new modification, and you did a second question/experiment to see if your new theory proved to be true. Do the same 4 trials and methodology you did on the original rocket with the new design. If you can somehow adapt the change to more than one of your old designs, that could also be interesting, to see how the change improved (or not) each other design, but now I'm doubling your work, so don't feel you need to do so. It does seem like one consequence of a larger fin would be more overall weight on the rocket at minimum, and maybe more drag - it would be interesting to see what happens - a possibility is something like more stability and less altitude and eventually a huge fin would obviously be more of a problem than a help. But you may not quite yet be in the "sweet spot" of best performance.

3. I'm probably committing some kind of Science Buddies sin saying this, but not all experiments have a control. The control group as a concept depends on your question. For example, if one of your designs was unmodified, like from the basic rocketry kit, and your question was "would another fin design do better" the original design would be the control. If instead you are asking something along the lines of "which fin design is best" with no idea of what "normal" is, the concept of a control doesn't apply - you try them all, enough times to be sure of consistent results, then crown the best the winner. In your second "step further" case, the original fin design that you've already tested is the control, and the new design would be the variable - you are saying "Ok, this is how the best design currently behaves, my question is is it better or worse if I make this change".