Help for a science project.

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Rockyt

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Hello all, and happy friday.

I am in need of a bit of assistance, my son is doing a science fair project on the impact of fins on a rocket's altitude reached. We have chosen the Apogee Avion (for a number of reason..not the least of which being hometown business) and have a solid plan, BUT, we are a bit lost on materials appropriate for a 4th grader to understand what is going on. The school is big on experiential learning, so the process is a big part of it (for this reason, my wife and I have decided to let the weights be variable aka, letting him realize more fins = more weight), so we are looking for materials that help him learn without spelling out what happens as fin # increases.

Any thoughts on where we can look?

just to answer some questions brewing in y'all- :)
The plan at this moment is to make 5 Avion's: versions with
2 fins,
3 fins,
4 fins,
5 fins, and
6 fins.
launching each 3 times, with a Jolly Logic altimeter on B4-4 (i'm thinking...A8-3 is enticing, but seems dangerous at higher fin counts, and our launch field isn't huge). Fire department has been contacted, and no permit needed at this time, just a Type A fire extinguisher, permission for field (easily obtained), and a courtesy call to local police as a 'headsup.' We'll launch as many as possible at a time to keep conditions stable as possible.

Charting the differences, and weighing each should help give him some decent results that, hopefully, are supported by research, rather than vice versa. Now I admit there are a few too many variable that can be at play here, but the idea, as stated earlier, is the experience, and since he came up with it on his own, and the teacher approved, we're running with it.
 
i'd skip the start with 3 fin and go up so you don't have stability or assymetric drag issues. You need to balast so you have constant mass between versions. You could save a bit of $$ by doing an 8 fin and six fin and then break off fins to get a 4 fin and 3 fin with the same airframe with some minor added balast to comprehend the fin mass.

Frank
 
@berkefj...
The 2 fin version is to test stability....the lack of ballast is to get him to correlate weight as a secondary variable that may effect altitude. Again, the activity of exploration, not necessarily application. Plus, his experiment..his ideas, his rules. (calling me a hippy that way.��)
@mccordmw...saw that, they have a nosecone too, but he was already approved for # rather than shape...so we are at a bit of a disadvantage on that. You k ow, hindsight and all.
 
As long as you know how to calculate stability to verify you aren't launching something unsafe. You said you were focusing on drag differences versus fin count. If you don't balast them the same it isn't a valid experiment with a single variable.
 
You should not need to test stability by launching. If you have a wind tunnel, that is an option, but launching something that has a high likelihood of flying in an unpredictable direction is unsafe.

You could consider card stock for the fins, which would allow relatively easy and consistent mass manufacturing. CA can be wicked onto the edges for additional hardness.

Unpainted will avoid issues with different drag levels caused from factors other than fins. If you want a finish, markers don’t change surface texture as considerably or variably. Oh, and did I mention the not having to paint part?
 
A 2 fin rocket will not be stable so you should not launch it.
 
I would suggest that you download OpenRocket, a free program, and run simulations of all the different versions of your build. If it turns out to be unstable, don't launch it.

Several people here can help you learn how to use it if necessary.
 
You click on the nosecone, body tube, and trapezoidal fin icons then enter numbers for dimensions and masses. You add parachutes and shock cords then wadding into body tube. You can set a body tube as a motor mount. Or you can have centering rings with another internal tube as a motor mount. You select the Estes A8-3. You set your fin profile by root,sweep,span, and tip lengths. Normally rockets like CG ahead of CP by a stability margin ratio of 1.5 to 2.0. You may want to add some lead shot or change a fin shape/size. If you make triangle fins it's easier to make. I'd make a bunch of same type of fins to keep experiment consistent. You can sim for apogee altitudes and max velocity and even get excel print outs of many parameters.
 
Trapezoidal fin profiles were slightly more stable than triangles. The CP position will correlate to the fin area and profile relatively. You can have a column of root, sweep, span height, and tip length in excel then multiply these values by a scaling number usually 1.1, 1.2 etc and start with span height near the tube diameter then you increment your fin design. It's not supersonic so I'm avoiding a the oblique shock waves and a Mach angle formula to make life easier for you. By adding more fins you increase the forward plate area and drag force increases. The rockets may want to weathercock into wind direction when stability margin exceeds 2. It will depend on launch rail exit velocity lower than 45 ft per s or wind velocity. There's a rough by word use on open rocket. Someone else can explain it better. Ch.5 Wing Design in Google has formulas for a the Mach angle sweep on fin design but I thought you don't need that for your application. Heck you could use a rectangle fin if you wanted.
 
So you could use your open rocket sims as a theoretical comparison to an actual altimeter recording then take a percent error of the data. Then you would have quantifiable results for this science project assuming your kid is doing it for school or whatnot. You graph that crap and answer a hypothesis and kid gets A. Lol. Seems doable.
 
When you look at the front end of a fin looking down the nosecone of rocket so it appears as a circle head on with thin plates, you have cross sectional area of the fins and the rocket's body tube. This cross sectional frontal area is used in a drag force calculation in a college level fluid mechanics class. Basically the more fins you use you gave more drag force. The density of fluid and velocity of rocket affect it. The open rocket will do its best to have a guess of a drag coefficient which also affects the drag force. Open rocket will automatically output the drag force into excel. However manually it's one line of algebra. There's a bit more constants you pull from fluid mechanic textbook tables or otherwise have experimental data.
like a wind tunnel test of the drag coefficient for that specific object. So you have Force drag = Cd*ro*A*v^2/2 But Cd is usually the reference table or experimentally found drag coefficient. Ro is fluid density. Area is cross sectional area of circle plus fin area (frontal) times number of fins. V is velocity. A softball has about Cd 0.43... It is a rough approximate for a 29mm diameter, half yard long rocket going M1.7 also... My Cd was slightly less on a senior design rocket project. Cd is tricky to find. Wind tunnel or computational fluid dynamics analysis on a CAD software. Or you ballpark a value from a text of similar objects. You could use the value OR gives you. Hopefully I didn't confuse crap out of you. You might have a grasp of how to explain it. NASA has more info on drag force equation.
 
You should not need to test stability by launching. If you have a wind tunnel, that is an option, but launching something that has a high likelihood of flying in an unpredictable direction is unsafe.

You could consider card stock for the fins, which would allow relatively easy and consistent mass manufacturing. CA can be wicked onto the edges for additional hardness.

Unpainted will avoid issues with different drag levels caused from factors other than fins. If you want a finish, markers don’t change surface texture as considerably or variably. Oh, and did I mention the not having to paint part?

Wind tunnel gets a drag coefficient. You can get a lift coefficient on bodies or airfoils too. That's a minor detail for one drag force. I would not worry at fourth grade level. Open rocket will do its best. Open rocket is plus minus ten percent, because of this. Stability is CEnter of gravity and center of pressure. Center of pressure is barrow mans equation and area related. Center of gravity is where rocket balances. A CFD program is cheaper than a wind tunnel. It was three weeks after I sent a three d model in at university. That level of detail is completely not needed for this project. If he compares altitudes recorded to altitudes predicted by rocketry program I think he will be okay if he can conceptual explain how drag force increases with more fins. The loss in altitude explains the higher drag force of adding more fins without having to calculate it. An increase in weight is only partly right. I have an approximate drag coefficient provided incase he needs to manually prove it. If he needs a source say it's roughly close to University Tennessee Chattanooga USRC SEDS drag coefficient from a project rocket tested by Dr. Sreenivas a hypersonic flow expert from Linux CFD. A fluid mechanics textbook has this value approximate a softball in air. It's not close to his scale of rocket but it's a way for him to finish the equation so the kid does well if they expect that. Open rocket will do its best. Your other points are very logical. Not many people understand what wind tunnels are for so I explained. You scale a prototype model to a full scale by unit less numbers in fluids before you build the real thing with similitude but that's way way off topic.
 
Andrew, buddy; that's five in a row again. Also, this is a kid's science project.

To echo previous advice: don't let the kiddo fly with only two fins, and do weigh all the fins. Should be a fun and attainable science project!
 
Well he doesn't need to calculate the drag force. The kid can explain how it works and decreases altitude as additional fins are added. Word a hypothesis like how does number of fins affect rocket performance? Then have predicted altitude versus time from open rocket per fin configuration export to excel. Then have real flights two per configuration with altitude recorded by altimeter versus time observed to altitude from launch to max alt seen. He could average the flights per configuration. Explain how increases in fin count adds additional weight force from mass increase and also additional drag force from increase in area. Maybe not calculate that part... Hehehe. Anyways the forces of drag and weight will counter a consistent thrust limited force therefore the conclusion will look like increasing fins decreases altitude for logic stated above and by these results we have experimentally tested against our predictions. If kid follows scientific method and can explain it decently kid should get an A. Anything extra is well icing or extra learning.
 
The only way I'd consider a 2 fin arrangement is if the fins were large and formed a classic Star Wars "TIE Fighter" arrangement. Even then, I'd still check it for stability before even thinking of launching it.
 
Ok, so 2fnc may be bad, got it. I'll research openrocket before we go there.
But, I am still in need a resource or two to help him identify the weight stability issues, etc. That isn't speaking above him. And just to reiterate, this is about discovery, so realizing more fins = more weight is a light bulb he can address in his experiment write up.
 
Ok, so 2fnc may be bad, got it. I'll research openrocket before we go there.
But, I am still in need a resource or two to help him identify the weight stability issues, etc. That isn't speaking above him. And just to reiterate, this is about discovery, so realizing more fins = more weight is a light bulb he can address in his experiment write up.

I have a two-finned rocket that flies great, but the fins are canted. The spin induced by the cant keeps it stable - not what you were planning, but I thought I would mention it...

IMG_2671.jpg


Sent from my iPhone using Rocketry Forum
 
Before launching, you can run a lot of scientific tests using a swing test. You can test:

CG adjustments
Fin count (including 2 fins)
Fin shapes
Nosecone shapes

The ones that are stable can be launched, and you can test altitude as another outcome.

The swing test:

[video=youtube;3S7_fg6ZCF4]https://www.youtube.com/watch?v=3S7_fg6ZCF4[/video]
 
Here's another great video on what makes a rocket stable. Good info to include in the report.

[video=youtube;AhS5fT_EE6M]https://www.youtube.com/watch?v=AhS5fT_EE6M[/video]
 
Someone give him the link to Neil_w's easy OpenRocket installer.

I'm on my phone otherwise I'd have done it.

Hope your boy has fun with this!
 
Videos look good, thank you.

Great idea Nytrunner,...but played with it this morning.

FYI...2FNC test at 60 ft altitude. Better than expected. String test will help.
 
Ok. So conceptually the kid is lost on drag force and maybe physics. Try to draw a free body diagram a picture of a rocket having a lift force from thrust as up arrow with weight and drag force arrows pointing down. If kid can understand changing angle of attack of hand stuck out a car window at interstate speed has more "felt force acting on kid's hand" by the drag force (with air resistance) increasing as frontal area of hand increases relative to velocity of hand moving through air. (The velocity of car relative to a static ground). When hand is parallel to wind smallest area. Hand perpendicular maximum area. He will feel the force change as pitch increases when rotating hand and have fingers touching in a wing shape. If you drive the car even faster the drag force is also greater. On the rocket the area of the fins increases with the fin count increasing which increases drag force by changing the Area value in the equation previously posted. The same equations govern both a hand or a fin. You can tell him a drag coefficient is just a number and its pixy dust without a wind tunnel. I suck at sarcasm. NASA should have some K-12 topics and those might help. It's algebra and units but if kid doesn't have the generic idea of it that can be the hardest thing to grasp. So you may have to try to word drag force or do a simple example into terms your kid can understand. I tried explaining the equations to you but I don't want that to go over your kid's head like previous post. If he can see it and feel it then it's more tangible than an abstract math equation likely at kid's level.

if can't understand relative velocity on kid level. Have kid sprint on a treadmill. No drag force (zero velocity) relative to ground. (Stationary). Then have kid sprint not on treadmill with felt drag force. (Relative velocity compared to stationary ground).
 
That's about as simple as I can explain fluid mechanics drag force equation, physics, and relative velocity without doing the algebra for you which would likely be meaningless garbage to a fourth grader. And I'm not trying to insult anyone or make it over complicated but if the kid can sense, see, feel a similar relationship of physics then it may make more sense such as the weight concept kid already grasps.
 
Quick update:
We built and flew 4 Apogee Avions:
3, 4, 5, 6 fin versions.
Flew all on B4-4 engines, recovered all, ran 3 sets of flights, but the 6 fin version did not survive past 2nd round; It lawn darted at a very awkward angle, bent body tube not air worthy. Acted as expected, otherwise. OR was great for creating fin alignment templates, and I gained respect for the estes fin alignment jig. He presents on thursday so we'll see. I'll keep you all updated.
 
Quick update:
We built and flew 4 Apogee Avions:
3, 4, 5, 6 fin versions.
Flew all on B4-4 engines, recovered all, ran 3 sets of flights, but the 6 fin version did not survive past 2nd round; It lawn darted at a very awkward angle, bent body tube not air worthy. Acted as expected, otherwise. OR was great for creating fin alignment templates, and I gained respect for the estes fin alignment jig. He presents on thursday so we'll see. I'll keep you all updated.

Good opportunity to learn about outliers.
 
A 2 fin rocket will not be stable so you should not launch it.
If you are adding ballast, to keep the mass of each rocket the same, the mass added into the two-fin rocket if added at the nose will likely make it stable. If you are going to test it go for at least double your minimum required launch distance for added safety.

Watch out for maximum launch weights so you have enough speed off the rod.

Remember that this is an experiment. If there are some failures they are part of the results too :) .
 
Dan, That is an ugly rocket.

I had an old NACA report that presented all the math to design 2 finned projectiles, but I lost it. It was an old war era report. Air dropped bombs can be packed more tightly with two fins than with three or more fins. The NACA guys figured out how to do it. However, the two finned bombs had more dispersion, so they were never used operationally. I'd really like to read that report again, but I can't remember the report number or the title. If someone would tell me the NACA Report number, I could download a copy.

Thanks, Alan
 
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