Fin Size Issues in Open Rocket

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SMarshall

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Hi all,

I am currently designing my own level 2 rocket and I am having issues finding a proper fin size that works.

I have read that generally you want your fins to have a root length of 2 calibers, a tip length of 1 caliber, and a height of 2 calibers

When I add fins of this size to my design in Open Rocket, my stability is way too high (over 5 calibers)
upload_2018-11-10_18-2-10.png


The only way to get a stability in the 1-3 range is to have fins that seem pretty small (see below)
upload_2018-11-10_18-3-26.png


These fins have a height of 1 caliber, which gives my rocket a stability of 2.36 caliber. My question is whether or not the latter fin size is a problem. With small fins, the rocket has a decent stability. This just goes against what I've seen on the forum about typical fin sizes.

I have attached the ork file if that helps at all.


Any tips would be greatly appreciated, thank you so much for the help!
 

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Ben Martin

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Hi all,

I am currently designing my own level 2 rocket and I am having issues finding a proper fin size that works.

I have read that generally you want your fins to have a root length of 2 calibers, a tip length of 1 caliber, and a height of 2 calibers

When I add fins of this size to my design in Open Rocket, my stability is way too high (over 5 calibers)
View attachment 366487

The only way to get a stability in the 1-3 range is to have fins that seem pretty small (see below)
View attachment 366488

These fins have a height of 1 caliber, which gives my rocket a stability of 2.36 caliber. My question is whether or not the latter fin size is a problem. With small fins, the rocket has a decent stability. This just goes against what I've seen on the forum about typical fin sizes.

I have attached the ork file if that helps at all.


Any tips would be greatly appreciated, thank you so much for the help!
I just flew my L2 with 3in span fins on an 3.25in diameter rocket. They performed perfectly and the rocket flew straight as an arrow. Stability margin of ~2cal is perfectly fine for long rockets like yours, in fact it's often recommended to have that high of a stability margin due to the CG/CP shifts that long tubes cause. Of course others may have opinions that conflict with mine, but I'm going off of my personal experience. I had the same concerns as you.
 

jjwb22101

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Those rules of thumb about fin size are really just a starting point, and generally result in grossly overstable rockets. The concern with making the fins too small is that, while the rocket may be statically stable, there's possible dynamic stability issues. Namely, the restoring force (while in the correct direction, which is all that static stability guarantees) may not be large enough to overcome flight asymmetries (rail drag, off-axis thrust or weight, etc), but I've never actually seen this happen except in a questionable design I made years ago for TARC, where I had something like 30% of the weight of the rocket stuffed in the nosecone, and didn't want to have to worry so much about weathercocking (that was a really questionable rocket...).

The bigger concern I see with this rocket design is how the HECK you plan to build a 4" diameter L2 rocket that weighs over 30 pounds! I'm working on a 4" diameter (and, quite frankly, WAY overbuilt) L3 project that's going to weigh about 25 lbs on the pad. I see you've got a payload right ahead of your motor - can I ask what that is?

Edit: Took a look through your OpenRocket file. Generally pretty good, but a couple things I noticed - If you're not planning to wind your own CF tubing, the standard size is 3.9" ID 4.024" OD (not 4.0"x4.066" and even if you are planning to make your own, it might be convenient to make it a standard size in case you need to buy things like centering rings, bulkheads, or couplers).
Centering rings - I'm a bit concerned by the thickness. 5/64" seems a bit thin (even for CF - the issue is having enough surface area for a strong bond to the airframe) for such a high-thrust motor, especially since you aren't using an appreciably thicker thrust plate to transfer the force to the airframe. I'd suggest moving to at least 3/32" centering rings, and perhaps getting/making a thicker thrust plate.
Motor Retention - I don't see it? that 16 grams in the thrust plate on the end sure isn't going to cover it.
Payload - Ah. There's the weight. I'll tell you right now - no payload in that volume is going to be 10.5 lbs. Simple math: the density is like 1.3 g/cm^3, so it's 30% heavier than if you filled that whole payload volume with water. Doesn't seem likely. I'd guess maybe half that at most if you've got a really dense payload (lots of batteries would probably be the biggest reason why)
Recovery - You may want to think about how you plan to pack an 8' diameter parachute into 16" of 4" diameter tubing, along with protection and the shock cord.
 
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Handeman

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Looking at how much your center of gravity changed when you made the fins slightly smaller, I would go and check weights of items. It may be the CG that is way off and not the CP.
 
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