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Ring Tail Issues in RockSim

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etsakimoto

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I picked up a couple of parts from a free parts bin at my local launch, and want to build a ring tail rocket with them. My current plan has a 4 inch body tube and a 5 inch ring tail that is 1 inch long. However, I am dubious of RockSim's accuracy on the matter. Adding a ring tail to the rocket makes it stable with zero nose weight (which seems iffy, given how short the rocket is), and perhaps more worrying is that the ring tail takes ~4000 feet off of the flight simulation altitude. I know ring tails are draggy, but this seems excessive.

So, would this rocket be stable, or is RockSim just being weird?

Rocket details:
4 inch tube, 45cm long
38mm motor mount


Thank you
 

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Dad Man Walking

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OpenRocket doesn't support ring fins, so it stripped those parts out of your design file and I'm flying blind here.

That being said, I have had decent results in OR simulating tube fin rockets by just representing them as rectangular fins with the height equal to the radius of the tube. Six tubes of BT80 = six rectangular fins that are 8" tall. Which is a whole lotta drag. A friend who built a tube-fin rocket for his L3 cert says that at higher speeds (transonic+) the tube fins start to behave like solid cylinders, but our tube fins were for TARC rockets that are way slower.

For yuks, you could try the the same thing -- model the ring as 3 or 4 rectangular fins with a total surface area equivalent to the ring, and add whatever support structure you are using (also stripped out by OpenRocket). See if that gets you close to the RockSim ring fin result.
 

David Schwantz

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I have a 16" dia, 44" long ring tail in Rocksim. It gives the stability as 1.73 with 36 oz in the NC. I at least HOPE it is right. But my ringtail extends past the rear transition by about 20".
 

kuririn

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I have a 16" dia, 44" long ring tail in Rocksim. It gives the stability as 1.73 with 36 oz in the NC. I at least HOPE it is right. But my ringtail extends past the rear transition by about 20".
Have you tried removing the ringtail and only using the supports for fins then running Rocksim simulations to see what difference it makes in the apogee?
 

David Schwantz

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No I have not. there was no need as it would never fly without the tail. This rocket relies on base drag only. What I had intended by my post was to the OP that I believe that Rocksim is correct in its simulations. I will let you know for sure when we get to fly again.
 

David Schwantz

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I'm guessing Rocksim is accurate. Take a look at tip #2 on this webpage:
http://store.heavenlyhobbies.com/53-021-7828.html
It's saying that removing the ringtail will double to triple the apogee.
I went and looked at the post #2. Mine is nothing like what is in the post. I have no fins and my ring tail is flat. Should be a lot of drag. I have a very large transition 20" in front of the tail.
 
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