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Old 03-02-2016, 07:30 PM
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LeeR LeeR is offline
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Default Open Rocket - Need Help with V-2 fins (tailcone issue)

I loaded a V-2 model I found in the Open Rocket desifn database, but it had no fins. I have created a fin profile using the freeform fin editor, but cannot get fins to drop from body tube diameter down to the tailcone. In the freeform editor, the graph of the fin shows a negative region on the graph, but points cannot be moved below the X-axis, or inward from the body tube to the tailcone.

I saw this issue raised by someone several years ago -- is it still an issue? Any workaround or suggestions appreciated. I'm pretty new to OR and have not had any issues building "conventional models", but this one has me stumped.
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Old 03-02-2016, 07:59 PM
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Rich Holmes Rich Holmes is offline
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I would love to be told I'm wrong, but I think OR simply cannot handle fins on boat tails. You can resort to trickery to make something that looks right, but it wouldn't sim right.
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Old 03-02-2016, 08:34 PM
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LeeR LeeR is offline
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Rich,

Thanks. I figured as much, but since I'm still learning, I was hoping there might be something I was overlooking. I thought about doing a stepped tailcone by using a series of smaller body tubes, and doing the fins in sections on their own body tubes, and then decided that was WAY more work than I wanted to do.

I trimmed the fins to scale, so I'll likely just add more nose weight to play it safe.
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Old 03-02-2016, 11:10 PM
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mwtoelle mwtoelle is offline
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OpenRocket, RockSim, and the commonly used Barrowman Equations will give incorrect CP information if you have fins on a transition. Someone else with the appropriate math and aero engineering background will need to work out the equations to handle fins on a transition properly. The other method would be to put the model in a wind tunnel and find the CP that way. Referring to the V-2, the rocket should balance about one-half of the main body diameter in front of the boattail if the model has scale sized fins in a flight-ready state.
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Old 03-03-2016, 07:09 AM
aeppel_cpm aeppel_cpm is offline
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If you're trying to get an accurate CP, then (as mentioned) OR won't do it. Rocksim has an essentially empirical adjustment that tries to handle it. YMMV. I like to build sims. But I don't trust them a great deal. People quickly forget that the sims were created with built-in assumptions that are easy to break.

That said, if you are trying to get a _visual_ rendering of a rocket, rather than a stability estimate, a number of tricks have been developed. You should really look at Jim/K'Tesh's work on the Voldemort Forum.
http://www.rocketryforum.com/showth...en-Rocket-files

To get a visual model of the V2, you put a 'phantom tube' after the body tube - zero length and mass. I happen to make it thick, too, with the ID matching the MMT OD. Then put an inner tube and the boat tail transition on the phantom tube. Put the fins on the phantom tube, too, sliding them into place. If you turn on the fins tabs, they will extend to the MMT. It seems funny, but the outside view should be good.

I've got an F25 Firelily in progress on another PC. I'll try to send it along later.
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Old 06-30-2017, 08:27 PM
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K'Tesh K'Tesh is offline
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Quote:
Originally Posted by aeppel_cpm
If you're trying to get an accurate CP, then (as mentioned) OR won't do it. Rocksim has an essentially empirical adjustment that tries to handle it. YMMV. I like to build sims. But I don't trust them a great deal. People quickly forget that the sims were created with built-in assumptions that are easy to break.

That said, if you are trying to get a _visual_ rendering of a rocket, rather than a stability estimate, a number of tricks have been developed. You should really look at Jim/K'Tesh's work on the Voldemort Forum.
http://www.rocketryforum.com/showth...en-Rocket-files

To get a visual model of the V2, you put a 'phantom tube' after the body tube - zero length and mass. I happen to make it thick, too, with the ID matching the MMT OD. Then put an inner tube and the boat tail transition on the phantom tube. Put the fins on the phantom tube, too, sliding them into place. If you turn on the fins tabs, they will extend to the MMT. It seems funny, but the outside view should be good.

I've got an F25 Firelily in progress on another PC. I'll try to send it along later.


Thanks for the plug.

He's right that at this time the only way to attach fins to a transition is to attach them to a PBT. You might be able to use the Sim for altitude and flight estimates, but I can't be certain that they will be accurate.
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Last edited by K'Tesh : 06-30-2017 at 09:04 PM.
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