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LeeR
03-02-2016, 08:30 PM
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.

Rich Holmes
03-02-2016, 08:59 PM
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.

LeeR
03-02-2016, 09:34 PM
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.

mwtoelle
03-03-2016, 12:10 AM
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.

aeppel_cpm
03-03-2016, 08:09 AM
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/showthread.php?123564-K-Tesh-s-Open-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.

K'Tesh
06-30-2017, 09:27 PM
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/showthread.php?123564-K-Tesh-s-Open-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.

LeeR
06-30-2017, 09:33 PM
Thanks for the info. I just happened to be reading new posts. Surprised to see I was the OP. :)

JumpJet
07-05-2017, 05:04 PM
Lee,

That's why I simply build my prototypes and fly them to see what happens. I use RockSim only after they have flown to see what the program says the Margin of Stability is and then add clay to get to a 1.5 that Estes requires. Of course non of the programs work for some of the stuff I've been designing these days so I am know to shot out those famous works. "This is a Heads Up Flight" when test flying prototypes out at my clubs field.



John Boren

LeeR
07-06-2017, 12:08 AM
John,

Yeah, I've done a number of very unusual futuristic builds, and most of the time I just decided that it "looked" stable! Mostly that has worked. But when I decided to try to model the V-2 in OpenRocket, I was surprised that fins on a tail cone wasn't a possibility l. When I heard of the workarounds required, I'd rather just use your approach -- "This is a heads up flight".

Looks like I'll have several gliders for the SCORE launch on the 15th. My wife reminded me today that we need to go get an easy-up canopy for shade. Looking forward to joining everyone in Pueblo.