Rocksim simulation trouble

I'm trying to sim a rocket that I built in Rocksim and whenever I click the 'launch' button, the little window that pops up when calculating the simulations. But the odd thing is that in the main box, it says 'Ignition of first engine(s)' and stay like that during the rest of the calculation. The weirder thing is that when the calculations is complete, all the numbers are zero except for 'Velocity at deployment' and 'Altitude at deployment'. They say 'nan'. Anyone know the problem?

Originally posted by jj94
all the numbers are zero except for 'Velocity at deployment' and 'Altitude at deployment'. They say 'nan'. Anyone know the problem?

Click to expand...

right-click on the sim in question, then select display details. It might
say something like 'the rocket never left the pad, use more thrust'...

Yup, that's what it said, but that's not the problem. First, I tried it on an F24 and it's only about 9 oz. Then I used a G55, and it said the same thing.

I suppose I could rebuild the rocket and make another file and see if it works, but I would do that when I'm getting desperate. It's a pretty complex design, and would take about 20 minutes just to recreate it. Any more ideas?

PING.... calling Teflonrocketry.... PING!



Just give it some time

Ben

Originally posted by jj94
I'm trying to sim a rocket that I built in Rocksim and whenever I click the 'launch' button, the little window that pops up when calculating the simulations. But the odd thing is that in the main box, it says 'Ignition of first engine(s)' and stay like that during the rest of the calculation. The weirder thing is that when the calculations is complete, all the numbers are zero except for 'Velocity at deployment' and 'Altitude at deployment'. They say 'nan'. Anyone know the problem?

Click to expand...

Please post the RockSim file! Just attache it to one of your posts if the file is too large use one of the compression programs like WinZip to create a zip file. I suspect that you have the Flight Events in the simulation set to something other than deploy at Max. ejection but without the file this is very difficult to troubleshoot.

This also could be a design without a nosecone. If you place a transition ahead of the nosecone or use a transition in place of a nose cone you can get this type of error.

Bruce S. Levison, NAR #69055

Umm...I have been asked not to post it in the public. I will however, change the file a bit. I'll post it when I get home...I'm at school right now. TIME FOR LUNCH!

Ok, I just got home from tennis practice. Tennis matches are tomorrow, Wed., and Thurs. Here's the modifed rocksim file. All scale dimensions were changed.

That is bizarre. I have never seen 'nan' in RockSim before, now there are two posts about it in a span of two days. (See this other thread.)

Summary: After a lot of trial and error, I determined that the problem is in your folding fins. Change the material from "Custom" to something else and the simulation will work.

Other recommendations:
1) Use the RockSim stability equations instead of Barrowman.
2) Do not use the Ring Tail Assembly in RockSim to simulate your ring tail. Instead use Bruce Levison's methods. I think Bruce's method is more accurate. RockSim adds a lot of drag for the ring tail, cutting the altitude by about 60%.


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Here's all the stuff I did while trying to track this problem:

I tried running the model that you posted. In the 2D Flight Profile with Details enabled, the rocket icon starts out in the horizontal position with a flight angle of 'nan' degrees. At the first simulation time step, most of the other flight variables become nan as well.

I tried changing the Simulation Controls settings from Explicit Euler at 800 samples/sec to 4th Order Runge-Kuta at 2000 samples/sec. No help.

Changed the Calculation Method and Simulation Method from Barrowman stability equations to RockSim stability equations. No help.

Changed the Launch angle on the Starting State tab. No help.

Deleted the Ring fin. No help.

Changed engines. Use C11, F21W, and G55. No help.

Deleted one set of fins in the mid airframe. No help.

Deleted the other set of fins on the mid-body. MAGIC! Flies great to 2442 feet on an F21W-6. Hmmmm!

Reloaded the original model file. Deleted hinge fins. No good.

Reloaded the original model file. Deleted folding fins. Works fine. 1594 feet on a G55.


Reloaded the original model file. Edited folding fins. Changed plan points. No good.
Noticed that Calculated Component Mass for the fin = 0.0000 oz. Also noted that Material = Custom. Changed that to plywood and everything works.

Oh, thanks. I didn't realize that the material was set to 'custom'. I thought I had changed it to 3/32" basswood. Oh well, must have missed it. Thanks for your help! Oh, and what is Bruce's method of simulating ring fins?

Originally posted by jj94
Oh, and what is Bruce's method of simulating ring fins?

Click to expand...

Bruce has written several articles that have appeared in the Apogee newsletter. Newsletter #27 covers ring fins. He has a related article that covers tube fins.

Thanks.

I don't exactly understand the E-zine. Could you please modify the rocksim file and repost it so I can see how you did it? (I'm tired right now... )

I put the ring tail in as per my original article. You can contact me by PM if you don't want to share a file on this forum.

Bruce S. Levison, NAR #69055

Originally posted by BobCox

2) Do not use the Ring Tail Assembly in RockSim to simulate your ring tail. Instead use Bruce Levison's methods. I think Bruce's method is more accurate. RockSim adds a lot of drag for the ring tail, cutting the altitude by about 60%.

Click to expand...

Does Rocksim properly calculate the CP with their built-in ring-fin function? I simulated a rocket and I don't think I believed the result, the CP was WAY further back than I expected it to be.

-Rick

I have my doubts about its ability to calculate the CP as well. When I used the RockSim tube fins to simulate a Quest Totally Tubular, it indicated that the rocket would be unstable regardless of which engine is installed. When I used one of the Levison methods, it said that the rocket would be stable with A8 or B6 motors but pretty marginal on a C6. That is exactly how the real rocket behaves.

Apogee says that they have wind-tunnel data to back up their calculations, but I Bruce's methods seem to match reality better.

I had the same problems as JJ94 with regards to the simulation not running properly after having a negative -ve value for some of the coordinates for my free form fins.

Ken

spikeyken,
Post your model here and Bruce or I will look at it.