Rocksim Altitude Off, Any Thoughts?

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Starfire73

OpenRocket Wannabe
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I just sim'd the Sirius Eradicator in Rocksim 10.5, ran a few launches on different motors and I'm consistently getting altitudes about 60% lower than what I would expect. For example, it's showing launches on some motors at 800' when they should easily be over 2,200'.

  • I've compared the altitudes to other larger and heavier rockets of mine which have gone much higher on the same motor.
  • I've loaded the exact same Rocksim file into Open Rocket and the OR file is about where I would expect, and of course, the Rocksim is far lower (again, about 60% less)
  • The Rocksim (and OR) Mass, CG, CP, etc., for the Eradicator are all where they should be

Something's wrong, but I can't figure out what. Any thoughts? Has this ever happened to you? I've been using Rocksim for probably about 20 years and I have never seen this happen.

The Rocksim file is attached.
 

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  • Eradicator S-0017_ds.rkt
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I don't know what's causing the problem. But, if you pick a simulation, right click and select "2d Flight profile...", and turn on the "Details >>", the initial Cd is 2.73! That seems huge. I'd guess (wildly) that one of the 8 strakes, 4 fins, or 2 ringtails that are causing the problem. You might try deleting them one at a time to see if the problem goes away.

When I load the RockSim file into OR the ringtails are dropped. That could be part of the difference between RS and OR results.

Good luck!
 
I just sim'd the Sirius Eradicator in Rocksim 10.5, ran a few launches on different motors and I'm consistently getting altitudes about 60% lower than what I would expect. For example, it's showing launches on some motors at 800' when they should easily be over 2,200'.

  • I've compared the altitudes to other larger and heavier rockets of mine which have gone much higher on the same motor.
  • I've loaded the exact same Rocksim file into Open Rocket and the OR file is about where I would expect, and of course, the Rocksim is far lower (again, about 60% less)
  • The Rocksim (and OR) Mass, CG, CP, etc., for the Eradicator are all where they should be

Something's wrong, but I can't figure out what. Any thoughts? Has this ever happened to you? I've been using Rocksim for probably about 20 years and I have never seen this happen.

The Rocksim file is attached.
Have you checked surface friction and fin edge prep?
 
I don't know what's causing the problem. But, if you pick a simulation, right click and select "2d Flight profile...", and turn on the "Details >>", the initial Cd is 2.73! That seems huge. I'd guess (wildly) that one of the 8 strakes, 4 fins, or 2 ringtails that are causing the problem. You might try deleting them one at a time to see if the problem goes away.

When I load the RockSim file into OR the ringtails are dropped. That could be part of the difference between RS and OR results.

Good luck!
Thanks for the suggestion and I think we've found our culprit. I don't know why (that remains a mystery) but the ring tails threw the Cd off by about 5x the value. I removed the ring tails and now I'm looking at some more expected numbers. That's a shame, I want the ring tails accounted for, but until I can figure out how to correct the issue, I'll have to leave them off.

Thanks for the help.
 
I added the ring tail pieces back on and un-checked the Cd override box which I usually leave checked by default (not sure why, or if I'm supposed to) and that seemed to return reasonable and expected results; results that were roughly in line with OR. That tells the sim to use a Cd of .75 instead of calculating it at simulation time.
 
BTW Sirius gives these altitudes for the various recommended motors.
How do these compare with Rocksim?
Altitiude ranges on recommended motors:

F62T-S 600' (183m)
F50-6T 1000' (305m)
G40-7W 1400' (427m)
G80-7T 1450' (442m)
G64-7W 1500' (457m)

High-Power reloadable motors:

H128W-M 1850' (563m)
H180W-M 2150' (655m)
 
BTW Sirius gives these altitudes for the various recommended motors.
How do these compare with Rocksim?
Altitiude ranges on recommended motors:

F62T-S 600' (183m)
F50-6T 1000' (305m)
G40-7W 1400' (427m)
G80-7T 1450' (442m)
G64-7W 1500' (457m)

High-Power reloadable motors:

H128W-M 1850' (563m)
H180W-M 2150' (655m)
Rocksim came in about 60% less in altitude than OR, and also what Sirius specified. The G40 in Rocksim gave me 585’, Sirius and OR showed about 1,400’. I got similar results with all the motors I ran… about 57% to 60% less altitude in Rocksim.

I noticed that in OR the plotted Cd at sim launch was .74; however, we have to keep in mind that OR does not support ring tails so it removed them from the Rocksim file when I loaded it in OR. The .74 Cd of OR is about the same as the uncalculated Cd that Rocksim provides as a default if you un-check the calculate at sim time box.
 
Hey Starfire, just out of curiosity I took a ring tailed rocket (Screaming Green Meanie) and ran sims with and without the ring tails.
with:
E12-4/567 ft.
F24-4/995 ft.
without:
E12-4/716 ft.
F24-4/1360 ft.
So the ring tails make a huge difference.
Now.........
Does that translate from the sims to actual flight?
Let us know. ;)
 
Hey Starfire, just out of curiosity I took a ring tailed rocket (Screaming Green Meanie) and ran sims with and without the ring tails.
with:
E12-4/567 ft.
F24-4/995 ft.
without:
E12-4/716 ft.
F24-4/1360 ft.
So the ring tails make a huge difference.
Now.........
Does that translate from the sims to actual flight?
Let us know. ;)
I'll have it ready to test this spring and I'll post the results here.

I took the two rings off it in both RS and OR, and put six fins with the same chord length as the ring length and made the height of the fins equal to the radius of the rings. I followed Levison's guidance in the Peak of Flight newsletters. That brought the altitudes into a reasonable and expected range. It appears that OR is a little bit closer to the altitudes provided in the Eradicator instructions. The G80 altitude was only 50' different than the instructions.

OR is showing it at 2,100' on an H135, RS is showing that same motor at 3,100'. So, the same rocket in OR vs RS is a thousand feet different. Huh
 
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