Open Rocket or RockSim

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Michael L

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I have both... one more time... I HAVE BOTH

:) I have RocSim 10 (paid version in other words) That's what I used to simulate my L1 cert rocket including the mods I made (LOC IV). The cert went fine. The sim max altitude was 5377.43' (1,639m). Actual altitude at apogee was 4,541' (1,384m) altitude from the AltusMetrum 3,0. The correct weight was entered (the rocket was weighed a lot times). The measured Cg was located with a string and I feel very confident about that. Weather wasn't "special". Winds were very low. No humidity to speak of. The rocket left the rail and appeared to track true (until we couldn't see it). I flew it on a Cesaroni I350SS. 836' (255m) seems like a lot of "error" to me. This was the first rocket that I've flown with electronics.

So now that we know that I have both pieces of software and that RocSim was off... make that could have been off... by 836' (255m), should I switch to Open Rocket?

To be fair I could have made an error setting up the simulation. Weight and Cg match actuals though. I used the LOC IV rkt file from LOC and only changed the weight and Cg and added the motor. 836' error (255m) 12. -15.5% difference. To me that's a big deal.

Since I'm not overly confident in RocSim I was thinking about switching to Open Rocket. The thing that's stopping me is physics is physics (or maybe fizzicks is fizzicks?). The equations used in both pieces of software are probably the same. Is there any benefit to changing? Or am I overlooking something that could have affected apogee by -15.5%?
 
Did you try opening your sim in OpenRocket and checking on its altitude estimate? That seems a better way to evaluate. OR can read the RKT file directly, but you might need to do a bit of cleanup.
 
A simulation is not reality. I have never used RockSim, but I will ask because settings in OR should be similar I would think.. What setting were you using for surface/paint condition? (no such thing as polished in reality) What was the setting for fin edges? (probably not an aerodynamic taper even though it is an option) Check the settings in the simulation
 
Both are a bit different, but both should have very similar and accurate results if you get everything dialed in properly. Being off by over 10% is a sign that something's wrong. I personally prefer Open Rocket and haven't bothered to upgrade my Rocksim account to the latest version as I didn't see the point.
 
First thing you need to do in RockSim is change the "finish" on each part from "polished" to something else. I find that, regardless of how I actually finish a rocket, "Matte" gives me results much closer to reality.

Try changing all the parts to "gloss" or "matt" and see if that gets your sim closer to the results you observed at the launch.

I have not been able to find a way to change the default from "polished" to "matt". If anyone knows how to do it, that would be a great piece of info to have.
 
I haven't tried it in OpenRocket yet. I didn't think I could use the RocSim file and I haven't had time to build it new in and learn how to use OpenRocket. What kind of altitude difference should I expect? I'm one of those people that likes numbers with 4 decimal places :) even when I know the last 3 don't matter.

I've attached the RocSim file
 

Attachments

  • PK-48 Loc-IV Modified.rkt
    56.5 KB · Views: 6
A simulation is not reality. I have never used RockSim, but I will ask because settings in OR should be similar I would think.. What setting were you using for surface/paint condition? (no such thing as polished in reality) What was the setting for fin edges? (probably not an aerodynamic taper even though it is an option) Check the settings in the simulation
First thing you need to do in RockSim is change the "finish" on each part from "polished" to something else. I find that, regardless of how I actually finish a rocket, "Matte" gives me results much closer to reality.

Try changing all the parts to "gloss" or "matt" and see if that gets your sim closer to the results you observed at the launch.

I have not been able to find a way to change the default from "polished" to "matt". If anyone knows how to do it, that would be a great piece of info to have.


I'll see if I can find that setting, I flew it in primer and due to a number of things that came up the primer wasn't sanded with 1500 grit like I normally would have. The fins were tapered to a fairly sharp edge (I went a little overboard when I was sanding)
 
After changing the fins to airfoil and finish to matte it's much better, 4,798.72 (1,463m). 257' (78m) difference. 5.37%. Not terrible.

Changing the fin to rounded = 4,798.75, Only .03' higher. It predicts higher (not much) with rounded fins
 
Changing the paint finish makes a huge amount of difference in Rocksim. I find it can often make more difference than structural changes to the rocket. A 5% variance is to be expected. On any given day temperature, wind, humidity, air pressure, butterfly wing flaps and duck farts will account for 5% at least.
 
I'm happy with it and I appreciate the help. You wouldn't happen to know why I can't get an Altus Metrum TeleBT to connect to Windows 10 laptop or PC would you. That's probably a question for another forum thread... but I'm tired of jacking with it. It's been months of frustration and no success.
 
Sorry, don't own one of those. Post it up in the "Rocketry Electronics and Software" forum - probably will get a bunch of good suggestions.
 
Bdale and Keith are in the loop. Keith doesn't do "windoze" but he has added a virus collector... oops... windoze laptop to his troubleshooting arsenal. It's a Bluetooth issue. The software sees it, lets me select it, will not talk to it. If I use a serial cable, it works. Sorry for the diversion.
 
As people are saying, dialing in the surface finish is important. If you get data from a flight (as you have), and then adjust surface finish until the simulation is accurate, it'll be pretty good at estimating behavior with other motors. At any rate this is true for OR; I assume it's also true for RS.
 
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