Rocsim-Openrocket discrepancy

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

kitchw8436

Well-Known Member
TRF Supporter
Joined
May 9, 2018
Messages
89
Reaction score
76
I'm trying to sim my 3" HPR model for an upcoming flight on a K805. Rocksim is giving me a apogee of 13,000 ft and OR an apogee of 9,100 ft. The big difference seems to be the CD each program is computing. The Rocsim CD is way lower than the Openrocket CD, 0.25 to 0.45 for Rocsim and 0.55 to 0.81 for OR. I've double checked the models and in each simulation and they're the same. Anyone else seen similar discrepancies? Recommendations?

- W
 
Post the OR file. If you are using square fin cross section, this can result in underprediction by OR. Also surface finish can be cause big differences between the two. Not knowing the weight of your rocket it's hard to say, but I find the OR result more believable.
 
As with most sim programs, there are enough even slightly divergent factors at play (fin cross section, surface finish) that the first launch is still only a 'best guess' until you get relavent real world data from said first flight.

From there, you can tweak your sims enough to match what happened in the real world. I use OR, and find that after 1 flight, I can get the sim to within 5%, after 5 flights to within 1%.
 
Are you using the Revised/Extended Barrowman aka RocSim method? Revised/Extended will give different results than OR which uses regular Barrowman method.
 
Yeah but that’s only for CP calc, right? Altitude discrepancy is a drag issue, as described above.
Do a plot of the flight, changing CP can affect weathercocking which can lower or increase altitude depending on how stability is affected. Drag has a lot to do with altitude(actually a whole lot) but its not the only thing that affects it, another thing to check is that the atmospheric conditions are the same for both sims (wind, temp, average humidity, etc).
 
Do a plot of the flight, changing CP can affect weathercocking which can lower or increase altitude depending on how stability is affected. Drag has a lot to do with altitude(actually a whole lot) but its not the only thing that affects it, another thing to check is that the atmospheric conditions are the same for both sims (wind, temp, average humidity, etc).
True, and absolutely worth checking all that stuff when trying to get consistent results between OR and Rocksim. But in this case the enormous Cd discrepancy reported by the OP is likely to be the primary issue here.
 
Appreciate the insights. The .ork & .rkt files are attached if anyone has a moment to look at them I'd appreciate it. There is some discrepancy in the motor thrust curves used. Total impulse is similar, but thrust vs time notably different. OR uses the eng curve and RS the rse curve. I set RS to use the traditional Barrowman equations so I could better compare the two simulations. I realize the sim needs to be calibrated and was expecting some difference. Just not this much.

- W


AT K805.png
 

Attachments

  • Prometheus V01.ork
    4.6 KB · Views: 0
  • Prometheus V02.rkt
    70.6 KB · Views: 0
I don't see anything obviously wrong with the ORK file. Don't have Rocksim so I can't check that one.

I note that the fins are set as rounded, paint finish is set to "Normal" for the ORK version. Here's the drag analysis:
1684166974366.png
That all looks reasonable to me.
 
In RockSim the fins have square edges and all external surfaces have a "polished" finish.
 
There is some discrepancy in the motor thrust curves used. Total impulse is similar, but thrust vs time notably different.
Not a huge difference, but on thrustcurve the Rocksim/RSE file note makes that file seem extremely questionable, see below. If you can force Rocksim to use the RASP file I'd try that. But I doubt this is the source of the mismatch.


Not K805G but designated K715G-P J Brent June 28, 2008
The K715G is a 1719 N-s motor; based on research data from 11-26-07 (preproduction). This is a 54/1706 motor. Data source was D.B. at AT. Weight data was assumed to be same as K805G since this was not provided. I will inquire with AT if this was a right approach and why the difference between the burn times (K715G at 2.4s and K805G at 2.2s).
 
Not a huge difference, but on thrustcurve the Rocksim/RSE file note makes that file seem extremely questionable, see below. If you can force Rocksim to use the RASP file I'd try that. But I doubt this is the source of the mismatch.


Not K805G but designated K715G-P J Brent June 28, 2008
The K715G is a 1719 N-s motor; based on research data from 11-26-07 (preproduction). This is a 54/1706 motor. Data source was D.B. at AT. Weight data was assumed to be same as K805G since this was not provided. I will inquire with AT if this was a right approach and why the difference between the burn times (K715G at 2.4s and K805G at 2.2s).
If you can't force Rocksim to use the RASP file, then you *can* force OR to use the RSE file, just to eliminate that as a source of discrepancy. Use the "Select thrust curve" pulldown; text color matches the color of the thrust curve in lower right of the "Show Details" tab.
1684168055232.png
 
We'll I triple check all the parts in each data file, made sure temp and pressure conditions were the same, and used same thrust curve in each. Still same discrepancies. Also ran both with no wind--same issue.

I was expecting 10 - 15% differences between RS & OR, but 40% surprises me.

I note the RS isn't showing any base drag. Is there something setting I've got wrong there?
If I fix the Cd to 0.6 in RS I get pretty good agreement between RS and OR.
 
Interesting stuff, kitchw8436

Based on real-world acceleration data, I am more inclined to believe the OpenRocket CD range of 0.55 to 0.81 for a rocket that will 'tickle' the Sound Barrier.

I looked at your .ork ( OR ) File and it looks OK to me.

I don't have RockSim so I can't look.

The only thing I might adjust in the OR Flight Sim Tab is to uncheck Internaltional Standard Atmosphere and enter a reasonable temperature and pressure for your Launch Site -- it might make a difference when a rocket flies transsonic ...

-- kjh

P.S. I am in Round Rock, north of Austin. Where do the West Texas Space Vaqueres Fly ?

West Texas Space Cowboys -- I love it :)

P.P.S. Thanks to neil_w for clicking on the [Motor Details] Tab in the OR [Motors and Config] Tab -- I never noticed it before -- NICE !
 
Interesting stuff, kitchw8436

Based on real-world acceleration data, I am more inclined to believe the OpenRocket CD range of 0.55 to 0.81 for a rocket that will 'tickle' the Sound Barrier.

I looked at your .ork ( OR ) File and it looks OK to me.

I don't have RockSim so I can't look.

The only thing I might adjust in the OR Flight Sim Tab is to uncheck Internaltional Standard Atmosphere and enter a reasonable temperature and pressure for your Launch Site -- it might make a difference when a rocket flies transsonic ...

-- kjh

P.S. I am in Round Rock, north of Austin. Where do the West Texas Space Vaqueres Fly ?

West Texas Space Cowboys -- I love it :)

P.P.S. Thanks to neil_w for clicking on the [Motor Details] Tab in the OR [Motors and Config] Tab -- I never noticed it before -- NICE !
WTSV Flies out of and abandon WWII auxiliary airfield south of Wall TX ( 31°19'52.65"N, 100°17'16.18"W). Have a waiver to 10K AGL. Next launch June 17. Come fly with us.

I'll be at the Texas Shootout in Seymour on Memorial Day weekend. Let me know if you're planning to come. I do show up at AARG launches on occasion.
 
I just discovered and interesting bug in RS. I notices that my RS simulation has zero base drag, which seemed really weird. I had configured the booster section as a sub-assembly, because I had been messing with different configurations of the payload bay. This configuration gave me no base drag. When I took all the parts out of the sub-assembly and put them in the model individually, the base drag appeared. The two simulations now differ by only 300' in apogee or a 3-5% difference. Now that's a number I can believe.

I keep finding all this bugs in RS. Not very reassuring.
 
I just discovered and interesting bug in RS. I notices that my RS simulation has zero base drag, which seemed really weird. I had configured the booster section as a sub-assembly, because I had been messing with different configurations of the payload bay. This configuration gave me no base drag. When I took all the parts out of the sub-assembly and put them in the model individually, the base drag appeared. The two simulations now differ by only 300' in apogee or a 3-5% difference. Now that's a number I can believe.

I keep finding all this bugs in RS. Not very reassuring.
Use OpenRocket!
 
WTSV Flies out of and abandon WWII auxiliary airfield south of Wall TX ( 31°19'52.65"N, 100°17'16.18"W). Have a waiver to 10K AGL. Next launch June 17. Come fly with us.

I'll be at the Texas Shootout in Seymour on Memorial Day weekend. Let me know if you're planning to come. I do show up at AARG launches on occasion.
kitchw8436 --

Thanks for the Invitation but I'll be out of town for work on Jun 17 and I found out about the TX Shootout too late to make plans.

I really like the looks of the Rocket Ranch and the WTSV Wall Site looks good too !

I plan to be at the next AARG Launch and my brother Mike from Orange, TX does the Hearn and NASA Launches regularly.

I imagine I'll see you sometime, somewhere :)

-- kjh
 
Last edited:
Back
Top