same sim, same motor, same everything, but different result - why?

[billdz]

Please have a look at the attached OR sim, in particular nos. 15 and 18. Both use the same motor, the H112J. All parameters are exactly the same. Yet on 15 there's a green check but 18 is a red warning "Stage began to tumble under thrust". On both, I have clicked "Run simulations" numerous times and the result is always the same -- green on 15, red on 18.

How can that possibly be, since both are identical?
Thanks,
Bill

 

[billdz]

Here are the time vs. stability graphs for both of the sims at issue. Sim 18, the one with the red warning, has negative stability at ignition, while sim 15 does not. But how can that be? It is the same motor on both.

 

[RainierWolfcastle]

Is there a difference in rod length or wind speed for either simulation? If I had to guess I'd be leaning toward the wind speed given the low launch velocity. If the wind speed is a range it could have one randomly picked lower and the other at the higher end?

 

[billdz]

No, the parameters are identical.

 

[mikec]

Post the ORK file if you want any real advice.

There's a random element to the wind speed and it might be that this is marginal enough that the seed for the random number generator is set differently for each sim. Try using a longer rod and see if the results are more consistent.

 

[billdz]

Here's the ork.

View attachment 54hornet.ork

 

[billdz]

Tried increasing the size of the rod to 6', same result. Sim 18 still red.

 

[markkoelsch]

You need a higher thrust motor. The sim is only predicting 9 meters/second of the rod/rail. This too far too low for it to be consistently stable. Heck, with zero wind that is really questionable.

If the rocket is going to slow the fins cannot generate much corrective force.

With motor this is nearly five pounds of rocket in the pad. I would not fly this on a motor with a thrust for the first second or so or less that about 130-150 Newton’s. More if you have wind.


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[billdz]

You need a higher thrust motor. The sim is only predicting 9 meters/second of the rod/rail. This too far too low for it to be consistently stable. Heck, with zero wind that is really questionable.

If the rocket is going to slow the fins cannot generate much corrective force.

With motor this is nearly five pounds of rocket in the pad. I would not fly this on a motor with a thrust for the first second or so or less that about 130-150 Newton’s. More if you have wind.


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Agreed, I would not fly the rocket with this motor. But I'm still puzzled about the software, why one green and one red with the identical motor? I've done sims before where stability was marginal and repeated clicking on Run Simulations would produce varying results, sometimes green, sometimes red. I was told this is because the program varies the wind somewhat each time the sim is run. But in this case 18 is always red and 15 is always green, every time.

 

[Buckeye]

When I open your file, I only see 5 simulations and none with H112J. Maybe you use custom motor files that I do not have.

Looks like you have non-zero values for wind standard deviation and turbulence intensity. This puts random variation to the wind speed every time you simulate and affects stability. If you want truly identical results every time, then make these value zero.

 

[billdz]

When I open your file, I only see 5 simulations and none with H112J. Maybe you use custom motor files that I do not have.

Looks like you have non-zero values for wind standard deviation and turbulence intensity. This puts random variation to the wind speed every time you simulate and affects stability. If you want truly identical results every time, then make these value zero.

That's odd, I'm not using custom files. Which sims are you seeing? Do you have the H112 in your data base?

 

[markkoelsch]

Agreed, I would not fly the rocket with this motor. But I'm still puzzled about the software, why one green and one red with the identical motor? I've done sims before where stability was marginal and repeated clicking on Run Simulations would produce varying results, sometimes green, sometimes red. I was told this is because the program varies the wind somewhat each time the sim is run. But in this case 18 is always red and 15 is always green, every time.

Perhaps the variation pattern is predictable so you end up like this.

Also, the programs and such seem to have issues predicting stability at really low air speed. At low sped the fins do not provide much corrective force.


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[Buckeye]

That's odd, I'm not using custom files. Which sims are you seeing? Do you have the H112 in your data base?

Yes, in fact two entries for H112, each with different properties. This may be part of the problem. Maybe each sim uses a different version of the H112?

Most simulation problems arise from bad motor files. Unfortunately, Thrustcurve, while an awesome resource, exacerbates the issue by including multiple versions of the same motor. There are .rse and .eng entries for H112. .rse and .eng are just formats. The data inside should be the same, but it isn't. They have different size, mass, impulse, and thrust curves. OpenRocket sucks in both, sees that they are different, and thus includes both in its database. Now it is up to the user to figure which one is more correct.

Oh, and setting up simulations with many different motors is painfully tedious in OR. Two steps needed - configuration, then simulation. Batch mode, please!

 

[mikec]

When I open your file, I only see 5 simulations and none with H112J.

Same here, using 14.06.

There are two versions of the H112J thrust curve, one with a peak thrust of ~100N and one with a peak of 125N. The higher-thrust version is from the AT instructions, the lower-thrust from the TMT cert data, so I'd be more inclined to trust the latter. The lower-thrust motor gives the tumble warning message but the altitudes are similar.

BTW, this rocket is pretty heavy for some of the motors in this file (e.g., G64.)

 

[billdz]

Yes, in fact two entries for H112, each with different properties. This may be part of the problem. Maybe each sim uses a different version of the H112?

Most simulation problems arise from bad motor files. Unfortunately, Thrustcurve, while an awesome resource, exacerbates the issue by including multiple versions of the same motor. There are .rse and .eng entries for H112. .rse and .eng are just formats. The data inside should be the same, but it isn't.

Yes, thanks, that's it, the two entries for H112 are substantially different.

 

[billdz]

The .ork file I posted earlier was an incorrect, earlier version. Here's the correct one. When mikec mentioned the G64, I realized I had posted the wrong one. All is good now, sim 15 is the higher-powered version of the H112, sim 19 is the lower-powered version. Thanks to all for helping me figure this out.

View attachment 54hornet.ork