OpenRocket tips?

I'm fairly new to rocketry, and my professor wanted us to first simulate the model rocket apogee on OpenRocket, then use an altimeter to check the actual apogee and compare the two. This was so we could use OpenRocket to design a higher power rocket in the future.
But on some occasions, the simulated apogee would be way lower than expected, or a fair margin higher than actual.
We made sure to check the parachute deployment time, as that was our first mistake, and we also checked the center or pressure being behind the center of gravity.
I feel like there's something I'm doing wrong and I don't know what it is, I see people saying OpenRocket is fairly accurate so it should just be something I'm messing up on. Any help is appreciated.

Here's a link to my tutorials on OR...

https://www.rocketryforum.com/threads/kteshs-openrocket-tutorials.62884/

hi, welcome to the forum. I think that you have run into the real world vs a simulation issue. in real life the performance of hobby grade rocket motors can vary by as much as 10% plus or minus from the published thrust data, in the simulation it doesn't, this can cause issues when trying to match sim data with actual flight results which means lots of test flights to average data.
Rex

Post an .ork that was norminal and one that wasn't with as much detail as you recorded.

AndrewL23 said:

But on some occasions, the simulated apogee would be way lower than expected, or a fair margin higher than actual.

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It's hard to draw any sort of conclusions without some more detail. What do "on some occasions", "way lower" and "a fair margin higher" mean? Is this the same rocket, showing a great amount of variability, or different rockets? Did the flights go up straight, or did they go up at an angle? That could lose you a bit of altitude. Was the rocket painted? Were the surface finish settings set appropriately in the OR model?

The ORK file plus all that data will give us a chance to sort it out.

To tack on, things like wind direction and speed, rocket finish, etc., affect results. If you used an altimeter in the actual flight, did you simulate the weight that the altimeter adds? Did you override the mass of the finished rocket, or just assume that the mass is correct based on simulated materials?

Bat-mite said:

Did you override the mass of the finished rocket, or just assume that the mass is correct based on simulated materials?

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Ooh that's an important one, can't forget that.

What I was going to say..

• weigh each piece, over-ride the values in open rocket.
• check drag of each piece.
• play with the environmental bits too.. air pressure & air temp can make a difference

There will be unaccounted for drag in the real world: buttons, ridges, fillets, slightly misaligned fins, etc.. Rod angle, wind at ground vs. at altitude, friction between the rod the butts, etc.. will also vary your results. Not to mention wind isn't necessarily linear, you have up & down drafts, eddies & such that can affect ascent..

You can fly the same rocket, on the same day, with the same motor, and get different results due to these (and other) variables..

Be careful overriding entire rocket mass and CG. This is always the mass and CG without motors, but make sure it is otherwise totally in the ready to fly configuration. Also don’t forget if you are using spacers in the motor you may also need to take this into account, which kind of makes overriding the entire rocket mass and CG impractical.

SpaceManMat said:

Be careful overriding entire rocket mass and CG. This is always the mass and CG without motors, but make sure it is otherwise totally in the ready to fly configuration. Also don’t forget if you are using spacers in the motor you may also need to take this into account, which kind of makes overriding the entire rocket mass and CG impractical.

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Not impractical at all. But, as you said, it has to be without the motor. You can always save multiple copies of the file with differing configurations -- with or without motor spacers, motor adapters, etc. Unless you know the exact mass of your adhesive and exactly where it is, and you can add that mass appropriately to your model, the only way to get stability right is to override total mass and CG.

Bat-mite said:

Not impractical at all. But, as you said, it has to be without the motor. You can always save multiple copies of the file with differing configurations -- with or without motor spacers, motor adapters, etc. Unless you know the exact mass of your adhesive and exactly where it is, and you can add that mass appropriately to your model, the only way to get stability right is to override total mass and CG.

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As you like, I personally don’t want to maintain a bunch of sim copies for my rockets. I just set the spacer mass object to 0 mass when I don’t use it. Actually I had been asking if we could get the motor files updated to allow spacer options so we didn’t have to worry about such things...

SpaceManMat said:

As you like, I personally don’t want to maintain a bunch of sim copies for my rockets. I just set the spacer mass object to 0 mass when I don’t use it. Actually I had been asking if we could get the motor files updated to allow spacer options so we didn’t have to worry about such things...

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I like that idea.

Hey All!

I was signing on to update my Goblin build post and saw this post and got side tracked...LOL

I'm new to rocketry too and am still figuring out Open Rocket as well.

I just updated my OR Goblin file last night to compare actual weight vs OR weight after last coat of primer and before paint. (just painted it last night)
Turns out it's almost 10 ounces heaver than OR is indicating...before final scuffing and before paint.
So, I checked to see if I could alter the finish weight to compensate....couldn't find a way to do it, so I calculated the overall surface area of the body, fins and nose cone, all separately so I can add weight at the right percentage in the right places.
When I get a final weight I will then divide the difference in weight between OR and divide it into tiny little pieces and then add it as a "mass" object, renaming it as "paint" (If anyone has a better easier way please let me know!).

Also I weigh EVERYTHING, I use a gun powder scale for the smaller pieces which weighs to 1/10th of a GRAIN.
That's 1/70,000th of a pound!!! Then I convert to ounces for Open Rocket.

I account for the epoxy by weighing each batch after mixing then weigh the remainder after application, and will apply as a "Mass" object in the proper location (re-named "epoxy") if I cant add the mass on the proper part in the proper place during assembly.
For example the epoxy fillets for the fin tabs to motor tube. I took that weight and added it as mass-centered between the mid and aft centering rings.
The weight wont be perfect because you have some epoxy loss that can't be weighed like on the mixing sticks, and wipe-off from the application sticks, but it will get you in the ball park and I like to round UP in weight on anything BELOW the center of gravity and round DOWN on any weight above.
That way the rocket will tend toward being MORE stabile than shown on Open Rocket instead of less.

It seems like a lot of work...but the saying is "Garbage In = Garbage OUT!

Another example is to weigh the forward motor mount CR and then weigh the chute strap hook that mounts to it and add that weight to that CR.

I'm NOT DONE YET, so it won't match the rocket setting on my table since again, I'm not done with painting, and I haven't added the Jolly Logic Chute release, but take a look and see how I try to make it accurate.
I try to make everything in OR be the proper weight and in the proper place....

Garbage in Garbage OUT!!! LOL