Question About Using Open Rocket

[mh9162013]

I'm building my first fiberglass model rocket, and I'm pretty sure that it's gonna need some nose weight. I hate to add non-functional weight, but I have no choice. The only real question is how much weight to add.

I'm using Open Rocket to try and figure out how much weight to add, but I have a few questions/issues that I'm hoping I can get some help with.

First, where can I get an estimated weight/mass of my rocket? Open Rocket says my rocket is 18 grams without the motors, which is very, very wrong. And when I change a component from say, cardboard to steel, the weight of the rocket stays the same. I don't understand what's going on here.

Second, I'm getting a warning in the lower right that says "Discontinuy in rocket body diameter." What does that mean?

Third, I'm adding a mass component to the top of the rocket, just below the nose cone. But after doing that, the CG remains unchanged.

If it matters, I created my sim by opening an Alpha III rocket .ork file, then adjusted the existing components, ie lengthening the main body tube, changing the main body tube's material to fiber glass, adjusting lengths and thicknesses of the components, adding a 4th fin, etc.

 

[neil_w]

Please post the file.

It sounds like the mass and CG are overridden at the top level. You'll need to remove those: double-click the Sustainer and uncheck the overrides there.

"Discontinuity in rocket body diameter" means.... well, what it says. There are consecutive components that don't have the same outer diameter where they join. The last part of the message gives the names of the two components that are discontinuous.

 

[mh9162013]

Please post the file.

It sounds like the mass and CG are overridden at the top level. You'll need to remove those: double-click the Sustainer and uncheck the overrides there.

"Discontinuity in rocket body diameter" means.... well, what it says. There are consecutive components that don't have the same outer diameter where they join. The last part of the message gives the names of the two components that are discontinuous.

That did it, thanks!

As for the discontinuity error, thank you for the further explanation. I'm not surprised I'm getting that error as I'm just using rough values concerning tube/coupler wall thicknesses, for example.

 

[neil_w]

That did it, thanks!

As for the discontinuity error, thank you for the further explanation. I'm not surprised I'm getting that error as I'm just using rough values concerning tube/coupler wall thicknesses, for example.

If you can, especially if you're just getting started with OR, use parts from the component library.

 

[lakeroadster]

First, where can I get an estimated weight/mass of my rocket?

Buy a small digital kitchen scale and weight your rocket. I use mine almost on a daily basis. About 10 dollars at Home Depot

​

 

[mh9162013]

Buy a small digital kitchen scale and weight your rocket. I use mine almost on a daily basis. About 10 dollars at Home Depot

View attachment 584373​

I was referring to getting the mass/weight in Open Rocket.

 

[lakeroadster]

I was referring to getting the mass/weight in Open Rocket.

That's weird... because you wrote

First, where can I get an estimated weight/mass of my rocket? Open Rocket says my rocket is 18 grams ...

So you saw the weight in Open Rocket was 18 grams...

 

[mh9162013]

That's weird... because you wrote



So you saw the weight in Open Rocket was 18 grams...

And I knew Open Rocket was wrong because I weighed the partially built rocket and it was already 50+ grams. Hence one of the reasons I made this thread...

 

[lakeroadster]

Alrighty then... have a great day.

 

[Handeman]

If you are just starting in OR, I would not worry too much about the weight OR says it will be. It will almost certainly be off from reality.

I use OR for CP. I weigh the rocket on a scale and determine the measured CG. Override the CG for the complete rocket with the measured values. Then and only then, after getting the CP and measured CG, the actual stability, will I ever add any nose weight. YMMV.

 

[OzHybrid]

And I knew Open Rocket was wrong because I weighed the partially built rocket and it was already 50+ grams. Hence one of the reasons I made this thread...

Open rocker was right. It was the person that accidentally overrode the mass/cg that was wrong....

 

[OzHybrid]

That did it, thanks!

As for the discontinuity error, thank you for the further explanation. I'm not surprised I'm getting that error as I'm just using rough values concerning tube/coupler wall thicknesses, for example.

In OpenRocket the components go from left to right. So the body tube is to the right of the nosecone. Create a basic nosecone and a body tube. If you double-click on the body tube you can select the automatic box and it will match the diameter to the component to the left. In this case the nosecone OD.

So in this case you are using your NoseCone as the primary OD definition with the body dependent on it. If you then go to the nosecone the Automatic box will be grayed out. Similarly, if you want to use the first body tube as the OD definer for OD, make sure the automatic is UN-ticked in the body tube and you can then go to the nosecone and tick automatic.
This methodology will generally prevent you from getting any body discontinuities in the first place.
The only other ones you'll get are from jagged fins and/or if you apply the base drag hack for short fat rockets.
If you don't do either of those things until you have a good error free simulation, you'll then know where the error being highlighted to you is coming from.

 

[neil_w]

If you're using library components, which is generally a good idea but especially for those just beginning with the program, sometimes there are very slight differences in dimensions that can cause discontinuities.

Here is another reminder that the warning tells you which two components are triggering it:


If you customize the names of each component, which is a good idea but often left undone, then you'll know exactly where the problem is. Otherwise, if you have several components named "Body Tube", then you'll have to do a bit of detective work.

 

[mh9162013]

Open rocker was right. It was the person that accidentally overrode the mass/cg that was wrong....

I took an already created .ork file and modified it, so whoever created that file implemented that override; hence why I wasn't aware of it.

I don't have much experience with Open Rocket, but I learn something new every time I use it.

I should probably start creating my own files...

 

[mh9162013]

If you're using library components, which is generally a good idea but especially for those just beginning with the program, sometimes there are very slight differences in dimensions that can cause discontinuities.

Here is another reminder that the warning tells you which two components are triggering it:
View attachment 584424

If you customize the names of each component, which is a good idea but often left undone, then you'll know exactly where the problem is. Otherwise, if you have several components named "Body Tube", then you'll have to do a bit of detective work.

I don't recall seeing that level of detail with my Open Rocket use earlier today. I am using an older version, I believe. I downloaded it 6 months ago?

 

[lakeroadster]

I took an already created .ork file and modified it, so whoever created that file implemented that override; hence why I wasn't aware of it.

I don't have much experience with Open Rocket, but I learn something new every time I use it.

I should probably start creating my own files...

Or start your file using one of the example rocket files that come with the program...

​

 

[neil_w]

I don't recall seeing that level of detail with my Open Rocket use earlier today. I am using an older version, I believe. I downloaded it 6 months ago?

Get the new one.

To clarify: the old one is 15.03, the new one is 22.02. You can tell which you're currently running in the "About" dialog.

 

[OzHybrid]

I took an already created .ork file and modified it, so whoever created that file implemented that override; hence why I wasn't aware of it.

I don't have much experience with Open Rocket, but I learn something new every time I use it.

I should probably start creating my own files...

when the mass is overridden, the Mass symbol is beside the component. If the CG position is overridden. that symbol shows up too.

 

[OzHybrid]

Us guessing what you're working with makes it difficult for everyone.
If you'd posted the file as requested in the first post after yours, we wouldn't have to guess. This will help us to help you.
https://www.rocketryforum.com/threads/question-about-using-open-rocket.180271/post-2442513

 

[mh9162013]

Us guessing what you're working with makes it difficult for everyone.
If you'd posted the file as requested in the first post after yours, we wouldn't have to guess. This will help us to help you.
https://www.rocketryforum.com/threads/question-about-using-open-rocket.180271/post-2442513

I didn't post the file because @neil_w immediately identified what my issue was.