CG Weirdness in OpenRocket

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lcorinth

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A few times I've noticed that the CG seems a bit too far forward in OpenRocket than on the real thing. I figured this had to do with the fact that it doesn't take glue into account or something.

But lately I've noticed that the CG seems farther forward than it should be, even without glue weight.

And I've really noticed this when you open a RockSim file.

I like that you can open either .rkt or .ork files with OR - it means I can download some premade sims of some of the rockets I have.

But check these out. Here's a picture of the Quest Big Dog, the RockSim file of which I downloaded from Apogee:

CG Weirdness - Big Dog.jpg

Now here's a more extreme example. I don't know what RockSim file I started with, but I used an already-designed rocket, played around with it a bunch, and ended up with this:

CG Weirdness - Quad Cluster.jpg

That CG is nuts!

I checked the mass of the components, and they're not too far off from the real thing, so I don't know what's going on here. And the weird thing is, even if I zero out the mass on the stuff on the front, and add a ton of mass on the stuff on the back (i.e., lead fins), the CG is still way forward.

I know you can override the CG of the rocket as a whole. But when I've done that, I found that when I add motors, the CG doesn't change - it stays where you put it.

I'm wondering if anybody else has noticed this, and what to do about it. Also, in files originally designed in OR, does anybody think the CG is a bit too far forward, even accounting for glue and such?

[Edit] I should note that I worry that if the CG is off, the CP may also be off, when importing RockSim files into OR - can anybody address this concern?
 
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How is a 1.2m rocket only 14.4oz?

Can you attach the OR files?
 
Iirc a mean machine is near that length and weighs about that much or less and has 12 calibers of stability.
 
I downloaded the quest big dog file from apogee and I see some diffs to your screenshot. My Length = 762mm and weight = 5.68oz. But the CG of 333mm looks ok given that the nose is set really heavy (1.6oz) compared to the fins (0.4 oz). When I add a motor it ends up at a couple of calibers of stability which I would expect due to the large fin span.

I'm using OR 15.0.3.
 
Another thing to make sure of is CG and mass overrides for individual components. A lot of time when I import a rocksim file it has those weird effects too, easy enough to fix with some patience.
 
View attachment Quad Cluster Mass 01.rkt
If you know your CG on the real rocket, you can override it in Open Rocket to match it to the real thing.

Hey, guys, sorry I didn't reply earlier. I've been busy the last week.

Here's the file:

View attachment Quad Cluster Mass 01.rkt

The nose cone is a bit heavy compared to some of the other components. But not so much that the CG should be as close to the nose as where it is in the above picture. And even if I override the mass on the individual components, the CG appears not to shift at all (example, as I was playing around with it, I overrid the mass of the nose to be 0.5 ounces and made the aft body tube 10 ounces, and no shift in CG).

Just to be clear, it's not this rocket in particular that I'm concerned about. It's just the most extreme example. This seems to happen when I open a RockSim file in OpenRocket. If I create a scratch file in OpenRocket, there appears to be no problem - the CG is a little far forward of where it should be, when you include glue, but otherwise it looks much more reasonable.
 
I was looking at the wrong file.

Buckeye is right - click on the "stage" section - you have a hard coded override on the entire stage.
 
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Total mass at the rear of the rocket = 0.84oz. (fins, motor mount, cr's etc.)
Total mass at the front of the rocket = 2.4oz (Nose and Chute).

CG looks reasonable in my open rocket - which version of OR do you have?

It doesn't matter very much what you do to the main body tube since the CG is relatively close to the middle of the tube.
It's more about the difference between front and rear parts.

I do see the CG shift when I change the mass of the NC (see attached screenshots).

I have version 15.03. I don't get any change when I override the mass of any parts. On the rocket I was referencing, it was made of two body tube sections. Even if I made the aft section very heavy (10 ounces) and the nose cone very light (0.5 ounce), the CG stayed put - very close to the nose of the rocket, and more than halfway forward on the forward body tube. Overriding the mass on the fins also had no effect. I could make them weigh two pounds, and the CG stayed where it was.

Maybe I've got a bug or something. If nothing like this seems to be happening to anybody else, maybe it's just my computer, or I need to delete and re-install OR. But for a CG to be that far forward, you'd have to have a very heavy object at the front, and very light objects at the back. The CG in my RockSim file sims is way forward regardless of how much I override parts.

Still, I find it useful for locating the CP. If I have a kit for which I can find a sim (on Apogee Components or RocketReviews.com), it helps me figure out whether I need to add nose weight, and calculate rough altitudes with different motors. I guess for now, I'll just override the CG to what I find on the real thing, until I can figure out what the deal is with my copy of OR. I was just wondering if anybody else noticed this weird thing, or if it was just me.
 
View attachment 276718

Hey, guys, sorry I didn't reply earlier. I've been busy the last week.

Here's the file:

View attachment 276718

The nose cone is a bit heavy compared to some of the other components. But not so much that the CG should be as close to the nose as where it is in the above picture. And even if I override the mass on the individual components, the CG appears not to shift at all (example, as I was playing around with it, I overrid the mass of the nose to be 0.5 ounces and made the aft body tube 10 ounces, and no shift in CG).

Just to be clear, it's not this rocket in particular that I'm concerned about. It's just the most extreme example. This seems to happen when I open a RockSim file in OpenRocket. If I create a scratch file in OpenRocket, there appears to be no problem - the CG is a little far forward of where it should be, when you include glue, but otherwise it looks much more reasonable.

Like I said, if you know the actual CG, you can override it in OR. In the list on the left hand side, click stage->edit->center of gravity. Input your value and make sure the check box next to it is checked off as well.

I don't use the calculated CG. I like to make sure I know the actual one, input it into OR and find out the stability margin based on that.
 
View attachment 276718

This seems to happen when I open a RockSim file in OpenRocket.

OR can't deal with everything in a Rocksim file. These are two different software, so don't expect perfect compatibility. Reading a rkt file in OR can save you from re-creating geometry and dimensions, but that is all I would trust it to do. You better double check everything.

Anyway, I read your rkt file into OR 14.11 and everything works fine with CG. You do know that the overall stage CG is overridden at 13.104 inches, right? This sets the CG of the whole rocket and supersedes any individual part overrides.
 
Gosh, guys, I'm a little embarrassed.

Buckeye: No, I hadn't noticed that the CG was overridden. That explains a lot.

T34zac: I knew I could override the rocket's CG, but in the original post, I thought that when I did that, the CG didn't change when I installed a motor. I can see now that isn't true. Perhaps I was remembering it wrong - when I changed the weight of a component, the CG didn't move (because it was overridden), but when I install a motor, it does.

I guess this thread was a little misguided. I should have played around with it a little more and maybe I'd have figured it out. Oopsie... I still feel like the CG sometimes looks like it's a little farther forward than it should be, but maybe glue is just heavier than I thought. :/

Anyway, thanks for the tips. I learned a few things from this thread, so I think it helped.

Happy Thanksgiving.
 
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