Open Rocket help - Mass and CG Override - when to use it?

[soopirV]

Hi everyone- I tried a search on here, and came close, but no cigar- can someone clarify for me when and how to implement mass and CG override? I'm trying to align my physical builds with the .rkt or .ork files from the vendors.
My process, tell me if I'm wrong:
1) I weighed the subcomponents (nose with bay, ebay, payload airframe with harness, etc)
2) I'm overriding the subcomponent mass in OR, but am not doing anything with CG override.
3) I'm also overriding all subcomponents, since my mass takes into account the actual materials, plus adhesive, fiberglass, paint, etc.

Is this kosher?

Thanks!

Dave

 

[Bat-mite]

That is exactly right. Weigh your parts, override mass.

You would only want to override CG in circumstances where you have an oddity that physically causes CG to shift. One example I could think of would be a nose cone with a metal tip. What most people do is add a mass component at the tip of the NC that accounts for the extra weight; but this makes your finished rocket look weird, and takes some finagling to get the mass just right.

The other way to do it would be to weight the entire NC with tip installed, and then use the CG override to move the CG slightly forward to compensate for the bulk of weight being at the tip. Make sense?

I used the CG override in my L3 project because my measure CG was about 1/2" different from the calculated CG. I wanted my stability sims to be spot on, so I modified the booster CG to get the calculated CG to match reality.

 

[neil_w]

When you're all finished, measure the mass and CG and enter them as overrides. All ORs calculations for those are useful at the design stage, but one you're built then Empirical results trump the estimates and calculations.

[edit] Just saw Bat-Mite's response... For really big stuff, it may be more convenient to use OR rather than measure... But if you're able to measure CG when you're done, it's good to compare it to the prediction, and use the override if there's a difference.

 

[Rex R]

yes but time consuming. that is pretty good for the planning steps. many of us will take the flight ready rocket (minus motor) weigh it, and find the cg point to set the overall (stage) mass & cg over rides, this gives you real world numbers to get more accurate simulations.
Rex

 

[soopirV]

Thank you all for your quick and excellent replies! Each of you raise great suggestions, and maybe I'm overthinking this, but...I haven't yet settled on parachute size, and was hoping to drop in the different masses (granted, many of these are multi-pound birds, and my largest chute is about 8oz, so...) to see what effect it had, and to check decent rates. I'm realizing as I type this that I'm trying to measure a fart in a hurricane, so will just weigh the assembled bird, override the stage mass, and call it a day!

@BatMite- thanks for the additional great explanation about CG override- makes perfect sense now that you've explained it!

 

[Banzai88]

yes but time consuming. that is pretty good for the planning steps. many of us will take the flight ready rocket (minus motor) weigh it, and find the cg point to set the overall (stage) mass & cg over rides, this gives you real world numbers to get more accurate simulations.
Rex

This

 

[ThirstyBarbarian]

Just to boil down what everyone else said:

Component overrides are for the planning process to more accurately estimate the built mass and CG of the rocket.

Mass and CG overrides for the whole stage using measured mass and CG of the built rocket are for accurate flight sims.

 

[TopRamen]

This

Agreed.
I sometimes don't even get around to playing with Openrocket til' after the first flight, and then just to see what the rocket can do on different motors that I know are to powerful for my flying locations.
It is fun to be meticulous on odd birds though, but then the Openrocket typically cannot simulate them accurately for flight sims anyhow, but it is still fun.
Today I'm angry at Openrocket, because it screwed up a save of an upscale, but I had already printed the PDF design report and preliminary sim figures, so I just barely got done throwing it back together, but this time with the figure from the printed PDF, instead of tying to upscale a current OR file. I have no Idea what caused the error, but when I upscaled it again to start over, I tried to save it, and it lost it again, so I just started from scratch, and saved it as soon as I had the cone, body mmt and transition put in, just to make sure I could get it back by "opening recent", and that it had saved it with the correct name and to the correct location.
I'll keep a stick drive handy and put my updated files there instead from now on, but the rocket is getting built regardless of Openrocket.