An alternative to whole-rocket mass and CG override in simulation

[neil_w]

I just posted this in another thread, thought I'd elaborate a bit here for posterity.

DISCLAIMER: I have no idea if this is common knowledge. If so I apologize.

The standard practice to get most accurate flight sim data from OR or Rocksim is to measure mass and CG of the completed rocket, and then apply those numbers as overrides. There is a problem with this approach, though: once you apply the overrides, further changes to the rocket are ignored with respect to mass and CG. Why do we care? Well, maybe you just finished construction of your rocket and you need to figure out how much nose weight to add for stability. You want to come up with an answer based on the actual constructed rocket, but then if you apply the whole-rocket overrides then you will not see the affects of the nose weight.

Here's an easy solution.

1. Measure the mass of the rocket and CG before nose weight is added.
2. Subtract the total rocket mass indicated in the sim. Usually your result will be positive, because actual builds are heavier than the sim. If this is not the case, or if the difference is very small, then make your sim lighter by overriding the mass of a significant component (the airframe is a good choice generally) to zero. Then subtract again, and you should get a bigger, positive number.
3. Add a new mass object equal to the difference you just measured.
4. Adjust the position of the mass object until the CG matches the value you measured in step 1.

Aaaaand you're done. Now you can add nose weight and play with its mass and position and see the results reflected in total rocket mass and CG.

I kind of like doing things this way all the time (at least, when I remember) because this leaves the sim ready for any future tweaks.

Enjoy!

Addendum: as pointed out below by @Buckeye, neither this method *nor* using the whole-rocket override provides accurate moment of inertia data. If that is important to you then you'll need to use a different approach.

 

[Buckeye]

Yes, I do this.

If you want moment of inertia data, then total vehicle override is not good, either. Need to override the individual components.

 

[neil_w]

When is accurate moment of inertia data important? I've not really thought about it.

 

[jqavins]

It is critical to dynamic flight performance. OK, so you're stable, meaning the rocket will tend to right itself. But how far will a wind gust swing it before is turns back toward straight, and how fast will it come back? How widely and how long will it oscillate? If you care about any of that then you need accurate mass distribution, not just total.

Consider this: the method you've described could be modified by the use of two new mass objects instead of one. Place one in the nose and another in the tail and adjust them together, keeping the total constant, until the CG lands in the right place. The flight performance will be different from the single mass object method. Whether or not it's different by an important amount depends on the specific details of the design and on what you call important.

 

[Buckeye]

When is accurate moment of inertia data important? I've not really thought about it.

Coning, pitch-roll coupling. So I have read. I have one rocket that wiggles off the rail. I reduced the MOI, I think, and it flys better. I am no expert on this and my data is limited.

 

[neil_w]

I guess I have never looked at this in my OR sim results (nor, for that matter, is it likely to be particularly significant for my LPR stuff, I think.)