Rocsim and CG question

[smoon]

I just finished building a Madcow 4" fiberglass Nike Smoke and modified Madcow's Rocksim file as I went with eights I measured and adding everything I put into the rocket.

I ended up with the most accurate Rocksim file I have ever created, but there is one problem. The CG Rocksim tells me I should have for my rocket with no motor is a full 6 inches plus further forward than my measured CG with no motor. I re-checked weights of the nose cone and sustainer, and they matched, as did the placement of all the parts where they are in the rocket.

Does anyone have any idea why the Rocksim CG is so far forward of the measured CG?

Thanks,

Steve

 

[EXPjawa]

If you manually entered the component mass numbers, did you also enter the CG locations correctly? IIRC, if the CG location is shown as "0" when the manual overrides are used, it'll assume that the CG is at the very front of the part. Also, I think that there is a manual override for the CG & mass of the entire rocket. Make sure that isn't checked.

 

[wighty44]

If you manually entered the component mass numbers... if the CG location is shown as "0" when the manual overrides are used, it'll assume that the CG is at the very front of the part. Also, I think that there is a manual override for the CG & mass of the entire rocket. Make sure that isn't checked.

That is correct. Whenever a mass override is used, you must also enter a CG for that specific part. For simple components with uniform material densities (like a BT, MMT, etc.) the CG is at their mid length point. But for more complex components (fins, NCs, etc) the CG is more challenging to compute. Of course there's always the string-balance method... :wink:

 

[smoon]

Thanks, I did not know I had to set the CG myself when overriding Rocksim's calculated component mass.

With so many components that I could not figure out the CG of, I will do what I have been doing. I will use Rockim's CP point and my measured CG.

 

[jahall4]

Thanks, I did not know I had to set the CG myself when overriding Rocksim's calculated component mass.

With so many components that I could not figure out the CG of, I will do what I have been doing. I will use Rockim's CP point and my measured CG.

Unless you think a kit is flawed somehow there is little reason to override mass and CG of the individual components. However, I think you'll find that using override for components is quite useful for custom designs, but then only for certain parts.

 

[wighty44]

Unless you think a kit is flawed somehow there is little reason to override mass and CG of the individual components...

This may be true for smaller kits, but for larger rockets I've found that depending upon how you build them the CG will change. I suspect that accounting for the weight of things like epoxy, parachutes, shock cords, etc. can make a difference.

Personally, I like the challenge of adapting a basic RockSim file to an as-built condition. In my experience, I have found many components in basic online RockSim files that have needed changes to their dimensions and weights. Most of my rockets have been bought from Madcow, Rocketry Warehouse (Proline), Sunward, and Dynastar. YMMV... :smile:

 

[jahall4]

This may be true for smaller kits, but for larger rockets I've found that depending upon how you build them the CG will change.

Certainly, but the comment you quoted was about INDIVIDUAL COMPONENTS not the whole rocket that you "build".

Personally, I like the challenge of adapting a basic RockSim file to an as-built condition. In my experience, I have found many components in basic online RockSim files that have needed changes to their dimensions and weights

I do too and you should if the dimension and mass change but if you can't dependably/readily determine the CG OF THE COMPONENT than you don't want to override just its mass.

 

[EXPjawa]

Certainly, but the comment you quoted was about INDIVIDUAL COMPONENTS not the whole rocket that you "build".

Going along with this, one thing that users should keep in mind is that for the individual components to translate into realistic numbers for a final assembled rocket is that the mass of paint and adhesives need to be added as an individual component as well. And it can be tough to judge the actual placement of these in relation to the rocket itself. I'd usually put the paint mass at the CG, since its likely to be relatively evenly distributed. But the glue/epoxy is almost always justified aft, since the motor mount, fin attachment and fillets are where most of it is concentrated. So, depending on how much you've used and were you place it, it can have a notable impact on actual CG location. What I do is estimate the mass of glue and paint based on size, and then weigh the finished model. The delta in predicted versus actual (almost always in the heavier direction) is then added, and the position shifted so that the measured CG matches the predicted CG. This usually works...

 

[jahall4]

Going along with this, one thing that users should keep in mind is that for the individual components to translate into realistic numbers for a final assembled rocket is that the mass of paint and adhesives need to be added as an individual component as well. And it can be tough to judge the actual placement of these in relation to the rocket itself. I'd usually put the paint mass at the CG, since its likely to be relatively evenly distributed. But the glue/epoxy is almost always justified aft, since the motor mount, fin attachment and fillets are where most of it is concentrated. So, depending on how much you've used and were you place it, it can have a notable impact on actual CG location. What I do is estimate the mass of glue and paint based on size, and then weigh the finished model. The delta in predicted versus actual (almost always in the heavier direction) is then added, and the position shifted so that the measured CG matches the predicted CG. This usually works...

Yes well said. I suspect that some using design software don't realize that using (or not using the mass override) for the WHOLE rocket is not either/or it is "and". You use both as part of the design and build process.

 

[rstaff3]

I don't have the patience to try to get everything right on every component at the design phase. I generally try to get the outside shape right, put in the motor mount(s), add a recovery device so the thing will sim. I will add nose weight to get the CG right and then override the weight and CG for the whole rocket. Usually there's some intermediate checks/iterations but that's the basic idea.

 

[jahall4]

I don't have the patience to try to get everything right on every component at the design phase. I generally try to get the outside shape right, put in the motor mount(s), add a recovery device so the thing will sim. I will add nose weight to get the CG right and then override the weight and CG for the whole rocket. Usually there's some intermediate checks/iterations but that's the basic idea.

I'm building an unusual design right know where the position of the mass (not just the CG) is critical to the Damping Ratio.

 

[Buckeye]

What is the difference? Position of the mass is the cg.

 

[Titan II]

https://apogeerockets.com/education/downloads/Newsletter197.pdf

 

[jahall4]

What is the difference? Position of the mass is the cg.

https://apogeerockets.com/education/downloads/Newsletter197.pdf

To summarize on the article Titan II referenced... where the "masses" are on the rocket does impact stability and performance. Are you familiar with the rolling ring vs the rolling disk physic demonstration?

 

[rstaff3]

Fair enough.

 

[Buckeye]

To summarize on the article Titan II referenced... where the "masses" are on the rocket does impact stability and performance. Are you familiar with the rolling ring vs the rolling disk physic demonstration?

Yes, and also how figure skaters conserve angular momentum.

From the Apogee newsletter, I assume you are referring to these statements:

"Move the heavy items closer to the middle of the rocket"

However, simulating the effect in RockSim is not mentioned. Does Rocksim actually calculate damping ratio with the component mass distributions? This would mean that you can never use the total vehicle mass override if you want the software to compute damping ratio.

 

[samb]

Fair enough.

Some guys need to science the s___ outta this. :grin:

 

[jahall4]

... I assume you are referring to these statements: "Move the heavy items closer to the middle of the rocket"

Not directly, but the quote would be an action one could take to alter the longitudinal moment and hence the damping ratio.

However, simulating the effect in RockSim is not mentioned.

RockSim's graphs and hence simulations are used throughout the whole series of articles on the "Basics of Dynamic Flight Analysis". Look at page 4

Does Rocksim actually calculate damping ratio with the component mass distributions?

Yes it appears to. I saw changes when mass locations changed; however, those changes can also affect the margin, which of course are directly related to each other. I have not attempted, yet, to isolate those types of changes. What was interesting was that my damping ratio increased (which was what I needed) when the margin narrowed.

This would mean that you can never use the total vehicle mass override if you want the software to compute damping ratio.

That's a good question. To avoid answering it I often add mass objects for adjustment purposes. I'm finding more and more that RockSim "does things" and makes assumptions without telling the user.