Using mass override in Rocksim question

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Bill S

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I'm working with Rocksim to design a rocket, and I have a question. I stumbled across some blog articles that talked about the weight of paint, and how it can affect stability/altitude achieved upon launch. As far as I can tell, Rocksim doesn't account for paint. I read in the articles that they guy's rocket actually got more stable as he added paint weight to it.

I ran some estimates of paint weight vs new stability numbers, and I'm not sure I'm using the program correctly. I've been using Barrowman numbers for stability - I was under the impression that they were more conservative. Though this may be an error as the design is nowhere near typical.

Baseline rocket with engine but without paint: 5.744 oz, 1.67 stability (Barrowman). Center of gravity is 16.025" from the front of the rocket. I estimated the added weight of paint to be 1.06 oz (rocket without engine is 4.24oz and I assumed 25% extra weight (1.06oz). Total weight of rocket with engine and paint should be 6.8 oz. I put that in the Mass override and enabled it, and now the rocket is stability 8.74 (overstable). I imagine that I should also override the center of gravity, setting that to be 16.025 (the original CG before adding paint weight)? If I do that, then the stability drops to .74 (marginal).

If I switch to Rocksim stability equations, then stability is 2.13 w/o paint, and 1.2 with paint, assuming the CG is forced to be the same as it was originally.

Am I doing this correctly? I wanted to get things sorted out before I built the rocket.

Picture of the rocket in question:model without markings (original condition).png
 

Big_Red_Daddy

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Definitely don't include motor weight if you override the mass. The sim programs will add the mass of the motor you select. You are correct to include actual CG (without motor). Paint should be pretty even all over and have a minimal effect on the CG. Paint should not affect the rocket as much as you are indicating but I suspect it's the additional motor weight throwing things off.
 

Bill S

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Thank you. I'll try re-running the program, not including the weight of the motor in the initial calculations.
 

rcktnut

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Complete the rocket paint and all, weigh the rocket with everything in place as ready for flight except the motor, find the CG, enter those figures, then sim. Trying to figure out the weight of each component is a waste of time, unless you have certain constraints on the design. The weight, CG is what it is when the rocket is complete.
 

JohnCoker

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Yep, I don't bother to model the internal structure in RockSim, just override the measured weight and C.G. (without motor) when you check stability and runs sims.
 

Big_Red_Daddy

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As a general rule, I use the kit components weight and add 50% to account for glue and paint, and electronics so I have an idea of final weight before I start. But actual mass as built is what you are going to get good sims with. It's rare that a manufacturer has correct mass listed for their kit. I built a LOC kit recently and the components weighed more than they had listed on the packaging.
 

Bill S

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I recalculated the stability with the motor removed. It turns out I was accidentally using the CG for the rocket with the motor installed, not the model without the engine. Once I forced the mass to the correct number and used the unloaded CG for the rocket, then loaded the engine into the simulation, it gave better stability numbers. (whew).

I definitely want to get a scale that can measure .1 gram increments so I can get a more accurate picture of what each part weighs, as well as the final weight of the rocket (without and with paint).

I plan to string test the rocket in any case for peace of mind before bringing it to the public launch. :)
 

Big_Red_Daddy

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You don't need very accurate mass for each item. Even if you build an accurate model you always have to override with actual mass and CG.
 

Bill S

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True. More for my personal info, and might help me figure out ways to make the rocket lighter or improve future designs. Much to learn, I have. :)
 

Buckeye

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You don't need very accurate mass for each item. Even if you build an accurate model you always have to override with actual mass and CG.
If you are interested in moment of inertia, then you need the mass of each component, not the override. I had a model that was showing signs of coning, and I fixed it (I think) with MOI analysis.

Uh, by definitiion, if you build an accurate model, then you don't need the override.
 
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Big_Red_Daddy

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If you are interested in moment of inertia, then you need the mass of each component, not the override. I had a model that was showing signs of coning, and I fixed it (I think) with MOI analysis.

Uh, by defiinitiion, if you build an accurate model, then you don't need the ovverride.
I build incredibly accurate models in Rocksim. I use calipers, and a scale In grams. Check my files on RocketReviews. But that doesn't account for adhesives and paint. So once the model is done it's best to override with actual CG and mass.
 

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ihbarddx

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A tangential point: Always check the actual CG just before launch, and always recompute your stability margin. Actual CG often depends on decisions you make during prep. (And, BTW, if the mass is wrong, chances are the CG is wrong.)

I can remember a 2005 N-class record-setting attempt that resulted in a loop-de-loop, evidently caused by wind shear and too much faith in the sim-calculated CG. Once propellant mass burned off, stability was restored, but it wasn't pretty.
 

Steve Shannon

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A tangential point: Always check the actual CG just before launch, and always recompute your stability margin. Actual CG often depends on decisions you make during prep. (And, BTW, if the mass is wrong, chances are the CG is wrong.)

I can remember a 2005 N-class record-setting attempt that resulted in a loop-de-loop, evidently caused by wind shear and too much faith in the sim-calculated CG. Once propellant mass burned off, stability was restored, but it wasn't pretty.
Keep in mind also that sometimes sliding mass objects such as parachutes shift aft during thrust, which of course shifts the Cg rearward.
 
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