Using mass override in Rocksim question

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:

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.

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

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.

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.

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.

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.

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.

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.

Big_Red_Daddy said:

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.

Click to expand...

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.

Buckeye said:

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.

Click to expand...

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.

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.

ihbarddx said:

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.

Click to expand...

Keep in mind also that sometimes sliding mass objects such as parachutes shift aft during thrust, which of course shifts the Cg rearward.

Good point, Steve.

I have a question on the mass override option. I have 4 fins which are heavier do to thick fillets. I click on Mass override and I add the grams for weight added to the fins. Show to change the mass I click on use the wt and cg entered below. I have added the grams but where to I pull the cg from. I'm assuming this is the cg location of the fins so is this printed in one of the graphs or do I measure it on the rocket, or is this the existing cg listed for the rocket in the side view lower screen. From where do I pull the cg for the mass I am adding to the 4 fins?

Just started using Rocksim so be nice.
Sterk03

If you are adding the CG location of the fins themselves just think of the fins as a separate entity and where their own CG would be. Probably somewhere in the middle of the fin. Although there’s not much point in doing that. Best to finish the rocket and override mass and CG for the whole rocket. That’s going to give you the most accurate sims.

Ok thanks , like I said I'm just starting out in Rocksim. So your saying if that is all I'm adding just override the new weight and let it calculate the new CP?CG? But don't I have to say where the new mass is entered in order for the sim to calculate the new cg/cp? I guess I missing something I don't have the new cg that is what the sim is figuring so I know Cp/CG if stable?

If you are talking about CG for just the fins, there’s no need to enter it. If you don’t enter it, it will default to something like 0 which means 0” from the forward end of the object. The important thing is to override mass and CG for the entire model. Balance the rocket on your finger and enter the CG in inches from the forward tip.

Yes Ok I got it. I forgot to mention I am using the Estes Vagabond Rocket and I added a 4th fin so I can add strap-on-boosters later. I already got the 4th fin added but I have added weight for too much filet material that is all I'm adding and wanted to see how that moved the CG/CP prior to then adding the strap-on-boosters to see if it remains stable. Thanks,

Sterk03