calculating CG

[graylensman]

is there a way to determine the CG of an unbuilt model design without an app like RocSim?

 

[rstaff3]

Of course. You can fall back to the good ol' Barrowman equations:

http://my.execpc.com/~culp/rockets/Barrowman.html

You can also get freeware like VCP or demo versions of the several common packages.

 

[cls]

Greylensman, did you really mean CG or CP? Barrowman calculates CP. to calculate CG, well it's easy if you have the components and a scale but it is a lot easier to have a computer do it. especially if you are doing a lot of "what-ifs".

 

[rstaff3]

oops, I read CP in the original post. You don't need a scale or a computer to find CG, just a ballance point such as a finger.

 

[graylensman]

Yes, I really did mean Center of Gravity. I've already got CP figured.

I'm intending to scratch-build a semi-scale down-n-dirty Saturn V - mostly because I've got a significant length of BT-60 sized tube and no other plans for it. Ran some quick calculations, and discovered that the rest of that rocket corresponds very neatly to BT-50 and BT-5 tubing, as well.

BUT: I'm trying to do this on the cheap (read: I'm flat broke) and thinking of making the fairings and nose from paper. So that affects the CG. Additionally I want to make sure that the fins will provide enough stability at scale size, or see how much I may need to scale them up.

I suppose I could cobble together tubes, MMT, motor, etc. and do a simple balance test. But I'd like to not have to cut tubes if the thing's not gonna work.

The double-whammy is I'm running a Mac... and all the sim apps are Windows.

 

[Stymye]

you can draw the rocket on paper full size ,you would have to know the weight of all the components you plan to use,insert the weights into their relative positions...divide the total weight by 2 ,measure back from either end untill you find that number.
alot of overlapping components will make it a little tricky

on the other hand if you have all the components to weigh and a scale..it would simply be easier to dry build the rocket and balance it

 

[graylensman]

Stymye,

I think you're right, I'm just gonna dry bulid it and go from there. A old Centuri tech report says for that method, don't worry about the fins; their weight is negligible.

Thanks to all for help. I'll keep you posted.

 

[cls]

you guys are right, it's easiest to just build it and balance it on your finger and add noseweight as required. I read the original post as "how do I figure out if this is going to work without actually cutting material" - in that case ya gotta calculate it.

anyways greylensman, want to post pix when you build whatever it is you are building?

 

[rstaff3]

Gee, first I answered the wrong question, then I followed up with a simplistic answer that deserves a "DUH". To follow up with another useless comment...stymie is right.

Now on to the build thread

 

[Elapid]

goes unpunished!

it's my version of Murphy's Law.

 

[graylensman]

Originally posted by cls
you guys are right, it's easiest to just build it and balance it on your finger and add noseweight as required...anyways greylensman, want to post pix when you build whatever it is you are building?

Oh, yeah, I will. It might fall under the "what the f- was I thinking" category, but what the heck.

 

[DynaSoar]

Originally posted by graylensman
Oh, yeah, I will. It might fall under the "what the f- was I thinking" category, but what the heck.

Fact is you could sy6nthesize it, either on paper or with stick-and-weight modeling. But you'd have to know not only the weight of each piece (or collection of identical pieces, such as fins) but also THEIR CG, so you'd know where to place things on the model/weights on the stick.

The more I build big birds with heavy duty MMTs, the more I'd like to be able to model CG first. I prefer to not add dead weight.

 

[graylensman]

Originally posted by DynaSoar
The more I build big birds with heavy duty MMTs, the more I'd like to be able to model CG first. I prefer to not add dead weight.

Exactly! I know I could just stuff a bag of lead weight in the nose and it'll be stable, but the model would probably stagger fifty feet into the air and fall like an anvil, even with a chute.

 

[Peter Alway]

Working on my little (2.04"-diameter) Saturn V, I've come to see that a little bit of fin oversizing does you a lot of good.

Peter Alway

 

[graylensman]

Peter, how oversized is a little oversizing? I'm working with BT-60 (1.6" dia) as the first two "stages".