Customizing tutorial please

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Jan 18, 2004
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I've read on some of these posts about increasing nose wieght and other stabilizing methods when increasing engine size or weight.Can anyone suggest a method for figuring how to properly figure these weight variances?
The mean machine is almost ready for primer and paint and will fly with a D.Then I want to go up to an E.Probably won't need to adjust this one but when I increase power on another rocket or build a custom I'd like to be safe.

John :cool:
There are a few different ways to figure the CG/CP relationship. You can use the search feature within the forum to bring up some really neat threads regarding this topic.
If you don't already have it, you can also get the Rocketeers Bible...Harry Stines Book."Handbook of Model Rocketry"
You'll find it very informative and a great book on the hobby.
You can get a copy at for about 15.00 (used)
Ditto what flying_silverad said.

In addition, if your concern is about the stability of an existing design with a bigger (heavier) motor is your concern, there's a very easy fix.

As you pointed out, something like the Mean Machine won't be affected as much by going from, say, a D12 to an E9, but shorter rockets and the like, *would* be.

You may also be in a situation where you're going from a single motor to a cluster, increasing the motor weight 2, 3 or even 4 times, which would have a big effect.

If you're modifying an existing stable rocket, here's the quick and easy fix for stability.

Adding weight to the tail moves the CG but does NOT affect the CP. Load the rocket up stock (with the recommended motor(s)) and balance it to locate the CG. Now, change the configuration to the new motor(s) and balance again to locate the new, or moved CG. Now, add nose weight to bring the CG back to where it was originally.

You are assured stability. The stability margin will be identical to the original kit/design.

hope this helps!
Like the others said, there are a few ways and tricks.
How complicated you want to get depends on how confident you are in your math and geometry skills.
No problem with the Mean Machine. Other kits that you modify you can use the method Jim mentioned.
Another easy thing to do is just search for the rocket here to see if anyone has already done it, and you'll be able to find out what you need to do.

Rockets of your own design are another story. You can use a program such as Rocksim. Or you can use the cardboard cutout method.

Personally, I figure out the surface area of the rocket to find the CP and then adjust the weight accordingly to get a desired CG/CP relationship.

Let us know if this isn't helping, or if you need more specifics.

If you like math there is a page created that explains the equations for finding the center of pressure for most regular rockets. I've done the math for several rockets to find CP as well as determining expected altitude from a given motor.
The Rocket Equations Page will give you the formulas and an explanation of how to use it.

I also want to Plug VCP which is a freeware program that has many useful features and calculates CP/CG relationships if the data is entered properly. The Downloads section of Cole Fired Rockets has VCP available for download.

Rocksim is a great program from what I've seen but like all simulation software they are only as good as the data you enter into it. The more precise you are the better the result would be especially when relying on the margin of stability.

I like Jim Flis' recomondation for modifying kits. Many commercial kits have a very good stability margin but if it's an oddroc I may be tempted to go a little extra. (Add length or fin maybe.) Lots of other good ideas in this thread already.

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