CG for Beefed-up Phoenix

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Mar 2, 2009
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So, I left the 24mm MMT in my Phoenix, but put in a Kevlar shock cord, a 9x9" Nomex square and a Skyangle classic 20" chute. Loaded with an F21-4W SU motor, the pad weight is a hair under 15 ounces. The question is this: If you fly a Phoenix this heavy, where do you put the CG?

(The directions say the fully loaded CG should be forward of the joint where the two body tubes meet, and mine is about an inch forward of that point, but I can't spin test because the fins are in the way.)
As long as you haven't changed the CP, then you want the CG to be in roughly the same spoat as before. If, as you say, it's an inch or so further forward than it was, with the motor you want to fly it on, then you should be good to go.
the tube junction sounds about right

heres a screen shot of the CP from rocksim, I shaded the 1 to 2 caliber area
So the fact that the thrust is originating from further aft doesn't matter? (the nozzle is further aft with the longer E size motor)
the center of pressure(cp) will not change
the center of gravity(cg or balance point) will change

so you want the rocket to balance around the tube coupler with everything loaded (motor ,chute,shock cord,chute protector)
you might have to add nose weight to change the balance point(cg) to that spot

does that help?
if you want to run a string test, one thing you can do is thread some twist tie wire (or just string) through the whole rocket (meaning through the mmt and bt, so it wraps around, and tie it together between the fins, then tie a string to the cg of it, and you can run a test normally... its just hard to get an engine fitting with the twist tie in the mmt
A quick rocketry 101 lesson.
As long as everything externaly is the same then arodynamics will not change, regardless of weight.
So that means your CP (center of pressure) is the same as stock.
Rule of thumb, CG (center of gravity or point where model balances at) should be at least 1 caliper (body tube diameter) ahead (toward the nose) of your CP.
If manufacturer suggest that the CG should be at the coupler then anywhere from the coupler forward should be a stable rocket (Note: balance rocket in flight ready condition with chute, motor, wadding ect... installed)
(Another note: typically 2 calipers is about the most you want without being overstable, which could result in excesive weathercocking)
Originally posted by rocketgroupmike
I can't spin test because the fins are in the way.

Yes you can. Take off the nose. Run a piece of cord through the rocket. Tie the cord in a loop. Replace the nose. Take your swinging cord and tie a slip knot in the end, around the loop cord through the rocket. Slide it back and forth on the loop until you get horizontal balance. Tape the slip knot tight on the swing line. Now you can swing it.

You can protect your tubes, and prevent the loop from sliding, by applying some masking tape where the cord enters the tubes.

I use a chain of (new! fresh! not chewed up!) rubber bands for my swinging cord. That way when I spin down, the rocket comes down at or very near my feet rather than running into something around me.

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