Calculating CG Over Flight

[SpaceBound2020]

Hey everyone,

I was wondering if anyone has come across a book or resource that talks about how to calculate the CG over the course of a flight. It can be a book that is really heavy on equations. Topics in Advanced Model Rocketry had some good information but if you know of anything else I would appreciate it.

Thanks in advance!

 

[jadebox]

Did you mean CP? The change in CG is usually pretty simple. The CG moves as the motor burns (reducing the mass of the propellant).

Assuming that nothing else leaves the rocket or shifts around inside it, the loss of mass in the motor is the only thing affecting the CG during the flight.

RockSim's .rse motor data files include the mass and motor's CG in addition to the thrust at each point of the recorded thrust curve. I am not sure if RockSim really uses the mass and CG in the files or how accurate the data would be anyway. I don't think it is hard for a simulation to approximate the mass and CG during the burn and with the relatively short burn times of most of our motors, a simple approximation is probably good enough.

CP, on the other hand ...

 

[dhbarr]

Measure CG full. Measure CG empty. Divide over intended burn profile.

CP now, that's shifty.

 

[Banzai88]

If I ever have questions like this, I just click the appropriate boxes in launch simulation mode in Open Rocket.....and all is revealed.

ETA: I understand the underlying math, but when I have the automated tool, I can go on to do other things with the times saved.

 

[GalantVR41062]

Center of gravity is easy, with the rocket ready to fly mark CG, then remove the consumables, propellant and delay grain, check again.

The center or pressure changes as you go past mach 1.

I found this picture on the WM Blackhawk 98 page.

~John

 

[SpaceBound2020]

Center of gravity is easy, with the rocket ready to fly mark CG, then remove the consumables, propellant and delay grain, check again.

The center or pressure changes as you go past mach 1.

I found this picture on the WM Blackhawk 98 page.

~John

I just saw that graph the other day! Thought it was neat.