Model Forum / General / Rockets / June 2006

kevin:

you are probably going to hav to generate this date yourself from used
engines . I once did this for A3-4T's and the CG moved around up to a
1/4".... same goes for loaded CG..although closer as the weight difference
was in 10th's of a gram...

terry dean

One important thing to keep in mind here, is that in an end-burning BP motor,
the CG will only move forward during the burn.  Therefore, any effect the motor's
CG change will have on the rocket's stability will be positive, except in the case
of certain esoteric designs (i.e., tractor rockets).

You may just have to measure the rocket's Cg with an unburnt motor and
then with a empty motor case and then interpolate. As mentioned before,
since Estes motors are end burning the Cg will progess forward at an
initially fast rate then will slow down as it reaches the end of the
burn. Core and c-slot engines, like Aerotech, probably won't change Cg
much, unless you are using the 29mm case with like a E or F reload.

I think, though, that the Cg of a motor is not going to be as critical
as the affect of the loss of propellant during flight on the model's
Cg. An end burning motor will move the Cg forward fasted than a
core/slot burning motor.

Tim Barr

Thanks to you all for your replies. At least I now know that I was searching
for this information in vain and can get on with experimenting rather than
searching.

We're new to rocket design and simulation and haven't yet built up enough
experience to know which factors really matter, which can be approximated
and therefore when we must chase a design factor down rigorously or just go
with the flow. So meanwhile, we're assuming we should find all the facts we
can.

Having flown Estes motor-based rockets so far we're just beginning to look
at RMS too and hence core-burners are on our horizon. And it hadn't occured
to me that the motor Cg positions would be affected differently from the
end-burners.

From here on we'll start keeping and testing our bunt-out motors to see what
happens to the Cgs experimentally.

Many thanks once again.

Kevin