"Jon S. Berndt" jsb.at.hal-pc-dot.org wrote in message
...
"Murray Anderson" wrote in message

The CG is way behind the CP at least until the SRB is close to burnout.
There isn't all that much weight up front. The specific gravity of a
Centaur
upper stage is something like .25 and the payload, including the payload
fairing, is even less dense. Even when the SRB is half burned the CG is
2
or
3 diameters behind the CP.

Is the CG really so far behind the CP - and for that long? Where do you
get
that data from? Remember 51-L? They _appeared_ neutrally stable, to me, as
they flew from 73 seconds to destruct - for some of that time flying nose
forward into Vinf. Add on an upper stage and a "20 metric ton" CEV and I'd
guess that the CG is ahead of the CP.

Might be an interesting "extra credit" problem for someone ...

Jon


The SRBs probably are dynamically stable when flying alone. They have that
skirt at the bottom to act as small tail feathers. Now suppose you put a
5-meter wide, 18 meter long upper stage on top, and a CEV under a payload
fairing on top of that (say 5 meters by 10 meters). Then if you trace the
projection onto a piece of cardboard, cut it out and balance it, you get a
rough approximation to the CP, which is way in front of the CG, even when
the SRB has burned half its propellant. The stuff up front destroys
stability if it's less dense than the stuff behind.

Murray Anderson