More on this nifty "center of mass" idea

Jeff Zeitlin ([email removed]) talked about center of mass as
an important, neglected consideration in Traveller & MT ship designs,
to which I reply from my sickbed with fevered brow and palsied typing
finger:

I agree that center of mass is a critical design consideration in
Traveller spacecraft, but not for quite the same reasons that you
raise.

F=ma is important, yes, and the design rules for *anti-grav* vehicles
take it into account, by guaranteeing that one has no more thrust than
one pays for.  The design evaluation step in MegaTraveller (and
presumably Striker, though, cheapskate that I am, I never got around
to buying it before the change-over) explicitly uses F=ma to determine
the maximum possible acceleration of a craft with a given
thrust-to-mass ratio, then applies black magic to convert the maximum
acceleration into a maximum safe speed in atmosphere.

However, as soon as we change over to "thrusters" as a means of
propulsion, we wave hands and make F=ma go away.  Instead, we
calculate the acceleration of a ship based on its thrust-to-*volume*
ratio.  This has never seemed entirely satisfactory to me--what it is,
is an attempt to make all those _High Guard_ ship designs vaguely
salvageable in MegaTraveller--and I've tried several times to juggle
the rules to make mass more important than volume in determining
thruster acceleration.  Unfortunately, doing so *demands* that ship
designers sit down with a spreadsheet (or a custom set of software for
their programmable calculator) and iterate, iterate, iterate until
they get a workable design.  If your players never want to design
their own ships (or you never give them the money to do so :-), and if
you have a computer and a lot of time to spare, this is all fine and
dandy, but as a general mechanic for the non-computer-oriented gaming
public, it just doesn't work.  IMHO.  (Whoops, that should probably be
at the top, shouldn't it....)

However, none of this is strictly relevant to center-of-mass.
Center-of-mass (or center-of-inertia, or kurtosis point, depending on
what power you raise the moment arm to) only comes into play when you
dig out the ol' 1 cm grid and start doing the deckplans.  Although
there's no intrinsically preferred attitude in vacuum, the Traveller
"thruster" drives have a preferred axis of thrust.  If this preferred
axis of thrust *doesn't* pass through the center of mass, you'll find
yourself spinning rather rapidly, since torque equals the
cross-product of radial displacement and force.  Or else you sacrifice
maneuver power by biasing the plates to thrust a bit off-axis.

Since Traveller has never had much more than a vague set of guidelines
for doing deckplans, there are no rules for dealing with
center-of-mass.  I take it into account in a vague sort of way in my
own deckplans (which are few and far between, since I'm an absolute
perfectionist), and then make players sweat about it when they load
and unload cargo.  It's nice to see the supercargo of a free trader
ordering the sweating, cursing, exhausted command crew to "push that
container just a liiiittle more to port...a liiiittle more...wait stop
that's too far" as he frantically fingers controls on his hand
computer.  It's *really* nice when a blown die roll by the supercargo
can mean a lost battle a few days later.  Makes his player feel wanted.

Seth "the Lesser"