More on this nifty "center of mass" idea

A thread of 3 messages, 29 November 1992 to 13 December 1992, in the order they were sent.

#4995 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"

#4999 More on this nifty "center of mass" idea

> Seth the Lesser [email removed] writes:
>
> 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:
>
> ...
> 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.
> ...


   I've never liked using thrusters that accelerate a volume instead of mass.
One problem is with relativity: if mass does not affect the acceleration, you
can approach and exceed c without penalty to the acceleration. Also, how does
the maneuver drive define the volume to be accelerated? Can you extend it out
beyond the hull? Why are inertial compensators needed if every part of the
ship is being accellerated equally?

   Using mass-based thrusters does add to the book keeping however; you'll
need accelerations defined for empty, average, and full loads. But, in the
interests of keeping things from becoming too complex, I would not worry
about center-of-gravity problems.

- --

               Edjs                    _
              ------                _ //  CI$  : 76427,662
   [email removed]                  \X/   GEnie: [email removed]

#5012 Replies to recent topics

Rob Dean [email removed] writes:
> of how thrusters work--I've always assumed that they _really_ work on
> F=ma, and that the drive ratings are simple approximations of what per-
> formance would be expected with a typical weight on a given hull size.

I'd like to see this change, one way or another, in TNE.  Either
everything should be calculated based on tons of thrust and tons of mass
(which has the advantage of making sense), or both accelleration and
agility should be computed on volume of the drive field (which has the
advantage of being easy to calculate).


[email removed] (Edward Swatschek) writes:
>    I've never liked using thrusters that accelerate a volume instead of mass.
> One problem is with relativity: if mass does not affect the acceleration, you
> can approach and exceed c without penalty to the acceleration. Also, how does
> the maneuver drive define the volume to be accelerated? Can you extend it out
> beyond the hull? Why are inertial compensators needed if every part of the
> ship is being accellerated equally?

Well there is an intertialess drive in the "literature".
It's called the Bergenholm, and it's an old and honourable stardrive
(created by Edward E. "Doc" Smith for his Lensman saga) with a long
history of science fiction behind it.

But it's not Traveller.


[email removed] (Scott S. Kellogg) writes:
> I was just making some orders to GDW, when, I got switched over to
> Loren Wiseman.  So, here's some news fer ya'll.  I haven't seen any
> advertizements of ImperialLines, but they've put up a few issues,
> I gather they've already sent out issue #3.  They do still have
> some back issues for those of you who didn't subscribe yet.

What a weird bit of synchronicity.  I spoke to Dave Nilsen, apparrently
on about the same date that you were talking to Loren.  The subject of
our discussion was made public on TML-Not! (and presumably the thread
will move over here).

I will simply have to subscribe to Imperial Lines.  No way around it.

> Astrogators Guide to Diaspora is also out.

I have it; it's apparrently been out for about a month now.  It is very
nice, and I reccommend it.  The price is right for the amount of data
provided.  I have only a few minor nits to pick.  First of all, I found
the large map to be overly "busy" - trying to code travel zone
(Frontier, Outland, Wilds) as well as everything else was confusing.
Since this information was already represented on the small map, I don't
see why it was coded twice.  The text also includes some minor errata
for Hard Times and Assignment: Vigilante (the Outlands/Wilds coding in
the Hard Times map legend is backwards, and some of the world UWPs in
Assignment: Vigilante are wrong).

I forget which one it was in (I bought Assignment: Vigilante and
Diaspora Sector at the same time), but one contained Imperial Lines #1,
while the other contained a multi-page, full-color, slick-paper ad for
(UGH!) Dangerous Journeys.  Save your advertising budget, folks; I'm not
interested.

> We chatted fer a bit, and I will confirm Metlay's statement that
> it's nice to connect a human voice to the writing on the computer.
> (Yes, folks, he's human, and... Even *I'm* human. :-)

I'll add that Dave Nilsen is a good guy, too.  It turns out that he is
from around here (Rockville, MD; near Washington, DC) and we chatted
about some things of local interest.


From: [email removed] (Scott S. Kellogg)
> Specifically, The inertia & conservation of energy problems
> are solved, allowing someone to teleport with far greater
> freedom and even up to interplanetary ranges.  To counteract
> this new advantage, he proposed that there is a delay time
> between when a teleport starts to form in the new environ
> and when they can actually move and act.  (A few seconds
> making teleportation similar to the transporter effects of
> Star Trek.)

I don't think that this is enough of a disadvantage to offset the
greater freedome and range of teleportation.  Although this is a start.
Let me see if I can come up with some logical limitations (I wouldn't
mind seeing teleport become more powerful, as long as it didn't
unbalance the game).  Such things are not necessicarily too powerful ...

> If someone can teleport across interplanetary ranges,
> Ship to ship combat would be useless in the Frontier Wars
> with the Zhos.  A Zho teleport commando would be able to
> teleport right to the bridge and wipe it out.

First of all, teleportation (and clairvoyance, and any other psionic
power) should not be able to cross an artifical psionic shield.  I
assumed that it was ordinary practice for the hulls of Imperial warships
(and for the secure areas of important Imperial installations) to have
integral psionic shields.  End of problem.

> Now, if you want to have a mechanical transporter
> either akin to Blake's Seven, or (Ohmygod) Star Trek
> Then nuclear mayhem will similarly be out of control.

Number One: make the transporter unable to beam through a nuclear damper
and/or a meson screen.  That makes it much less useful for large-ship
actions.

I used a "Blake's Seven" style transporter in one of my early campaigns.
It had this limitation (and also couldn't beam through superdense or
bonded superdense materials).  Finally, it had a really significant
power draw (about 4ep, in High Guard terms, which would now be what,
about 1,000Mw) and a relatively short range (low orbit at most) and was
limited in the amount of material it could beam through.

Yes, it did give the players a big advantage.  On the other hand,
because they had the device, they got a lot of the otherwise
"impossible" assignments.  It all comes out in the end.

For many player character groups, this would be too much.  As a referee,
"Star Trek" sized gimmics go with "Star Trek" sized responsibilities.

Here's one for all of the psionics experts out there.  What are the
range and conservation of energy effects for a teleport going from one
artifical gravity field to another, or from one artifical gravity field
to a world's surface?


[email removed] (Michael A. Surman) writes:
> Chrysler or Chevrolet has installed a basic HUD as an option in
> some of their models. It only displays your speed but it does so
> by projecting the information onto the windshield.

Then again, one of the plants that GM will be closing is it's Willow Run
facility, which makes automatic transmissions.  Willow Run was
originally constructed by Ford in 1941 to build B-24 bombers for the
USAAF.  At 2.5 million square feet, it was the largest in the world, and
by 1944, it was producing a finished aircraft at the rate of one an
hour.  After the war it was sold to Kaiser-Fraiser to produce
automobiles and C-119 cargo planes; GM bought the facility in 1953.
Some of the original equipment installed by Ford (including heavy
machinery like the overhead cranes) is still in regular use.

Late TL-5 or very early TL-6.


[email removed]
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