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SV: FTFB Turn Arcs

From: "Oerjan Ohlson" <oerjan.ohlson@t...>
Date: Fri, 27 Nov 1998 19:55:24 +0100
Subject: SV: FTFB Turn Arcs

Chris Lowrey asked:

> >Because the timescales are different. One game turn in FT is
> >agreed upon" as ~15-20 minutes, whereas one EFSB turn is maybe 60
> >seconds. 
> >
> Where did you find these numbers at?

The FT time scale was derived from the MT orbital mechanics a long time
ago (...Thomas Barclay, wasn't it? Or have I mixed you up again? Can't
find the post now, but it is somewhere in the archives.) I put the
"commonly agreed upon" within quotation marks - they're a rough
rather than written in stone, and there are many (Laserlight, for
example) who have derived other scales as well.

The EFSB time scale is derived from comparing how fast ships are
destroyed in the show with how fast they die in the game :-)

Laserlight wrote:

> I still think 450 seconds (7.5 minutes) is the way to go.

Not in EFSB - or, rather, not if you compare EFSB battles with battles
from the show - especially the amount of damage they inflict on one
another :-/ For FT, your scale works as well as anyones as long as the
size of an MU and the g rating of a thrust point matches :-)

> > "Realistically", a change of facing for a starship takes on the
> > one minute to execute.
> Based on what?  I'm not arguing, just asking.  It would make more
> me that a dreadnought isn't going to be able to flip as fast as a
>  More mass, and a longer axis to rotate.

The longer axis doesn't matter that much - or, rather, it also increases
the lever (and therefore the torque) which helps compensate. The main
importance of the length of the ship is that the structural integrity
needs to be better the bigger the ship is, since the shear forces
increase. You need to assume that the thruster strength is proportional
to the size of the ship, but the FT and FB design systems do that


Assume that you want to turn a ship around 180 degrees in one minute.
fast must the bow and stern be accelerated sideways to achieve this?


Phi be the angle turned by the ship
a the angular acceleration ( = the 2nd time derivative of Phi)
t the time in which the ship turns 

If you start turning with constant a from a standstill, you get the

Phi = 0.5 * a * t^2 

a is what we're looking for here, so for a given Phi and t we get a = 2
Phi / t^2.

We accelerate the rotation half the way (90 degrees) and then reverse
thrust. a is constant throughout the acceleration and through the
retardation, but is reversed inbetween the two, so I'll just calculate
how large a needs to be in order to turn the ship 90 degrees in 30
seconds (leaving it with a respectable angular velocity). This gives 

a = 2 * 90 / 30^2 = 0.2 degrees/second^2

which isn't very much. If your ship is 1000 meters long and has its
center of mass roughly half-way along its length, an angular
of 0.2 degrees/s^2 means that the bow and stern must accelerate sideways
by about 0.9 m/s^2, or less than 0.1 g.

Using the "usual" game scale, 1 thrust point is IIRC about 0.125 g. A
thrust-2 ship has maneuvering thrust 1, but this is thrust 1 sideways
the *entire* ship - you usually need more force to accelerate the ship
sideways (or forward, or whatever direction you like) than to set it
spinning, depending on the shape and mass distribution of the ship. You
never need *more* force to set it spinning than to push it sideways
though, so even a 1-km long thrust-2 battleship is capable of turning
degrees in a minute or less.

If your background assumes that ships are more than 1 km long (Renegade
Legion, for example), you may have to adjust the assumptions. However,
even a Shiva-class BB (2.7 km long IIRC) wouldn't need more than a
maneuvering thrust of 2 to flip around in a minute using the "usual"

So yes, unless you assume *very* big ships they won't have any problems
turning to any new heading in a minute or less. Depending on what
assumptions you make for the time scale of one game turn, this may or
not be too fast to reduce the amount of main thrust available; and EFSB
as written doesn't use the same assumptions as the FB.


Oerjan Ohlson

"Life is like a sewer.
  What you get out of it, depends on what you put into it."
- Hen3ry

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