Combining replies:

But if most of the fuel is consumed before jump to supply the necessary
energy, with only a fraction of that required (as "displacement mass")
to keep the jump bubble inflated, then (IMO) a ship with a relatively
small "surge tank" could jump with external power and use the contents
of the tank to fill the volume.  And if it's not, where does it all go?
Why does a ship with a black globe need all the fuel that it would
normally use for a jump, if its capacitors are full?  And since they
/are/ already full, what happens to the fuel?

Yes, I'm trying to break the rules and/or the setting.  That's my point:
  that the rules are arbitrary, gamist (rather than simulationist), and
logically inconsistent.  We're back to "because" and/or "because
starships were only ever meant to (1) shoot at each other and (2)
deliver the PCs to places where they can shoot at things" - a universe
made of wood and cardboard and canvas, merely painted to look like
functioning technology.

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Kelly St. Clair
xxxxxx@efn.org