Why we don't see methane or ammonia-fuel starships

Ok, went home and thought about it and cranked some numbers.  Figured out why
you don't want to use different fuels.

The average starship has much if not most of its fuel volume as jump fuel.
Now, from Starship Operators Guide [yes, it's not GDW but they approved it so
i figure it's official], the Zuchai energy storage capacitors in a Jump drive
can't be held charged for more than two hours safely; three max.  Thus, the
jump powerplant must generate its power within [let's say for safety's sake]
an hour.  And it needs enough fuel to run for an hour...

Now, if we're going to purify something else on the fly, this means that we
need six times the usual purification plant [which will take 6 hours].  At
TL 15, the purification plant is 20% of the fuel volume for the 6hr version.
For our faster requirements, it rises to 120%.  This means that the total
of jump fuel volume and purification plant is now 220% of what we started
with...and even with a fuel of relative hydrogen density 1.9 we are still
16% over the volume of pure hydrogen.  You Lose...

Even if we're optomistic, and assume that we can push the two-hour lower
limit on crystal charge, we're at 160% volume, divided by 1.9 to 84% of the
origional volume.  We only gain 16%, and in the process lose reliability
[if the purif plant breaks we're stuck], safety [how long do you WANT to hold
those crystals charged???], and convenience [ya want WHAT instead of hydrogen?]
compared to hydrogen.  Plus, the added cost of the purification plant [a not
inconsiderable 150kCr/kl ] makes this a touchy option.

It is no longer the wonder-fuel.  Go home, people, and have a good night.  If
you really want to trade off the 16% for the added headache i won't mind if
I'm GMing, but i warned ya what's going to go wrong :-)

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George William Herbert              JOAT For Hire: Anything, Anywhere: My Price
   UCB Naval Architecture undergrad: Engineering with a Bouyant Attitude :-)
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"Who?" the man managed.                    Whip me, Beat Me, Make me learn C...
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