Mark Simmons (scorpio@best.com)
Mon, 27 Sep 1999 13:43:00 -0700


Matt Hanyok asks,

>well, that explains it. But, I thought the MS's used a kind of fusion
>engine? I don't kow my science, but aren't those nuclear powered or
>something, so why would they need fuel?

Chris Beilby observes,

>Exactly. The proper term isn't actually fuel, but rather 'reaction Mass."
>While the Mobile Suits as seen use some sort of combustible oxidizing
>reaction mass, it is entirely conceviable that they could even use water...

And Paul Fields responds,

>Actually Chris while you're right they could
>use water, Mark would slap your hand on this
>one... they use Helium 3 from Jupiter, hence
>the dependence on the Jupiter Energy fleet
>for fuel in Gundam...

  Actually, Chris is quite right. Helium 3 is used for _reactor_ fuel,
and a little of it goes a long way. But using this stuff for bulk
propellant would be a terrible waste of this precious substance, when
just about any common gas or liquid would do.

  But what substance works best? Since we're only interested in this
stuff for its raw physical properties, it doesn't matter whether it's
combustible or not. In fact, where thermonuclear rockets of this kind are
concerned, there's only one factor that determines its "fuel efficiency":
The atomic weight of the propellant, the lower the better.* And any
susbstance that works as propellant can do double duty as coolant, since
both functions involve carrying heat away from the reactor.

  While liquid oxygen would be a stinky propellant, water is more
attractive - the efficiency of the hydrogen somewhat balances out the
inefficiency of the oxygen, giving you an overall efficiency of one-third
that of hydrogen. Plus, water is easy to handle, store, and pump. It's
also implied that regular helium might be used in space mobile suits
(witness the Gundam's "helium control core" boxes, which have also been
identified as cooling equipment), and this would have fully half the
efficiency of pure hydrogen. Perhaps there's a lot of regular helium left
over from helium-3 processing...

-- Mark

* This is because the temperature of the propellant equates to the
average kinetic energy of each atom. Kinetic energy is proportional to
atomic mass x velocity^2, so the velocity increases when the atomic mass
is smaller. Hydrogen, with an atomic weight of 1, is the most efficient
propellant; pure oxygen, with an atomic weight of 16, has only a quarter
of hydrogen's exhaust velocity at any given temperature. As a result, a
kilogram of hydrogen gives you four times the thrust of a kilogram of oxygen.

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