🚀 go-pugleaf

B> Date:         Fri, 10 Oct 86 10:22:46 EDT
B> From: ST401385%BROWNVM.BITNET@wiscvm.wisc.edu
B> To: space digest <space-incoming@s1-b.arpa>
B> Subject:      Specific Impulse

B>      Yes, but with an ion drive, one is typically more concerned
B> about getting the most performance out of the ENERGY source, which
B> is usually the limiting factor instead of the reaction mass.
B> A fixed voltage ion drive will give a fixed energy per unit charge, E.
B> E=1/2 mv**2, so momentum transfer (mv) is SQRT (2mE).
B> To optimize this per unit reaction mass, we want the smallest molecular
B> weight possible.  To optimize this per unit ENERGY, we want the
B> HIGHEST molecular weight possible.  That's why mercury or cesium is
B> typically used.

I don't believe this. In deep space, you have months to reach your
target. During that time, your solar collector or atomic pile can
generate more energy than you need for your delta-vee, but the only
fuel you can carry with you is what you could launch, which is limited
by your launch booster. (Assuming you don't have a space station with
re-fueling center, which would allow a different strategy of
accumulating fuel from lots of launches to power a single deep-space
probe.)  Therefore if there were no engineering problems you would
pick the lightest ion so you could pack the largest number of
molecules in your ion-fuel tank for a given Earth-launch mass. But
there are indeed engineering problems: Hydrogen requires pressure
containers whereas mercury is a compact liquid at normal temperature
and pressure. Other atomic materials are like Hydrogen, or solid.
Mercury is the only atomic liquid available. Compounds would have to
be broken into parts (they'd break apart anyway if you tried to use
them as ion fuel) and you'd have to get rid of the parts you're not
using or else have equipment to handle all the parts with their
different particle masses and different ionization characteristics, a
big hassle equipmentwise.

I have no experience in this area, this is just brainstorming, would
like to hear from an expert to judge our difference of opinion.