Steve Higginbotham commented that my numbers for
Fuel Usage on a fusion rocket were sensible for
what we can forsee now, but not for what we can
do in 3000 years. This is not strictly true.
There's only so much energy available when you
take four hydrogen atoms and make helium.
That energy is the top-end, 100% best that you'll
ever get out of a fusion rocket. The corrected
numbers for TL9 fusion rockets I gave are running
a significant fraction (50ish percent, if I remember
my source material right) of that maximum limit.
Engineering only lets you get closer to that limit,
not to pass it....
[note that the potential Isp of an antimatter rocket
is a whole lot more fun to work with 8-) ]
- -george