In article .com,
Allen Thomson wrote:
It would be interesting to know if there is currently
any propulsion approach available that would allow
significantly faster than Hohmann trips for humans
to other planets/moons/major asteroids... "Currently available"
can be interpreted to mean "available by 2025 at a development
+ procurement cost of no more than $10G in 2004 dollars
per year between now and then."

Yes: orbital assembly/fueling will let you do faster-than-Hohmann trips
for small expeditions with chemical propulsion. You need an orbital fuel
depot, and lots of fuel launches, but the former is fairly straightforward
if you don't insist on using LH2, and the latter provides high flight
rates for RLVs and a large competitive market for launchers of all sorts.

Double yes: if you're willing to spend a bunch on R&D to reduce launch
rates -- which is probably a bad deal, but is undeniably attractive to
organizations that specialize in R&D -- solid-core nuclear rockets can
considerably improve the picture, speeding things up further or permitting
larger expeditions or both. Rover/NERVA solved most of the major
technical problems of a first-cut version in the 60s, and demonstrated
that a fast-paced program could improve the state of the art remarkably
quickly in this area. You can start with NERVA derivatives, and pursue
more ambitious designs in parallel with the first expeditions. The one
big hassle is low-emissions test facilities, and it's one that should
yield quickly to substantial amounts of money -- no breakthroughs are
required.

Liquid-core or nuclear-lightbulb is substantially better, and gas-core
is much better, although they are longer-term options with significant
development issues.

Equally intresting would be to know about the technology
for life support systems that would reasonably reliably
sustain a half-dozen people for two or more years in
space without help from Earth.

Adequate water recycling -- the big issue -- has been demonstrated, on
a modest scale. (Air is a minor side issue by comparison.) The simplest
way to address the food loop is not to try, given that freeze-dried food
weighs less than half a ton per man-year. Generally, much the simplest
and most reliable way to tackle a lot of the smaller recycling/repair
issues is brute force: more mass, and more fuel to push it, is cheaper
than major engineering R&D.

Of course, trying to sell that approach to R&D-oriented organizations is a
bit of a challenge. "Anything which they do not wish to do is always
lacking in technology. Whether single stage to orbit or Mars missions,
the technology is never quite ready..." (Jim French)

--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |