Pat Flannery wrote:

Since ISS needs fairly frequent resupply missions via Shuttle/Progress
(and wait till they get it up to a six-person crew, water recycling or
not) a pretty low orbit made sense as the supply vehicles could
reboost it.

BTW, I couldn't figure out why the water usage amounts per ISS crew
member per day were so high; it's not due to its use as drinking water
or for sanitation, its due to the fact that that's how the oxygen for
the station is being generated. Water is being broken down into hydrogen
and oxygen:
http://www.space-travel.com/reports/..._To_ISS.h tml
With the hydrogen being jettisoned into space - very similar to the
system used on a nuclear submarine.
Which is dumb.
What's needed is a system that breaks the CO2 the crew exhales down into
oxygen and carbon (they used something like this on Mir involving
red-hot metal plates that the carbon got deposited on and could be
scrapped clean of.), then the oxygen recirculated for breathing again.
Of course there will be a net loss of oxygen as you go along due to the
amount of energy extracted from it the body uses.
But tossing the hydrogen overboard is wasteful - when it could be
recombined with oxygen to produce electrical power in a fuel cell, or
used as rocket propellant of some sort (say in a small nuclear-thermal
engine based on a RTG for orbit boost?).
In regards to that, this was one mighty slick idea:
http://www.astronautix.com/engines/rd0410.htm
The reason that thing has so much plumbing involved with it, and has the
big ring around its base, is that it's a dual phase nuclear engine.
Crank it up full tilt and it heats hydrogen flowing through it like
NERVA, but reduce it to idle power, and it just heats up a liquid to a
vapor state to drive a turbine and generate electrical power via a
dynamo in a closed-cycle system - with radiators to cool the vapor back
into a liquid state.
That is downright brilliant.

Pat