From: schillin@spock.usc.edu
"Allen Thomson" writes:
>Michael Smith wrote:
>> It would be interesting to work out how much of a
>> spacecraft you would have with a couple of submarine
>> style fission reactors and as many ion or hall thrusters
>> as you had power for.
>> Given the lack of enthusiasm for this approach I can
>> only assume that it doesn't deliver transit times short
>> enough to be safe for humans.
>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. (Our moon
>excepted, of course.) "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."
If by "significantly faster" you mean a factor of two or
so, that can be done.
A minimum-energy transfer to Mars takes somewhere between
240 and 260 days, depending on what launch window you use.
Looking at payload delivery from low Earth orbit to low
Mars orbit using various propulsions systems, we get:
LOX-LH2 Chemical Rocket: 240 days 25% payload fraction
Nuclear Thermal Rocket: 240 days 45% payload fraction
Nuclear Electric Drive: 300 days 45% payload fraction
Solar Electric Drive: 300 days 40% payload fraction
"Nuclear Thermal", means running liquid hydrogen through a
hot reactor core and exhausting the gas through a nozzle -
same principle as a chemical rocket, but different energy
source and lighter working fluid. This has been tested
on the ground, back when open-air nuclear reactors were
an acceptable thing, but never flown.
"Electric Drive" refers to an ion or plasma thruster system
similar to what I described in an earlier post, using either
a nuclear reactor or advanced solar arrays as a power source.
The longer trip time comes from the necessary acceleration
period using low-thrust propulsion, and these are systems
that have flown at a smaller scale and on solar power.
If we're looking for a factor of two improvement in speed,
we can use moderately high energy orbits using any of these
propulsion systems.
LOX-LH2 Chemical Rocket: 120 days 10% payload fraction
Nuclear Thermal Rocket: 120 days 30% payload fraction
Nuclear Electric Drive: 180 days 30% payload fraction
Solar Electric Drive: 180 days 20% payload fraction
So, even if we are stuck using chemical rockets, we can get
four-month trips if we are willing to accept 10:1 mass
ratios. And we can do better than that if we are willing
to go nuclear, using reasonably well established but of
course politically controversial technology. Even if we
have to use fluffy green solar power, we can still beat
chemical rocketry and the Hohmann orbit by a fair margin
If you want weeks instead of months, no go using any
technology we can forsee for the next two decades. And
note that for all of these, launch windows open every
2.15 years. Interplanetary travel without regard for
launch windows is another thing we aren't going to be
doing in this generation, though in the case of Mars
missions you can frequently get an off-year window at
tolerable cost by using a Venus flyby.
>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.
For a mission of that scale, you'd keep it simple and use
industrial chemistry to close the air and water loops.
Roughly speaking, the human body turns clean water into
dirty water, water content of food into dirty water, and
dry food plus air into carbon dioxideand clean water with
a little bit of solid waste on the side.
That last step is critical, because it means there is
a surplus of water in the output stage which can be
used directly to make up for inefficiencies in your
water recycling system or electrolyzed to make up for
inefficiencies in your oxygen recycling system.
A physiochemical life support system for a two- to
three-year mission would consist of six major elements:
A vapor distillation unit to turn dirty water into
clean water, with the impurities concentrated in a
brine that is vented overboard (along with a little
bit of water, but as noted we can make that up).
A molecular sieve or other regenerable physiochemical
system for extracting carbon dioxide and other trace
impurities from the cabin atmosphere
A Sabatier reactor for turning carbon dioxide plus
hydrogen into methane plus water. The methane we
might be able to make use of or might just vent,
the water we for sure can make use of.
A water electrolysis unit to turn the surplus water
(both from human respiration and the Sabatier reactor)
into oxygen and hydrogen. The hydrogen feeds back
into the Sabatier reactor, and the oxygen goes into
the cabin air.
An incineration or other oxidation unit to reduce
the solid waste to ash and recover what water and
carbon dioxide we can.
And a stockpile of canned, dehydrated, frozen, or
otherwise preserved food, details yet to be determined.
If it isn't fully dehydrated, that's still more surplus
water that can be used to compensate for inefficiencies
elsewhere.
The hardware would mass about 200-500 kilograms per man
with current technology, and consume about one kilowatt
per man of electric power.
Stored consumables would ammount to about two kilograms
per man per day, mostly food. Minor consumables would
be nitrogen to make up for atmospheric leakage, hydrogen
for the Sabatier reactor (that loop can't be fully closed
without a very large excess of water to electrolyze),
maybe ammonia or hydrazine in place of seperate nitrogen
and hydrogen if the mass balance is right, various personal
and environmental hygene supplies, filters and other parts
for the machinery, and packaging for all of the above.
With a bit of effort and austerity, it might be possible
to get the consumables requirement down to 1 kg/man-day,
but almost certainly no further unless you start growing
your own food. And that's not worth the bother for only
a few people and a few years.
--
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