I have been enjoying the Mars landing and the "sojourner" vehicle's exploits.
I understand that the vehicle, which is regularly described as being about the
size of a microwave oven, was originally constructed that size as a 1/8 scale
model version of the one they really wanted to send. 

When this project including the full-sized rover began to encounter escalating
costs, and the project was jeopardized for lack of funding, this scale model
became the "real thing", replacing its prototype in a scaled daown project,
with more manageable costs, which we are enjoying the fruits of now.

Modern internet technology has allowed them to share their pictures nearly
instantly with the whole wired world.

There is an article on the CNN website about the scientisit behind this. It
is entitle "Revenge of the Science Nerds".
http://cnn.com/TECH/9707/11/jpl.scientists/index.html

Big science captures the world's imagination, and this is a good thing, but I 
have been pleased with the science itself, too. Not that it isn't based
on a whole hell of a lot of good engineering.

54 responses total.

It sort of annoys me how lots of commentators, especially James
Van Allen, point to Mars Pathfinder as "proof" that people are
superfluous in space and only robots ought to go.
 
Consider the truly pathetic aggregate performance of the last
several Mars probes.  All but this one failed quite spectacularly
and suddenly and returned no data at all.
 
Now look at Pathfinder/Sojourner.  Pathfinder is rooted to one
spot on the planet.  True, it is an interesting spot, bearing
rocks washed from very different places and showing much about
Mars we didn't know... but it is *one* spot.  It can't move
under its own power to look at anything else.  It can't go to
examine the places where these rocks originated, and it could
never have landed in a great many places where the history of
Mars is revealed in greater detail.
 
Now look at Sojourner.  Much has been made of its ability to
move up to rocks and get some data about their composition.  But
really, is this anything to crow about?  It takes all day to get
an aggregate chemical composition on *one* rock.  One geologist 
would take a camera and a rock hammer and have samples of literally
dozens of specimens inside of an hour, and be able to tell far more
about crystal types just by looking.  Sojourner can't go any
farther than about 1600 feet from Pathfinder.  A man in a suit
can stroll 1600 feet in about 10 minutes; they moved quite a bit
faster than that on the Moon.
 
Pathfinder has to be recognized for what it is:  a very timid
first (okay, second) step on the Martian surface.  Yes, it's nice
to have something there, but really... we had the hardware, and
could have sent people there 20 years ago.  If we took advantage
of what we've learned since then, we could do it for relatively
little today.  When are we finally going to go?
 
If you have WWW access, look at the Mars Direct home page.

This response has been erased.

How many members of the robots-only crowd think that humans are needed on the
planet Earth?  

It is not a question of whether people will go to Mars, it is a question 
of when. I question Russ's statement that if we took advantage of what 
we've learned we could go relatively cheaply. I'd certainly like to know 
how we can go relatively cheaply.

It is mainly the cost that is keeping people from these really long 
space flights. I'm not disappointed, at last for now, that we're 
focusing manned space activities on closer-to-home stuff like space 
stations.

s/last/least/

Re #2:  Valerie, I can't tell you offhand what the URL is, but it's
easy enough to find.  There are only a few pages on the whole Net with
the string "Zubrin" in them, and if you do an Alta Vista search on
it, you'll hit it.  Since I'm not on the WWW right now, I'll put in
a plea here for anyone who does the search to post the URL.

Re #4:  There is an *amazing* amount of stuff which can be done in
space, far cheaper than we're doing it now.  Mars Direct is a full-blown
exploration and colonization program which could be done for something
less than what we're spending to maintain an aging fleet of Shuttles,
according to Bob Zubrin.  Look up that URL for the figures, I'm not
willing to go out on a limb and trust my memory on this one.
 
Delta Clipper is one way that we could have done our space program
much cheaper.  However, NASA has sunk the Clipper, first destroying
the sub-scale test vehicle during a flight test (disconnected landing
gear release lines, oversight or sabotage?  You decide) and then
pushing a vehicle concept with a *lower* payload potential, higher
development costs and a much more drawn-out schedule.  (Guess where
the money is?  R&D and Shuttle operations.)
 
Another example of "better, faster, cheaper" was the proposal brought
forth by LLNL a few years ago for an inflatable space station aimed
at support of life-sciences and exploration work.  It would have cost
about a billion bucks from the word go to the whole enchilada on-orbit,
inflated and *spinning* for artificial gravity, including two (if memory
serves) ground test articles.  The whole shebang would have launched on
one Titan V.  Modules could have served on the Moon or Mars.
 
(LLNL used some of their discretionary research funds to come up with this
revolutionary cost-saving concept.  The reaction of Congress was to take
away such discretionary funding.  Apparently, it gored someone's ox.)
 
So, I think there are many ways we could be doing things cheaper and
faster in space.  Unfortunately, pork at home and keeping Russian rocket
scientists employed doing space shots overseas (instead of making missiles
for Iran) are the driving forces behind our spending priorities.

I found a URL for Mars Direct:  http://www.magick.net/mars (maybe ~mars).

The Mars Direct site is indeed http://www.magick.net/mars.
 
I've just stumbled across some other net.resources on Mars.  (All this
stuff on American things, from a British magazine, "New Scientist".  I
recommend this publication *very* highly.  However, it is very expensive
at $140/year.  They are having a half-price sale right now, though.)
 
http://nssdc.gsfc.nasa/gov/planetary/viking.html    Viking landers
http://cmex-www.arc.nasa.gov/                               General Mars
info

And for some of NASA's ideas of what a manned Mars mission would look like,
 
http://www-wn.jsc.nasa.gov/explore/DATA/MImages/MImages.htm
 
The 6/28/1997 issue of "New Scientist" covers some of the concepts for
manufacturing fuel for Mars missions.  It doesn't cover the original
Mars Direct idea very well, though, so read the WWW stuff for more detail.
 
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
 
Zubrin's basic idea (and the cornerstone of Mars Direct) is to avoid
having to ship the fuel for the return trip on the outbound leg.  One
really expensive part of a mission is launching it.  The less weight
that has to be launched, the cheaper the mission.  The fuel required
to return a vehicle to Earth from Mars is several times the mass of
the vehicle itself (how much depends on the fuel).  Eliminating the
return fuel from the mass to be launched cuts mission cost dramatically.
 
Zubrin noted something very important:  Mars has an atmosphere which is
composed largely of carbon and oxygen, which are two major constituents
of many rocket fuel compositions.  Hydrogen is in short supply in what
passes for air on the red planet, but it is also the lightest element
and costs relatively little to launch.  So, with the Martian atmosphere
and some hydrogen and energy, you can make rocket fuel on-site.
 
I'll give people a chance to read the Mars Direct stuff themselves before 
going much further with this.

This response has been erased.

The Mars atmosphere has lots of carbon dioxide - not free carbon and free
oxygen (just to keep the facts clear). Carbon dioxide makes great - burnt
fuel. The trouble with Mars is, no signs of reducing agents, such as free
carbon. I agree that the most economical reducing agent to send to Mars is
hydrogen - but then, you need an oxidizing agent too. There isn't much of
that on Mars either. Oxygen is mostly locked up in carbon dioxide or
iron oxides, which makes it pretty unavailable. What fuel mixture is being
proposed to be made on Mars?

My impression is that you're always burning an H2/O2 mix; you take enough
H2 with you for the round trip (light weight stuff) and only enough O2
(much heavier) to get TO Mars.  You also take a very good electrical power
supply (solar or nuclear), a dry ice maker designed to work from the Martian
atmosphere, a CO2 splitter, and LOX maker that will work off the splitter.
As soon as you get to Mars, you set up your moonshiner operation to make
your oxidizer for the trip home. 

Getting oxygen from CO2 is pretty difficult. However with H2 and CO2 (and
energy) you can make H2O, which you can electrolyze to recover your H2 and
make O2. Hmmm... H2 + CO2 = H20 + CO. Reject the CO and split the water
as H2O = H2 + 0.5O2. OK. In principle....except for inefficiencies.

What's gravity like on Mars?  Half that of earth? How about wind loading?
If these are all low relative to earth, larger / weaker structures are
possible.  How about solar flux?  With all the dust in the atmosphere, is
it significantly higher than that on earth?  (Better pack lots of Sun
_Screen if it is ;-)  If so, even our current technology 10% efficient
solar cells might be good enough to break up H2O.  Plus, with less,
gravity, it would take less energy to escape its pull = less fuel.

Re# 13:  Your scheme falls within the +/- details of my knowledge of the
plan.  Similarly, dry ice is a poor form of CO2 for the next step, etc.

Both gravity and wind loads are much lower on Mars.  Even if the atmosphere
is calm as usual (little dust), the solar flux will be poor due to the 
greater distance from the sun.  (Yea, SPF of your skin lotion ain't so
important when you're wearing a space suit.)  Given the problems (night,
unpredictable dust storms, etc.) with solar, I'd rather take a RELIABLE
(preferably no moving parts, etc.) nuclear power source if it was my 
return ticket on the line.  I'd also be inclined to look real seriously
at an ion drive for the interplanetary legs.

Re #11:  It is obvious that you have not read the Mars Direct pages,
else you would already know Zubrin's answer to your question.  }:->
 
I am going to give people a bit more time to look them up before I
post many details here.  Again, the URL is http://www.magick.net/mars.
 
The gist of Zubrin's idea is this:  Mass is expensive to ship, but energy
is cheap.  Only difficult-to-obtain materials need to be sent with the
mission; given launch costs, anything which can be obtained on-site
("living off the land") probably should be.  He has some excellent
examples of this philosophy which convincingly argue his point.

I've been following the Pathfinder mission on
http://mpfwww.jpl.nasa.gov/default1.html
which I haven't seen mentioned here yet. All of the scientific mineralogical
data, and tentative interpretations, are available there. All of the images
are also. They have also put together a Quicktime VR movie, by which you
can scan 360 degrees around the lander at the speed you wish.

Twin Peaks is about 0.86 km away, and 90 degrees counter clockwise
from that, at a distance of 2 km, is a 1 km diameter impact crater
that looks quite fresh. I would think that a lot of rocks found at
the lander site are ejecta from that crater. The tilt of the rocks, which
in some places has been identified as due to water flow, is also
consistent with having been blown out of the crater. 

I have looked at the Zubin site, and scanned the fuel production proposal.
It is feasible (and was feasible even before they spent $47,000 to run
a demo). It does require transporting H2 from earth, and they propose to
synthesize methane (CH4) as fuel, from the H2 and CO2. It was not stated
in the fuel synthesis document why they want to make methane: H2 itself
if a fine fuel, and converting it to methane would seem to be an unncessary
step (especially considering all the additional equipment required, which
has to be transported to Mars. 

The far-higher boiling point of methane (vs. H2) may have a lot to do
with it.  If all their O2 made on Mars comes from CO2 + 2H2 => CH4 + O2,
they'll waste half the H2 they take to Mars, so I doubt its for reasons
of economical synthesis.

How much of an issue is dual-fuel rockets (H2/O2 outbound, CH4/O2 inbound),
or do they get around that some way?

What you wrote is the overall result of the use of the Sabatier reaction,
CO2 + 4H2 = CH4 + 2H2O followed (electrolytically) 2H2O = 2H2 + O2. 
However the mix they get of their fuel mixture of CH4/O2 =1, does not
provide complete combustion. They would have to discard CH4. Is that what
you meant? 

Since the plan is to *take* loads of hydrogen to Mars, they are not going
to get any additional specific impulse from converting that to CH4. In
fact, they have to take with them enough H2 to get back *and* enough to
make oxygen. Therefore the most economic way to make oxygen is the key.
The reaction H2 + CO2 = H2O + CO --> 1/2 O2 is just as good as the methane
route (1/2 O2 per H2) and, as you said, they would not have to throw away
H2 in excess CH4, or carry the equipment to make and store CH4. (The
higher boiling point of CH4 can't be important as they would already have
the H2 technology with them.)

Yes, I figured that wasting half the H2 hauled to Mars (at great expense)
would not be good technique.  In general, the O2 production process must
be extremely efficient (in kg of O2 produced per kg of materials, equipment,
and chemical feedstocks hauled to Mars).

If O2 is very cheap on Mars, then CH4 might be a more efficient way to go.
(in m/s delta-v homebound per kg of H2 hauled outbound.)  I don't know 
enough to say without pulling down reference books, looking up specific
impulses, etc.

Yes, they're arriving with tanks of liquid O2 (near empty) and H2 (plenty
left).  Storing cryogenic liquids is easy in orbit (minimal condensation,
insulation, etc. problems in a vaccuum, and you can spread a featherweight
mirror between the tanks and the sun.  Storing them on the surface in an
atmosphere (even Martian) is another matter.  Look at the boiling points:
CO2  -78C
CH4  -164C
O2   -183C
H2   -253C
(Yes, CO2 sublimes and all these figures are for 15psi pressure.)
Figuring that your desired liquids boil higher due to pressure in your
tanks and that CO2 condenses lower due to being at Martian atmospheric
pressure, H2 looks like a problem child.  Substantial extra effort will
be needed to insulate it, re-condense boil-off, prevent CO2 frosting of
anything holding it, etc.  Converting it to CH4 may be the way to go.

Re #18:  The higher boiling point of CH4 is essential, because
ultra-cryogens like LH2 cannot be stored for long without active
cooling and heavy insulation.  In space, vacuum and shade make it
reasonably easy to hold hydrogen for a while.  In an atmosphere....
 
Oxygen can be made from CO2 using either thermal cracking with
zirconia cells to separate the oxygen, or the reverse-water-gas-shift
reaction and electrolysis (CO2 + H2 -> CO + H2O, H2O + e -> H2 + 1/2 O2).
All hydrogen converted to water during fuel synthesis is at least
potentially recyclable.

Re #17: The driving philosophy behind Mars Direct is *leverage*,
multiplying the benefit of every unit of resource shipped.  (Zubrin's
example from the 19th century is a pair of Northwest Passage expeditions,
one of which carried salt beef (?) and the other which carried dogsleds
and dogs for getting around, and rifles for hunting the northern caribou.
Guess which expedition lived the best and accomplished the most?)
 
Until all-nuclear rockets which can use e.g. liquid CO2 as propellant
are available, we'd have to use chemical rockets to lift anything off
of Mars.  The atmosphere there is extremely dry, so the cost of extracting
anything other than carbon and oxygen (in energy and complexity) is probably
too high for the first missions.  So, given the availability of carbon
and oxygen, what can be made, and what does it gain?
 
The on-site power source would have to be shipped regardless, so let's
ignore its weight.  Zubrin's laboratory test rig had enough capacity to
fuel a Mars Direct return mission and only weighed a few hundred pounds,
tops, so lets ignore its weight too. 
 
Returning from Mars to Earth requires about 5.4 km/sec of delta-V.  The
figure of merit for a proposed fuel/oxidizer mixture, all else being
equal, is the amount of payload which can be returned per ton of material
shipped.  Anything which can be made on-site is a freebie.  This analysis
assumes that oxygen can be made without any net input of hydrogen.
 
                           Exhaust      Mass ratio      Tons of return
1 ton LH2  Fuel            velocity     to achive       vehicle to Earth
makes      mixture         m/sec        5.4 km/sec      per ton H2
 
1t LH2     H2/O2 1:3.5      4307        3.50:1          1.28
1t LH2     H2/O2 1:5        4326        3.48:1          1.72
1t LH2     H2/O2 1:6        4281        3.53:1          1.98
5t CH4     CH4/O2 1:3      ~3400        4.90:1          3.27
 
As you can see, specific impulse isn't everything; the least-potent
H2/O2 mixture returns the greatest payload because it gets more mileage
out of each unit of hydrogen.  And methane, which exploits the available
carbon from the Martian atmosphere, allows about 65% more vehicle mass
on the return trip than even the best hydrogen/oxygen mix.  Methane is
the preferred fuel due to the greatest leverage.
 
Potentially, carbon monoxide and LOX could be used as rocket fuel.
This allows fuel production unconstrained by imports; the fuel plant
never runs out of raw materials.  However, CO/O2 does not have a very 
impressive specific impulse and burns very hot, complicating engine
design.  I think it would be worth looking at for a first stage,
especially if inflatable foam-insulated tanks could be used to reduce
the bulk of the return vehicle during shipment from Earth.  I have
spoken with Zubrin about this, and he does not agree.  C'est la vie.

Are those ratios in mass units? If so - why would you want to run below
stoichiometric O2? You are using H2 as a diluent, reducing the temperature
and velocity. Note the improved performance as you approach
stoichiometric. 

You cannot split CO2 to produce O2, except by the laborious chemical paths
being discussed. Assuming O2 can be made without consuming H2 is not
realistic. The problem remains that producing CH4 does not also yield
enough O2 to burn it completely, and in addition produces only 1 O2 per 4
H2 consumed. The reaction in #19 (which is the water gas shift)  requires
only 2 H2 per O2 produced - assuming 100% efficiencies in either case, of
course. 

I acknowledge that storing H2 is a little more difficult than storing CH4.
However you still have to store all the H2 you took with you, until you
can convert it to CH4, which will not be fast, during which time you have
to store both. 

My impression is that Zubrin (nor anyone else) has done all the tradeoffs
properly. Their "demonstration" of producing CH4 from H2 was such a
ham-fisted operation (I do give them credit for admitting their stupid
mistakes). Is the whole proposed chemical scenario avialable somewhere on
the web?

CO is not a good fuel (and does not burn "very hot", compared to H2 or
CH4).

All in all, my initial impression is that both components for a return
flight need to be taken, at least for early trips and until colonization
and Martian industry is established.

<WHOA!!!  I make a bunch of guesses & russ come back and confirms them.
Should I be standing proud or running scared?>

Are there issues with dual-fuel rocket engines here, or is the plan to use
H2/O2 optimized engines outbound and carry along whatever/O2 optimized ones
for the return trip?

I'd agree with russ that the greatest virtue of CO/O2 is that it's cheap.
His statement that it burns hot enought to complicate engine design does
strike me as doubtful.

Especially if you plan to use CO/O2 as first stage fuel taking off from 
Mars, would it be worth leaving most of your H2 in Martian orbit and pick
it up on the way home?  Certainly it complicates things, but the thrust
used to land it on Mars then launch it back into orbit are completely
wasted otherwise.  

If "free C" makes CH4 a better return fuel than H2, then why not go further?
C2H6, C2H4, C2H2, C3H8, etc. are all possible.  (Cripes, the bottom stage of
the old Saturn V burned kerosese if I recall right.)  What's optimal?

I'm no chem. engin. expert, but I doubt that (in context) making O2 from
CO2 is as difficult as rcurl makes it out to be.

I goofed up a number and the table formatting in my last response.
Here's the corrected table:
 
                           Exhaust      Mass ratio      Tons of return
1 ton LH2  Fuel    velocity     to achive       vehicle to Earth
makes      mixture         m/sec        5.4 km/sec      per ton H2
 
1t LH2     H2/O2 1:3.5      4307        3.50:1          1.28
1t LH2     H2/O2 1:5        4326        3.48:1          1.72
1t LH2     H2/O2 1:6        4281        3.53:1          1.98
5t CH4     CH4/O2 1:4      ~3400        4.90:1          5.10
 
Incidentally, the formula I'm using to calculate the return vehicle
mass is (total propellant mass per ton H2)/(mass ratio).  I should
also note that I'm guesstimating the exhaust velocity of CH4/O2 by
extrapolating from the figures for kerosene/oxygen engines operating
in vacuum, but I know I'm not too far off.

Re #23:  "When declaring something is impossible, be certain not to
tell the person who is doing it."
 
>You cannot split CO2 to produce O2, except by the laborious chemical paths
>being discussed. Assuming O2 can be made without consuming H2 is not
>realistic.
 
Are you saying that the zirconia oxygen-ion pump is a laborious chemical
path?  You should be informed that it is basically an automotive exhaust
oxygen sensor run in reverse.  It's hard to get simpler than that, and
there is no hydrogen involved whatsoever.
 
I know that Zubrin prefers the zirconia cell concept, but if solar-electric
power (as opposed to nuclear) is mandated by political requirements, it
becomes difficult due to thermal cycling.  But please tell me, what is so
difficult about removing the water and hydrogen from the reaction products
of the reverse-water-gas-shift?  And after telling me, tell Zubrin.
 
Storing H2 is not a problem in hard vacuum, as previously noted.  After
landing on Mars, the boiloff can be processed to methane and water as
fast as desired; the chemical reaction runs downhill, and boiling the
hydrogen consumes more than enough heat to liquefy the methane (according
to Zubrin; I don't have the thermodynamic tables to check this).  Water
condenses nicely of its own accord at Martian temperatures, so all the
hydrogen can be converted to easily-storable liquids shortly after landing.
 
You know, Rane, it is very charitable of you to characterize Zubrin's
experiment as "ham-fisted" when he admits very clearly in the extensive
description that he is not a chemical engineer.  (Did you miss a thought
there?  You appear to have left a sentence fragment.)  Despite this, the
setup was constructed and proven for a pittance.  It is a characteristic
of robust technologies that anyone can use them, and this one appears
to fit the definition.  If a half-baked effort works in the lab, then it
is very likely that an expertly-designed system will function on site.
 
Finally, if CO does not burn "very hot", why is its combustion temperature
almost the same as hydrogen/oxygen?  The reason it is not such a good
rocket fuel is the high molecular weight, which cuts exhaust velocity.
 
My impression is that you are criticizing off-the-cuff without doing
any analysis whatsoever, and you're wrong more often than right.

Re #24:  Proud or scared?  Your call, Walter. ;-)
 
I doubt (my opinions here) that any dual-fuel engines would be used.
Before Mars lift-off, the last time a high-impulse engine would be 
required is at trans-Mars injection.  The upper stage engines and
tankage would be discarded along the way, and most of the braking at
Mars is done with an aeroshell and parachutes.  Given concerns about
contamination with dust, I would expect the engines on the lander to
be sealed until it is time to use them.  If they are only used once,
they do not need to handle different fuel mixtures.
 
The opinion about CO burning hot enough to complicate an engine is not
mine, but Zubrin's.  You may contest it, just don't attribute it to me. 
It appears to have some basis but I'm slightly skeptical myself.
 
The idea of leaving H2 in Martian orbit may solve the problem of
boiloff, but you also lose the multiplier effect of adding carbon to
the hydrogen left in orbit.
 
Worst, you have to perform a rendezvous in order to get home.  If the
rendezvous fails, you're stuck in Mars orbit (and probably dead).
If you send an unmanned return vehicle which lands and fuels itself
before anyone ever ships out from Earth to meet it, you know that
your return ticket is good before you start.  All you have to do
is put down close enough to the return vehicle, and as we proved on
the Moon (putting down close enough to a robot Surveyor lander to
get to it and bring its camera head home), this can be done with 60's
technology.
 
(I suppose I should clarify, for those who have not read the Mars
Direct pages yet, that the first Mars Direct mission has only one
vehicle:  an ummanned return craft which lands on Mars and deploys
a nuclear reactor by tele-operation [much like Sojourner], and over
the next 2 years uses the energy from the reactor to convert its
payload of hydrogen and Martian air into liquid methane and oxygen.
Each mission thereafter sends 2 vehicles:  one with a crew, who land
near the return vehicle and investigate anything interesting in the
area, and the return vehicle for the next mission.  If the crew lands
someplace too far from their return vehicle, the one launched along
with them can put down in their vicinity and give them a backup ticket.
If both options fail, they have food, nuclear power, and other amenities
sufficient to hold out for the next one.  It looks like the crew is
pretty safe even if multiple things fail; I'd be willing to go anytime.)

Read further, Russ. You are describing the "Direct" option, which Zubrin
has rejected in favor of the "Semi-Direct" or "Hybrid" options. Those
don't look any better to me, though they save a lot of fuel. 

The only direct way to get O2 from CO2 is 2CO2 = 2CO + O2. The equilibrium
improves as T is increased. What a fesible maximum T from that nuclear
reactor? 1000 F? At 1000 Kp = E-26. That's the partial pressure of O2
available. Zirconia or any other catalyst can't change this. 

The water-gas shift produces a mixture of all reactants. It is easy to
condense the water, but it is hard to recover the H2 from the CO + CO2.
The equilibrium for H2 + CO2 = H2O + CO is about 5E-4 at 1000 F (the shift
reaction is run the other way in industry - to make H2). H2O can be
condensed out, but efficient use of the H2 will be difficult.

If you make CH4 via 4 H2 + CO2 = CH4 + 2H2O, you use two moles of H2 to
make one mole of CH4. However the heat of vaporization of CH4 is nearly
10x that of H2, so for every one mole of methane you make, you will have
to evaporate 10 moles of H2 to condense it. What do you do with all that
gaseous H2?

The boiling points of H2 and CH4 are 20.4 K and 111.7 K - at one
atmosphere.  At Mars pressure ( 7 mb), they are 10.5 K and 70 K. These are
so low that the difference from ambient is about the same for both, so
both would have about the same heat transfer rate. However the heat of
vaporization of H2 is only 1/10 of that of methane, so H2 boils off faster
(in molar terms). This is the real reason for thinking of using CH4
instead of H2, not the temperatures.

<The downside to being on russ's side:  properly distancing yourself with
dignity when russ gets into attack mode>

Leaving the H2 in Mars orbit - I was more interested in saving the impulse
otherwise needed to get it down to the surface & back up (along with the
big & bulky tankage, etc.)  (And the aerobraking you mention couldn't do
the boil-off rate any good...)

Your "impression"?  I thought I'd made it very clear that I was working from 
the seat of my pants and deliberately avoiding the numbers.

I would think that the H2, CH4, etc. would generally be stored under 
substantial pressure instead of at ambient.  Why the lower pressure, when
the insulation, cooling, etc. problems are so much easier at higher?

What's the big difficulty with separation of a H2 / CO / CO2 mix?  My
impression is that they have quite different condensation points that are
not very hard to achieve on Mars (though that of CO looks a bit low for
good economics)?  (I assume no need to condense the purified H2.) 

Are high-thrust engines really useful for the interplanetary legs?  Given
the same exhaust velocity, I'd think that a smaller (lighter) engine would
be preferable (within limits).

Given enough energy, CO2 can be cracked (cripes, use Na to reduce the C,
then electrolize NaOH and H2O to complete the cycle if you want to go
REALLY brute force).  What's the best way to do it (on Mars, with cheap
energy & dear materials) is the question at hand.  (Though I've nothing
against brewing O2 from the rocks if that works better.  Certainly NASA's 
space habitat design studies had no reservations about using various
carbochlorination/electrolysis processes to reduce lunar rock to metals,
oxygen, carbon, etc. on a far large scale than Mars direct has even the
faintest ambition of doing.)

We saw at the beginning that CO2 can be cracked - by water-gas shift and
electrolysis.Where do you get Na? Well, Na doesn't react with CO2 anyway -CO2
is a good fire extinguishing agent for Na fires. 

The problem with separation of gas mixtures by condensation is, first,
that *mixtures* condense, not pure substances, and second, as the partial
pressure of the component you are trying to condense gets lower, lower and
lower temperatures are required to condense it. This makes all separations
by condensation of those gases imperfect and incomplete. 

Russ implied storing condensed gases at ambient pressure. You can do it at
higher pressure, but that doesn't help much until you get to *very* high
pressures. Noticee that a pressure ratio of about 130 made only 10-30
degree difference in boiling point. You then have a containment problem,
as you have to carry lots of heavy metal spheres. 

Zubrin is something like a scientific creationist. He knows enough science
to sound plausible and even, in some cases, to propose *alternatives*,
which is how engineering works. Some of the alternatives are better than
others, but they still need a LOT of work to determine or create
feasibility.  Details are glossed over, as well as contradictions.
Meanwhile, he is selling his book for $28, which could be the real
objective. 

I figured that something as reactive as Na could crack CO2 by come cycle
or other.  If not, doesn't Mg burn in CO2?  My point was that it could be
cracked somehow.  (I'm not much interested in whether Zubrin has the
details right or not.)

I thought that liquid N2, O2, etc. were made commercially by distilling
liquid air (separation by differing boiling points).  Is the problem 
here that many condense/evaporate stages are needed to get high purity
(too many kg of plumbing to carry to Mars), or what?  Would it be more
practical to brew O2 from rock, or what?

From figures seen earlier, I understood that ambient vs. 1 atmosphere made
a substantial difference in boiling points.  Again, a "whatever works best"
detail.

Yes, Mg burns in CO2 - but that doesn't get you O2, as MgO is more stable
than CO2. You also don't have Mg...and why do you want to make C?

Yes, air is separated by distillation. It requires more and more "plates
as the separation becomes more difficult. The separation is not by different
boiling points per se, but by different vapor pressures. 

So, the problem remains to get O2 on Mars. If you don't take any bulk chemical
there, I think you would have to electrolyze the water that is already
there. Some automated exploration for permafrost and water ice caps is
in order. Then you just mine the water and electrolyze it with your nuclear
power generator. Certainly it would be stupid to not determine whether this
is feasible before creating a vastly more complex and expensive alternative
technology.

Alternatively, there *might* be deposits of oxides that can be decomposed
by heat to produce O2. This is not too likely as it appears that the reduced
iron in the volcanic rocks would tend to scavange accessible O2 - but also
worth looking for. For example, a deposit of Ag2O, HgO, PbO2 or KNO3....
though all very unlikely, not impossible as long as some kinds of
differentiation has occurred on Mars.

I gave some comparisons of the boiling points of CO2 and H2 at 7 mb and
1 bar. Those temperature differences are only a few degrees (though at
quuite low temperatures). You get a bigger change of boiling point with
pressure for water because water is a polar compound and has a high heat of
vaporization. 

My understanding is that MgO -> MgCl2 is practical, after which (basically)
the same electrolysis process used to make Mg commercially recycles the
Mg & Cl.  Getting from 2H2 + C to CH4 I'm not so sure about.  Given the
miserably energy inefficiency of just the Mg steps of this cycle, I can't
imagine it actually being used.

Whether economic enough for Martian rocket oxidizer production or not, 
chemical gadgets able to do CO2 + energy -> C + O2 have obvious applications
in space for life support systems.  Any promising scheme for that use?

There was a big "hydrogen economy" fad a few years back, which led to
proposals for splitting water thermochemically (by cycling reagents
between high and lower temperatures in a closed system with only water
going in and H2 and O2 coming out - avoiding the inefficient generation of
electricity)). In fact, I am the coinventor of a couple of patents for a
large class of such reaction schemes. None ever became practical, however,
for a slew of reasons. Doing the same with CO2 looks even more difficult. 
Water reacts with lots of things - CO2 with very few. I also looked into
splitting water thermomagnetically. Since O2 is paramagnetic, the
equilibrium for water decomposition is shifted slightly toward being
favorable, in a magnetic field. I calculated the magnetic field required
and published a paper describing such a process. The required field is
astronomical, however (the magnetic field energy per cubic centimeter
approaches the energy release of atomic explosions - well, it was fun
doing the calculation). CO2 would also be more favored to decompose in a
magnetic field - but I think the conditions would be even worse than for
water (you can't even electrolyze CO2 to make C and O2). 

Re #29:  Rane, the zirconia cells operate at about 1800 F.  This
changes your equilibrium constant somewhat, as does the presence of a
catalytic electrode.  Further, it only affects your cell voltage; the
voltage across a concentration cell is proportional to the *log* of the
ratio of the concentrations.  Going from 1e-6 to 1 is only half as
energy-intensive as going from 1e-12 to 1, not one-millionth.
 
Who says "you can't even electrolyze CO2 to make C and O2" (save for
the technical nit, that CO is not C)?  Unless I am badly mistaken,
these things have been tested and they work.  If so, all the objections
you have raised are specious.
 
>If you make CH4 via 4 H2 + CO2 = CH4 + 2H2O, you use two moles of H2 to
>make one mole of CH4. However the heat of vaporization of CH4 is nearly
>10x that of H2, so for every one mole of methane you make, you will have
>to evaporate 10 moles of H2 to condense it. What do you do with all that
>gaseous H2?
 
Use it to condense CH4, as it warms up to CH4's boiling point.  (I found
a reference.)  Each kg of CH4 takes 1/4 kg of H2 to make and another 1/4
kg of H2 to form water, for 1/2 kg of H2 overall.  At STP, boiling it takes
223,500 joules; heating it from 20 K to 109 K takes 184 kJ, total 407 kJ.
The latent heat of evaporation of methane is about 510 kJ/kg at atmospheric.
Since the gas constant of hydrogen is almost 8 times that of methane,
there is plenty of extra cooling capacity available, e.g., by expanding
the hydrogen through a turbine after heating it to 100 K.  Cooling
requirements can be reduced by increasing the tank pressures.
 
(Using high-pressure tankage may be a good idea; if the return rocket
motors are pressure-fed rather than pump-fed, a pump breakdown can't
strand the explorers.  KISS.)
 
Once all the LH2 is converted to liquid methane and water, all of the
materials are easily storable with a bit of insulation and some cooling
gear.  If you have enough power available to generate oxygen, a bit to
run a Stirling chiller isn't going to set you back much.  Foam insulation
can have thermal conductivities of 5 mW/m*K.  For a 10-cm layer around a
4 m diameter tank and a 170 K delta-T, this adds up to 430 watts of heat
leakage.  Pumping up a 3:1 temperature gradient with 30% efficiency makes
that 4.3 KW to run the 'fridge.  A nuclear reactor (or even a 15 KWe
isotope power supply, as in Mars Semi-direct) can manage this.

Re #29:  I didn't think I was attacking you, Walter, just commenting
on the points raised.  (If an idea isn't fair game for analysis in
the Science conference, then where?)
 
Let's see, leaving the H2 in Mars orbit....  What does it buy you?
 
First, you still have to brake the hydrogen from the transfer orbit to
the parking orbit.  If you don't use rockets (huge mass penalty, hauling
the fuel from Earth) you are aero-braking again, about half as much as you
need to actually land.  If heating during the braking process is a problem
for boil-off, it's hard to see why landing is the greater of evils.
 
While in orbit, the hydrogen tank is vulnerable to meteoroids and such.  
Neither can H2 in orbit be used to leverage the lift-off from Mars itself.
It takes roughly 3.6 km/sec to get to Mars orbit.  If CO/O2 is assumed to
have an impulse of 190 seconds, a mass-ratio of about 7 is needed just to
get back to the hydrogen tank.  Compared to the 4.5 mass-ratio to escape
from Mars burning methane, it's not very attractive even before you
consider that the 7:1 doesn't include the oxygen needed to burn the
hydrogen.  You'd need a vehicle with pretty big tanks.
 
Unlike Rane, I do not think that separating H2, H2O, CO and CO2 is a big
problem.  Water and CO2 freeze out easily.  H2 can be chemically separated
using hydride-forming metal alloys, leaving CO; regenerating the alloy and
H2 gas is done with heat.  There are doubtless better methods.
 
>Are high-thrust engines really useful for the interplanetary legs?
 
I think so, at Mars at least.  First, you have a planet to get off of,
and that means an engine with thrust greater than the fuelled vehicle
weight; the greater the margin, the smaller the G losses will be.
Second, you get more feet/second out of your fuel if you can burn it
lower in a gravity well, meaning before you get very far.
 
CO/O2 might be useful for stretching CH4/O2; if the first 1 km/sec can
be gained burning CO/O2, the remaining 4.4 km/sec only requires a mass
ratio of about 3.6:1.  This increases the payload by a further 25% per
ton of H2.

Re #30:  Rane, you are really going over the line.
 
First, I mentioned nothing about the storage pressure of gases.  Your
inferences are yours.  Implying that others intended them is dishonest.
 
Second, higher pressures can buy you quite a bit; at the critical
pressure, the heat of evaporation drops to *zero*.  The issue then
is the weight penalty of the tankage (which may in turn be offset by
being able to dispense with pumps).  If it isn't worth paying, so
be it, but you cannot use your authority to dismiss the issue without
analysis and retain any intellectual integrity.
 
Last, if you are going to substitute slurs like "scientific
creationist" for reasoned discourse, you should move over to a
conference devoted to raw opinion.  May I suggest Politics?

Re #31:  Air is indeed separated by fractional distillation.  Oxygen
plants commonly run with a feed of dry compressed air at about 4 atm,
and nothing else.
 
These plants output nearly pure nitrogen and oxygen, with some noble
gases mixed with the oxygen.  At atmospheric pressure, nitrogen boils
at 77 K, oxygen at 90 K.  It is preposterous to claim that it would
be difficult to use this method to separate carbon monoxide (BP 82 K)
and hydrogen (20.4 K), but that is what Rane is clearly implying when
he says "This makes all separations by condensation of those gases
imperfect and incomplete."  This stuff is for fuel, not analytical
grade materials, and small impurities and losses are both acceptable.
 
Other methods are likely to be preferred for reasons of cost, weight,
and reliability, but to argue or imply (like Rane) that it can't be
done is simply wrong.  More difficult separations are done every day,
by the hundreds and thousands of tons.  There's nothing to it.

Russ, I resent your personal tone in a technical discussion, so I'm opting
out.

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss