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Hey Void,

The remaking of a planet is a gigantic project, orders of magnitude beyond anything humans have ever done in its scale.  What this means, in my opinion, is that we need to be looking for ways to leverage our capabilities to get the planet to do much of the work for us.  It's very much an open question what the best things to do are, although there have been lots of interesting ideas put out there.

To the extent that I question or challenge you idea it's because I think it's worth engaging with.

So, here we go:

Correct me if I am wrong: Your proposal is to keep the water in the upper atmosphere by condensing it into clouds, right?

Clouds are not gaseous.  They are suspensions of liquid water or water ice in atmospheric gas.  What this means (to the best of my understanding) is that its absorption/transmission properties in the infrared are more like water and ice.  On net it's my understanding that clouds actually cool a planet because the greenhouse effect associated with them is cancelled out by the degree to which they reflect sunlight away from the planet.

Creating methane is good but at a guess (a pretty uninformed guess) the actual rates will be low.  Methane has a higher global warming potential than CO2 (around 25 to 100 vs. 1 for CO2*).  This is higher, but not so much higher that small amounts can make a big difference for climate.  I would look towards seeding the upper atmosphere with fluorine as a better way to generate warming: CF4 is an extremely powerful greenhouse gas, with a GWP around 5,000 to 10,000 times higher than CO2.

*Global Warming Potential is a measure of how much warming comes from each tonne of gas when released into Earth's atmosphere as compared to CO2.  It is a function of a lot of variables and the precise number is really only meaningful for Earth and for a specified period of time (100 years is the standard).  When looking at GWP, I would group gases into four basic categories: Weaker than CO2 or negligible warming effect (Nitrogen, Oxygen, and the noble gases), similar to CO2 (CO2 and water), mildly stronger than CO2 (methane and ammonia), and much stronger than CO2 (chlorofluorocarbons etc).

I would agree that there's some historical value in retaining the Moon's current appearance.  But I would argue that there's much more value in creating a Moon's worth of habitable land.  Per my post above, the Moon has has much land area as the US, Canada, China, the EU, and India combined.   The value of that much land is simply immense, incalculably so, both in the sense of profitability and in the sense of its value to humanity.  And on top of that it's just three days' journey from here to there on a minimum energy trajectory, less if you have better engines (it's vaguely conceivable, although not with current technology, that you could go to the Moon for a weekend).

It's true that the Moon is quite beautiful as it is, but it's also true that living worlds are quite beautiful. 

It's true that there are huge challenges in remaking the Moon (as there are in terraforming any world), including "known unknowns" (Where is the water and gas inventory to come from? What do you do about the low gravity and slow rotation?) and "unknown unknowns".  However, it's such an incredible project that it will surely be worth it if you set your time horizons long enough.

Methyl formate is an option, sure.  If we're going that route though I would think we might go for Methanol.  Methanol is produced by reacting H2 with CO over a zinc catalyst at elevated temperature and pressure.  It freezes at 175 K and boils at 338 K (under one atmosphere).  Of course you still need an oxidizer, which is the real problem.  LOX boils around 90 K, way below Martian ambient.  You'd probably want something more like the kind of oxidizer used in solid rockets: A peroxide, perchlorate, or nitrate.  The problem is that all of these are complicated to make, corrosive, and use up a lot of energy.  You could store Oxygen as a compressed gas.  At Martian temperatures and 100 atm, the density would be around 175 kg/m^3 according to the ideal gas law, which isn't awful.  Worth mentioning that Oxygen will be a supercritical fluid at this temperature/pressure so the ideal gas law is likely to be way off.  High-pressure gas tanks are heavy and expensive too.

I would say the Democratic party is generally favorable towards him, seeing as he's the electric car/solar power billionaire.  The Democratic Party is distinct from the entire universe of left-of-center Americans, of course.  My understanding is that the farthest leftward people (perhaps 1% of the actual population or less although disproportionately loud online) dislike him.  I would expect continuing support from elected dems unless spaceflight becomes a polarized political issue.

Hey all,

Sorry for my long absence in this thread.  I was travelling over the holiday and unable to get the time together to have a look at things on the forum and reply.

Anyway, I've gone and had a look at some of the stuff you guys posted about sCO2 as a working fluid.  I think it's a promising technology with a substantial range of applications that may or may not be optimally suited for Mars.  In this post, I will go into more detail on why.

First, the good: The use of sCO2 in a thermodynamic engine has the potential to increase efficiency and decrease size at the same time.  This is a rare case where a new development seems to be strictly better than what came before under all circumstances.  And I want to be clear that under most circumstances, particularly on Earth, I really do think this is better.  However, I don't think it's necessarily better under all circumstances.  To explain why, I will start with two simple numbers: The pressure and temperature of CO2's critical point:

305 K (32 C and 89 F) and 7.4 MPa (74 atm).  No portion of the thermodynamic cycle can operate below that temperature or below that pressure, else you lose the benefits of using a supercritical fluid.

These references all correctly note that denser fluids require less energy to compress, which is a big deal.  The low-temperature engine I described above is not exactly typical, but I'm sure yo all noticed the fact that the low efficiency was due in large part to the work required to freeze CO2 out of the air.

I have three additional observations about this sort of system, one neutral and two drawbacks. 

Neutrally, it seems that much of the efficiency gains come from a higher operating temperature.  There is certainly nothing wrong in the abstract with operating at a higher temperature but it helps to make clear where the efficiency gains are really coming from.

Negatively, the pressures used in this system are likely to be enormous, far larger than comparable systems in use today and maybe even higher than a rocket engine like the SSME.  If the low-pressure portion at the system is at 7.4 MPa, the high pressure portion will be at a substantially higher temperature.  The Sandia report cited a maximum pressure of 14 MPa and the GE report mentioned operating pressures above 25 MPa.  For reference even the SSME operates around 20 MPa.

Also negatively, such high pressures and temperatures operating in a small volume generate a huge challenge in designing a turbine to withstand them.  Turbines already typically operate with advanced materials; this combination of high temperature, high pressure, and extreme force (large changes in pressure over short distances) are going to be really challenging to deal with, possibly at the very limits of materials as they currently exist.  As a point of comparison, the surface of Venus is around 9 MPa and 450 C, which is to say it's actually a less hostile environment in many ways than the interior of such an engine.

I don't mean to claim that it's impossible to build such an engine, or even that it's excessively difficult.  It's not.  What I do want to point out is that the extreme operating conditions will produce an engine that uses advanced materials and requires thorough (read: expensive) design and testing to operate safely and reliably. It's precisely the kind of engine that you would consider for grid power on Earth, where efficiency is critical and system lifetimes are measured in decades.

This is what makes me question whether it's really the best choice for off-planet use.  This scCO2 engine appears to approach a rocket engine in its conditions and complexity, and I'm sure we're all well-aware of the extent to which launches are a critical failure point.  Efficiency, in general, takes a backseat to reliability for pioneering missions or settlements.  This is particularly true of something as critical as an electrical generator.  I am also curious if it would be possible to build a generator small enough to work in the range of our power needs without big losses in efficiency.

When looking at thermodynamic engines, as is often the case, there is no one correct answer but there are many wrong ones.  I would say this falls within the group of reasonable options--the biggest concern to me being that it is not now nor has it in the past been in common use.  Chicken and egg, I know.

I'm sorry, what?

Planning to have a look at the rest of your references tonight

Hey SpaceNut,

That's an interesting device that would work for some applications but probably isn't adequate for this one.  The problem here is volume.  A 50 kW overnight system is going to need something in the range of 15-20 tonnes per day of LCO2.  A batch process in which you need to manually scrape frost off of a plate just doesn't have the capacity or the efficiency that would be needed to make this workable.

There are two related technical issues that I see with the process of freezing CO2 out of the atmosphere.

The first is the low density, and therefore low rate of heat transfer, of the Martian atmosphere.

The second is that CO2 frost on solid surfaces will be hard to recapture for use and will reduce the rate of heat transfer from the refrigeration unit to the atmosphere.  This is a problem because the CO2 will tend to frost on precisely the surfaces being used to freeze it.

I believe I may have a solution which addresses both problems: Use the ambient CO2 atmosphere as your working fluid.

I mentioned refrigeration very briefly in post #26.  I would like to go into somewhat more detail here.  Refrigerators (or heat pumps, which is the more general term) are thermodynamic engines very similar to the power generating cycles we've been discussing in this thread.  However, instead of generating power while transferring heat from hot to cold, they transfer heat from cold to hot while consuming power.  Here's how a refrigeration cycle works, idealized and in the abstract.  Note that the "fluid" in this case usually means a gas:

1. Isentropic compression: In this stage, the working fluid is compressed with no heat transfer.  Compression causes the fluid to heat up to above the ambient temperature.

2. Heat Rejection: In this stage, the working fluid is cooled by releasing heat to the environment

3. Isentropic Expansion: In this stage, the working fluid expands through a piston or turbine and cools, returning some (but not all) of the work done in stage 1.  At the end of this stage, the fluid is cooler than it was originally.

4. Heat Absorption: In this stage, the working fluid absorbs heat from the cold side.  At the end of this stage the fluid is at the same temperature and pressure as it was at the beginning of stage 1.  The cycle then repeats.

In general, the system works best when the "cold" temperature corresponds to the condensation of the working fluid at the operating pressure of the system.  The reason for this is that this creates a uniform temperature on the cold side and increases the density of the fluid.  All else being equal, both of these improve heat transfer.

This drives the choice of fluid for the system.  Historically, some of the first refrigerators used Ammonia.  Later, new gases were developed: Chlorofluorocarbons (CFCs).  These were banned for environmental reasons and replaced with fluorocarbons (FCs) and hydrofluorocarbons (HFCs).  These gases also raise some environmental concerns and lately there has been a movement to consider some other gases.

One refrigerant gas which saw some use in the early years and sees increasing use today is Carbon Dioxide (which is known as R744 when it's being used in a refrigerator/heat pump).  This makes it sorta convenient for our use: We can, in some senses, use Carbon Dioxide to refrigerate itself.

Here's how I propose to do that, in the abstract:

1. Start with ambient air and filter out the fines to the extent that it's possible to do so.  An electrostatic system seems like the best way to get the really fine particles out to me.  This air isn't intended for human consumption so it's a question of machine wear vs. filtering cost.

2. Compress the air using a turbine.  In this case the pressurization we're looking at is pretty mild: As an example, assuming you want to pressurize from 230 K to 270 K (and assuming CO2 behaves like an ideal gas) the pressure differential will be by a factor of two.  In the case of Martian ambient, that means you're pressurizing from 0.7 kPa to 1.4 kPa.

3. Cool the gas back down to ambient: Use a heat exchanger that radiates to the surrounding environment to cool the gas back down towards 230 K.  Ammonia might be a good working fluid for this purpose because of its condensation temperature/pressure curve.

4. Expand the gas through a second turbine and allow it to cool.  Ideally the parameters of this first stage would be such that the gas would cool to the sublimation temperature of 195 K

5. Compress the gas a second time and remove the heat a second time

6. Expand the gas through a turbine.  In the first instance, the gas ended up at a lower temperature than when it entered.  In this instance, the temperature can't fall because the gas already started at its sublimation temperature.  Instead, some of the gas condenses out as dry ice.  By cleverly designing the gas flow you can get this dry ice to be deposited at certain select points, where it can be moved batch by batch into the melting chambers.

7. No more than roughly 10% of the gas can condense out in any one cycle for the turbine to work correctly.  Of the remaining gas, some is tapped off to be used as feedstock for N2 and Ar [this will be more energy intensive because the CO2 needs to be removed entirely but will also produce some dry ice along with useful quantities of Nitrogen and Argon buffer gases] or released to the atmosphere (this is necessary to prevent the mass of gas in the system from increasing forever as N2 and Ar build up) and most is redirected back to the input for the second stage, where it is mixed in an appropriate ratio with gas from the first stage.

My knowledge of refrigeration and chemical process (this system straddles the boundary) is not great, but I think this is the best way to meet our requirements.

It means that you need to know the precise location of the spacecraft better, in order to use such a gentle force over such a long period of time to put it on the right course given the ongoing effect of various gravitational fields.  It's a heavier computational load too.

I don't know exactly where our capabilities lie here but it should be doable.

No substance in nature is pure. There is dry ice at the poles and there is water ice at the poles. Any attempt to mine the one will therefore also produce the other.  Admittedly, I do not know what the ratio of the one to the other will be, but I'm fairly confident that whatever you mine will be less than 100% pure.  Water admixed into the dry ice presents both potential benefits and potential challenges.

Only the real world will tell, but competition between different technologies only works when the two technologies are close enough to parity that they can both be profitable at the same time.  For example, it's possible to get from New York to Los Angeles on foot, on bike, by plane, by train, and by car. People do the latter three to varying degrees, but only do the former extremely rarely because it's so much harder.  Nobody will open a business to bring cargo to LA by bike because it couldn't possibly be profitable.

It's important to mention that mining by teleoperation is not a labor-free activity.  In general the use of robots requires substantial setup and oversight.  Just because the workers aren't out on the cap with a pickaxe doesn't mean their labor doesn't count.

Hey tahanson,

It's certainly conceivable that mining dry ice at the poles could be economical, but it's hard (for me) to imagine that it would actually make sense unless the end user is actually at the poles.

You used coal as an analogy, and it's a pretty decent one.  However, rather than showing the potential value of CO2 mining I believe it shows why it probably won't be worthwhile.

I believe the appropriate comparison is the system we were discussing before: a refrigeration system that uses energy to freeze CO2 out of the atmosphere.  Here are the costs and benefits of such a system:

Costs:

• Requires industrial equipment

• Consumes energy

• System has not been designed fully yet

Benefits:

• Generates ~1 kg per kWh of inert gas (primarily Nitrogen and Argon)

• Works equally well anywhere on Mars

Now, by comparison, here are the costs and benefits of pole-mining:

Costs:

• Requires an ongoing supply of human labor (All mining does)

• Consumes energy

• Requires shipment over thousands of kilometers

• Requires industrial equipment

• System for mining a substance that sublimes at 195 K has never been built and would require some tweaks to existing techniques

Benefits:

• Will produce water as a byproduct

In my view, the question is: Is more labor and thousands of km of shipping worth saving yourself 150 kJ/kg?  In my opinion, it is not.  It's good to compare this to coal:  Coal on Earth has an energy content of 30 MJ/kg, 200 times greater than the energy you save.  Because the energy content is so high, it really is worth pulling out out of the ground and shipping it to the point of use (or at least it was in times past).  I question whether mining CO2 from the poles would save any energy at all.

It would be incredibly frustrating if the FCC felt the need to block people from using certain frequencies in certain ways, but that's life I suppose.

Anyway I don't know a ton about this stuff. If you were creating a network for Martian communications with Earth on a minimum budget, how would you do it?

Hey guys!

I took the engine we were talking about in the "Steam Engines on Mars" thread and wrote it up into an article on Newmars:

http://newmars.com/2018/11/on-mars-air- … r-storage/

Hopefully more to come!