Author Topic: necessity of methane production on Mars  (Read 11791 times)

Offline TomH

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Re: necessity of methane production on Mars
« Reply #40 on: 01/14/2017 12:59 AM »
Sorry for the confusion..

Me too. I learned something from this. Next time I will ask for clarification as a first step.

Offline sdsds

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Re: necessity of methane production on Mars
« Reply #41 on: 01/16/2017 08:02 AM »
What about much larger synthetic hydrocarbons? C12H20 / TH-dimer / tetrahydromethylcyclopentadiene / RJ-4 as an example.

Another example would be RJ-5, discussed here: https://forum.nasaspaceflight.com/index.php?topic=19457.0

More generally http://www.biofuelsdigest.com/bdigest/2014/06/18/9-advanced-molecules-that-could-revolutionize-jet-and-missile-fuel/ lists a bunch.

Are any of these appropriate targets of synthesis on Mars?
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Offline guckyfan

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Re: necessity of methane production on Mars
« Reply #42 on: 01/16/2017 08:31 AM »
The first step beyond methane would likely be ethylene. It is the base of many chemical products. There is also extensive research going on to transfer the production of ethylene from methane from lab status to practical application. This is driven by the fact that many remote oil production facilities still burn methane byproduct because of transport problems. If methane can be processed on site to ethylene this waste would go away. Incidentally it would be extremely useful for Mars.

Offline sdsds

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Re: necessity of methane production on Mars
« Reply #43 on: 01/16/2017 08:39 AM »
Yes but don't all the small hydrocarbons (e.g. methane, ethane, propane, butane) have fairly high vapor pressures, i.e. aren't they gasses under standard conditions? Bigger ones are liquids....
« Last Edit: 01/16/2017 08:39 AM by sdsds »
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Offline TomH

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Re: necessity of methane production on Mars
« Reply #44 on: 01/17/2017 04:57 AM »
Yes but don't all the small hydrocarbons (e.g. methane, ethane, propane, butane) have fairly high vapor pressures, i.e. aren't they gasses under standard conditions? Bigger ones are liquids....

Gasoline is a mixture of quite a few hydrocarbons and has a fairly low molar enthalpy of vaporization. #1 and #2 (diesel) fuel oils take longer. Being mixtures, they lose the lighter volatiles and thicken in viscosity. You mention "standard conditions." The 0.0059 atmospheric density on Mars allows almost any liquid to quickly evaporate. OTOH, any synthesis is going to be in closed tanks, not open vats, so I would think that tanks with the necessary strength eliminate the problem by allowing pressurization. Getting LOX cold enough on Mars is what I see as the biggest hurdle. (I am assuming methane as the fuel, not LH2.) As it is compressed, a lot of heat will have to be dumped and you have to get it to -183C (and the methane -162C or so) at STP to liquify and even colder to densify. You can do that, but I just wonder about the mass requirement for such hardware on Mars. When you look at down to 20K for LH2, how much more mass is required for the necessary hardware? Not only do you have to make the stuff, you have to make it faster than it boils off in the ITS tanks and you have to keep it cold. I do not see how you have storage tanks that are separate from the on board prop tanks. IDK, but my WAG is that the requirements for all this are well beyond simple.
« Last Edit: 01/17/2017 05:02 AM by TomH »

Online john smith 19

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Re: necessity of methane production on Mars
« Reply #45 on: 01/17/2017 07:23 AM »
so I would think that tanks with the necessary strength eliminate the problem by allowing pressurization.
Keep in mind all tanks on Earth are "pressurized" to 1 atmosphere absolute. The fact that we breath that pressure normally means we forget about it.
Quote
Getting LOX cold enough on Mars is what I see as the biggest hurdle. (I am assuming methane as the fuel, not LH2.) As it is compressed, a lot of heat will have to be dumped and you have to get it to -183C (and the methane -162C or so) at STP to liquify and even colder to densify. You can do that, but I just wonder about the mass requirement for such hardware on Mars. When you look at down to 20K for LH2, how much more mass is required for the necessary hardware?
LH2 could be more viable on Mars. On Earth LH2 is difficult because it chills LOX out of air and creates a fire and explosion hazard. On Mars it would chill CO2 to dry ice (eventually)

In principle refrigeration is easier on Mars as the difference between outside air temperature and the target is smaller, needing less power to put a refrigerant through its compression/expansion cycle. OTOH the atmosphere is 1/160 that of Earth. Not a vacuum but I suspect actual radiation would be a bigger part of how a radiator would dump heat on Mars. A nice convenient glacier would be a useful heat sink at this point.
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Offline Norm38

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Re: necessity of methane production on Mars
« Reply #46 on: 01/17/2017 05:20 PM »
In short, there is no way a hydrolox rocket massing 1.6t dry will boost the same payload as a methalox rocket massing 4t dry. It simply cannot hold enough propellant. (Upper stages are different since they do not have to lift their own mass and thus can have tiny engines and tiny thrust structures. They also don't have to store LH2 for more than a few hours).

I don't want to derail this thread any, but has anyone ever worked up what the Space Shuttle would have looked like as methalox instead of LH2?  That would be instructive.

Offline envy887

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Re: necessity of methane production on Mars
« Reply #47 on: 01/17/2017 06:02 PM »
In short, there is no way a hydrolox rocket massing 1.6t dry will boost the same payload as a methalox rocket massing 4t dry. It simply cannot hold enough propellant. (Upper stages are different since they do not have to lift their own mass and thus can have tiny engines and tiny thrust structures. They also don't have to store LH2 for more than a few hours).

I don't want to derail this thread any, but has anyone ever worked up what the Space Shuttle would have looked like as methalox instead of LH2?  That would be instructive.

The Shuttle+ET was effectively an upper stage. Hydrolox made sense there since the SSMEs burned through almost 10 km/s of delta-v and didn't have to lift the entire stack off the pad.

Offline TomH

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Re: necessity of methane production on Mars
« Reply #48 on: 01/18/2017 02:20 AM »
Keep in mind all tanks on Earth are "pressurized" to 1 atmosphere absolute. The fact that we breath that pressure normally means we forget about it.

As a former physics teacher, I am quite aware of what you are talking about. I disagree, however, because an open container has the same pressure on the inside and outside walls, therefore the pressure differential is zero. If you boil a tiny bit of water in metal can, then screw the cap on immediately after removing it from heat, a partial vacuum will begin to form within the can as the steam condenses back to liquid water, and the outer pressure will almost completely crush the can. On Mars, you have almost no exterior pressure. To condense vaporous CH4 or O2 into liquid, the container will need to be pressurized internally, but there will be almost no compensating external pressure. Of course most containers can withstand far more internal pressure than external pressure (think COPV issues).

OTOH the atmosphere is 1/160 that of Earth. Not a vacuum but I suspect actual radiation would be a bigger part of how a radiator would dump heat on Mars.

That is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into space. If you could afford the mass, you could drill vertical shafts into the ground and make an earth-coupled (Mars-coupled??) heat-pump with tubes full of ethylene or propylene glycol, ammonia, or some class of freon and dump the heat into the ground. If you had a trencher or backhoe you could lay out tubes in a back and forth pattern a couple of meters down. My WAG is that something like the ammonia radiators on ISS would be required.

I fear chilling and storing prop on Mars may be more complicated than many people realize due to these issues.

I personally think the ideal mission for the first Red Dragon (other than landing) would be a basic experimental exercise in making and storing LCH4 and LOX. That is the most important thing that has to happen to get back off The Red Planet.
« Last Edit: 01/18/2017 02:33 AM by TomH »

Offline Hotblack Desiato

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Re: necessity of methane production on Mars
« Reply #49 on: 01/18/2017 08:29 AM »

I fear chilling and storing prop on Mars may be more complicated than many people realize due to these issues.

Yes, storing methane and LOX for extended periods of time could be difficult.

The LOX requires constant cooling, so that the oxygens partial pressure stays low. It will form a dry ice layer on the outside (just frozen martian atmosphere). Which could be actually an advantage. It adds an extra insulation layer. Just keep it in the shadow duing the day (It will still require constant cooling).

On the fuel side, things are more interesting.

We don't know yet, if methane is the only liquid fuel the raptor engine can consume. I'm thinking about propane, butane and pentane, which have very interesting melting and boiling points. Butane melts at 139K and boils at 274K, propane EDIT pentane (my bad) melts at 144K and boils at 309K. This will shift a bit to lower temperatures due to lower temperatures due to the lower pressure on Mars, but it could still be good enough for thin walled metal tanks to store large quantities of fuel at outside temperatures.

As a nice bonus, these fuels are much more dense than methane, not requireing extensive subcooling of the fuel for densification, yet they behave similarly to methane in terms of viscosity and energy density (actually, energy density is better).

The fuel could be easily produced (requires CO and H2O via the Fischer Tropsch Process) in situ, similar to the Haber Bosch Process, which only gives methane. Actually, Fischer Tropsch would give a mix of different hydrocarbons (propane to hexane), which can be adjusted even better for the required melting and boiling points, since hydrocarbons mix very well.

EDIT: as a funny side note, wikipedia mentiones a large FT-reactor was operated from 1951 to 57 in Brownsville, TX.
« Last Edit: 01/20/2017 10:48 PM by Hotblack Desiato »

Offline Paul451

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Re: necessity of methane production on Mars
« Reply #50 on: 01/19/2017 08:40 PM »
propane melts at 144K and boils at 309K

I think you meant pentane.

Offline CuddlyRocket

Re: necessity of methane production on Mars
« Reply #51 on: 02/10/2017 11:42 PM »
SpaceX plan to produce Methane on Mars but to do so, they need to harvest water and break it.

Step one - produce hydrolox
 2H2O -> 2H2 + O2

Step two - produce methalox
 2H2 + O2 + CO2  ->  CH4 + 2O2

My back of the envelope (attached) shows that using the HydroLox from stage one gives almost as much dV as the methalox of both steps, which makes me wonder, does it worth the extra effort of producing methane?

Step two is not how it is usually proposed to produce methalox on Mars. It's generally not a good idea to conflate steps as this can lead you astray.

The Sabatier reaction is 4H2 + CO2  ->  CH4 + 2H2O. The water goes back into step one. The entire process does not produce sufficient LOX for methalox propellant so additional LOX is obtained by electrolysing CO2 directly. People tend to concentrate on the fuel component, but it's the LOX that dominates the mass of the propellant. Plus Oxygen has other uses!

Offline guckyfan

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Re: necessity of methane production on Mars
« Reply #52 on: 02/11/2017 06:28 AM »
The Sabatier reaction is 4H2 + CO2  ->  CH4 + 2H2O. The water goes back into step one. The entire process does not produce sufficient LOX for methalox propellant so additional LOX is obtained by electrolysing CO2 directly. People tend to concentrate on the fuel component, but it's the LOX that dominates the mass of the propellant. Plus Oxygen has other uses!

You forgot the O2 from electrolysis. The complete process produces O2 at stochiometric value so there is an excess of oxygen because the rocket engines always run fuel rich.

Edit: You need O2 from CO2 if you assume feed H2 brought in from earth.
« Last Edit: 02/11/2017 06:35 AM by guckyfan »

Offline dror

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Re: necessity of methane production on Mars
« Reply #53 on: 02/11/2017 07:26 AM »
SpaceX plan to produce Methane on Mars but to do so, they need to harvest water and break it.

Step one - produce hydrolox
 2H2O -> 2H2 + O2

Step two - produce methalox
 2H2 + O2 + CO2  ->  CH4 + 2O2

My back of the envelope (attached) shows that using the HydroLox from stage one gives almost as much dV as the methalox of both steps, which makes me wonder, does it worth the extra effort of producing methane?

Step two is not how it is usually proposed to produce methalox on Mars. It's generally not a good idea to conflate steps as this can lead you astray.

The Sabatier reaction is 4H2 + CO2  ->  CH4 + 2H2O. The water goes back into step one. The entire process does not produce sufficient LOX for methalox propellant so additional LOX is obtained by electrolysing CO2 directly. People tend to concentrate on the fuel component, but it's the LOX that dominates the mass of the propellant. Plus Oxygen has other uses!

That's a schematic of their plan. The source of H is ISRU water ice, not H2 from earth.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf
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Online john smith 19

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Re: necessity of methane production on Mars
« Reply #54 on: 02/13/2017 07:40 AM »
OTOH the atmosphere is 1/160 that of Earth. Not a vacuum but I suspect actual radiation would be a bigger part of how a radiator would dump heat on Mars.

That is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into space. If you could afford the mass, you could drill vertical shafts into the ground and make an earth-coupled (Mars-coupled??) heat-pump with tubes full of ethylene or propylene glycol, ammonia, or some class of freon and dump the heat into the ground. If you had a trencher or backhoe you could lay out tubes in a back and forth pattern a couple of meters down. My WAG is that something like the ammonia radiators on ISS would be required.
I'd be careful here. While roughly 640Pa is nothing by human standards that's nowhere near hard vacuum. Anyone who's tried to draught proof a house will know that even small draughts can carry a lot of heat.

On an odd physics note a pipe hanging down the side of a narrow hole should look quite a lot like an ideal black body radiator.

The balance will shift a lot on Mars but it's still a balance, unlike the Moon or LEO, where it's strictly radiation only.
Quote
I personally think the ideal mission for the first Red Dragon (other than landing) would be a basic experimental exercise in making and storing LCH4 and LOX. That is the most important thing that has to happen to get back off The Red Planet.
Yes. When someone said it'd take about 50  000 tonnes of propellant to propel ITS back to Earth I wondered if people realize just how big a system they are talking about.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline R7

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Re: necessity of methane production on Mars
« Reply #55 on: 02/13/2017 02:04 PM »
That is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into space
I'd be careful here. While roughly 640Pa is nothing by human standards that's nowhere near hard vacuum. Anyone who's tried to draught proof a house will know that even small draughts can carry a lot of heat.

Yep, heat transfer coefficient isn't linear to pressure. Martian atmosphere carries heat surprisingly well.

https://www.researchgate.net/profile/Alvaro_Soria-Salinas/publication/299600459_Convective_Heat_Transfer_Measurements_at_the_Martian_Surface/links/570361ee08aedbac126f4bc4/Convective-Heat-Transfer-Measurements-at-the-Martian-Surface.pdf

Estimates h of almost 10 W/m2K for 4m/s wind. Forced convection would increase that a lot.

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Online john smith 19

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Re: necessity of methane production on Mars
« Reply #56 on: 02/13/2017 04:06 PM »
Yep, heat transfer coefficient isn't linear to pressure. Martian atmosphere carries heat surprisingly well.

https://www.researchgate.net/profile/Alvaro_Soria-Salinas/publication/299600459_Convective_Heat_Transfer_Measurements_at_the_Martian_Surface/links/570361ee08aedbac126f4bc4/Convective-Heat-Transfer-Measurements-at-the-Martian-Surface.pdf

Estimates h of almost 10 W/m2K for 4m/s wind. Forced convection would increase that a lot.
Something to keep in mind. ITS will need a fair bit of insulation to keep temperatures reasonable.  The upside is "radiators" will be much more efficient than in deep space and the high delta t will translate to actual useful improvements in radiator size reduction.

That still leaves you with an awful lot of water and CO2 to move to refuel.
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Offline Kaputnik

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Re: necessity of methane production on Mars
« Reply #57 on: 02/13/2017 09:16 PM »
The first step beyond methane would likely be ethylene. It is the base of many chemical products. There is also extensive research going on to transfer the production of ethylene from methane from lab status to practical application. This is driven by the fact that many remote oil production facilities still burn methane byproduct because of transport problems. If methane can be processed on site to ethylene this waste would go away. Incidentally it would be extremely useful for Mars.

It's a few years since I last read it, but didn't Zubrin make a strong case for ethylene, as a sort of footnote to Mars Direct?
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Offline Dalhousie

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Re: necessity of methane production on Mars
« Reply #58 on: 02/14/2017 12:24 AM »
Yes. When someone said it'd take about 50  000 tonnes of propellant to propel ITS back to Earth I wondered if people realize just how big a system they are talking about.

Do you have a link? Back of the envelope suggests that for a minimum energy unmanned return ~1500 tonnes should be enough.  Which is still a substantial amount to make on Mars.
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Offline envy887

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Re: necessity of methane production on Mars
« Reply #59 on: 02/14/2017 12:35 AM »
Yes. When someone said it'd take about 50  000 tonnes of propellant to propel ITS back to Earth I wondered if people realize just how big a system they are talking about.

Do you have a link? Back of the envelope suggests that for a minimum energy unmanned return ~1500 tonnes should be enough.  Which is still a substantial amount to make on Mars.

50,000 tonnes of propellant at the Earth's surface is about what it will take to land a single loaded ITS on Mars.

Getting it back empty could probably be done for only ~800 tonnes the Mars surface.

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