necessity of methane production on Mars

[dror]

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?

[RonM]

Long term storage of liquid hydrogen is difficult. Methane is easier to handle.

[philw1776]

Yes, it's worth the effort because your Raptor engines run on methalox, not hydrolox. Apologies to Gen One BO.

Methane is denser meaning smaller fuel tanks, less mass.

Methane is much, much easier to store and handle than sneaky, leaky hydrogen.  Less massive failure prone cooling.

[oiorionsbelt]

In the making life multi-planetary presentation this chart was shown.

[dror]

Long term storage of liquid hydrogen is difficult. Methane is easier to handle.

So do you think that the extra effort in producing methane is overall better because of storage?
Do you think that that's what drives SpaceX to do so?
Looking at that table above  it does seem like the biggest showstopper

[envy887]

It appears your calculations assume the same dry mass for a hydrolox rocket capable of storing the same amount of propellant and producing the same thrust as a methalox rocket: (m_dry = 0.05*m_fuel).

Since tank mass and propulsion mass both scale with bulk density, and a rocket's mass is primarily tankage and propulsion, and hydrolox bulk density is ~half of methalox, this is a poor assumption.

[Robotbeat]

Ignoring mass fraction is one of the biggest mistakes I see people make again and again in rocketry.

[dror]

It appears your calculations assume the same dry mass for a hydrolox rocket capable of storing the same amount of propellant and producing the same thrust as a methalox rocket: (m_dry = 0.05*m_fuel).

Since tank mass and propulsion mass both scale with bulk density, and a rocket's mass is primarily tankage and propulsion, and hydrolox bulk density is ~half of methalox, this is a poor assumption.

I put it as 0.05*m_fuel which is different for LH2LO2 (36 ton in that example) and for LCH4LO2 (80 ton).
Only the payload is the same for both (10 ton). The two rockets doesn't produce the same thrust nor store the same mass.
The volume may be similar but I assumed that the tank's wheight is dominated by the propelant's mass and not by it's volume.

[dglow]

The volume may be similar but I assumed that the tank's wheight is dominated by the propelant's mass and not by it's volume.

That part is backwards.

[Lar]

Another factor is water consumption. By using CO2 as one of your feedstocks your IMLMO is higher per litre of water consumed. We believe water to be abundant but it's not as abundant as CO2.

[envy887]

It appears your calculations assume the same dry mass for a hydrolox rocket capable of storing the same amount of propellant and producing the same thrust as a methalox rocket: (m_dry = 0.05*m_fuel).

Since tank mass and propulsion mass both scale with bulk density, and a rocket's mass is primarily tankage and propulsion, and hydrolox bulk density is ~half of methalox, this is a poor assumption.

I put it as 0.05*m_fuel which is different for LH2LO2 (36 ton in that example) and for LCH4LO2 (80 ton).
Only the payload is the same for both (10 ton). The two rockets doesn't produce the same thrust nor store the same mass.

In terms of relative mass, they do. A hydrolox engine sized to loft 47.6t will mass about as much as a methalox engine sized to loft 94t. A fuel tank sized to hold 36t of hydrolox will mass about as much as a tank sized to hold 80t of methalox, because the hydrogen tank needs more insulation.

The volume may be similar but I assumed that the tank's wheight is dominated by the propelant's mass and not by it's volume.

A pressure vessel's mass scales linearly with volume, at a constant pressure... generally regardless of the density of it's contents. LH2 is particularly hard (read: mass intensive) to store long-term, since it tends to leak and boil-off much worse than LCH4 or LOX.

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).

And you're still 10% short on dV, which is a big deal, despite the unrealistic mass fractions.

[TomH]

The volume may be similar but I assumed that the tank's wheight is dominated by the propelant's mass and not by it's volume.

That part is backwards.

It surely is. Hydrogen's ISP density would require the fuel tanks on the ITS to be multiple times larger.

[oiorionsbelt]

According to Elon "the energy cost on Mars, of producing and storing hydrogen is very high."

[dror]

According to Elon "the energy cost on Mars, of producing and storing hydrogen is very high."

My point was that hydrogen production on mars is necessary but methane production is not.
The hydrogen is turned into methane in order to get a better fuel and slightly better performance, which you all seem to belive is worth the effort.

[Robotbeat]

/liquid/ hydrogen production on Mars is not necessary, though. Going to methane solves that problem. And then you can use equipment that's actually easier than the oxygen liquifier.

Also, methane is a feedstock in many chemical processes, including some ways of growing single celled protein.

[MATTBLAK]

Methane production is very useful on Mars. But depending on your propulsion systems; not essential. I think LOX production is equally if not more important - you can use LOX with hydrazine, kerosene, LH2, propane, LNG, etc. And even with Carbon Monoxide, too - albeit with poor specific impulse.

[dror]

What about solid carbon as fuel?
Theoretically, powdered coal will burn just fine with O2 and may be tweaked to produce thrust in the right configuration. That can be produced without water mines.

[TomH]

What about solid carbon as fuel?
Theoretically, powdered coal will burn just fine with O2 and may be tweaked to produce thrust in the right configuration. That can be produced without water mines.

Coal is an organic fossil fuel-rocks formed from living plants from hundreds of millions of years ago, metamorphed through all those years into peat, lignite, bituminous, then anthracite. None of it is pure carbon nor pure hydrocarbon. All fossil fuels are hydrocarbons, never pure carbon, and even anthracite has impurities in it. The conditions never existed on Mars for coal to be made there. The stuff isn't just organic; it's a product of biochemistry, followed by immense prolonged geological processes. Also, there is no practical way to manufacture solid carbon on Mars. Solid fuels cannot use pure oxygen as an oxidizer anyway, at least not under these circumstances. You have to mix solid fuel and solid oxidizer as liquid/paste like substance and carefully cast in a casing. Solid rockets have relatively poor ISP anyway. Liquid engines are the only real possibility, and when it comes to making fuel on Mars, methane is the only practical fuel. Sorry, but this is a real dead end.

[envy887]

Don't forget that returning from Mars requires far more than getting to LMO. The return vehicle needs storable propellants for RCS, trajectory corrections, and landing at Earth. The mass penalty for storing liquid hydrogen in space for that long is much higher than LOX or LCH4.

[Robotbeat]

What about solid carbon as fuel?
Theoretically, powdered coal will burn just fine with O2 and may be tweaked to produce thrust in the right configuration. That can be produced without water mines.

Coal is an organic fossil fuel-rocks formed from living plants from hundreds of millions of years ago, metamorphed through all those years into peat, lignite, bituminous, then anthracite. None of it is pure carbon nor pure hydrocarbon. All fossil fuels are hydrocarbons, never pure carbon, and even anthracite has impurities in it. The conditions never existed on Mars for coal to be made there. The stuff isn't just organic; it's a product of biochemistry, followed by immense prolonged geological processes. Also, there is no practical way to manufacture solid carbon on Mars. Solid fuels cannot use pure oxygen as an oxidizer anyway, at least not under these circumstances. You have to mix solid fuel and solid oxidizer as liquid/paste like substance and carefully cast in a casing. Solid rockets have relatively poor ISP anyway. Liquid engines are the only real possibility, and when it comes to making fuel on Mars, methane is the only practical fuel. Sorry, but this is a real dead end.

Most of this post is missing the point and/or wrong, although I agree with the conclusion that liquid fuels are a much better way to go.

Solid carbon CAN be produced synthetically (in spite of your claim otherwise), I don't know why you brought up several sentences about fossil fuels just because he mentioned the fact that powdered coal burns well (he only used that as a supporting statement... High grade coal is almost all carbon).

And where did he mention a solid rocket? Solid fuel and liquid oxidizer is also clearly a thing. It's called a hybrid rocket, of course, and it's what Virgin Galactic is using for suborbital flights.

Why do people like to jump in quickly to say something is impossible without doing any research? Solid carbon is actually a byproduct of certain closed loop life support systems designed to pull all the oxygen out of exhaled CO2.

But I agree with the conclusion. Liquid fuels are best since you would have to manufacture a hybrid fuel grain on Mars versus just a liquid process, especially because liquid is far better suited for reuse.

In answer to dror, carbon monoxide actually makes a good enough fuel. It has lower Isp, but not THAT low. A pump-fed CO/O2 rocket could get better than 300s in vacuum and the bulk density is higher than methane/LOx. And 300s is better Isp than most first stage engines on Earth, so I'm not sure why people dismiss it so easily. Additionally, the flip side of lower specific energy (i.e. essentially related to lower Isp) is the fact that the it takes less energy to produce in the first place, AND can be done in a single step (unlike methane which needs Sabatier to combine CO2 and H2, thus introducing losses). Remember, with ISRU for a single stage vehicle we're not really limited by propellant mass, but instead by dry mass, energy required to make the propellant, and perhaps water needed to be harvested. So if you consider the ACTUAL constraints, CO/O2 looks good. Not only that, but it'll be the first ISRU technology to be tested on Mars (that we know of). The ISRU demo flying on the 2020 rover will produce not only oxygen but also carbon monoxide (which is usually discarded).

And one more thing: if you're trying to minimize energy to reach orbit in a single stage, you don't want to maximize Isp. At some point, if you have higher Isp, it'll actually take MORE energy to get to orbit. I believe the optimal exhaust velocity for a given delta-v is about 60-80% of the delta-v. So for a 4km/s delta-v needed to reach orbit, CO/O2's 3km/ exhaust velocity is pretty close to optimal.

So yeah, I do think there's something to be said of using a fuel that doesn't need water. But not solid carbon, but liquid CO.