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You imply that we can produce and use all that hydrogen with net zero carbon, yet methane is impossible to somehow? We have to consider whole life cycle and inf

by rini17 3y ago

You imply that we can produce and use all that hydrogen with net zero carbon, yet methane is impossible to somehow? We have to consider whole life cycle and infrastructure, not only the point of combustion at the nozzles. Then the hydrogen has its drawbacks too.

credit_guy 3y ago
It's two things actually. First, yes, "green" methane is not impossible, but it's much more expensive than green hydrogen for the simple reason that green methane is green hydrogen plus a few more extra steps. And those few extra steps are very costly, in particular that one step where you scrub CO2 from the atmosphere and split it in C and O2. Second, the way the Starship works, they don't just burn CH4 in O2 and emit CO2 and H2O, but they also release a lot of CH4 too. In other words their mix is "fuel rich" rather than stoichiometric or oxygen rich. I did run the calculations, and the end result is that you end up with about twice the greenhouse emissions, because CH4 is a much more potent greenhouse gas than CO2. So, if you want to be net carbon neutral, you not only need to manufacture green CH4, but also to scrub once more the required amount of carbon from the atmosphere. Some people may retort that Musk's plan was to manufacture methane on Mars all along. He even mentioned the Sabatier process, so he must know what he's talking about. And he does. But you need to keep in mind that a very small fraction of the Starship trips will end on Mars, most will go from Earth to LEO. And for those that make it to Mars, the Sabatier process is the dirt cheap alternative to bringing their return fuel from Earth. But something can be at the same time dirt cheap on Mars and exorbitantly expensive on Earth. Plus, in the original plan (made by Robert Zubrin), the Mars rocket was supposed to bring the hydrogen from Earth. In a stoichiometric mix of CH4 and O2, the hydrogen is only one eight of the mass, and in an oxygen-rich mix it could be much less. So that was the plan until they discovered water on Mars. And it most likely continues to be the plan for many cases, because water exists in quite few places on Mars, not everywhere.
- rini17 3y ago
  On he other hand, hydrogen requires thicker insulation including on the starship/booster itself, or pressure vessels everywhere, better cooling etc., All kinds of additional things that need to be manufactured and maintained and that emits plenty of CO2 in today's economy and near future. Manufacturing green hydrogen is very akin to green CH4, so that cancels out. It's very likely we will need huge CO2 capture capacity anyway for other reasons, so offsetting rocket launches will then become marginal. And releasing CH4 on launch looks like a technical problem, not a fundamental one. Release additional oxygen so that everything burns completely further below the engine. I don't think Mars manufacturing will make any change in Earth greenhouse gases emissions, or do i miss something?

by rini17 3y ago

credit_guy 3y ago

It's two things actually. First, yes, "green" methane is not impossible, but it's much more expensive than green hydrogen for the simple reason that green methane is green hydrogen plus a few more extra steps. And those few extra steps are very costly, in particular that one step where you scrub CO2 from the atmosphere and split it in C and O2. Second, the way the Starship works, they don't just burn CH4 in O2 and emit CO2 and H2O, but they also release a lot of CH4 too. In other words their mix is "fuel rich" rather than stoichiometric or oxygen rich. I did run the calculations, and the end result is that you end up with about twice the greenhouse emissions, because CH4 is a much more potent greenhouse gas than CO2. So, if you want to be net carbon neutral, you not only need to manufacture green CH4, but also to scrub once more the required amount of carbon from the atmosphere. Some people may retort that Musk's plan was to manufacture methane on Mars all along. He even mentioned the Sabatier process, so he must know what he's talking about. And he does. But you need to keep in mind that a very small fraction of the Starship trips will end on Mars, most will go from Earth to LEO. And for those that make it to Mars, the Sabatier process is the dirt cheap alternative to bringing their return fuel from Earth. But something can be at the same time dirt cheap on Mars and exorbitantly expensive on Earth. Plus, in the original plan (made by Robert Zubrin), the Mars rocket was supposed to bring the hydrogen from Earth. In a stoichiometric mix of CH4 and O2, the hydrogen is only one eight of the mass, and in an oxygen-rich mix it could be much less. So that was the plan until they discovered water on Mars. And it most likely continues to be the plan for many cases, because water exists in quite few places on Mars, not everywhere.

rini17 3y ago
On he other hand, hydrogen requires thicker insulation including on the starship/booster itself, or pressure vessels everywhere, better cooling etc., All kinds of additional things that need to be manufactured and maintained and that emits plenty of CO2 in today's economy and near future. Manufacturing green hydrogen is very akin to green CH4, so that cancels out. It's very likely we will need huge CO2 capture capacity anyway for other reasons, so offsetting rocket launches will then become marginal. And releasing CH4 on launch looks like a technical problem, not a fundamental one. Release additional oxygen so that everything burns completely further below the engine. I don't think Mars manufacturing will make any change in Earth greenhouse gases emissions, or do i miss something?