For some reason I never saw anyone inquiring about the climate change implications of the Starships.
Elon is envisioning a rate of one million Starship launches per year. That makes the numbers very round: every launch produces 5000 tons of CO2-equivalent, so that's 5 gigatons of CO2e. For comparison the whole of the US emitted 4.7 GT of CO2e in 2022.
Starship uses methane, which can be produced in a carbon neutral way. If the launch frequency really becomes that big, I'd expect regulators to demand green-ification of used fuel.
> methane, which can be produced in a carbon neutral way
Maybe it can, but will it? And we are not talking about producing a bit of methane, we are talking about gigatons. For comparison, green hydrogen can be produced many times cheaper than "green" methane, but nobody hopes that we'll get to gigaton scale anytime soon. Here's the relevant quote from the IEA summary [1]:
> If all projects currently in the pipeline were realised, by 2030 the production of low-emission hydrogen could reach 16-24 Mt per year, with 9-14 Mt based on electrolysis and 7-10 Mt on fossil fuels with CCUS.
Well, SLS uses hydrogen and solid fuel boosters. I'm not sure if there ever was a rocket that only used hydrogen and oxygen to go to space.
But if we are to get to a million launches per year, I think we should go with hydrogen/oxygen only, otherwise the climate impact is going to be gigantic.
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.
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.
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?
Elon is envisioning a rate of one million Starship launches per year. That makes the numbers very round: every launch produces 5000 tons of CO2-equivalent, so that's 5 gigatons of CO2e. For comparison the whole of the US emitted 4.7 GT of CO2e in 2022.