Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

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Emissions calculation for Starship:

- each starship burns 4600 tons of methalox fuel

- the mixture ratio of the propellent is 78% O2 and 22% CH4 [1]

- that means that each 100 tons of methalox results in 53.6 tons of CO2 and 2.5 tons of CH4

- generally in the climate science it is accepted that one ton of CH4 is equivalent to 25 tons of CO2

- overall, each 100 tons of propellent will produce 116 tons of CO2-equivalent emissions

- a single launch of 4600 tons propellent will result in 5.34 kT CO2 emissions

- if we get to 5000 launches per year we end up with about 25 MT of CO2-equivalent emissions

- the current worldwide level of emissions is 45 GT CO2-equivalent as of 2017 [2]

- that means 5000 launches will increase our global emissions by 0.06%

[1] https://twitter.com/elonmusk/status/1258580078218412033

[2] https://en.wikipedia.org/wiki/List_of_countries_by_greenhous...

This doesn’t take into account that the methane used in fuelling can be extracted from the atmosphere and that much of the fuel / oxidizer mixture actually is removed from Earth entirely.

> the propellent is O2 and CH4 [1]

> that results in CO2 and CH4

The chemical reaction of O2 + CH4 -> CO2 + CH4 does not compute.

More likely is the reaction 2 O2 + CH4 -> CO2 + 2 H2O.

So no methane emissions, that's the actual fuel. Only CO2 (and water) emissions.

Sorry, I skipped some steps in my calculation.

The chemical reaction is indeed CH4 + 2 O2 -> CO2 + 2 H2O. So, for each 16 grams of CH4 you get 64 grams of O2, that's a stoichiometric mass ratio of 1:4.

The fuel ratio for the Raptor engine is 22:78, according to Elon Musk's tweet. So, for each 100 g of propellant mixture you get 19.5 g of CH4 to burn using 78 g of O2 and you are left with 2.5 g unburned CH4.

I simply neglected the H2O resultant from the reaction because, while H2O is a very potent greenhouse gas, it has a fast cycle in nature (rain, etc). Also, I have no idea how to account for it as a greenhouse gas.

From what I've heard there is a significant fraction of carbon monoxide in the exhaust, so there will be a lot less CH4 in the exhaust than you have calculated. CO is only a weak greenhouse gas.

That is very interesting. Do you happen to have a link? I'm not doubting you, I think you are right, but a link that goes into this type of detail would be a very interesting one to read.

I probably heard that here: https://everydayastronaut.com/rocket-pollution/

Otherwise it would have been in a comment on an Eric Berger Ars Technica article.

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