[ylere]

Methane has 28% more energy per kg than kerosene and also produces slightly less CO2 (2.75kg CO2/kg burned vs 3.00 for kerosene) when burned [1]. SpaceX uses a 78:22 LOX to CH4 ratio, so for 34M kg of fuel burned, 20.57M kg of CO2 are produced (34×0.22×2.75).

[1] https://www.engineeringtoolbox.com/co2-emission-fuels-d_1085... [2] https://x.com/elonmusk/status/1298426245991063554?lang=en

[aw1621107]

Somewhat tangential, but as far as rocket fuels go energy per volume is also an important metric to consider. It's one of the (several) reasons hydrogen isn't quite as good for rockets in practice as it is in theory - while hydrogen has tons of energy per unit mass (120-142 MJ/kg for hydrogen vs. 50-55 MJ/kg for methane and 43.1-46.2 MJ/kg), it has a far lower density (70.85 kg/m3 vs. 422.8 kg/m3 for liquid methane and 820 kg/m3 for kerosene). As a result, you need quite a bit more tankage for a given amount of energy from hydrogen compared to what methane/kerosene requires.

IIRC there's a tradeoff between efficiency and thrust as well. Heavier fuels aren't quite as energy-efficient, but it's easier for them to develop a lot of thrust, which is important for the initial stages of launch. If I'm remembering events described in Ignition! correctly this led to "thrust density" being something that was optimized for - to the point that there were experiments with mixing mercury into the fuel!

[0]: https://world-nuclear.org/information-library/facts-and-figu...

[1]: https://en.wikipedia.org/wiki/Kerosene

[ylere]

Correction (can't edit the post anymore): As another commenter pointed out, the rocket can carry 3.4M kg of fuel, not 34M kg.

[jbu]

I do wonder if the cost of capturing that carbon has been included in the published cost of a launch? at $0.5/kg that's $10,000,000.