[gmanley]

Could you simply separate hydrogen from the water, using electrolysis, or other methods and then use the Sabatier reaction? [1] Carbon dioxide in bulk may be harder to come by on the moon, however.

Also, the Lunar Gateway [2] is a key part of the plans by NASA/SLS. It's not about settling on the moon, it's about making it a stop off point to refuel or pickup supplies before going on to a further off destination like Mars. Instead of having to have all your fuel and payload when taking off from Earth, you can have a lot of your supplies and weight on the Moon. This means your trip off Earth can be cheaper. Getting out of Earths gravity and getting to escape velocity is the hard part. Getting off the moon is a lot easier.

[1] https://en.wikipedia.org/wiki/Sabatier_reaction [2] https://en.wikipedia.org/wiki/Lunar_Gateway

[dhosek]

The Sabatier reaction relies on carbon dioxide. For the concept of using it to generate rocket fuel, there's a reliance on atmospheric CO₂ which is viable on Mars but not the Moon (which is why the section in the Wikipedia article is called “Manufacturing propellant on Mars”). And the Lunar Gateway is irrelevant to what I'm arguing, which is that there's not really any reason to settle off-planet.