system
Given the following post, respond as if you're commenting on Reddit:
human
Do rockets use fossil fuels? Is there danger of running out of rocket fuel as we deplete oil reserves in the next 50-200 years? If so, are there alternative fuels that have the necessary power to take us into space?
gpt
We are in no danger of running out of liquid oxygen or liquid hydrogen, as they can be extracted from water. You can make a workable rocket with just those fuels, although a big fat first stage full of low density hydrogen has penalties. 

Realistically, if absolute supply of kerosene or methane is an issue, we'll be too far gone to have the technical infrastructure to support spaceflights. 

Helium is the real supply danger. It's irreplaceable as a pressurant and coolant and it is a nonrenewable resource. It's plentiful in space, but in limited supply on earth.