Late to the party but... we don't need fossil fuels to launch rockets.
Aside from synthetic fuels, you can launch rockets on methane. Can't make methane? (You are a robot society by now, because it would mean there is no biosphere anymore). Still fine! We can convert our oceans into fuel (hydrogen + oxygen), given energy input. Energy can come from renewables, nuclear, etc.
No, we are not going to run out of fuel for as long as we have water.
Aside from synthetic fuels, you can launch rockets on methane. Can't make methane? (You are a robot society by now, because it would mean there is no biosphere anymore). Still fine! We can convert our oceans into fuel (hydrogen + oxygen), given energy input. Energy can come from renewables, nuclear, etc.
No, we are not going to run out of fuel for as long as we have water.