I looked at potassium nitrate + sugar as rocket fuel but this doesn't really yet provide enough of
a specific impulse to escape earth and into space.

I then found out about ethanol + water, as used in the V2 rocket.

I thought about using air instead of liquid oxygen as an oxidiser but realised that once the air
had run out (in space), the fuel would no longer burn.

Submarines use liquid oxygen, presumably because oxygen as a gas runs out too quickly.

In order for a space craft to support human life, it needs oxygen anyway. This probably means, at
least for now, that liquid oxygen + ethanol + water is close to the best combination for space
exploration possible.