I'm making a small game about travelling deep space on limited resources. One thing I'm interested in having is the idea that the craft is scavenging what it can from planets as it travels through entirely uninhabited locations.
The idea is this is a sufficiently advanced society that they could feasibly extract elements out of common materials to make a fuel from what they came across. So my question is, what element would they design around finding the most? I know earth's atmosphere is largely Nitrogen but if scientists were to make their best assessment, what would they bank on being the most copious element available to fuel the craft?