Deuterium gets depleted really fast on stars and you won't find any considerable amount to scoop in order to fuel an engine.
Tritium doesn't occur from fusion,but fission, thus you wouldn't be able to scoop any from stars to begin with.
It can't be much else than protium. The whole scooping feature restricts the possibilities to that.
Oh well, forget it - they'll have to find some other way to balance the game a bit then.