It looks like these ships are designed only for quick jaunts out to the asteroid belt, scan a couple of rocks, and come home. The fuel load in each case is about enough that the ship can fly out and then turn around once the deployment clock runs out.
Usually you wouldn't do this since even meager NTE ships can have months or years of endurance easily, but in this case I think the OP is modeling after the
Buran-class orbiter which had a 15-20 day mission profile i.e. deployment time, so this is purely an RP design.