The biggest thing that I learned from Orbiter Space Simulator was how to accurately represent space travel realistically in a RPG.
The two key terms you need to know it delta-vee and time.
This represent a change in velocity. For example to get into orbit you need to have enough fuel and thrust to go from zero to 7.8 km/sec. Sure if you doing it for real you have to have the right ascent trajectory so that when you hit the 7.8 km/sec mark everything is right to have a stable orbit. But for gaming all you care about is whether the character makes his piloting roll (or navigation depending on the system) and whether there is enough Delta-Vee to make that change in velocity.
All space maneuvers have a cost in delta-vee. So if you rate your fuel in delta-vee (instead of liters or kg) then you know how many of these maneuvers you can make. So if you on the moon of Alpha Leonis and want to goto Alpha Leonis. It may take a DV of 2.7 km/s to get into lunar orbit. Another 1.5 km/s to a trans-lunar trajectory to get to Alpha Leonis, and then a 3.5 km/s maneuver to achieve stable orbit around Alpha Leonis. So if you have fuel with a total of 7.7 km/s Delta-Vee in it then you have enough to make it the trip.
Many Sci-fi systems have antigrav or some other superscience means of propulsion. In this case the limiting factor is time not fuel. In that you can only change your delta-vee by so much in X time. Normally this isn't a factor but if you are trying to catch someone that trying to escape that in a different orbital plane then it becomes critical.* Or if you coming in hot and trying to make a stable orbit in 2 minutes you may finding that you wished that you bought that 6G maneuver drive back on Efate.
*Changing the plane of your orbiter (the degree it is tilted to the planet's equator) requires 10 times the amount of delta-vee then simply raising and lower your orbit's altitude. This is why when there are mission to other planets or the ISS there is a window for the launch time. Mission Control is waiting for the orbital plane of the target to cross the launch site. Generally there is a couple of minute leeway on both sides. But the further you are off the more fuel (or time) it will take.
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