Ships burn fuel after movement. If the ship has less fuel, nothing bad will happen, but the ship will end up without fuel afterwards.

Fuel_burned_per_turn =
   Ceil(Hull_mass * FuelUsagePerTurnFor100KT / 100)

Ships burn fuel after each fight. If the ship has less fuel, nothing bad will happen, but the ship will end up without fuel afterwards (and must thus stop fighting).

Fuel_burned_per_fight =
   Ceil(Hull_mass * FuelUsagePerFightFor100KT / 100)
This is a testing version.
It may be incomplete, and have more bugs (or features) than the public live version at planetscentral.com.