To calculate the fuel consumption in tons, use this formula: fuel consumption(ton/s) = Thrust/10 SI That's the thrust of the engine divided by ten times the specific impulse. This will give you a number in tons per second of fuel used. There is 200 fuel units per ton I believe and assuming a "unit" is a liter, you can use: fuel consumption(L/s) = 20L*Thrust/SI to get the useage in L/s. It works out with your example of the NERVA engine. The thrust number is the number you see in game, so 200 or 300 or 60 or whatever.