X fuel can get Y payload to Z speed. To get that payload to 2 * Z speed, you need 2 * X + (X * (X / Y)) fuel, because the fuel counts as part of your payload fo

X fuel can get Y payload to Z speed. To get that payload to 2 * Z speed, you need 2 * X + (X * (X / Y)) fuel, because the fuel counts as part of your payload for the first Z of speed. If X is 500t and Y is 5t, you now need 51,000t of fuel to achieve 2 * Z speed.