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.
Maybe this is obvious, but two rockets can get twice as much payload to orbit using twice as much fuel. You can that up linearly all day long. (It holds for "twice as much", but the same argument doesn't work for "twice as fast" or other metrics.)