See also: Main page
In order to move, a ship must expend delta vee. Each ship has a limited amount of DV that it can use. One point of DV will accelerate the ship by 1 movement point. However, each DV point does not represent the same amount of fuel used. A ship which is fully loaded with fuel will be much more massive than a ship which has very little fuel left. In the first case, X amount of fuel will only accelerate the ship by a small amount, whereas in the second case X amount of fuel will accelerate it by a larger amount.
Since the drive system is limited by the amount of fuel it can burn in a turn, it will be able to accelerate much faster in the second case.
A ship has a number boxes representing DV, which are crossed off as they are used. These boxes are divided into five equal parts - each representing 20% of the ship's DV. Each turn, the ship may spend up to a set number of DV points, the maximum being defined for each grouping.
For example, for the first 20%, the ship might have a thrust of 3, for the second 20% it might have a thrust of 4, for the third 5, for the fourth 7 and the fifth 10.