F=ma Force equals mass times acceleration (I'm a physics student). But, in space, If an astronaut is not attached to the space shuttle and he drifts too far away, he can take that backpack looking thingy off his back and push off of it. Whatever amount of force the astronaut exerts on the backpack, the same amount of force pushes him in the opposite direction. So, if a giant space cruiser fires its thrusters in one direction, the ship moves in the opposite direction with whatever amount of force the thrusters exert. It would just take a big push on the other side of the ship to stop the thing from turning, because of the big mass. But in space, that amount of force isn't very hard to come by since no other forces are interrupting the system (gravity, friction, etc.). Exactly, so mass is still a determining factor. If we transpose the equation it becomes: A = F/M And in the case of Def's ship, modifying an existing ship by reducing its mass but keeping the same amount of force it produces (ie. the engine) the acceleration would be improved. Obviously this is all science fiction, and as such we can't slot anything into this equations - but on a completely theoretical level it still works. After all, a ship measuring 300 hundred odd metres, equipped with massive engines, thick armour for combat, weapons, personal etc. it's going to have a rather large mass..ok, huge mass - requiring massive thrust blah blah blah. But, like Elvis said - it's just science fiction Doesn't matter if it's perfect, it's fun.