Gravity Engine

An engine that works by creating a microsingularities in front of the propelled object, thus 'pulling' it forward which is equal to generating thrust. It works much better in theory that in practice, with the instability of the created singularity being a main problem. Noone is insane enough to board ship equipped with an engine that has around 5% chance of ripping apart - or even downright annihilating - the ship it is supposed to accelerate.
On the other hand, the insane acceleration it can cause makes them perfect method of propulsion for rockets. They have no crew and exist only for a while, not to mention that losing 5% of rockets to singularities is more than acceptable considering how much faster they can go and how much it increases the chances of piercing through antirocket defense. It is still an extremely costly and difficult technology, so it is used only in most technologically advanced Outer Colonies.