The Gravitonic Accelerator is a device built into the hull of the Br4 Class Gunship, the Br5 Kaye Class Torpedo Boat and the Meteor Class Blockade Runner. This device allows a ship to travel twice the distance of normal ship at the same warp factor.
For example, most ships travel 81 light-years at warp 9, but a ship with a Gravitonic Accelerator will travel 162 light-years at warp 9. This is done with the same amount of fuel that a normal ship with the same total mass and engines would require to move 81 light-years.
See also: Out of Fuel.