The Inertialess Drive is a non-Newtonian, reactionless form of voidcraft propulsion used by the Necrons that allows a vessel equipped with it to move through space without any form of inertia. This allows the voidcraft to actually travel faster than the speed of light in a vacuum in realspace without resorting to travel through an extradimensional medium like the Immaterium.
How this is accomplished without violating the known laws of physics is a complete mystery to Imperial tech-adepts.
The Inertialess propulsion system is utilised exclusively by the Necrons, who are capable of interstellar travel without the need to enter the Warp when using this technology. Like much about Necron technology, its inner workings are completely arcane to Imperial science.
Once activated, the Inertialess Drive unleashes a flare of relativistic energy.
In addition to faster-than-light interstellar travel, this same technology can be used by the Necrons to initiate more short-ranged jumps by their vehicles on a planetary battlefield.
The Necrons have several other technologies available for them to facilitate FTL travel without resorting to the Warp, including long-range teleportation or "phasing" technology that does not use the Immaterium as the medium of transport and the Dolmen Gates used to enter the Webway.
Sources[]
- Battlefleet Gothic: Armada (Blue Book), pg. 74
- Warhammer Twitch Channel - Index Xenos: Necrons - 36:51
- Shield of Baal: Exterminatus (7th Edition), "Death of Worlds"
- Battlefleet Gothic Armada II (PC Game)