Reactionless drive spaceships uses exotic matter and negative energy to manipulate space so that the very fabric of space-time becomes an integral part of the stardrive. These warp bubbles are either contained within the drive nodes, or, in the most advanced and expensive forms, envelope the ship as a whole. The whole economy of voidbubbles is tied up with exotic matter godtech and trade with goddwellers and other eccentric factions
The Diametric or Displacement drive ship is the cheapest and most basic Reactionless Drive. They’re called Displacement drive as their engines are displaced out of normal space. In the Displacement drive configuration, warp bubble based drive nodes are enclosed within one or more magnetic containment vessels aboard the ship. These are magnetically linked to the vessel within the containment volumes and react against it as they move, effectively pushing or pulling the ship across space with no ejection of reaction mass.
The Halo drive is a more advanced system which uses an external halo of warp bubbles. These are internally much larger than the ones used in the Diametric drive, meaning they contain more mass. The combination of the large mass and small size combines to give the bubbles a small but very steep gravity gradient. The intense gravity gradient couples them to the hull, allowing much higher acceleration.
In the Alcubierre drive or Warp Ship (the entire ship is entirely enclosed within a Void Bubble. Here the entire ship is contained in a single giant warp bubble. This allows still greater acceleration and maximum velocity, although it is also more energy expensive
Finally, the most advanced form of reactionless drive is the Voidship, in which the hull itself is replaced by a spacetime bubble.
Material on Reactionless drive adapted from the Orion’s Arm worldbuilding project.