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Lightspeed drive that warps the geometry of spacetime in front of a vessel — the engine first hinted at in Yun Tianming's fairy tales, built in secret by the Halo Group under Wade and AA, and humanity's only escape from the dual-vector foil.
Curvature propulsion is the trilogy's only true escape. The Council bans it because the slowed-light wake is itself a Dark Forest signal, exposing the trajectory of any ship that uses it. Halo Group develops it anyway — quietly, aggressively, partly murderously — under Wade's direction. AA decoded its principle from the soap-paper boat in Yun Tianming's first fairy tale. Halo's curvature drive is what carries Cheng Xin and AA out of the Solar System minutes before the dual-vector foil arrives.
There is no drive bell or visible exhaust — a curvature ship simply moves. From outside, the most visible evidence is the slowed-light wake left in its path: a long luminous trail that stays behind for centuries, like a chalk line drawn across the dark. Halo itself is a small cylindrical vessel, almost ornamental, around the actual curvature core.