The ZFR Horizon Accelerator Engine, also known informally as the ZFR Drive, is a system of interstellar starship propulsion that was the great invention of the Tau Earth Caste and allowed the Tau Empire to expand rapidly during the Second Sphere Expansion. An ingenious design, this powerful new mechanism allowed Tau starships to attain near-light speed. It is the primary means of Tau interstellar travel, and is used by all of the vessels of the Tau Navy, the Kor'vattra.
The rise of the Tau can be seen to develop through three distinct phases, periods of intense growth known to the Tau as "Spheres of Expansion." Each of these waves of colonisation is marked by a long building up of resources, after which continual waves of exploratory missions are launched, followed, where needed, by military campaigns to solidify territorial gains. Once a Tau colony transforms itself into a stable settlement, it then serves as a jumping-off point for the next expansion.
By the end of the millennia-long First Sphere Expansion, as it later came to be called, the Tau Empire had unfurled across the heavens and consisted of eight fully-settled star systems known as Septs. Named after its prime or "Sept World," a Sept can include any number of additionally colonised planets or moons, as well as other holdings such as listening posts, sensor fields, shield satellites, orbital cities, and mining operations. Everything is connected, both by a series of space stations and a massive net of communications and sensor relays strung between major locations. Although it might take many generations to establish itself, each Sept is unique, with its own cultural nuances and varying proportions of the different Tau castes and alien populations.
Several factors combined to end the Tau's first great period of rapid growth. Firstly, despite the terrific population explosion their race had been experiencing, their numbers were being stretched too thin, with need for more of every caste being felt. Fire Caste warriors were in particularly high demand; the wars fought to conquer new planets had proven costly, and ongoing conflicts still raged in outlaying areas. The second reason the First Sphere Expansion came to a halt was simply due to the distances between systems. After colonising the many dense star clusters near their homeworld of T'au in the Eastern Fringe of the galaxy, the expanses between worlds became greater. At that time, it was already impossible to travel the span of the Empire in a single Tau lifetime using the slow sublight drives available to Tau voidcraft, and crossing the black gulfs that surrounded their star systems would take many, many generations. The Tau clearly needed to innovate new methods of interstellar space travel.
The first problem was solved by the integration of the Kroot race into the Tau Empire following the War in the Place of Union, where the Tau and Kroot first allied to defeat a massive assault by the Orks upon the Kroot of the world of Krath and then went on to free other Kroot enclaves under Greenskin assault. In a short period of time, the Fire Caste armies were swollen by billions of additional Kroot warriors to aid the fight for the Greater Good, albeit in a more mercenary fashion than was considered ideal by the ruling Tau Ethereals.
It was only the Earth Caste which had failed to reach their prescribed goal of developing a new method for more rapid interstellar travel. With engineering centres in every Sept working diligently, the Earth Caste provided innumerable innovations for the Tau military and society, but the invention demanded by the Ethereals -- faster propulsion-technology to drive starships -- eluded them. At last, the quantum leap came from Fal'shia Sept, where its Earth Caste scientists and engineers finalised development of the ZFR Horizon Accelerator Engine. An ingenious design, this powerful new mechanism allowed Tau starships to attain near-light speed. It was this device that would usher in the next phase of progress, as now Tau vessels could reach new star systems in only a few objective Terran years, while their crews experienced very little subjective passage of time due to the relativistic effects of moving at such high speeds.
- Codex: Tau Empire (6th Edition), pp. 11-13