I'm starting to seriously think about 'Not One Step Back' now, and one thing that is apparent is that the next book will be giving the FTL system a lot more of a serious workout. As a result, I decided to codify complete the 'rules' of the faster-than-light drive I'm using in what I think I have decided to call the 'Triplanetary' setting. First of all, above is a map of 'colonised space' as of 'Price of Admiralty', I'd meant to post it before, but this seems like an appropriate place for it. I'd like to thank the Internet Stellar Database for this one, and for the work I've been able to do for future books in the same series – it is a lot of fun to play around with that for a few hours, generating starmaps! (The map is made with 'yEditor', which I can also recommend.)
The map above focuses on worlds with permanent human presence, with the addition of Lalande 21185 (the setting for most of 'Price of Admiralty'). The bolded lines are systems with worlds inhabitable by humans; the dotted lines indicate a world that is marginally suitable for human habitation. For a little background – primarily, the interest is asteroid mining. The nature of FTL made it about as profitable to utilise asteroid belts around other stars as to exploit those around Sol, and during the era of the Interplanetary Wars, the Asteroid Belt was enough of a battleground to cause a lot of the big mining companies to move their operations to the stars. There has been little in the way of a systematic exploration of interstellar space at this point; all stars within one 'jump' of Sol have been visited, as well as most stars within two, but only a handful of further stars have been explored.
Proxima Centauri is the primary site for a lot of this mining, with Barnard's Star and Sirius also exploited – the former more for political reasons, the latter because the young star has some interesting exotic elements present in its asteroid belt. There are nascent colonies at Tau Ceti and Epsilon Eridani; at Epsilon Eridani II an event analogous to the oxygen revolution took place, and humanity got there early enough afterward that native life is still struggling to adapt – whilst Terran lifeforms are better suited to the environmental conditions, and are thriving. Tau Ceti IV is a water world, with a complicated native ecosystem; while humans can breathe the air and, with a little sterilizing, drink the water, all food must be grown. Long-term, Epsilon Eridani is regarded as the better prospect for colonisation. Both of these worlds were settled by the United Nations.
As for the drive itself; the Evans Drive was the result of a decades-long research program designed to find a way out of Sol System. The breakthrough was made at Cardiff University in 2087, a way to access hendecaspace, eleventh-dimensional space. The catch was that transition was only possible at points of gravitational stability – which effectively meant Trojan points. The bigger the better, as it turned out; Triton turned out to be around the low point for the planetary mass. (This gave Titan, Callisto and Triton accessible 'egress points'; the other Galilean moons technically have points, but Jupiter's radiation makes them out of bounds for humans.) Initially, travel took around three days for each light-year travelled, but rapidly refinements in calculating hendecaspace entry got that down to twenty-seven hours, which thus far has proven to be the fastest possible speed. The catch is that no more than a little over ten days can be spent in hendecaspace without the ship involved never coming out again – there are hundreds of theories on that, most focusing around a 'loss of dimensional stability'. This has equated to a limit of 9.1 light-years on trips made using the Evans Drive. Naturally, this is also extremely power intensive. As an example, the Thermopylae-class Battlecruiser carries sufficient Helium-3 to fuel three transitions, and takes roughly seven days to charge sufficient power in its SMES systems. Early ships were extremely limited in terms of tonnage, restricted essentially to small survey-ships, with a high risk factor; mastering the dimensional transition proved an extremely difficult task.
Four out of five of the alien races contacted are reported to use similar drive technology to that operated by humanity, though with different tricks – but all are extremely secretive regarding this technology, and have resisted any efforts to investigate their drives. Research is based largely on sensor data, as well as some reports from the few who have boarded their vessels. (The fifth alien race, encountered only once at Barnard's Star, used only STL-drive, operating large ships converted from asteroids. The Maktari were present in-system for nineteen months, only accepting limited contact, before returning to the stars having conducted mining operations in the outer parts of the system.)
While naturally work continues on refining the drive, progress was badly interrupted by the Interplanetary War; both sides targeted FTL researchers and installations, attempting to prevent the other side getting an advantage. Despite lagging significantly behind the United Nations originally, the Lunar Republic is now thought to be at the cutting-edge of technical research in this field, having offered safe haven to several key scientists during the war years.