I never accepted that Khan Noonian Singh's Botany Bay actually launched in the 1990's; only that the beginnings of DY-class technology were in that era. McCoy rolled his eyes when he mentioned "the Eugenics Wars", which could've started in the '90's by killer Al Quaeda Doctor Ayman al-Zawahiri, for all we know. Word is, he's hiding out in Pakistan... Anyway, the notion of "Eugenics Wars" suggests a string of conflicts, which could've started with leftover Cold War conflicts including Al Quaeda. Maybe the final conflict was in the 2050's. Khan's ship could've left anytime between now and then.
The thing is, we have to accept divergence at some point. We sure as hell won't have artificial gravity, let alone warp drive, in 2063. We will be lucky to have an interstellar mission by then. Warp might follow a hundred years from now, or perhaps a thousand, if ever. Why not accept divergence in the mid-20th century? Warp and gravity manipulation as fictionally portrayed in Trek are technologies that require a major breakthrough in physical theory - not steady progress in technology. Both could emerge today, or could have emerged back in the days of Dirac or Maxwell. And an Earth capable of antigravity and advanced interplanetary flight in 1980 would not look different enough for us to see anything amiss in ST4... After all, Apollo flights didn't happen on the streets, either. Timo Saloniemi
I agree that we have to acknowledge that Star Trek's timeline is not our own... the technology level as originally portrayed was much more optimistic than our own technology ended up being and like Timo said it will probably be a long time before our civilization is anywhere near achieving what Trek has us achieving in just a few decades. With that said, as far as the Phoenix goes, I'd say that it has inertial dampers and artificial gravity, which are probably outgrowths of a shared breakthrough. I'd also say that the warp nacelles (and possibly the warp core itself) were probably built in a separate research facility, say Los Alamos, that were later relocated, with Cochrane, to Bozeman for protection during WWIII. Cochrane and Sloane then had the more difficult to build components already built for them, and were able to use readily accessible A/G and IDF components stripped from other sources (along with their scrounged titanium) to convert the missile payload into the Phoenix.