Re: Excelsior Technical Manual - Revived!
I think initially it was only a throwaway source to make the Excelsior seem like a considerable leap up in technology, but several contemporary sources (Mr. Scott's Guide and the FASA RPG) had it as an experimental combination between the warp nacelles and the transporter, based on the dimensional interphase from "Tholian Web." IIRC, the FASA Starship Construction Manual also describes transwarp as being a threshold issue, but I'd have to look at mine to see exactly what it says.
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