Grammatron wrote:What's a Transwarp drive and why do they no longer use it?
Ok, taking these on in the order received... that's for YOU De_Bunk!
A sticky question never really resolved on screen. Various printed materials have provided three speculations.
The FASA RPG and technical writings of Shane Johnson suppose that the transporters are beaming the warp bubble in front of the ship. ETA: Ludicrous, in my opinion.
Another body of work known as "Technical Fandom" -- the most consistent and prolific set at one time, known for containing blueprints and ship registries... goes by the definition in a book called the Officer Requirements Manual, "...an experimental system based upon harnessing the reduction of space/time fabric resistance experienced by a vessel in a wormhole situation". Basically they're exploiting the "wormhole effect" seen in Star Trek: The Motion Picture.
Another theory that got some play in the Pocket Books novels is shaky in my memory, but I seem to recall it being a new kind of warp field discovered by studying some of the interphase effects near Tholian Space.
As for why it was discontinued, the ORM has a few ideas, mostly due to the impracticality of finding a tunnel of open space long enough to sustain ultra-high warp, the inability to maneuver while in high warp, and construction issues making the nacelles capable of both Transwarp and regular warp drive. Another book called the "Ingram Class Blueprints" suggests that it never performed as well as hoped for without sacrificing safety factors, and a Pocket Books novel, "Battlestations!" mentions that its functioning requires "trilithium" crystals, which are rare and discussed the difficulties of producing/using that material.
ETA: By some reckoning it was a great success, and was added to NCC-1701A.. which might explain how they got to the center of the galaxy so quickly in Star Trek V... but I think that's mainly more Shane Johnson/FASA/SFC crap.
Boy that was a mouthful. HTH.
Gotta go though, other's questions to be answered later.