The Engineering section of the refit Enterprise was supposed to be lodged on Decks 14 and 15, with controls and the horizontal intermix chamber located on the latter. To the port side of Main Engineering is a dilithium reactor room, where Spock died in Star Trek II: The Wrath of Khan.
Production designer Harold Michelson was largely responsible for reworking the engineering set on Star Trek: The Motion Picture to give it a much grander scale. “We wanted to give the ship a new kind of engine, something never seen before,” he told Starlog magazine in January 1980.
It had to suggest limitless power. We built it three stories tall; and it was as if it extended all the way down one of the nacelles.
“The engine was a Plexiglas thing,” Michelson explained to Star Trek: The Magazine in 2001, “and I assumed that it would go all the way back through the ship and up through the two diagonal supports to the nacelles.”
I did it forced perspective so that I had a five foot man, a four foot man and a three foot kid standing on the set to create the illusion of depth. We were able to send this energy through that thing and up the tubes, and it worked out very well.