The main engine room of the TOS Enterprise NCC-1701 was never precisely located in the filmed content of the series; although it is generally considered to be situated in the secondary hull of the vessel, which was sometimes referred to as the engineering hull, as opposed to the saucer section. In the first season episode "The Enemy Within", when Kirk and Spock speculate on where Kirk's double might go to elude a mass search, it is stated on "the lower levels ... the engineering deck". Additionally, in the third season episode "The Day of the Dove", when the good humor between the Klingons and Starfleet officers drives the alien energy entity to hurriedly flee the starship, it is seen leaving the main engineering section and emerging into space from the center of the secondary hull of the ship. Some speculate that Main Engineering was located at the lower rear portion of the primary hull or saucer section, to be physically close to the impulse engines. In the first season episode "Court Martial", Ben Finney is located by Spock via sensor scan to be on "B Deck", in or near engineering, which would support a saucer section location assuming the Bridge is on "A Deck". Most of the TOS filmed references, however, imply a secondary hull location.
Apart from the Bridge set, the Main Engineering set was perhaps the largest and most elaborate of the permanent sets, followed closely by Sickbay. It underwent extensive refurbishment prior to filming the 2nd season of TOS; with the addition of a second tier or mezzanine level of control stations as well as detailed offices and secondary control rooms adjacent to the main area.
When initially constructing the Main Engineering, the production team made used of "forced perspective" to generate the appearance of a much larger amount of floor space. As discussed in "Inside Star Trek: The Real Story" by Herb Solow and Bob Justman ...
Justman and Jefferies squeezed enough money from the budget to pay for one major new permanent set, the engine room of the Starship Enterprise. Knowing that a large set was required and hurting for lack of stage space, Justman turned to forced perspective once more. Jefferies designed the set with a heavy screen separating the foreground work area from the ship's massive engines in the background. Behind the screen, the engine conduits were large closest to camera and tapered radically to a much smaller size in the rear. The film crew used pulsating reddish lighting in the background, which "emanated" from the energy generated by what appeared to be an enormous power plant -- just so long as it was photographed from the proper angle.
As was the case with the other permanent TOS sets, main fixtures in engineering could be easily relocated to allow for the different camera and lighting positions necessary for filming. On occasion, the main engineering deck was refitted for use as the shuttlecraft hangar bay. Curved doorways could also easily be replaced with triangular doorways and the background lighting color could be modified to transform the set into an alien control room.
Many of the complex shapes or devices attached to decorate the bulkheads and panels were the result of innovative, cost savings measures on the part of the design staff. Again, from "Inside Star Trek" ...
Whenever Jefferies and Justman -- of "Jefferies and Justman Interiors", as Solow referred to them -- walked together through the studio streets, they vied with each other to find serendipitous items. A successful walk meant finding Styrofoam packing pieces in refuse bins. Someone's garbage was someone else's treasure. The Styrofoam pieces were painted in primary colors and glued to the ship's interiors, making them look oddly modern. And the actors were cautioned, "Don't touch that piece on the aft bulkhead; it'll crumble in your hand!"
And now, some episode screenshots that illustrate the design evolution of the Main Engineering set.
From the first season ...
The brown panel display is the Computer Systems Monitoring Board, according to the Starfleet Technical Manual (Franz Joseph, Ballantine Books, 1975) ...
The large grey tank-like structures on the floor in the following picture make up the Impulse Syncrotron Unit ...
A view into the Impulse Engine Power Unit at far right, with radiating red background lighting ...
