During the making of the Star Trek III movie, the production designers built several study models of the USS Excelsior to show to the director, Leonard Nimoy. He chose a design that we are now familiar with. This model based on one of the study models that was not chosen. There is only one picture of this particular concept (shown with the model) and I have made several modifications. Firstly, I have completely redesigned the secondary hull into a more squared of version of the Excelsior type. Secondly, I have made the warp engines the same basic shape, but I have added grilles that are intended to look like an step in between those of the Enterprise refit and the Oberth class. Thirdly, I have thickened the pylons into sturdier versions and added impulse engines to the rear and the neck sections. I have also added several hull details seen on the final Excelsior model, such as the RCS thrusters and ventral saucer sensor dome. I have also added several phaser turrets strategically around the vessel and forward and aft torpedo tubes to provide an effective arc of fire. The registry comes from the 'Excelsior Technical Manual - Revived!' written by Praetor on the TrekBBS chat boards. As he writes:
"The initial design, SV-20A, completed in 2268, was quite unconventional in design. It featured a saucer-shaped primary hull in line with a flattened, stepped engineering section. A horizontal “wing” supported four twin warp nacelles, two mounted above and two mounted below each at the end of the wing. Warp power would have been provided by a horizontal intermix chamber. Impulse engines were mounted aft, between warp nacelle pylons. The warp nacelles looked radically unique but were nearly the same size as those that would be installed aboard the refit Enterprise. The ship also featured an unconventional navigation deflector “pod” mounted on the ventral side of the engineering hull."
http://calamitysi.deviantart.com/art/USS-Excelsior-Study-Model-Named-USS-Xavier-485764366?ga_submit_new=10%253A1412191987&ga_type=edit&ga_changes=1&ga_recent=1
"The initial design, SV-20A, completed in 2268, was quite unconventional in design. It featured a saucer-shaped primary hull in line with a flattened, stepped engineering section. A horizontal “wing” supported four twin warp nacelles, two mounted above and two mounted below each at the end of the wing. Warp power would have been provided by a horizontal intermix chamber. Impulse engines were mounted aft, between warp nacelle pylons. The warp nacelles looked radically unique but were nearly the same size as those that would be installed aboard the refit Enterprise. The ship also featured an unconventional navigation deflector “pod” mounted on the ventral side of the engineering hull."
http://calamitysi.deviantart.com/art/USS-Excelsior-Study-Model-Named-USS-Xavier-485764366?ga_submit_new=10%253A1412191987&ga_type=edit&ga_changes=1&ga_recent=1
