And more specfically, it was being a proposed as a replacement for the S-IVB, the third stage of the Saturn V. Early maps (like the one shown at https://www.hq.

And more specfically, it was being a proposed as a replacement for the S-IVB, the third stage of the Saturn V. Early maps (like the one shown at https://www.hq.nasa.gov/office/pao/History/SP-4204/ch11-8.html https://www.hq.nasa.gov/office/pao/History/SP-4204/ch11-8.ht... ) show the Nuclear Assembly Building, along with three pads, at Launch Complex 39, which launched the Saturn and Space Shuttle, and now launches the Falcon rocket.