Originally Posted by Uplinker
"Does that mean that the Shuttle and its fuel tank could have used the Saturn V first stage instead of the boosters?
I presume the boosters were much easier and vastly cheaper to manufacture?"
The Saturn V S-IC first stage was considered as a shuttle booster:
https://joema.smugmug.com/Aerospace/...er/i-TmrdjpS/A
However the initial goal was a fully-reusable two-stage vehicle. Unfortunately the optimum staging velocity for lowest total mass is around Mach 12, which means a reusable first stage would essentially be a hypersonic transport in the 3-4 million pound gross weight class. This would have entailed extremely high development cost and risk. Former shuttle program manager Robert F. Thompson said even had they been given the money, he didn't think it would have been possible.
This iteratively drove the design process to an expendable booster (the above Saturn S-IC was one of many concepts), which then led to an orbiter with external propellant, which then led to an expendable tank with semi-reusable solid rocket boosters.
See "The Space Shuttle Decision":
The Space Shuttle Decision: Chapter 6
It was understood early on the shuttle would likely not be economical to operate. This was covered in the 1972 GAO report to Congress:
http://archive.gao.gov/f0302/096542.pdf
There was also a RAND study in that period which had similar statements about projected per-flight cost, also reviewed in the Jenkins book, "Space Shuttle: History of the National Space Transportation System", p. 173. It was known in the early 1970s even at 60 (!) flights per year, the shuttle payload cost per pound would only be about 38% cheaper than expendable boosters. If that flight rate could not be achieved it would be (and was) more expensive.
The vehicle was never designed to reach that flight rate, e.g, the maximum production rate of external tanks using three shifts at the Michoud facility was only 24 tanks per year.
The shuttle was an amazing vehicle but in hindsight there was no possible design in the early 1970s that would have permitted achieving the conflicting performance, safety, cost and flight rate goals.