Since only one Space Shuttle orbiter was ever in space at a time, it seems logical that only two would be needed: one for the mission and the other as a backup for an emergency rescue. Perhaps a third could prove useful if one is in for repairs while the other two are mission-ready. Why, then were four orbiters—Columbia, Challenger, Discovery, and Atlantis—initially built?
As projected development costs for the shuttle climbed, the only way for it to appear to be more cost-effective than expendable launcher options was if it flew a very high mission tempo, about one launch per week. With such rapid turnaround times, four orbiters probably wouldn't have been enough, and more would have been ordered.
In the end, launch tempo was more like 7 launches a year, which was appropriate for a four-ship fleet. Maintenance of each orbiter between launches was extensive, taking at least two to three months, so it wouldn't have been impossible to need one on-mission, one being stacked for the next mission, one undergoing normal between-flight refurbishment, and one undergoing more extensive repair if anything went wrong.
$\begingroup$ Which means that, ironically, although Endeavour was built as a replacement for Challenger, it - or its equivalent - would have been built and flown much earlier had OV-099 not suffered a launch failure! $\endgroup$– VikkiAug 12, 2019 at 2:18
1$\begingroup$ @Sean Unlikely; it was clear from the start that the operational tempo would never reach the level of the original proposal. Construction of OV-105 was begun in 1982 but halted in 1983, long before the loss of Challenger. $\endgroup$ Jul 16, 2020 at 18:00
The Space Transportation System was intended to provide the United States with a low-cost, reliable means of transportation into low Earth orbit. The expectation was that all United States space launches would take place on the STS fleet. Four Orbiters were required to meet the expected demand and flight rate.
The official US government policy in 1982 was that STS would be the primary launch system for both military and civil space missions. It stated that "Expendable launch vehicle operations shall be continued by the United States Government until the capabilities of the STS are sufficient to meet its needs and obligations." (italics mine, showing that expendable launch vehicles were to be phased out)
None of this played out as planned.
8$\begingroup$ I think the last line is probably the most important. $\endgroup$– T.J.L.Jan 3, 2018 at 16:03
$\begingroup$ A) your link seems to have rotted, it directs me to a page about Theodore Roosevelt. B) what strikes me as wonky about that concept is — were they planning on every satellite launch being crewed? Or were they planning on the majority of shuttle launches eventually being uncrewed? $\endgroup$– hobbsMay 10, 2020 at 2:35
$\begingroup$ @hobbs thanks, sorry it took so long but I fixed the link. All shuttle launches were to be crewed unless they went to a Shuttle-C type vehicle. $\endgroup$ Oct 13, 2020 at 16:34