posted 08-04-2017 04:22 PM               

Calling missions that were simply delayed, like STS-4, as canceled because their original launch date target was missed by two years does not qualify to me as canceled.

However, scroll down to STS-17, which is said to have been originally scheduled as the first spaceflight of the Enterprise (and STS-20 and STS-22)! I was told by Fred Haise it would have needed so many modifications to make it spaceflight worthy that it would have been impractical. He used references I do not recall exactly, but he said something similar to there having been two or three "levels" to rate the structures of the orbiters and the Enterprise was something like "level 1" for lack of the correct terminology.

posted 08-04-2017 04:50 PM               

Per Dennis Jenkins' "Developing An Icon":

...several things convinced NASA to change its plan. As OV-101 was being built, engineers learned numerous lessons about the design and the materials used in its construction. All subsequent orbiters (STA-099 and subs) used wings and mid-fuselages significantly stronger than those installed on OV-101 and some aluminum castings in other areas of the fuselage were changed to titanium to save weight. Given the extent of the changes, the wings would need to be returned to Grumman in New York, the mid-fuselage to Convair in San Diego, and the aft-fuselage to Downey – all expenses the program did not have funds to cover.

The extensive modifications required to retrofit OV-101 led NASA to modify the structural test article (STA-099) into the second flight vehicle since most of the structural and materials changes had been incorporated during production. In addition, since STA-099 was never completely assembled, there would not be as much rework required. As a bonus, there was no simulated TPS on STA-099, saving a great deal of effort to strip the foam off the airframe.