Being the first space-rated orbiter produced, Space Shuttle Columbia was the heaviest and therefore rarely selected for high inclination trajectories like the ISS near the end of the program.
Which components were still heavier for Columbia compared to the other shuttles by 2003?
The tl;dr answer is "structure, mainly in the wings".
I have struggled to find references giving details, though. I particularly wanted to find comparison diagrams, but I did not.
There are several references that give little parts of the story.
The differing weights of the orbiters reflected improvements in design that had accrued in the course of development. Though people spoke of “production” of shuttle orbiters, this was not at all like the massive aircraft orders of World War II. Every orbiter amounted to an individual procurement, and there was ample opportunity to change the blueprints from one to the next. Weight reduction was essential, for a lighter orbiter could carry correspondingly heavier payloads. It was achieved through use of titanium in the primary structure, as that metal was denser than aluminum but considerably stronger. Boron-epoxy composite, particularly light light in weight, also began to appear. This indicated a clear trend: early orbiters being heavy, even after refurbishment, and later ones being considerably less so:
- Enterprise (as flight vehicle) 160,000 pounds
- Columbia 158,000 pounds
- STA-099 (as Challenger) 155,000 pounds
- Discovery 151,000 pounds
- Atlantis 151,000 pounds
Heppenheimer, Development of the Space Shuttle, 1972-1982, p. 348
Additionally, the wings were modified to incorporate lessons learned from the static testing just completed. Challenger would end up some 2,889 pounds lighter than the earlier Columbia.
Jenkins, Space Shuttle (1992 edition), page 166
After the initial design of Challenger and Columbia, NASA initiated a weight-savings program for the follow-on vehicles—Discovery, Atlantis, and Endeavour. The space agency achieved weight savings through optimization of aluminum structures and replaced the aluminum spar webs in the wing with a graphite/epoxy laminate.
Multiple authors, Wings in Orbit, Structural Design chapter, p. 275
I would love it if someone can come up with more details.
