Originally,
Enterprise had been intended to be refitted for orbital flight, which would have made it the second space shuttle to fly after
Columbia.
[2] However, during the construction of
Columbia, details of the final design changed, particularly with regard to the weight of the fuselage and wings. Refitting
Enterprise for spaceflight would have involved dismantling the orbiter and returning the sections to subcontractors across the country. As this was an expensive proposition, it was determined to be less costly to build
Challenger around a body frame (STA-099) that had been created as a test article.
[2] Similarly, Enterprise was considered for refit to replace Challenger after the latter was destroyed, but
Endeavour was built from structural spares instead.
[2][3]