Which Boeing aircraft was converted into NASA’s first Shuttle Carrier Aircraft?

The Boeing 747-100 was converted into NASA’s first Shuttle Carrier Aircraft.

NASA acquired a Boeing 747-123 from American Airlines and modified it to carry Space Shuttle orbiters on its back. The aircraft received structural reinforcements, attachment hardware, additional vertical-tail surfaces, and other changes needed to support the orbiter during ferry flights.

The modified aircraft was known as NASA 905. It carried the prototype orbiter Enterprise during atmospheric approach-and-landing tests in 1977. Those flights allowed NASA to study how the unpowered orbiter handled after being released from the 747.

The Shuttle Carrier Aircraft later transported orbiters between landing sites, processing facilities, and launch locations. NASA eventually operated two modified 747s, with the second based on a Boeing 747-100SR. The first aircraft is distinct from the Boeing 747 used in the widely known NASA ferry operation generally described as the Shuttle Carrier Aircraft.