What aircraft model did NASA modify as the Shuttle Carrier Aircraft?

NASA modified Boeing 747 airliners to serve as the Shuttle Carrier Aircraft.

The Shuttle Carrier Aircraft carried orbiters between landing sites, testing locations, and Kennedy Space Center. NASA used two heavily modified 747s: N905NA, a 747-100 originally operated by American Airlines, and N911NA, a 747-100SR acquired from Japan Airlines after the Challenger disaster.

Engineers strengthened the fuselage, installed mounting struts on top, modified the avionics, and added vertical stabilizers to improve handling with an orbiter attached. The shuttle sat above the aircraft rather than inside it, creating substantial drag and reducing range. Ferry flights therefore required careful planning and frequent refueling.

The first SCA supported the 1977 Approach and Landing Tests with the prototype orbiter Enterprise. Enterprise was released from the 747 in flight so astronauts could evaluate the shuttle’s unpowered glide and landing characteristics. The Lockheed C-5 Galaxy was considered, but NASA selected the 747’s low-wing layout and the ability to own the aircraft outright.