Why Did NASA Choose A Boeing 747 To Piggyback The Space Shuttle?

When NASA needed a taxi service for its roughly 100-ton space shuttles, it couldn't exactly call an Uber. The agency needed a plane powerful enough for the job. It dug around, and the two candidates were selected as the ridiculously massive Lockheed C-5 Galaxy and the venerable Boeing 747. NASA ultimately went with the latter, mainly because of its design. The 747's airframe placed the wings below the main fuselage. This made it a much better platform for the role. Meanwhile, the C-5 featured a high-wing design, which could have made mounting a shuttle on top difficult.

Power was another major factor, and the 747 delivered in that department. The plane was a four-engine jumbo jet that had just entered commercial service in 1969. It was the largest commercial jet when it made its maiden flight. To this day, it remains one of Boeing's most acclaimed creations. These planes were designed as intercontinental-range workhorses with swept wings and massive power. Each of their total of four Pratt & Whitney engines could pump out 48,600 pounds of thrust. This power helped these jets haul a maximum gross taxi weight of 713,000 pounds. The specific 747 unit used for the job wasn't lightweight either, weighing well over 300,000 pounds when empty.

NASA ultimately modified two 747s for the job. The first, dubbed NASA 905, was a Boeing 747-123 model picked up from American Airlines in 1974 for a cool $15,601,192. Then it acquired a second plane in 1989 – a 747-100SR-46 from Japan Air Lines, which became the NASA 911. Both jets underwent a serious transformation. Boeing added three massive struts to the fuselage to hold the shuttle, which required significant internal structural strengthening. Of course, strapping a shuttle atop worsened aerodynamics. To handle that, two extra vertical stabilizers were bolted to the tail fins for better directional stability. Even small aerodynamic tweaks mattered, since the orbiter's thermal protection tiles already made its surface uniquely delicate.

Engineers also added instrumentation to monitor the orbiter's electrical loads in flight. For the initial 1977 tests, NASA 905 even had a wild crew escape system with pyrotechnics. Actually getting the shuttle on top was a whole production. It required enormous structures called Mate-Demate Devices. These would hoist the orbiter high off the ground, leaving room for the 747 to be driven underneath to be attached. As for actual missions, the NASA 905 ran the show alone for over a decade. But the NASA 911 joined in late 1990, and the two jets shared the duty thereafter.

When the shuttle fleet retired, the SCAs had one last job. The NASA 905 flew three final ferry missions to deliver the orbiters to their museum homes. It famously delivered Discovery to the Smithsonian's National Air and Space Museum with a celebratory loop around Washington D.C. at about 1,500 feet. The NASA 911 was retired in February 2012 and is now displayed at the Joe Davies Heritage Airpark. The NASA 905 retired later that year and became part of a massive exhibit at Space Center Houston.