The Boeing 747 is a great aircraft and has been popular with passengers and airlines for decades. As well as passenger and cargo airline use, it has also seen many other specialized, private, and VIP uses. One of the more unusual was as a ‘piggyback’ aircraft for the NASA Space Shuttle. Two 747s underwent many modifications to make this possible.
The NASA Shuttle Carrier Aircraft
NASA introduced the re-usable Space Shuttle in 1977. After landing, the shuttle vehicle needed to be returned to its base at the Kennedy Space Center in Florida. It was unable to do this under its own power, of course, so a transport aircraft was needed. After considering the Lockheed C-5 Galaxy for the role, NASA selected the 747-100 as the aircraft to modify for this role (the main reason being its low rather than high wing design.
The first Shuttle Carrier Aircraft (SCA) was a second-hand 747-100 from American Airlines, with registration N905NA. This had started service with American Airlines in 1970 and was taken on by NASA in 1974. After extensive modification, it started shuttle service in 1977. Amazingly, it kept part of its distinctive American Airlines livery until 1983, when it was repainted in NASA livery.
A second aircraft was procured in 1988 and entered service in 1990. This was a 747SR previously with Japan Airlines.
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Modifying the airframe
There were plenty of changes made to the aircraft to handle the shuttle. Much of the interior was stripped out (including all seats except a small part of the first class at the front). The fuselage was strengthened to take the weight placed on it, and mounting struts were added on top of the fuselage to hold the shuttle.
Major changes were made to the tail, with vertical stabilizers added to the main horizontal stabilizers. This was to counter the change in the center of gravity when the shuttle was mounted. When flying without the shuttle, ballast weight had to be added to maintain the center of gravity.
Attaching the shuttle
Once modifications were made to the fuselage, the shuttle could be lifted onto the top of the fuselage and latched into place. This was no simple operation – with some reports claiming it took up to a week for the crew to prepare and attach the shuttle prior to a flight mission.
There were three struts used for attachment. The single forward strut was atop the 747 upper deck. And the two aft struts were midway along the fuselage. These attached to the shuttle’s external fuel tank.
Shuttle flight tests
The SCA and its attachment struts also served another purpose besides ferry flights of the shuttle. The Space Shuttle prototype Enterprise went through an extensive series of tests in 1977. These tests, and modifications, only applied to the first SCA – not the modified 747SR added later.
These tests included flights attached to the SCA to establish safe operation. But it also involved free test flights of the shuttle. For this, the shuttle was carried to altitude by the SCA, then released to allow a free flight glide and return to land.
The SCA and its struts were specially modified for these tests. A longer forward strut was used, which increased the shuttle’s angle of attack relative to the SCA. And explosive bolts were added, which would sever the connection and leave the shuttle free.
To safely achieve release, the paired aircraft started a high-speed shallow dive. The increased lift from the shuttle led to the situation where it was supporting the SCA beneath it (with the forces closely monitored by sensors). At this point, the bolts were exploded, and the shuttle would effectively drop the 747 beneath it.
An obvious question is how the aircraft could fly with such addition on top of the fuselage. This would totally change the aerodynamic and performance capabilities of the original design.
Drag was a major issue, and there is little that can be done to change this. With the increased drag and the shuttle’s weight, the aircraft was much more inefficient and burnt more fuel. Its range was reduced to around 1,025 NM (1,900 kilometers). Aerial refueling capability (as was added for Air Force One VC25A aircraft) was considered but rejected. Instead, the SCA had to make frequent stops on longer flights.
The SCA aircraft were great examples of the abilities of both the 747 aircraft and NASA engineers. They remained in service until 2012, and both aircraft remain preserved on display. Feel free to discuss their modifications or mission history further in the comments.