Shuttle Carrier Aircraft (SCA) are two Boeing 747-type airliners heavily modified by NASA and used to transport the Space Shuttle orbiter. One of these is based on the 747-100 model, while the other is a short-range version called 747-100SR. SCAs are used to return Shuttles to the Kennedy Space Center whenever they are forced to land in a runway other than the NASA Shuttle Landing Facility. In the first test flights of the shuttle, the SCAs served to bring the prototype of the orbiter to altitude and then release it in order to test the landing. SCAs were also used to transport orbiters to their designated destinations after retirement. The orbiter is placed on the back of the aircraft fuselage through a special structure.