NASA's two Boeing 747 Shuttle Carrier Aircraft (SCA) are seen here nose to nose at Dryden Flight Research Center, Edwards, California. The front mounting attachment for the Shuttle can just be seen on top of each. The SCAs are used to ferry Space Shuttle orbiters from landing sites back to the launch complex at the Kennedy Space Center, and also to and from other locations too distant for the orbiters to be delivered by ground transportation. The orbiters are placed atop the SCAs by Mate-Demate Devices, large gantry-like structures which hoist the orbiters off the ground for post-flight servicing, and then mate them with the SCAs for ferry flights. Features which distinguish the two SCAs from standard 747 jetliners are; three struts, with associated interior structural strengthening, protruding from the top of the fuselage (two aft, one forward) on which the orbiter is attached, and two additional vertical stabilizers, one on each end of the standard horizontal stabilizer, to enhance directional stability. The two SCAs are under the operational control of NASA's Johnson Space Center, Houston, Texas.