Famously, NASA heavily modified two Boeing 747 Jumbo jets to carry its Space Shuttles (they are called the Space Shuttle Carrier Aircraft). Both of these aircraft are now retired and on display. The Space Shuttle Carriers are easily recognizable from the extra stabilizers placed on the tail of the 747s as well as the struts on top (where the orbiters were attached). Some other converted 747 aircraft (like Air Force One) are not particularly obvious from the outside. Perhaps the most striking converted 747s are the Boeing Dreamlifters (747-400 Large Cargo Freighter) built from the Boeing 747-400 airliner.