A
supersonic transport (SST) is a civilian
airliner designed to travel at
supersonic speeds. Supersonic flight is much more expensive than subsonic flight, due to the heavier aircraft structures (the empty weight per seat of a Concorde is more than three times that for a Boeing 747), and the need for
afterburning jet engines. For this reason, only two SSTs have been built, the Anglo-French
Concorde, and the
Soviet Union's
Tupolev Tu-144. The
United States designed the
Boeing 2707 SST, which would have been larger and faster than the European and Soviet SSTs, but this was cancelled due to its cost, and due to opposition from
environmentalists.