A supersonic aircraft is an aircraft able to fly faster than the speed of sound (Mach number 1). Supersonic aircraft were developed in the second half of the twentieth century and have been used almost entirely for research and military purposes. Only two, Concorde and the Tupolev Tu-144, were ever designed for civil use as airliners. Fighter jets are the most common example of supersonic aircraft, although they do not always travel at supersonic speed.