The Boeing 2707. "Two seven oh seven". It sounds as cool as it looks. The hydrocarbon fueled 2707 supersonic transport would have cruised at Mach 2.7 at an altitude of 20 kilometers, well above the mid-latitude tropopause. Boeing had 115 airframe orders on the books when congress cancelled development in 1971. Will supersonic transport ever be viable? What does "viable" mean when speaking of the stratosphere?