Modern jet aircraft

Modern airliners cruise at speeds of 0.75 to 0.85 Mach, or 75% to 85% of the speed of sound

Virgin Atlantic Airbus A340-600 landing

(420 to 580 mph/ 680-900 km/h). The speed of sound predominantly depends on air temperature (hardly at all on pressure), so the Mach number for the speed of a jet also varies with atmospheric conditions.

Aerodynamics

Bahrain Royal Flight Boeing 747SP

Many jet aircraft fly at high speeds, either supersonic or speeds just below the speed of sound ("transonic"). Aerodynamics is therefore an important consideration.

Jet aircraft are usually designed using the Whitcomb area rule, which says that the cross-section of the aircraft at any point must be approximately the same as the Sears-Haack body. This minimises the production of shockwaves which would waste energy.

Other jets

Most people use the term 'jet aircraft' to denote gas turbine based airbreathing jet engines, but rockets and scramjets are both also propelled by them.

The fastest airbreathing jet aircraft is the unmanned X-43 scramjet at around Mach 9-10. The fastest manned (rocket) aircraft is the X-15 at Mach 6.85.

The Space Shuttle, while far faster than the X-43 or X-15, is not regarded as an aircraft during ascent, nor during reentry and landing (as it is unpowered during this phase of operation)