Modern Jet Aircraft

Modern airliners cruise at speeds of 0.75 to 0.85 Mach, or 75% to 85% of the speed of sound (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. NASA and the US Federal Aviation Administration have been promoting Very Light Jets: small general aviation aircraft seating 4 to 8 passengers.

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 a jet aircraft during ascent, nor during reentry and landing (as it is unpowered during this phase of operation).

The shape of most airliners is usually designed to have nearly the same cross-sectional area at each point along its length as the Sears-Haack body

Aerodynamics

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.

Jet Engines

The types are used for different aircraft. Turbojets are seldom used, but was used on Concorde; it has a high exhaust speed and low frontal cross-section, and so is best suited to high-speed flight. Low bypass turbofans have a lower exhaust speed than turbojets and are used for transonic and low supersonic speeds. High bypass turbofans are used for subsonic aircraft and are quite efficient and are widely used for airliners.

Rockets have extremely fast exhaust speeds and are mainly used when high speeds or extremely high altitudes are needed.

Source : Wikipedia