Jets

Jets refers to several related concepts involving the expulsion or movement of a fast stream of gas or liquid, as well as other uses of the term. In aviation and propulsion, jet propulsion describes engines that produce thrust by ejecting a high-velocity jet of gas. The most common modern jet engines are turbojets and turbofans, which use a gas turbine to compress air, mix it with fuel, and exhaust the combustion products through a nozzle. Turbofans include a large front fan and are generally more fuel-efficient at subsonic speeds, while turbojets emphasize higher exhaust velocity for greater thrust at higher speeds. Other concepts include ramjets and scramjets, which have no onboard compressor and rely on the aircraft’s forward speed to compress intake air; ramjets operate best at high subsonic to hypersonic speeds, while scramjets function at hypersonic speeds. Jet engines power most current commercial and military aircraft and can employ afterburners to boost thrust for certain missions.

Jet streams are narrow bands of fast-moving air in the upper atmosphere, typically around 9 to 16 kilometers above the Earth’s surface. They arise from temperature contrasts between air masses and are strongest near the polar and subtropical regions. Speeds can be substantial, and jet streams influence weather patterns, storm tracks, and aviation by shaping flight routes and fuel use.

Water jets are propulsion systems for boats and submarines that draw water from an intake and eject it rearward at high speed to generate thrust. They offer high maneuverability, shallow draft, and reduced risk of hull damage, and are used on high-speed craft, rescue vessels, and some military platforms. Some designs include steerable nozzles or reversing vanes for direction control.

Jet is also the name of a black gemstone, a fossilized lignite used in jewelry. As a mineraloid, it lacks a crystalline structure and is typically cut as cabochons or beads; it has historical associations with Victorian fashion and is sometimes treated to enhance color.