Jet propulsion

[3] Specific impulse (usually abbreviated Isp) is a measure of how effectively a rocket uses propellant or jet engine uses fuel.

This effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.

Two types of jet engines, the turbojet and turbofan, employ axial-flow or centrifugal compressors to raise the pressure before combustion and turbines to drive the compression.

Pulse jets also omit the compressors and turbines but can generate static thrust and have limited maximum speed.

The pump-jet, used for marine propulsion, uses water as the working fluid, pressurized by a ducted propeller, centrifugal pump, or a combination of the two.

[1] Sea hares (gastropod molluscs) employ a similar method, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.

[1] Some teleost fish have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.

[7]: 201 In some dragonfly larvae, jet propulsion is achieved by the expulsion of water from a specialised cavity through the anus.