Jet propulsion

Specific impulse (usually abbreviated I_sp) is a measure of how effectively a rocket uses propellant or jet engine uses fuel. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed[4] and is dimensionally equivalent to the generated thrust divided by the propellant mass flow rate or weight flow rate.[5] If mass (kilogram, pound-mass, or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound-force) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g₀) converts specific impulse from the mass basis to the weight basis.[5]

A propulsion system with a higher specific impulse uses the mass of the propellant more effectively in creating forward thrust and, in the case of a rocket, less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation.[4][6] In rockets, this means the engine is more effective at gaining altitude, distance, and velocity. 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.

Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air.

Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship I_sp = 1/(g_o·SFC) for SFC in kg/(N·s) and I_sp = 3600/SFC for SFC in lb/(lbf·hr).

From the definition of specific impulse thrust in SI units is:

${\displaystyle F={\dot {m}}V_{e}}$

where V_e is the effective exhaust velocity and ${\displaystyle {\dot {m}}}$ is the propellant flow rate.

A jet engine is a reaction engine which uses ambient air as the working fluid and converts it to a hot, high-pressure gas which is expanded through one or more nozzles. Two types of jet engine, the turbojet and turbofan, employ axial-flow or centrifugal compressors to raise the pressure before combustion and turbines to drive the compression. Ramjets operate only at high flight speeds because they omit the compressors and turbines, depending instead on the dynamic pressure generated by the high speed (known as ram compression). Pulse jets also omit the compressors and turbines but can generate static thrust and have limited maximum speed.

The rocket is capable of operating in the vacuum of space because it is dependent on the vehicle carrying its own oxidizer instead of using the oxygen in the air, or in the case of a nuclear rocket, heats an inert propellant (such as liquid hydrogen) by forcing it through a nuclear reactor.

Plasma thrusters accelerate a plasma by electromagnetic means.

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.