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 Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 3600/SFC for SFC in lb/(lbf·hr). Thrust From the definition of specific impulse thrust in SI units is: where V is the effective exhaust velocity and is the propellant flow rate. Types of reaction engine Reaction engines produce thrust by expelling solid or fluid reaction mass; jet propulsion applies only to engines which use fluid reaction mass. Jet engine 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. Rocket engine 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 engine Plasma thrusters accelerate a plasma by electromagnetic means. Pump-jet 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. Jet-propelled animals Cephalopods such as squid use jet propulsion for rapid escape from predators; they use other mechanisms for slow swimming. The jet is produced by ejecting water through a siphon, which typically narrows to a small opening to produce the maximum exhalent velocity. The water passes through the gills prior to exhalation, fulfilling the dual purpose of respiration and locomotion. Sea hares (gastropod molluscs) employ a similar method, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily. Some teleost fish