TypeDescriptionAdvantagesDisadvantagesWater jetSquirts water out the back through a nozzleCan run in shallow water, powerful, less harmful to wildlife, (indeed used by
squid)Can be less efficient than a propeller, more vulnerable to debris
MotorjetMost primitive airbreathing jet engine. Essentially a
supercharged piston engine with a jet exhaust.Higher exhaust velocity than a propeller, offering better thrust at high speedHeavy, inefficient and underpowered
TurbojetGeneric term for simple turbine engineSimplicity of design, efficient at supersonic speeds (~M2)Basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy.
TurbofanMost common form of jet engine in use today. Used in airliners like the Boeing 747 and military jets, where an afterburner is often added for supersonic flight. First stage compressor greatly enlarged to provide bypass airflow around engine core.Quieter due to greater
mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperatureGreater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to
FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine.
RocketCarries all propellants and oxidents onboard, emits jet for propulsionVery few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust/weight ratio over 100, no complex air inlet, high compression ratio, very high speed (
hypersonic) exhaust, good cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works best exoatmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, and no turbine in hot exhaust stream.Needs lots of propellant- very low
specific impulse — typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidiser onboard which increases risks. Extraordinarily noisy.
RamjetIntake air is compressed entirely by speed of oncoming air and duct shape (
divergent)Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed), cooling much easier than turbojets as no turbine blades to cool.Must have a high initial speed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, usually limited to a small range of speeds, intake flow must be slowed to subsonic speeds, noisy, fairly difficult to test, finicky to keep lit.
Turboprop (
Turboshaft similar)Strictly not a jet at all — a gas turbine engine is used as powerplant to drive propeller shaft (or Rotor in the case of a Helicopter)High efficiency at lower subsonic airspeeds (300 knots plus), high shaft power to weightLimited top speed (aeroplanes), somewhat noisy, complex transmission
Propfan/Unducted FanTurboprop engine drives one or more propellers. Similar to a turbofan without the fan cowling.Higher fuel efficiency, potentially less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortagesDevelopment of propfan engines has been very limited, typically more noisy than turbofans, complexity
PulsejetAir is compressed and combusted intermittently instead of continuously. Some designs use valves.Very simple design, commonly used on model aircraftNoisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly
Pulse detonation engineSimilar to a pulsejet, but combustion occurs as a
detonation instead of a
deflagration, may or may not need valvesMaximum theoretical engine efficiencyExtremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use
Air-augmented rocketEssentially a ramjet where intake air is compressed and burnt with the exhaust from a rocketMach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets.
ScramjetSimilar to a ramjet without a diffuser; airflow through the entire engine remains supersonicFew mechanical parts, can operate at very high
Mach numbers (Mach 8 to 15) with good efficiencies
[2]Still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, very poor thrust/weight ratio (~2), extreme aerodynamic complexity, airframe difficulties, testing difficulties/expense
TurborocketA turbojet where an additional
oxidizer such as
oxygen is added to the airstream to increase maximum altitudeVery close to existing designs, operates in very high altitude, wide range of altitude and airspeedAirspeed limited to same range as turbojet engine, carrying oxidizer like
LOX can be dangerous. Much heavier than simple rockets.Precooled jets /
LACEIntake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet or turbojet engine. Can be combined with a rocket engine for orbital insertion.Easily tested on ground. Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel.Exists only at the lab prototyping stage. Examples include
RB545,
SABRE,
ATREX