JET ENGINES
Water jet
Description
For propelling boats; squirts water out the back through a nozzle
Advantages
Can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable at all boat speeds, high speed efficiency, less vulnerable to damage from debris, very reliable, more load flexibility, less harmful to wildlife
Disadvantages
Can be less efficient than a propeller at low speed, more expensive, higher weight in boat due to entrained water, will not perform well if boat is heavier than the jet is sized for
Motorjet
Description
Most primitive air breathing jet engine. Essentially a supercharged piston engine with a jet exhaust
Advantages
Higher exhaust velocity than a propeller, offering better thrust at high speed
Disadvantages
Heavy, inefficient and underpowered. Examples include: Coandă-1910 and Caproni Campini N.1
Turbojet
Description
A tube with a compressor and turbine sharing a common shaft with a burner in between and a propelling nozzle for the exhaust. Uses a high exhaust gas velocity to produce thrust. Has a much higher core flow than bypass type engines
Advantages
Simplicity of design, efficient at supersonic speeds (~M2)
Disadvantages
A basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy
Turbofan
A turbofan is a type of jet engine, similar to a turbojet. It essentially consists of a large ducted fan with a smaller diameter turbojet engine mounted behind it that provides propulsion and also powers the fan. Part of the airstream from the ducted fan passes through the turbojet, providing oxygen to burn fuel to power the turbojet. But part, usually most, of the flow bypasses the turbojet, and is accelerated by turbine blades acting like a propeller. The combination of these two processes produces thrust more efficiently than other jet designs.[1]
A few designs work slightly differently and have the fan blades as a radial extension of an aft-mounted low-pressure turbine unit.
Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6.
All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft.
Low-bypass Turbofan
Description
One- or two-stage fan added in front bypasses a proportion of the air through a bypass chamber surrounding the core. Compared with its turbojet ancestor, this allows for more efficient operation with somewhat less noise. This is the engine of high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, the McDonnell Douglas DC-8, and their derivatives
Advantages
As with the turbojet, the design is aerodynamic, with only a modest increase in diameter over the turbojet required to accommodate the bypass fan and chamber. It is capable of supersonic speeds with minimal thrust drop-off at high speeds and altitudes yet still more efficient than the turbojet at subsonic operation
Disadvantages
Noisier and less efficient than high-bypass turbofan, with less static (Mach 0) thrust. Added complexity to accommodate dual shaft designs. More inefficient than a turbojet around M2 due to higher cross-sectional area
High-bypass Turbofan
Description
First stage compressor drastically enlarged to provide bypass airflow around engine core, and it provides significant amounts of thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the high-bypass turbofan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft
Advantages
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 temperature. Less noisy and exhibit much better efficiency than low bypass turbofans
Disadvantages
Greater complexity (additional ducting, usually multiple shafts) and the need to contain heavy blades. Fan diameter can be extremely large, especially in high bypass turbofans such as the GE90. More subject to FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine. Thrust lapse at higher speeds, which necessitates huge diameters and introduces additional drag
Rocket
Description
Carries all propellants and oxidants on-board, emits jet for propulsion
Advantages
Very 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
Disadvantages
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 oxidizer on-board which increases risks. Extraordinarily noisy
Ramjet
Description
Intake air is compressed entirely by speed of oncoming air and duct shape (divergent), and then it goes through a burner section where it is heated and then passes through a propelling nozzle
Advantages
Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all air-breathing jets (thrust / weight ratio up to 30 at optimum speed), cooling much easier than turbojets as no turbine blades to cool
Disadvantages
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)
Description
Strictly not a jet at all — a gas turbine engine is used as a powerplant to drive a propeller shaft (or rotor in the case of a helicopter)
Advantages
High efficiency at lower subsonic airspeeds (300 knots plus), high shaft power to weight
Disadvantages
Limited top speed (airplanes), somewhat noisy, complex transmission
Propfan / Unducted Fan
Description
Turbojet engine that also drives one or more propellers. Similar to a turbofan without the fan cowling
Advantages
Higher fuel efficiency, potentially less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortages
Disadvantages
Development of propfan engines has been very limited, typically noisier than turbofans, complexity
Pulsejet
Description
Air is compressed and combusted intermittently instead of continuously. Some designs use valves
Advantages
Very simple design, commonly used on model aircraft
Disadvantages
Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly
Pulse detonation engine
Description
Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves
Advantages
Maximum theoretical engine efficiency
Disadvantages
Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use
Air-augmented rocket
Description
Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket
Advantages
Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4
Disadvantages
Similar 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
Scramjet
Description
Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic
Advantages
Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies
Disadvantages
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
Turborocket
Description
A turbojet where an additional oxidizer such as oxygen is added to the airstream to increase maximum altitude
Advantages
Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed
Disadvantages
Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous. Much heavier than simple rockets
Precooled jets / LACE
Description
Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and / or turbojet and / or rocket engine
Advantages
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
Disadvantages
Exists only at the lab prototyping stage. Examples include RB545, SABRE, ATREX. Requires liquid hydrogen fuel which has very low density and heavily insulated tankage