JET ENGINES

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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