Simple jet propulsion system

There are a large number of different types of jet engines, all of which achieve forward thrust from the principle of jet propulsion.

JET ENGINE

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The term “jet engine” is often used as a generic name for a variety of engines, including the turbojet, turbofan, turboprop, and ramjet. These engines all operate by the same basic principles, but each has its own distinct advantages and disadvantages. All jet engines operate by forcing incoming air into a tube where the air is compressed, mixed with fuel, burned, and exhausted at high speed to generate thrust.

The key to making a jet engine work is the compression of the incoming air. If uncompressed, the air-fuel mixture won’t burn and the engine can’t generate any thrust. Most members of the jet family employ a section of compressors, consisting of rotating blades, that slow the incoming air to create a high pressure. This compressed air is then forced into a combustion section where it is mixed with fuel and burned. As the high-pressure gases are exhausted, they are passed through a turbine section consisting of more rotating blades. In this region, the exhausting gases turn the turbine blades which are connected by a shaft to the compressor blades at the front of the engine. Thus, the exhaust turns the turbines which turn the compressors to bring in more air and keep the engine going. The combustion gases then continue to expand out through the nozzle creating a forward thrust. The above explanation describes a simple turbojet, as illustrated below.

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Diagram of an axial-flow turbojet

The turbojet (and the turbofan) can also be fitted with an afterburner. An afterburner is simply a long tube placed in between the turbine and the nozzle in which additional fuel is added and burned to provide a significant boost in thrust. However, afterburners greatly increase fuel consumption, so aircraft can only use them for brief periods.

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Comparison of a turbojet and a turbojet with an afterburner

A further variation on the turbojet is the turbofan. Although most components remain the same, the turbofan introduces a fan section in front of the compressors. The fan, another rotating series of blades, is also driven by the turbine, but its primary purpose is to force a large volume of air through outer ducts that go around the engine core. Although this “bypassed” air flow travels at much lower speeds, the large mass of air that is accelerated by the fan produces a significant thrust (in addition to that created by the turbojet core) without burning any additional fuel. Thus, the turbofan is much more fuel efficient than the turbojet. In addition, the low-speed air helps to cushion the noise of the jet core making the engine much quieter.

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Comparison of a low-bypass turbofan with long ducts and a high-bypass turbofan with short ducts

Turbofans are typically broken into one of two categories–low-bypass ratio and high-bypass ratio–as illustrated above. The bypass ratio refers to the ratio of incoming air that passes through the fan ducts compared to the incoming air passing through the jet core. In a low-bypass turbofan, only a small amount of air passes through the fan ducts and the fan is of very small diameter. The fan in a high-bypass turbofan is much larger to force a large volume of air through the ducts. The low-bypass turbofan is more compact, but the high-bypass turbofan can produce much greater thrust, is more fuel efficient, and is much quieter.

A concept similar to the turbofan is the turboprop. However, instead of the turbine driving a ducted fan, it drives a completely external propeller. Turboprops are commonly used on commuter aircraft and long-range planes that require great endurance like the P-3 Orion and Tu-95.

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Schematic of a turboprop engine

The turboprop is attractive in these applications because of its high fuel efficiency, even greater than the turbofan. However, the noise and vibration produced by the propeller is a significant drawback, and the turboprop is limited to subsonic flight only. In a typical turboprop, the jet core produces about 15% of the thrust while the propeller generates the remaining 85%.

Another noteworthy variation on the turbojet is the ramjet. The idea behind this type of engine is to remove all the rotary components of the engine (i.e. fans, compressors, and turbines) and allow the motion of the engine itself to compress incoming air for combustion.

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Simple schematic of a ramjet

However, the price of this simplicity is that the ramjet can only produce thrust when it is already in motion. Instead of using a compressor to draw in air and compress it for combustion, the ramjet relies on the motion of the aircraft to ram air into the engine at high enough speed that it is already sufficiently compressed for combustion to occur. Since ramjets typically cannot function until reaching about 300 mph (485 km/h) at sea level, they have been rarely used on manned aircraft. However, the ramjet is more fuel efficient than turbojets or turbofans starting at about Mach 3 making them very attractive for use on missiles. Such missiles are typically launched using rocket motors that accelerate the vehicle to high-subsonic or low-supersonic speeds where the ramjet is engaged.

Finally, let us talk briefly about the turboshaft, a version of the jet engine that powers nearly every helicopter built today. As the below image illustrates, the turboshaft utilizes many of the same components as a turbojet.

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Schematic of a turboshaft engine

Air is drawn in through an inlet, compressed by low- and high-pressure compressor blades, mixed with fuel and burned in a combustion chamber, passed through turbine blades, and exhausted through a nozzle. The key difference between the turboshaft and previously discussed engines is that the turbine not only drives the compressors, but the shaft is also connected to a gear box that drives a helicopter’s rotor blades. Although the engine shaft rotates about the horizontal, the gear box contains a sequence of gears that transform that motion to a rotation about the vertical axis as required by a helicopter main rotor. Helicopters also typically operate at much lower altitudes than aircraft where dust, sand, and other debris can easily be sucked into the engine. To address this problem, most turboshaft engines are equipped with a particle separator that filters out and expels the unwanted dust before the air flow reaches the compressor.

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Schematic of a turboshaft engine particle separator

While the turboprop is still popular on aircraft where low fuel consumption is vital, nearly all aircraft today employ some version of the turbofan, usually high-bypass turbofans. The high thrust, low fuel consumption, and low noise levels of these engines make them well-suited to both military and commercial applications. Today, about the only use for turbojets and ramjets is in missiles. Air-breathing, long-range, subsonic missiles like the Tomahawk use turbojets since these are small, relatively low-cost systems that provide much greater range than is possible with a rocket of comparable size. Ramjets find applications on air-breathing, long-range, supersonic missiles for similar reasons. Turboshafts, of course, have displaced the piston engine as the primary powerplant used on helicopters. To continue learning more about aircraft propulsion, be sure to check out NASA’s Learning Guide on Propulsion for a wealth of information, animations, and interactive applets about rockets, propellers, ramjets, and gas turbine engines.

Type

Description

Advantages

Disadvantages

Water jet

For propelling water rockets and jetboats; squirts water out the back through a nozzle

In boats, 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

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

Works like a turbojet but instead of a turbine driving the compressor a piston engine drives it.

Higher exhaust velocity than a propeller, offering better thrust at high speed

Heavy, inefficient and underpowered. Examples include: Coandă-1910 and Caproni Campini N.1.

Turbojet

A tube with a compressor and turbine sharing a common shaft with a burner in between and a propelling nozzle for the exhaust.[15] Uses a high exhaust gas velocity to produce thrust. Has a much higher core flow than bypass type engines

Simplicity of design, efficient at supersonic speeds (~M2)

A basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy.

Low-bypass Turbofan

One- or two-stage fan added in front bypasses a proportion of the air through a bypass duct straight to the nozzle/afterburner, avoiding the combustion chamber, with the rest being heated in the combustion chamber and passing through the turbine.[16] 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.

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.

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

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.[16] Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft.

Quieter around 10 to 20 percent more than the turbojet engine due to greater mass flow and lower total exhaust speed and 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.

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

Carries all propellants and oxidants on-board, emits jet for propulsion[17]

Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 5.0 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 on-board which increases risks. Extraordinarily noisy.

Ramjet

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[18]

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.

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 a powerplant to drive a propeller shaft (or rotor in the case of a helicopter)

High efficiency at lower subsonic airspeeds (300 knots plus), high shaft power to weight

Limited top speed (aeroplanes), somewhat noisy, complex transmission

Propfan/Unducted Fan

Turbojet engine that also 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 shortages

Development of propfan engines has been very limited, typically more noisy than turbofans, complexity

Pulsejet

Air is compressed and combusted intermittently instead of continuously. Some designs use valves.

Very simple design, commonly used on model aircraft

Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly

Pulse detonation engine

Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves

Maximum theoretical engine efficiency

Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use

Air-augmented rocket

Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket

Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4

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

Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic

Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies[19]

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

A turbojet where an additional oxidizer such as oxygen is added to the airstream to increase maximum altitude

Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed

Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous. Much heavier than simple rockets.

Precooled jets / LACE

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.

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, Reaction Engines SABRE, ATREX. Requires liquid hydrogen fuel which has very low density and heavily insulated tankage.

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