JET PROPULSION Jet propulsion is a thrust produced by passing a jet of

matter (typically air or water) in the opposite

direction to the direction of motion.

Turbojet engines were the first type of an air breathing jet engine, used in aircraft.

Working Air Intake

An intake, or tube, is needed in front of the compressor to help direct the incoming

air smoothly into the moving compressor blades. Older engines had stationary

vanes in front of the moving blades. These vanes also helped to direct the air onto

the blades. The air flowing into a turbojet engine is always subsonic, regardless of the

speed of the aircraft itself.

Compressor

The compressor is driven by the turbine. It rotates at high speed, adding energy to

the airflow and at the same time squeezing (compressing) it into a smaller space.

Compressing the air increases its pressure and temperature. The smaller the

compressor the faster it turns. At the large end of the range the GE-90-115 fan

rotates at about 2,500 RPM while a small helicopter engine compressor rotates at

about 50,000 RPM.

Compressor types used in turbojets were typically axial or centrifugal.

Early turbojet compressors had low pressure ratios up to about 5:1. Aerodynamic

improvements including splitting the compressor into two separately rotating parts,

incorporating variable blade angles for entry guide vanes and stators and bleeding

air from the compressor enabled later turbojets to have overall pressure ratios of

15:1 or more.

Combustion chamber: In a turbojet the air and fuel mixture burn in the combustor and pass through to

the turbine in a continuous flowing process with no pressure build-up. Instead

there is a small pressure loss in the combustor. Further compressor air is

introduced which completes the combustion process and reduces the

temperature of the combustion products to a level which the turbine can accept.

Less than 25% of the air is typically used for combustion, as an overall lean mixture

is required to keep within the turbine temperature limits.

Turbine: Hot gases leaving the combustor expand through the turbine. Typical materials for

turbines include inconel and Nimonic. The hottest turbine vanes and blades in an

engine have internal cooling passages. Air from the compressor is passed through

these to keep the metal temperature within limits. The remaining stages don't need

cooling.

In the first stage the turbine is largely an impulse turbine (similar to a pelton

wheel) and rotates because of the impact of the hot gas stream. Later stages are

convergent ducts that accelerate the gas. Energy is transferred into the shaft

through momentum exchange in the opposite way to energy transfer in the

compressor. The power developed by the turbine drives the compressor as well as

accessories, like fuel, oil, and hydraulic pumps that are driven by the accessory

gearbox.

Nozzle:

After the turbine, the gases expand through the exhaust nozzle

producing a high velocity jet.

In a convergent nozzle, the ducting narrows progressively to a throat.

The nozzle pressure ratio on a turbojet is high enough at higher

thrust settings to cause the nozzle to choke.

If, however, a convergent-divergent de Laval nozzle is fitted, the

divergent (increasing flow area) section allows the gases to reach

supersonic velocity within the divergent section. Additional thrust is

generated by the higher resulting exhaust velocity.

Pros: Relatively simple design, Capable of very high speeds, Takes up

little space

Cons: High fuel consumption, Loud, Poor performance at slow speeds

Turbojets have been replaced in slower aircraft

by turboprops because they have better range-specific fuel

consumption.

Heinkel He 178, the world's first turbojet aircraft

The turbofan or fanjet is a type of airbreathing jet engine that is widely used

in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and

"fan":

The turbo portion refers to a gas turbine engine which achieves mechanical

energy from combustion, and the fan, a ducted fan that uses the mechanical

energy from the gas turbine to accelerate air rearwards.

Thus, whereas all the air taken in by a turbojet passes through the turbine

(through the combustion chamber), in a turbofan some of that air bypasses

the turbine. A turbofan thus can be thought of as a turbojet being used to

drive a ducted fan, with both of those contributing to the thrust.

The ratio of the mass-flow of air bypassing the engine core compared to the

mass-flow of air passing through the core is referred to as the bypass ratio.

The engine produces thrust through a combination of these two portions

working in concert; engines that use more jet thrust relative to fan thrust

are known as low-bypass turbofans, conversely those that have considerably

more fan thrust than jet thrust are known as high-bypass. Most commercial

aviation jet engines in use today are of the high-bypass type and most

modern military fighter engines are low-bypass.

Turbofans work by attaching a ducted fan to the front of a turbojet engine. The fan

creates additional thrust, helps cool the engine, and lowers the noise output of engine.

Step 1 - Inlet air is divided into two separate streams. One stream flows around the

engine (bypass air), while the other passes through the engine core.

Step 2 - Bypass air passes around the engine and is accelerated by a duct fan,

producing additional thrust.

Step 3 - Air flows through the turbojet engine, continuing the production of thrust.

Pros: Fuel efficient, Quieter than turbojets, They look awesome

Cons: Heavier than turbojets, Larger frontal area than turbojets, Inefficient at very high

altitudes

Boeing 777-300ER engine, can produce over 115,000

pounds of thrust

Rolls-Royce Trent 1000 turbofan

powering a Boeing 787 Dreamliner

Turbofans are the most

efficient engines in the range

of speeds from about 500 to

1,000 km/h (310 to 620 mph),

the speed at which most

commercial aircraft operate

A turboprop engine is a turbine engine that drives an

aircraft propeller. In contrast to a turbojet, the engine's exhaust

gases do not contain enough energy to create significant thrust, since

almost all of the engine's power is used to drive the propeller.

In its simplest form a turboprop consists of an

intake, compressor, combustor, turbine, and a propelling nozzle.

Air is drawn into the intake and compressed by the compressor. Fuel

is then added to the compressed air in the combustor, where the fuel-

air mixture then combusts.

The hot combustion gases expand through the turbine. Some of the

power generated by the turbine is used to drive the compressor. The

rest is transmitted through the reduction gearing to the propeller.

Further expansion of the gases occurs in the propelling nozzle, where

the gases exhaust to atmospheric pressure. The propelling nozzle

provides a relatively small proportion of the thrust generated by a

turboprop.

•Unlike the small diameter fans used in turbofan jet engines, the propeller has a

large diameter that lets it accelerate a large volume of air. This permits a lower

airstream velocity for a given amount of thrust.

•As it is more efficient at low speeds to accelerate a large amount of air by a

small degree. Propellers lose efficiency as aircraft speed increases, so

turboprops are normally not used on high-speed aircraft

Pilatus PC-12

ATR-72, a typical modern

turboprop aircraft.

Fairchild F-27

Pros: Very fuel efficient, Most efficient at

mid-range speed between 250-400 knots,

Most efficient at mid-range altitudes of

18,000-30,000 feet

Cons: Limited forward airspeed, Gearing

systems are heavy and can break down

A ramjet, sometimes referred to as a flying stovepipe or

an athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses

the engine's forward motion to compress incoming air without an axial compressor.

A ramjet uses this high pressure in front of the engine to force air through the tube,

where it is heated by combusting some of it with fuel. It is then passed through a

nozzle to accelerate it to supersonic speeds. This acceleration gives

the ramjet forward thrust.

Because ramjets cannot produce thrust at zero airspeed, they cannot move an

aircraft from a standstill.

A ramjet-powered vehicle, therefore, requires an assisted take-off like a rocket

assist to accelerate it to a speed where it begins to produce thrust.

Ramjets work most efficiently at supersonic speeds around Mach 3 (2,300 mph;

3,700 km/h). This type of engine can operate up to speeds of Mach 6 (4,600 mph;

7,400 km/h).

Ramjets can be particularly useful in applications requiring a small and simple

mechanism for high-speed use, such as missiles. Weapon designers are looking to use

ramjet technology in artillery shells to give added range; a 120 mm mortar shell, if

assisted by a ramjet, is thought to be able to attain a range of 35 km (22 mi).They

have also been used successfully, though not efficiently, as tip jets on the end

of helicopter rotors.

Jet engines scoop air in such speed so, the inlet is designed as a rapidly tapering nozzle,

so that it compresses the incoming air automatically, without either a compressor or a

turbine to power it. Engines that work this way are called ramjets, and since they need

the air to be travelling fast, are really suitable only for supersonic and hypersonic

(faster-than-sound) planes. Air moving faster than sound as it enters the engine is

compressed and slowed down dramatically, to subsonic speeds, mixed with fuel, and

ignited by a device called a flame holder, producing a rocket-like exhaust similar to that

made by a classic turbojet. Ramjets tend to be used on rocket and missile engines but

since they "breathe" air, they cannot be used in space.

Leduc 0.10 one of the

first ramjet-powered

aircraft to fly in 1949.

A scramjet (supersonic combusting ramjet) is a variant of a ramjet airbreathing

jet engine in which combustion takes place in supersonic airflow.

As in ramjets, a scramjet relies on high vehicle speed to forcefully compress

the incoming air before combustion (hence ramjet), but a ramjet decelerates

the air to subsonic velocities before combustion, while airflow in a scramjet is

supersonic throughout the entire engine. This allows the scramjet to operate

efficiently at extremely high speeds.

Artist's conception of

the NASA X-43 with scramjet

attached to the underside

Advantages

Does not have to carry oxygen

No rotating parts makes it easier to manufacture than a turbojet

Has a higher specific impulse (change in momentum per unit of propellant) than a rocket engine; could provide between 1000 and 4000 seconds, while a rocket typically provides around 450 seconds or less.

Higher speed could mean cheaper access to outer space in the future

Disadvantages

Difficult / expensive testing and development

Very high initial propulsion requirements

The compression,

combustion, and

expansion regions of:

(a) turbojet, (b) ramjet,

and (c) scramjet

engines.

A rocket engine is a type of jet engine that uses only stored rocket propellant mass for forming its high speed propulsive jet.

Rocket engines are reaction engines, obtaining thrust in accordance with Newton's third law.

Most rocket engines are internal combustion engines, although non-combusting forms (such as cold gas thrusters) also exist.

Vehicles propelled by rocket engines are commonly called rockets. Since they need no external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel spacecraft and ballistic missiles.

Compared to other types of jet engines, rocket engines have the highest thrust, are by far the lightest, but are the least propellant efficient (have the lowest specific impulse).

The ideal exhaust is hydrogen, the lightest of all gases, but chemical rockets produce a mix of heavier species, reducing the exhaust velocity.

Rocket engines become more efficient at high velocities (due to greater propulsive efficiency).

Since they do not require an atmosphere, they are well suited for uses at very high altitude and in space.

•Rocket engines produce thrust by the expulsion of an exhaust fluid which has

been accelerated to a high speed through a propelling nozzle.

•The fluid is usually a gas created by high pressure (150-to-2,900-pound-per-

square-inch (10 to 200 bar)) combustion of solid or liquid propellants, consisting

of fuel and oxidiser components, within a combustion chamber.

•The nozzle uses the heat energy released by expansion of the gas to accelerate

the exhaust to very high (supersonic) speed, and the reaction to this pushes the

engine in the opposite direction.

•Combustion is most frequently used for practical rockets, as high temperatures

and pressures are desirable for the best performance, permitting a longer nozzle,

giving higher exhaust speeds and better thermodynamic efficiency.

Viking 5C

rocket engine RS-68

A solid-propellant rocket or solid rocket is a rocket with a rocket engine that uses solid

propellants (fuel/oxidizer). The earliest rockets were solid-fuel rockets powered

by gunpowder.

All rockets used some form of solid or powdered propellant up until the 20th century,

when liquid-propellant rockets offered more efficient and controllable alternatives. Solid

rockets are still used today in model rockets and on larger applications for their simplicity and

reliability.

Since solid-fuel rockets can remain in storage for long periods, and then reliably launch on

short notice, they have been frequently used in military applications such as missiles. The

lower performance of solid propellants (as compared to liquids) does not favor their use as

primary propulsion in modern medium-to-large launch vehicles customarily used to orbit

commercial satellites and launch major space probes.

Solid rockets are used as light launch vehicles for low Earth orbit (LEO) payloads under 2

tons or escape payloads up to 500 kilograms

A liquid-propellant rocket or liquid rocket is a rocket engine that uses liquid propellants.

Liquids are desirable because their reasonably high density allows the volume of the

propellant tanks to be relatively low, and it is possible to use lightweight

centrifugal turbopumps to pump the propellant from the tanks into the combustion

chamber, which means that the propellants can be kept under low pressure.

An inert gas stored in a tank at a high pressure is sometimes used instead of pumps in

simpler small engines to force the propellants into the combustion chamber.

These engines may have a lower mass ratio, but are usually more reliable and are

therefore used widely in satellites for orbit maintenance.

Liquid rockets can be monopropellant rockets using a single type of

propellant, bipropellant rockets using two types of propellant, or more

exotic tripropellant rockets using three types of propellant.