UTJT: Universal Twin Jet
A jet engine is an engine that discharges a fast moving jet of fluid to
generate thrust in accordance with Newton's third law of motion. This broad
definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse
jets and pump-jets, but in common usage, the term generally refers to a gas
turbine Brayton cycle engine, an engine with a rotary compressor powered by a
turbine, with the leftover power providing thrust. Jet engines are so familiar
to the modern world that gas turbines are sometimes mistakenly referred to as a
particular application of a jet engine, rather than the other way around. Most
jet engines are internal combustion engines but non combusting forms exist also.
History
Jet engines can be dated back to the first century AD, when Hero of Alexandria
invented the aeolipile. This used steam power directed through two jet nozzles
so as to cause a sphere to spin rapidly on its axis. So far as is known, it was
little used for supplying mechanical power, and the potential practical
applications of Hero's invention of the jet engine were not recognized. It was
simply considered a curiosity.
Jet propulsion only literally and figuratively took off with the invention of
the rocket by the Chinese in the 11th century. Rocket exhaust was initially used
in a modest way for fireworks but gradually progressed to propel formidable
weaponry; and there the technology stalled for hundreds of years.
In Ottoman Turkey in 1633 Lagari Hasan ?elebi took off with what was described
to be a cone shaped rocket and then glided with wings into a successful landing
winning a position in the Ottoman army. However, this was essentially a stunt.
The problem was that rockets are simply too inefficient at low speeds to be
useful for general aviation. Instead, by the 1930s, the piston engine in its
many different forms (rotary and static radial, aircooled and liquid-cooled
inline) was the only type of powerplant available to aircraft designers. This
was acceptable as long as only low performance aircraft were required, and
indeed all that were available.
However, engineers were beginning to realize that the piston engine was
self-limiting in terms of the maximum performance which could be attained; the
limit was essentially one of propeller efficiency. This seemed to peak as blade
tips approached the speed of sound. If engine, and thus aircraft, performance
were ever to increase beyond such a barrier, a way would have to be found to
radically improve the design of the piston engine, or a wholly new type of
powerplant would have to be developed. This was the motivation behind the
development of the gas turbine engine, commonly called a "jet" engine, which
would become almost as revolutionary to aviation as the Wright brothers' first
flight.
The earliest attempts at jet engines were hybrid designs in which an external
power source first compressed air, which was then mixed with fuel and burned for
jet thrust. In one such system, called a thermojet by Secondo Campini but more
commonly, motorjet, the air was compressed by a fan driven by a conventional
piston engine. Examples of this type of design were Henri Coand?'s Coand?-1910
aircraft, and the much later Campini Caproni CC.2, and the Japanese Tsu-11
engine intended to power Ohka kamikaze planes towards the end of World War II.
None were entirely successful and the CC.2 ended up being slower than the same
design with a traditional engine and propeller combination.
simulation of the Jet Engine AirflowThe key to a practical jet engine was the
gas turbine, used to extract energy from the engine itself to drive the
compressor. The gas turbine was not an idea developed in the 1930s: the patent
for a stationary turbine was granted to John Barber in England in 1791. The
first gas turbine to successfully run self-sustaining was built in 1903 by
Norwegian engineer ?gidius Elling. The first patents for jet propulsion were
issued in 1917. Limitations in design and practical engineering and metallurgy
prevented such engines reaching manufacture. The main problems were safety,
reliability, weight and, especially, sustained operation. In 1923, Edgar
Buckingham of the US National Bureau of Standard published a report saying that
jet propulsion was unlikely to ever be competitive with prop driven aircraft.
The W2/700 engine flew in the Gloster E.28/39, the first British aircraft to fly
with a turbojet engine, and the Gloster Meteor.In 1929, Aircraft apprentice
Frank Whittle formally submitted his ideas for a turbo-jet to his superiors. On
16 January 1930 in England, Whittle submitted his first patent (granted in
1932). The patent showed a two-stage axial compressor feeding a single-sided
centrifugal compressor. Whittle would later concentrate on the simpler
centrifugal compressor only, for a variety of practical reasons. Whittle had his
first engine running in April 1937. It was liquid-fuelled, and included a
self-contained fuel pump. Whittle's team experienced near-panic when the engine
would not stop, even after the fuel was switched off. It turned out that fuel
had leaked into the engine and accumulated in pools. So the engine would not
stop until all the leaked fuel had burned off. Whittle was unable to interest
the government in his invention, and development continued at a slow pace.
In 1935 Hans von Ohain started work on a similar design in Germany, unaware of
Whittle's work. His first engine was strictly experimental and could only run
under external power, but he was able to demonstrate the basic concept. Ohain
was then introduced to Ernst Heinkel, one of the larger aircraft industrialists
of the day, who immediately saw the promise of the design. Heinkel had recently
purchased the Hirth engine company, and Ohain and his master machinist Max Hahn
were set up there as a new division of the Hirth company. They had their first
HeS 1 engine running by September 1937. Unlike Whittle's design, Ohain used
hydrogen as fuel, supplied under external pressure. Their subsequent designs
culminated in the gasoline-fuelled HeS 3 of 1,100 lbf (5 kN), which was fitted
to Heinkel's simple and compact He 178 airframe and flown by Erich Warsitz in
the early morning of August 27, 1939, from Marienehe aerodrome, an impressively
short time for development. The He 178 was the world's first jet plane.
Meanwhile, Whittle's engine was starting to look useful, and his Power Jets Ltd.
started receiving Air Ministry money. In 1941 a flyable version of the engine
called the W.1, capable of 1000 lbf (4 kN) of thrust, was fitted to the Gloster
E28/39 airframe specially built for it, and first flew on May 15, 1941 at RAF
Cranwell.
A picture of an early centrifugal engine (the DH Goblin II) sectioned to show
its internal componentsOne problem with both of these early designs, which are
called centrifugal-flow engines, was that the compressor worked by "throwing"
(accelerating) air outward from the central intake to the outer periphery of the
engine, where the air was then compressed by a divergent duct setup, converting
its velocity into pressure. An advantage of this design was that it was already
well understood, having been implemented in centrifugal superchargers, then in
widespread use on piston engines. However, given the early technological
limitations on the shaft speed of the engine, the compressor needed to have a
very large diameter to produce the power required. This meant that the engines
had a large frontal area, which made it less useful as an aircraft powerplant
due to drag. A further disadvantage was that the air flow had to be "bent" to
flow rearwards through the combustion section and to the turbine and tailpipe,
adding complexity and lowering efficiency. Nevertheless, Whittle's engines had
the major advantages of light weight, simplicity and reliability, and
development rapidly progressed to practical airworthy designs.
Austrian Anselm Franz of Junkers' engine division (Junkers Motoren or Jumo)
addressed these problems with the introduction of the axial-flow compressor.
Essentially, this is a turbine in reverse. Air coming in the front of the engine
is blown towards the rear of the engine by a fan stage (convergent ducts), where
it is crushed against a set of non-rotating blades called stators (divergent
ducts). The process is nowhere near as powerful as the centrifugal compressor,
so a number of these pairs of fans and stators are placed in series to get the
needed compression. Even with all the added complexity, the resulting engine is
much smaller in diameter and thus, more aerodynamic. Jumo was assigned the next
engine number in the RLM numbering sequence, 4, and the result was the Jumo 004
engine. After many lesser technical difficulties were solved, mass production of
this engine started in 1944 as a powerplant for the world's first jet-fighter
aircraft, the Messerschmitt Me 262 (and later the world's first jet-bomber
aircraft, the Arado Ar 234). A variety of reasons conspired to delay the
engine's availability, this delay caused the fighter too arrive too late to
decisively impact Germany's position in World War II. Nonetheless, it will be
remembered as the first use of jet engines in service. Following the end of the
war the German jet aircraft and jet engines were extensively studied by the
victorious allies and contributed to work on early Soviet and US jet fighters.
The legacy of the axial-flow engine is seen in the fact that practically all jet
engines on fixed wing aircraft have had some inspiration from this design.
A cutaway of the Junkers Jumo 004 engine.Centrifugal-flow engines have improved
since their introduction. With improvements in bearing technology the shaft
speed of the engine was increased, greatly reducing the diameter of the
centrifugal compressor. The short engine length remains an advantage of this
design, particularly for use in helicopters where overall size is more important
than frontal area. Also, its engine components are robust; axial-flow
compressors are more liable to foreign object damage.
Although German designs were more advanced aerodynamically, the combination of
simplicity and advanced British metallurgy meant that Whittle-derived designs
were far more reliable than their German counterparts. British engines also were
licensed widely in the US (see Tizard Mission), and were sent to the USSR in a
technology exchange, with the Nene going on to power the famous MiG-15. American
and Soviet designs, independent axial-flow types for the most part, would not
come fully into their own until the 1960s, although the General Electric J47
provided excellent service in the F-86 Sabre in the 1950s.
By the 1950s the jet engine was almost universal in combat aircraft, with the
exception of cargo, liaison and other specialty types. By this point some of the
British designs were already cleared for civilian use, and had appeared on early
models like the deHavilland Comet and Canadair Jetliner. By the 1960s all large
civilian aircraft were also jet powered, leaving the piston engine in niche
roles here as well. Relentless improvements in the turboprop has since pushed
the piston engine out of the mainstream entirely, leaving it serving only the
smallest general aviation designs, and some use in drone aircraft. The ascension
of the jet engine to almost universal use in aircraft use took well under twenty
years.
Types
There are a large number of different types of jet engines, all of which achieve
propulsion from a high speed exhaust jet.
Type Description Advantages Disadvantages
Water jet Squirts water out the back through a nozzle Can run in shallow water,
powerful, less harmful to wildlife, (indeed used by squid) Can be less efficient
than a propeller, more vulnerable to debris
Motorjet Most primitive airbreathing jet engine. Essentially a supercharged
piston engine with a jet exhaust. Higher exhaust velocity than a propeller,
offering better thrust at high speed Heavy, inefficient and underpowered
Turbojet Generic term for simple turbine engine Simplicity of design, efficient
at supersonic speeds (~M2) Basic design, misses many improvements in efficiency
and power for subsonic flight, relatively noisy.
Turbofan Most 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 temperature Greater 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.
Rocket Carries all propellants and oxidants onboard, emits jet for propulsion
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. 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.
Ramjet Intake 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 weight Limited top speed (aeroplanes), somewhat noisy, complex
transmission
Propfan/Unducted Fan Turboprop 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 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 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 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. Requires liquid hydrogen fuel which has
very low density and heavily insulated tankage.
Type comparison
Comparative suitability for (left to right) turboshaft, low bypass and turbojet
to fly at 10 km attitude in various speeds. Horizontal axis - speed, m/s.
Vertical axis displays engine efficiency.
Specific impulse as a function of speed of different Jet types. Although
efficiency plummets with speed, greater distances are covered, it turns out that
efficiency per unit distance (per km or mile) is roughly independent of speed
for Jet engines as a group; however airframes become inefficient at supersonic
speeds
Dependence of the energy efficiency (η) from the exhaust speed/airplane speed
ratio (c/v) for airbreathing jets
Dependence of the energy efficiency (η) upon the vehicle speed/exhaust speed
ratio (v/c) for rocket enginesThe motion impulse of the engine is equal to the
air mass multiplied by the speed at which the engine emits this mass:
I = m c
where m is the air mass per second and c is the exhaust speed. In other words,
the plane will fly faster if the engine emits the air mass with a higher speed
or if it emits more air per second with the same speed. However, when the plane
flies with certain velocity v, the air moves towards it, creating the opposing
ram drag at the air intake:
m v
Most types of jet engine have an air intake, which provides the bulk of the gas
exiting the exhaust. Conventional rocket motors, however, do not have an air
intake, the oxidizer and fuel both being carried within the airframe. Therefore,
rocket motors do not have ram drag; the gross thrust of the nozzle is the net
thrust of the engine. Consequently, the thrust characteristics of a rocket motor
are completely different from that of an air breathing jet engine.
The air breathing engine is only useful if the velocity of the gas from the
engine, c, is greater than the airplane velocity, v. The net engine thrust is
the same as if the gas were emitted with the velocity c-v. So the thrust is
actually equal to
S = m (c-v)
The turboprop has a wide rotating fan that takes and accelerates the large mass
of air but only till the limited speed of any propeller driven airplane. When
the plane speed exceeds this limit, propellers no longer provide any thrust (c-v
< 0).
The turbojets and other similar engines accelerate much smaller mass of the air
and burned fuel, but they emit it at the much higher speeds possible with a de
Laval nozzle. This is why they are suitable for supersonic and higher speeds.
From the other side, the propulsive efficiency (essentially energy efficiency)
is highest when the engine emits an exhaust jet at a speed that is the same as
the airplane velocity. The exact formula, given in the literature, is
The low bypass turbofans have the mixed exhaust of the two air flows, running at
different speeds (c1 and c2). The thrust of such engine is
S = m1 (c1 - v) + m2 (c2 - v)
where m1 and m2 are the air masses, being blown from the both exhausts. Such
engines are effective at lower speeds, than the pure jets, but at higher speeds
than the turboshafts and propellers in general. For instance, at the 10 km
attitude, turboshafts are most effective at about 0.4 mach, low bypass turbofans
become more effective at about 0.75 mach and true jets become more effective as
mixed exhaust engines when the speed approaches 1 mach - the speed of sound.
Rocket engines are best suited for high speeds and altitudes. At any given
throttle, the thrust and efficiency of a rocket motor improves slightly with
increasing altitude (because the back-pressure falls thus increasing net thrust
at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling
density of the air entering the intake (and the hot gases leaving the nozzle)
causes the net thrust to decrease with increasing altitude. Rocket engines are
more efficient than even scramjets above roughly Mach 15.
Turbojet engines
A turbojet engine, in its simplest form is simply a gas turbine with a nozzle
attachedM Turbojet
A turbojet engine is a type of internal combustion engine often used to propel
aircraft. Air is drawn into the rotating compressor via the intake and is
compressed, through successive stages, to a higher pressure before entering the
combustion chamber. Fuel is mixed with the compressed air and ignited by flame
in the eddy of a flame holder. This combustion process significantly raises the
temperature and volume of the air. Hot combustion products leaving the combustor
expand through a gas turbine, where power is extracted to drive the compressor.
This expansion process reduces both the gas temperature and pressure but
sufficient fuel is burnt so that both parameters are usually still well above
ambient conditions at exit from the turbine. The gas stream is then expanded to
ambient pressure via a propelling nozzle, producing a high velocity jet as the
exhaust. If the jet velocity exceeds the aircraft flight velocity, there is a
net forward thrust upon the airframe.
Under normal circumstances, the pumping action of the compressor prevents any
backflow, thus facilitating the continuous-flow process of the engine. Indeed,
the entire process is similar to a four-stroke cycle, but with induction,
compression, ignition, expansion and exhaust taking place simultaneously, but in
different sections of the engine. The efficiency of a jet engine is strongly
dependent upon the overall pressure ratio (combustor entry pressure/intake
delivery pressure) and the turbine inlet temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller
engines. Turbojet engines take a relatively small mass of air and accelerate it
by a large amount, whereas a propeller takes a large mass of air and accelerates
it by a small amount. The high-speed exhaust of a turbojet engine makes it
efficient at high speeds (especially supersonic speeds) and high altitudes. On
slower aircraft and those required to fly short stages, a gas turbine-powered
propeller engine, commonly known as a turboprop, is more common and much more
efficient. Very small aircraft generally use conventional piston engines to
drive a propeller but small turboprops are getting smaller as engineering
technology improves.
The turbojet described above is a single-spool design, in which a single shaft
connects the turbine to the compressor. Higher overall pressure ratio designs
often have two concentric shafts, to improve compressor stability during engine
throttle movements. This High Pressure (HP) spool consists of the outer high
pressure shaft which connects the high pressure compressor to the high pressure
turbine. This HP Spool, with the combustor, forms the core or gas generator of
the engine. An inner shaft within the HP shaft connects the Low Pressure (LP)
compressor to the LP Turbine to create the LP Spool. Both spools are free to
operate at their optimum shaft speed. (Concorde used this type).
Turbofan engines
M Turbofan
Most modern jet engines are actually turbofans, where the low pressure
compressor acts as a fan, supplying supercharged air not only to the engine
core, but to a bypass duct. The bypass airflow either passes to a separate 'cold
nozzle' or mixes with low pressure turbine exhaust gases, before expanding
through a 'mixed flow nozzle'.
Turbofans are used for airliners because they give an exhaust speed that is
better matched to subsonic airliner's flight speed, conventional turbojet
engines generate an exhaust that ends up travelling very fast backwards, and
this wastes energy. By emitting the exhaust so that it ends up travelling more
slowly, better fuel consumption is achieved. In addition, the lower exhaust
speed gives much lower noise.
In the 1960s there was little difference between civil and military jet engines,
apart from the use of afterburning in some (supersonic) applications. Civil
turbofans today have a low exhaust speed (low specific thrust -net thrust
divided by airflow) to keep jet noise to a minimum and to improve fuel
efficiency. Consequently the bypass ratio (bypass flow divided by core flow) is
relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage
is required, because a low specific thrust implies a low fan pressure ratio.
Today's military turbofans, however, have a relatively high specific thrust, to
maximize the thrust for a given frontal area, jet noise being of less concern in
military uses relative to civil uses. Multistage fans are normally needed to
reach the relatively high fan pressure ratio needed for high specific thrust.
Although high turbine inlet temperatures are often employed, the bypass ratio
tends to be low, usually significantly less than 2.0.
An approximate equation for calculating the net thrust of a jet engine, be it a
turbojet or a mixed turbofan, is:
where:
intake mass flow rate
fully expanded jet velocity (in the exhaust plume)
aircraft flight velocity
While the term represents the gross thrust of the nozzle, the term represents
the ram drag of the intake.
Major components
Basic components of a jet engine (Axial flow design)The major components of a
jet engine are similar across the major different types of engines, although not
all engine types have all components. The major parts include:
Cold Section:
Air intake (Inlet) — The standard reference frame for a jet engine is the
aircraft itself. For subsonic aircraft, the air intake to a jet engine presents
no special difficulties, and consists essentially of an opening which is
designed to minimise drag, as with any other aircraft component. However, the
air reaching the compressor of a normal jet engine must be travelling below the
speed of sound, even for supersonic aircraft, to sustain the flow mechanics of
the compressor and turbine blades. At supersonic flight speeds, shockwaves form
in the intake system and reduce the recovered pressure at inlet to the
compressor. So some supersonic intakes use devices, such as a cone or ramp, to
increase pressure recovery, by making more efficient use of the shock wave
system.
Compressor or Fan — The compressor is made up of stages. Each stage consists of
vanes which rotate, and stators which remain stationary. As air is drawn deeper
through the compressor, its heat and pressure increases. Energy is derived from
the turbine (see below), passed along the shaft.
Common:
Shaft — The shaft connects the turbine to the compressor, and runs most of the
length of the engine. There may be as many as three concentric shafts, rotating
at independent speeds, with as many sets of turbines and compressors. Other
services, like a bleed of cool air, may also run down the shaft.
Hot section:
Combustor or Can or Flameholders or Combustion Chamber — This is a chamber where
fuel is continuously burned in the compressed air.
Turbine — The turbine acts like a windmill, gaining energy from the hot gases
leaving the combustor. This energy is used to drive the compressor (or props, or
bypass fans) via the shaft, or even (for a gas turbine-powered helicopter)
converted entirely to rotational energy for use elsewhere. Relatively cool air,
bled from the compressor, may be used to cool the turbine blades and vanes, to
prevent them from melting.
Afterburner or reheat (chiefly UK) — (mainly military) Produces extra thrust by
burning extra fuel, usually inefficiently, to significantly raise Nozzle Entry
Temperature at the exhaust. Owing to a larger volume flow (i.e. lower density)
at exit from the afterburner, an increased nozzle flow area is required, to
maintain satisfactory engine matching, when the afterburner is alight.
Exhaust or Nozzle — Hot gases leaving the engine exhaust to atmospheric pressure
via a nozzle, the objective being to produce a high velocity jet. In most cases,
the nozzle is convergent and of fixed flow area.
Supersonic nozzle — If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient
Pressure) is very high, to maximize thrust it may be worthwhile, despite the
additional weight, to fit a convergent-divergent (de Laval) nozzle. As the name
suggests, initially this type of nozzle is convergent, but beyond the throat
(smallest flow area), the flow area starts to increase to form the divergent
portion. The expansion to atmospheric pressure and supersonic gas velocity
continues downstream of the throat, whereas in a convergent nozzle the expansion
beyond sonic velocity occurs externally, in the exhaust plume. The former
process is more efficient than the latter.
The various components named above have constraints on how they are put together
to generate the most efficiency or performance. The performance and efficiency
of an engine can never be taken in isolation; for example fuel/distance
efficiency of a supersonic jet engine maximises at about mach 2, whereas the
drag for the vehicle carrying it is increasing as a square law and has much
extra drag in the transonic region. The highest fuel efficiency for the overall
vehicle is thus typically at Mach ~0.85.
For the engine optimisation for its intended use, important here is air intake
design, overall size, number of compressor stages (sets of blades), fuel type,
number of exhaust stages, metallurgy of components, amount of bypass air used,
where the bypass air is introduced, and many other factors. For instance, let us
consider design of the air intake.
Air intakes
See also: Inlet cone
Subsonic inlets
Pitot intake operating modesPitot intakes are the dominant type for subsonic
applications. A subsonic pitot inlet is little more than a tube with an
aerodynamic fairing around it.
At zero airspeed (i.e., rest), air approaches the intake from a multitude of
directions: from directly ahead, radially, or even from behind the plane of the
intake lip.
At low airspeeds, the streamtube approaching the lip is larger in cross-section
than the lip flow area, whereas at the intake design flight Mach number the two
flow areas are equal. At high flight speeds the streamtube is smaller, with
excess air spilling over the lip.
Beginning around 0.85 Mach, shock waves can occur as the air accelerates through
the intake throat.
Careful radiusing of the lip region is required to optimize intake pressure
recovery (and distortion) throughout the flight envelope.
Supersonic inlets
Supersonic intakes exploit shock waves to decelerate the airflow to a subsonic
condition at compressor entry.
There are basically two forms of shock waves:
1) Normal shock waves lie perpendicular to the direction of the flow. These form
sharp fronts and shock the flow to subsonic speeds. Microscopically the air
molecules smash into the subsonic crowd of molecules like alpha rays. Normal
shock waves tend to cause a large drop in stagnation pressure. Basically, the
higher the supersonic entry Mach number to a normal shock wave, the lower the
subsonic exit Mach number and the stronger the shock (i.e. the greater the loss
in stagnation pressure across the shock wave).
2) Conical (3-dimensional) and oblique shock waves (2D) are angled rearwards,
like the bow wave on a ship or boat, and radiate from a flow disturbance such as
a cone or a ramp. For a given inlet Mach number, they are weaker than the
equivalent normal shock wave and, although the flow slows down, it remains
supersonic throughout. Conical and oblique shock waves turn the flow, which
continues in the new direction, until another flow disturbance is encountered
downstream.
Note: Comments made regarding 3 dimensional conical shock waves, generally also
apply to 2D oblique shock waves.
A sharp-lipped version of the pitot intake, described above for subsonic
applications, performs quite well at moderate supersonic flight speeds. A
detached normal shock wave forms just ahead of the intake lip and 'shocks' the
flow down to a subsonic velocity. However, as flight speed increases, the shock
wave becomes stronger, causing a larger percentage decrease in stagnation
pressure (i.e. poorer pressure recovery). An early US supersonic fighter, the
F-100 Super Sabre, used such an intake.
An unswept lip generate a shock wave, which is reflected multiple times in the
inlet. The more reflections before the flow gets subsonic, the better pressure
recoveryMore advanced supersonic intakes, excluding pitots:
a) exploit a combination of conical shock wave/s and a normal shock wave to
improve pressure recovery at high supersonic flight speeds. Conical shock wave/s
are used to reduce the supersonic Mach number at entry to the normal shock wave,
thereby reducing the resultant overall shock losses.
b) have a design shock-on-lip flight Mach number, where the conical/oblique
shock wave/s intercept the cowl lip, thus enabling the streamtube capture area
to equal the intake lip area. However, below the shock-on-lip flight Mach
number, the shock wave angle/s are less oblique, causing the streamline
approaching the lip to be deflected by the presence of the cone/ramp.
Consequently, the intake capture area is less than the intake lip area, which
reduces the intake airflow. Depending on the airflow characteristics of the
engine, it may be desirable to lower the ramp angle or move the cone rearwards
to refocus the shockwaves onto the cowl lip to maximise intake airflow.
c) are designed to have a normal shock in the ducting downstream of intake lip,
so that the flow at compressor/fan entry is always subsonic. However, if the
engine is throttled back, there is a reduction in the corrected airflow of the
LP compressor/fan, but (at supersonic conditions) the corrected airflow at the
intake lip remains constant, because it is determined by the flight Mach number
and intake incidence/yaw. This discontinuity is overcome by the normal shock
moving to a lower cross-sectional area in the ducting, to decrease the Mach
number at entry to the shockwave. This weakens the shockwave, improving the
overall intake pressure recovery. So, the absolute airflow stays constant,
whilst the corrected airflow at compressor entry falls (because of a higher
entry pressure). Excess intake airflow may also be dumped overboard or into the
exhaust system, to prevent the conical/oblique shock waves being disturbed by
the normal shock being forced too far forward by engine throttling.
Many second generation supersonic fighter aircraft featured an inlet cone, which
was used to form the conical shock wave. This type of inlet cone is clearly seen
at the very front of the English Electric Lightning and MiG-21 aircraft, for
example.
The same approach can be used for air intakes mounted at the side of the
fuselage, where a half cone serves the same purpose with a semicircular air
intake, as seen on the F-104 Starfighter and BAC TSR-2.
Some intakes are biconic; that is they feature two conical surfaces: the first
cone is supplemented by a second, less oblique, conical surface, which generates
an extra conical shockwave, radiating from the junction between the two cones. A
biconic intake is usually more efficient than the equivalent conical intake,
because the entry Mach number to the normal shock is reduced by the presence of
the second conical shock wave.
A very sophisticated conical intake was featured on the SR-71's Pratt & Whitney
J58s that could move a conical spike fore and aft within the engine nacelle,
preventing the shockwave formed on the spike from entering the engine and
stalling the engine, while keeping it close enough to give good compression.
Movable cones are uncommon.
A more sophisticated design than cones is to angle the intake so that one of its
edges forms a ramp. An oblique shockwave will form at the start of the ramp. The
Century Series of US jets featured several variants of this approach, usually
with the ramp at the outer vertical edge of the intake, which was then angled
back inward towards the fuselage. Typical examples include the Republic F-105
Thunderchief and F-4 Phantom.
Concorde intake operating modesLater this evolved so that the ramp was at the
top horizontal edge rather than the outer vertical edge, with a pronounced angle
downwards and rearwards. This design simplified the construction of intakes and
allowed use of variable ramps to control airflow into the engine. Most designs
since the early 1960s now feature this style of intake, for example the F-14
Tomcat, Panavia Tornado and Concorde.
From another point of view, like in a supersonic nozzle the corrected (or
non-dimensional) flow has to be the same at the intake lip, at the intake throat
and at the turbine. One of this three can be fixed. For inlets the throat is
made variable and some air is bypassed around the turbine and directly fed into
the afterburner. Unlike in a nozzle the inlet is either unstable or inefficient,
because a normal shock wave in the throat will suddenly move to the lip, thereby
increasing the pressure at the lip, leading to drag and reducing the pressure
recovery, leading to turbine surge and the loss of one SR-71.
Compressors
Axial compressors
Compressor stage GE J79Axial compressors rely on spinning blades that have
aerofoil sections, similar to aeroplane wings. As with aeroplane wings in some
conditions the blades can stall. If this happens, the airflow around the stalled
compressor can reverse direction violently. Each design of a compressor has an
associated operating map of airflow versus rotational speed for characteristics
peculiar to that type (see compressor map).
At a given throttle condition, the compressor operates somewhere along the
steady state running line. Unfortunately, this operating line is displaced
during transients. Many compressors are fitted with anti-stall systems in the
form of bleed bands or variable geometry stators to decrease the likelihood of
surge. Another method is to split the compressor into two or more units,
operating on separate concentric shafts.
Another design consideration is the average stage loading. This can be kept at a
sensible level either by increasing the number of compression stages (more
weight/cost) or the mean blade speed (more blade/disc stress).
Although large flow compressors are usually all-axial, the rear stages on
smaller units are too small to be robust. Consequently, these stages are often
replaced by a single centrifugal unit. Very small flow compressors often employ
two centrifugal compressors, connected in series. Although in isolation
centrifugal compressors are capable of running at quite high pressure ratios
(e.g. 10:1), impeller stress considerations (i.e. T3, NH implications) limit the
pressure ratio that can be employed in high overall pressure ratio engine
cycles.
Increasing overall pressure ratio implies raising the high pressure compressor
exit temperature (i.e. T3). This implies a higher high pressure shaft speed, to
maintain the datum blade tip Mach number on the rear compressor stage. Stress
considerations, however, may limit the shaft speed increase, causing the
original compressor to throttle-back aerodynamically to a lower pressure ratio
than datum.
Combustion chamber GE J79
Combustors
Great care must be taken to keep the flame burning in a moderately fast moving
airstream, at all throttle conditions, as efficiently as possible. Since the
turbine cannot withstand stoichiometric temperatures, resulting from the optimum
combustion process, some of the compressor air is used to quench the exit
temperature of the combustor to an acceptable level. Air used for combustion is
considered to be primary airflow, while excess air used for cooling is called
secondary airflow. Combustor configurations include can, annular, and
can-annular.
Turbines
Turbine Stage GE J79Because a turbine expands from high to low pressure, there
is no such thing as turbine surge or stall. The turbine needs fewer stages than
the compressor, mainly because the higher inlet temperature reduces the deltaT/T
(and thereby the pressure ratio) of the expansion process. The blades have more
curvature and the gas stream velocities are higher.
Designers must, however, prevent the turbine blades and vanes from melting in a
very high temperature and stress environment. Consequently bleed air extracted
from the compression system is often used to cool the turbine blades/vanes
internally. Other solutions are improved materials and/or special insulating
coatings. The discs must be specially shaped to withstand the huge stresses
imposed by the rotating blades. They take the form of impulse, reaction, or
combination impulse-reaction shapes. Improved materials help to keep disc weight
down.
Turbopumps
M Turbopump
Turbopumps are centrifugal pumps which are spun by gas turbines and are used to
raise the propellant pressure above the pressure in the combustion chamber so
that it can be injected and burnt. Turbopumps are very commonly used with
rockets, but ramjets and turbojets also have been known to use them.
Afterburners (reheat)
M afterburner
Due to temperature limitations with the gas turbines, jet engines do not consume
all the oxygen in the air ('run stochiometric'). Afterburners burn the remaining
oxygen after exiting the turbines, but usually do so inefficiently due to the
low pressures existing at this part of the jet engine; however this gains
thrust, which can be useful.
Nozzles
Afterburner GE J79The primary object of a nozzle is to expand the exhaust stream
to atmospheric pressure, thereby producing a high velocity jet, relative to the
vehicle. If the fully expanded jet has a higher impulse than the moving
aircraft, there will be a forward thrust on the airframe.
Simple convergent nozzles are used on many jet engines. If the nozzle pressure
ratio is above the critical value (about 1.8:1) a convergent nozzle will choke,
resulting in some of the expansion to atmospheric pressure taking place
downstream of the throat (i.e. smallest flow area), in the jet wake. Although
much of the gross thrust produced will still be from the jet momentum,
additional (pressure) thrust will come from the imbalance between the throat
static pressure and atmospheric pressure.
Many military combat engines incorporate an afterburner (or reheat) in the
engine exhaust system. When the system is lit, the nozzle throat area must be
increased, to accommodate the extra exhaust volume flow, so that the
turbomachinery is unaware that the afterburner is lit. A variable throat area is
achieved by moving a series of overlapping petals, which approximate the
circular nozzle cross-section.
At high nozzle pressure ratios, the exit pressure is often above ambient and
much of the expansion will take place downstream of a convergent nozzle, which
is inefficient. Consequently, some jet engines (notably rockets) incorporate a
convergent-divergent nozzle, to allow most of the expansion to take place
against the inside of a nozzle to maximise thrust. However, unlike the fixed
con-di nozzle used on a conventional rocket motor, when such a device is used on
a turbojet engine it has to be a complex variable geometry device, to cope with
the wide variation in nozzle pressure ratio encountered in flight and engine
throttling. This further increases the weight and cost of such an installation.
Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a
Boeing F-18The simpler of the two is the ejector nozzle, which creates an
effective nozzle through a secondary airflow and spring-loaded petals. At
subsonic speeds, the airflow constricts the exhaust to a convergent shape. As
the aircraft speeds up, the two nozzles dilate, which allows the exhaust to form
a convergent-divergent shape, speeding the exhaust gasses past Mach 1. More
complex engines can actually use a tertiary airflow to reduce exit area at very
low speeds. Advantages of the ejector nozzle are relative simplicity and
reliability. Disadvantages are average performance (compared to the other nozzle
type) and relatively high drag due to the secondary airflow. Notable aircraft to
have utilized this type of nozzle include the SR-71, Concorde, F-111, and Saab
Viggen
For higher performance, it is necessary to use an iris nozzle. This type uses
overlapping, hydraulically adjustable "petals". Although more complex than the
ejector nozzle, it has significantly higher performance and smoother airflow. As
such, it is employed primarily on high-performance fighters such as the F-14,
F-15, F-16, though is also used in high-speed bombers such as the B-1B. Some
modern iris nozzle additionally have the ability to change the angle of the
thrust (see thrust vectoring).
Iris vectored thrust nozzleRocket motors also employ convergent-divergent
nozzles, but these are usually of fixed geometry, to minimize weight. Because of
the much higher nozzle pressure ratios experienced, rocket motor con-di nozzles
have a much greater area ratio (exit/throat) than those fitted to jet engines.
At the other extreme, some high bypass ratio civil turbofans use an extremely
low area ratio (less than 1.01 area ratio), convergent-divergent, nozzle on the
bypass (or mixed exhaust) stream, to control the fan working line. The nozzle
acts as if it has variable geometry. At low flight speeds the nozzle is unchoked
(less than a Mach number of unity), so the exhaust gas speeds up as it
approaches the throat and then slows down slightly as it reaches the divergent
section. Consequently, the nozzle exit area controls the fan match and, being
larger than the throat, pulls the fan working line slightly away from surge. At
higher flight speeds, the ram rise in the intake increases nozzle pressure ratio
to the point where the throat becomes choked (M=1.0). Under these circumstances,
the throat area dictates the fan match and being smaller than the exit pushes
the fan working line slightly towards surge. This is not a problem, since fan
surge margin is much better at high flight speeds.
Thrust reversers
M Thrust reversal
These either consist of cups that swing across the end of the nozzle and deflect
the jet thrust forwards(as in the DC-9), or they are two panels behind the
cowling that slide backward and reverse only the fan thrust (the fan produces
the majority of the thrust.)This is the case on many large aircraft such as the
747,C-17,KC-135,etc.
Cooling systems
All jet engines require high temperature gas for good efficiency, typically
achieved by combusting hydrocarbon or hydrogen fuel. Combustion temperatures can
be as high as 3500K (5841F), above the melting point of most materials.
Cooling systems are employed to keep the temperature of the solid parts below
the failure temperature.
Air systems
A complex air system is built into most turbine based jet engines, primarily to
cool the turbine blades, vanes and discs.
Air, bled from the compressor exit, passes around combustor and is injected into
the rim of the rotating turbine disc. The cooling air then passes through
complex passages within the turbine blades. After removing heat from the blade
material, the air (now fairly hot) is vented, via cooling holes, into the main
gas stream. Cooling air for the turbine vanes undergoes a similar process.
Cooling the leading edge of the blade can be difficult, because the pressure of
the cooling air just inside the cooling hole may not be much different from that
of the oncoming gas stream. One solution is to incorporate a cover plate on the
disc. This acts as a centrifugal compressor to pressurize the cooling air before
it enters the blade. Another solution is to use an ultra-efficient turbine rim
seal to pressurize the area where the cooling air passes across to the rotating
disc.
Seals are used to prevent oil leakage, control air for cooling and prevent stray
air flows into turbine cavities.
A series of (e.g. labyrinth) seals allow a small flow of bleed air to wash the
turbine disc to extract heat and, at the same time, pressurize the turbine rim
seal, to prevent hot gases entering the inner part of the engine. Other types of
seals are hydraulic, brush, carbon etc.
Small quantities of compressor bleed air are also used to cool the shaft,
turbine shrouds, etc. Some air is also used to keep the temperature of the
combustion chamber walls below critical. This is done using primary and
secondary airholes which allow a thin layer of air to cover the inner walls of
the chamber preventing excessive heating.
Exit temperature is dependent on the turbine upper temperature limit depending
on the material. Reducing the temperature will also prevent thermal fatigue and
hence failure. Accessories may also need their own cooling systems using air
from the compressor or outside air.
Air from compressor stages is also used for heating of the fan, airframe
anti-icing and for cabin heat. Which stage is bled from depends on the
atmospheric conditions at that altitude.
Rocket engines
M Rocket engine
Rocket engines have extreme cooling requirements, due to the simultaneous
combination of both high pressures (typically 20-200 bar) and high temperatures
(typically ~3500 K) found in the combustion chamber.
Rocket engines often use liquid coolant, typically the propellant is passed
around the hot parts of the engine (regenerative cooling); but other techniques
such as radiative cooling or dump cooling can be used.
In addition, the chamber is normally designed so that the injectors provide for
cooler gas at the circumference (curtain cooling) or cool liquid: film cooling
however these techniques reduce performance somewhat due to incompletely burnt
propellant being ejected, but are nevertheless used by many engines as it
permits higher chamber pressures and thus a more efficient nozzle can be
employed.
Fuel system
Apart from providing fuel to the engine, the fuel system is also used to control
propeller speeds, compressor airflow and cool lubrication oil. Fuel is usually
introduced by an atomized spray, the amount of which is controlled automatically
depending on the rate of airflow.
So the sequence of events for increasing thrust is, the throttle opens and fuel
spray pressure is increased, increasing the amount of fuel being burned. This
means that exhaust gases are hotter and so are ejected at higher acceleration,
which means they exert higher forces and therefore increase the engine thrust
directly. It also increases the energy extracted by the turbine which drives the
compressor even faster and so there is an increase in air flowing into the
engine as well.
Obviously, it is the rate of the mass of the airflow that matters since it is
the change in momentum (mass x velocity) that produces the force. However,
density varies with altitude and hence inflow of mass will also vary with
altitude, temperature etc. which means that throttle values will vary according
to all these parameters without changing them manually.
This is why fuel flow is controlled automatically. Usually there are 2 systems,
one to control the pressure and the other to control the flow. The inputs are
usually from pressure and temperature probes from the intake and at various
points through the engine. Also throttle inputs, engine speed etc. are required.
These affect the high pressure fuel pump.
Fuel control unit (FCU)
This element is something like a mechanical computer. It determines the output
of the fuel pump by a system of valves which can change the pressure used to
cause the pump stroke, thereby varying the amount of flow.
Take the possibility of increased altitude where there will be reduced air
intake pressure. In this case, the chamber within the FCU will expand which
causes the spill valve to bleed more fuel. This causes the pump to deliver less
fuel until the opposing chamber pressure is equivalent to the air pressure and
the spill valve goes back to its position.
When the throttle is opened, it releases i.e. lessens the pressure which lets
the throttle valve fall. The pressure is transmitted (because of a back-pressure
valve i.e. no air gaps in fuel flow) which closes the FCU spill valves (as they
are commonly called) which then increases the pressure and causes a higher flow
rate.
The engine speed governor is used to prevent the engine from over-speeding. It
has the capability of disregarding the FCU control. It does this by use of a
diaphragm which senses the engine speed in terms of the centrifugal pressure
caused by the rotating rotor of the pump. At a critical value, this diaphragm
causes another spill valve to open and bleed away the fuel flow.
There are other ways of controlling fuel flow for example with the dash-pot
throttle lever. The throttle has a gear which meshes with the control valve
(like a rack and pinion) causing it to slide along a cylinder which has ports at
various positions. Moving the throttle and hence sliding the valve along the
cylinder, opens and closes these ports as designed. There are actually 2 valves
viz. the throttle and the control valve. The control valve is used to control
pressure on one side of the throttle valve such that it gives the right
opposition to the throttle control pressure. It does this by controlling the
fuel outlet from within the cylinder.
So for example, if the throttle valve is moved up to let more fuel in, it will
mean that the throttle valve has moved into a position which allows more fuel to
flow through and on the other side, the required pressure ports are opened to
keep the pressure balance so that the throttle lever stays where it is.
At initial acceleration, more fuel is required and the unit is adapted to allow
more fuel to flow by opening other ports at a particular throttle position.
Changes in pressure of outside air i.e. altitude, speed of aircraft etc are
sensed by an air capsule.
Fuel pump
Fuel pumps are used to raise the fuel pressure above the pressure in the
combustion chamber so that the fuel can be injected. Fuel pumps are usually
driven by the main shaft, via gearing.
Turbopumps are very commonly used with liquid-fuelled rockets and rely on the
expansion of an onboard gas through a turbine.
Ramjet turbopumps use ram air expanding through a turbine.
Engine starting system
The fuel system as explained above, is one of the 2 systems required for
starting the engine. The other is the actual ignition of the air/fuel mixture in
the chamber. Usually, an auxiliary power unit is used to start the engines. It
has a starter motor which has a high torque transmitted to the compressor unit.
When the optimum speed is reached, i.e. the flow of gas through the turbine is
sufficient, the turbines take over. There are a number of different starting
methods such as electric, hydraulic, pneumatic etc.
The electric starter works with gears and clutch plate linking the motor and the
engine. The clutch is used to disengage when optimum speed is achieved. This is
usually done automatically. The electric supply is used to start the motor as
well as for ignition. The voltage is usually built up slowly as starter gains
speed.
Some military aircraft need to be started quicker than the electric method
permits and hence they use other methods such as a turbine starter. This is an
impulse turbine impacted by burning gases from a cartridge. It is geared to
rotate the engine and also connected to an automatic disconnect system. The
cartridge is set alight electrically and used to turn the turbine.
Another turbine starter system is almost exactly like a little engine. Again the
turbine is connected to the engine via gears. However, the turbine is turned by
burning gases - usually the fuel is isopropyl nitrate stored in a tank and
sprayed into a combustion chamber. Again, it is ignited with a spark plug.
Everything is electrically controlled, such as speed etc.
Most Commercial aircraft and large Military Transport airplanes usually use what
is called an auxiliary power unit or APU. It is normally a small gas turbine.
Thus, one could say that using such an APU is using a small jet engine to start
a larger one. High pressure air from the compressor section of the APU is bled
off through a system of pipes to the engines where it is directed into the
starting system. This "bleed air" is directed into a mechanism to start the
engine turning and begin pulling in air. When the rotating speed of the engine
is sufficient to pull in enough air to support combustion, fuel is introduced
and ignited. Once the engine ignites and reaches idle speed, the bleed air is
shut off.
The APUs on aircraft such as the Boeing 737 and Airbus A320 can be seen at the
extreme rear of the aircraft. This is the typical location for an APU on most
commercial airliners although some may be within the wing root (Boeing 727) or
the aft fuselage (DC-9/MD80) as examples and some military transports carry
their APU's in one of the main landing gear pods (C-141).
The APUs also provide enough power to keep the cabin lights, pressure and other
systems on while the engines are off. The valves used to control the airflow are
usually electrically controlled. They automatically close at a pre-determined
speed. As part of the starting sequence on some engines fuel is combined with
the supplied air and burned instead of using just air. This usually produces
more power per unit weight.
Usually an APU is started by its own electric starter motor which is switched
off at the proper speed automatically. When the main engine starts up and
reaches the right conditions, this auxiliary unit is then switched off and
disengages slowly.
Hydraulic pumps can also be used to start some engines through gears. The pumps
are electrically controlled on the ground.
A variation of this is the APU installed in a Boeing F-18; it is started by a
hydraulic motor, which itself receives energy stored in an accumulator. This
accumulator is recharged after the right engine is started and develops
hydraulic pressure, or by a hand pump in the right hand main landing gear well.
Ignition
Usually there are 2 igniter plugs in different positions in the combustion
system. A high voltage spark is used to ignite the gases. The voltage is stored
up from a low voltage supply provided by the starter system. It builds up to the
right value and is then released as a high energy spark. Depending on various
conditions, the igniter continues to provide sparks to prevent combustion from
failing if the flame inside goes out. Of course, in the event that the flame
does go out, there must be provision to relight. There is a limit of altitude
and air speed at which an engine can obtain a satisfactory relight.
For example, the General Electric F404-400 uses one ignitor for the combustor
and one for the afterburner; the ignition system for the A/B incorporates an
ultraviolet flame sensor to activate the ignitor.
It should be noted that most modern ignition systems provide enough energy to be
a lethal hazard should a person be in contact with the electrical lead when the
system is activated, so team communication is vital when working on these
systems.
Lubrication system
A lubrication system serves to ensure lubrication of the bearings and to
maintain sufficiently cool temperatures, mostly by eliminating friction.
The lubrication system as a whole should be able to prevent foreign material
from entering the plane, and reaching the bearings, gears, and other moving
parts. The lubricant must be able to flow easily at relatively low temperatures
and not disintegrate or break down at very high temperatures.
Usually the lubrication system has subsystems that deal individually with the
pressure of an engine, scavenging, and a breather.
The pressure system components are an oil tank and de-aerator, main oil pump,
main oil filter/filter bypass valve, pressure regulating valve (PRV), oil
cooler/by pass valve and tubing/jets.
Usually the flow is from the tank to the pump inlet and PRV, pumped to main oil
filter or its bypass valve and oil cooler, then through some more filters to
jets in the bearings.
Using the PRV method of control, means that the pressure of the feed oil must be
below a critical value (usually controlled by other valves which can leak out
excess oil back to tank if it exceeds the critical value). The valve opens at a
certain pressure and oil is kept moving at a constant rate into the bearing
chamber.
If the engine speed increases, the pressure within the bearing chamber also
increases, which means the pressure difference between the lubricant feed and
the chamber reduces which could reduce slow rate of oil when it is needed even
more. As a result, some PRVs can adjust their spring force values using this
pressure change in the bearing chamber proportionally to keep the lubricant flow
constant.
Advanced designs
J-58 combined ramjet/turbojet
The SR-71's Pratt & Whitney J58 engines were rather unusual. They could convert
in flight from being largely a turbojet to being largely a compressor-assisted
ramjet. At high speeds (above Mach 2.4), the engine used variable geometry vanes
to direct excess air through 6 bypass pipes from downstream of the fourth
compressor stage into the afterburner. 80% of the SR-71's thrust at high speed
was generated in this way, giving much higher thrust, improving specific impulse
by 10-15%, and permitting continuous operation at Mach 3.2. The name coined for
this configuration is turbo-ramjet.
Pre-cooled turbojets
An idea originated by Robert P. Carmichael in 1955 is that hydrogen fuelled
engines could theoretically have much higher performance than hydrocarbon
fuelled engines if a heat exchanger were used to cool the incoming air. The low
temperature allows lighter materials to be used, a higher mass-flow through the
engines, and provides lower temperatures which permits combustors to inject more
fuel without overheating the engine.
This idea leads to plausible designs like SABRE, that might permit
single-stage-to-orbit, and ATREX, that might permit jet engines to be used up to
hypersonic speeds and high altitudes for boosters for launch vehicles.
Nuclear-powered ramjet
Project Pluto was a nuclear-powered ramjet, intended for use in a cruise
missile. Rather than combusting fuel as in regular jet engines, air was heated
using a high-temperature, unshielded nuclear reactor. This raised the specific
impulse of the engine by stupendous amounts, and the ramjet was predicted to be
able to fly for months at supersonic speeds (Mach 3 at tree-top height).
However, there was no obvious way to stop it once it had taken off, which is a
great disadvantage. Unfortunately, because the reactor was unshielded, it was
dangerous to be in or around the flight path of the vehicle (although the
exhaust itself wasn't radioactive).
Scramjets
M Scramjet
Scramjets are an evolution of the ramjet that are able to operate at much higher
speeds than ramjets (or any other kind of airbreathing engine) are capable of
reaching. They share a similar structure with ramjets, being a specially-shaped
tube that compresses air with no moving parts through ram-air compression.
Scramjets, however, operate with supersonic airflow through the entire engine.
Thus, scramjets do not have the diffuser required by ramjets to slow the
incoming airflow to subsonic speeds.
Scramjets start working at speeds of at least Mach 4, and have a maximum useful
speed of approximately Mach 17 . Due to aerodynamic heating at these high
speeds, cooling poses a challenge to engineers.
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Trik.com continues IJFG.com's
success, but Trik.com has more to offer. Trik Topsite can be found at
Trik Topsite; the TopSite is a great addition if you want to find the best
MMO RPG site(s) or raise your site in the rankings. Trik.com also has a
viciously competitive Arcade. If you want to be the #1 Arcade on Trik, then come
prove yourself at Trik.com arcade:
Trik arcade. Trik.com ?Trik.com/topsite ?Trik.com/forum/arcade.php
With the rising popularity of
commercial MMORPG games came the desire from ardent players of these games to
run their own servers beside the ones run by the game's creator. Since the
original server software is not usually available, the behavior of the server
has to be re-engineered. This can be done by analyzing the data stream with the
original server, or by disassembling and analyzing the client which is
available.
Ultima Online was one of the first
large MMORPGs. Due to its openness in implementation, server emulators arose
very quickly, even during the beta stage of development. The destination to
which the client connects was changeable by simply editing a text file. In beta
stage the client-server data stream was not encrypted yet. The term server
emulator became known through Ultima Online server reimplementation such as UOX,
which was the pioneer. Many forks and reimplementations followed UOX, because
its source code was released under the GNU General Public License relatively
early. RunUO is today the most widely used UO-server emulator. After RuneScape
implemented anti-cheating measures, many gamers left and started their own
private servers. The best place to discuss the private server is at
Trik- The Master of Private Server.
Another useful site is
Rune
Web ruwb.com . This site is about more serious RuneScape gold trading,
account exchange, gold for real life cash and many services. It includes tips on
how to avoid getting lured/scammed while using the marketplace. For programming,
visual basics, java, C/C++, scar and all other languages such as PHP, HTML, ASP,
Delphi. There are also sections for graphics talents, plus many cool videos and
fun stuff.
A defining moment in internet
gaming history was when a group of gamers called (hygo 7) decided to start an
ultimate game forum, which they named
hygo.com. It has the best financial backing, the friendliest game community,
and the highest quality of information. Currently Hygo.com has entered a new
phase...Hygo.com is offering the best private server game. With thousands of
members, Hygo.com is your next place to visit, as they have an amazing game with
a community and economy.
Hygo.com - The Online Adventure Game. is definitely one of the top sites you
want to join right now!
Ezud.com is now the powerhouse of
Runescape bugs and glitches. All and any rs2 bugs that anyone could ever
want are now found at the
Ezud forum. From a range of infinite running in runescape, to rs item
duping, ezud truly is an amazing glitching site.
Ezud has an excellent administration, and a great
moderating team. When everyone strives to make ezud.com a better place….it
becomes just that: a better place. Everyone contributes, and helps
Ezud strive.
So come on down to the new type of runescape 2 cheating:
runescape bugging. This is Ezud…this is
RuneScape 2 Bug Abuse.
Contact Information
Call our office today to set up an appointment. Learn more about how we can
help you, and learn more about the other services that we can offer you. All
messages we receive will be answered as soon as possible. We look forward to
hearing from you.
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