The rotating propeller pulls the aircraft forward. But a jet engine throws hot exhaust gases backward at high speed and thereby creates a jet thrust force directed forward.
Types of jet engines
There are four types of jet or gas turbine engines:
Turbojet;
Turbofans- such as those used on Boeing-747 passenger liners;
Turboprop where they use propellers driven by turbines;
And Turboshaft that put on helicopters.
turbofan engine consists of three main parts: a compressor, a combustion chamber and a turbine that provides energy. First, air enters the engine and is compressed by a fan. Then, in the combustion chamber, the compressed air mixes with the fuel and burns to form a gas at high temperature and high pressure. This gas passes through the turbine, causing it to rotate at a tremendous speed, and is thrown back, thus creating a forward thrust force.
Image is clickable
Once in turbine engine The air goes through several stages of compression. The pressure and volume of the gas increase especially strongly after passing through the combustion chamber. The thrust generated by the exhaust gases allows jet aircraft to travel at altitudes and speeds far in excess of those available to piston-engined rotorcraft.
IN turbojet engine air is taken in from the front, compressed and burned along with the fuel. resulting from combustion traffic fumes generate reactive thrust.
Turboprops connect jet thrust exhaust gases with forward thrust generated by the rotation of the propeller.
ABSTRACT
ON THIS TOPIC:
Jet Engines .
WRITTEN: Kiselev A.V.
KALININGRAD
Introduction
Jet engine, an engine that creates the traction force necessary for movement by converting the initial energy into the kinetic energy of the jet stream of the working fluid; as a result of the expiration of the working fluid from the nozzle of the engine, a reactive force is formed in the form of a reaction (recoil) of the jet, which moves the engine and the apparatus structurally associated with it in the direction opposite to the outflow of the jet. Various types of energy (chemical, nuclear, electrical, solar) can be converted into the kinetic (speed) energy of a jet stream in a rocket engine. A direct reaction engine (direct reaction engine) combines the engine itself with a mover, that is, it provides its own movement without the participation of intermediate mechanisms.
For creating jet thrust used by R. d., you need:
the source of the initial (primary) energy, which is converted into the kinetic energy of the jet;
working body, which in the form of a jet stream is ejected from the R. d .;
R. D. himself is an energy converter.
The initial energy is stored on board an aircraft or other apparatus equipped with RD (chemical fuel, nuclear fuel), or (in principle) it can come from outside (solar energy). To obtain a working fluid in R. d., a substance taken from the environment (for example, air or water) can be used;
the substance which is in tanks of the device or directly in the R.'s chamber of d.; a mixture of substances coming from the environment and stored on board the apparatus.
In modern R. d., chemical is most often used as the primary
Missile firing tests
engine Space Shuttle
Turbojet engines AL-31F aircraft Su-30MK. belong to the class jet engines
energy. In this case, the working fluid is incandescent gases - combustion products of chemical fuel. During the operation of a rocket engine, the chemical energy of the burning substances is converted into the thermal energy of the combustion products, and the thermal energy of the hot gases is converted into the mechanical energy of the translational motion of the jet stream and, consequently, the apparatus on which the engine is installed. The main part of any R. d. is the combustion chamber in which the working fluid is generated. The end part of the chamber, which serves to accelerate the working fluid and obtain a jet stream, is called a jet nozzle.
Depending on whether the environment is used or not during the operation of R. D., they are divided into 2 main classes - air- jet engines(WFD) and rocket engines (RD). All WFDs are heat engines, the working fluid of which is formed by the oxidation reaction of a combustible substance with atmospheric oxygen. The air coming from the atmosphere makes up the bulk of the working fluid of the WFD. Thus, an apparatus with a WFD carries a source of energy (fuel) on board, and draws most of the working fluid from the environment. Unlike the WFD, all components of the working fluid of the RD are on board the apparatus equipped with the RD. The absence of a propeller interacting with environment, and the presence of all components of the working fluid on board the apparatus make the RD the only one suitable for work in space. There are also combined rocket engines, which are, as it were, a combination of both main types.
History of jet engines
The principle of jet propulsion has been known for a very long time. Heron's ball can be considered the ancestor of R. d. Solid rocket engines - powder rockets appeared in China in the 10th century. n. e. For hundreds of years, such missiles were used first in the East, and then in Europe as fireworks, signal, combat. In 1903, K. E. Tsiolkovsky, in his work "Investigation of World Spaces with Reactive Devices", was the first in the world to put forward the main provisions of the theory of liquid-propellant rocket engines and proposed the main elements of a liquid-propellant rocket engine. The first Soviet liquid rocket engines - ORM, ORM-1, ORM-2 were designed by V. P. Glushko and created under his leadership in 1930-31 at the Gas Dynamics Laboratory (GDL). In 1926, R. Goddard launched a rocket using liquid fuel. For the first time, an electrothermal RD was created and tested by Glushko at the GDL in 1929-33.
In 1939, missiles with ramjet engines designed by I. A. Merkulov were tested in the USSR. The first diagram of a turbojet engine? was proposed by the Russian engineer N. Gerasimov in 1909.
In 1939, the construction of turbojet engines designed by A. M. Lyulka began at the Kirov Plant in Leningrad. The tests of the created engine were prevented by the Great Patriotic War of 1941-45. In 1941, a turbojet engine designed by F. Whittle (Great Britain) was first installed on an aircraft and tested. The theoretical works of the Russian scientists S. S. Nezhdanovsky, I. V. Meshchersky, and N. E. Zhukovsky, the works of the French scientist R. Enot-Peltri, and the German scientist G. Oberth were of great importance for the creation of R. D.. An important contribution to the creation of the VRD was the work of the Soviet scientist B. S. Stechkin "Theory of an air-breathing engine", published in 1929.
R. d. have a different purpose and the scope of their application is constantly expanding.
R. d. are most widely used on various types of aircraft.
Turbojet engines and dual-circuit turbojet engines are equipped with most military and civil aircraft around the world, they are used in helicopters. These rocket engines are suitable for flights at both subsonic and supersonic speeds; they are also installed on projectile aircraft; supersonic turbojet engines can be used in the first stages of aerospace aircraft. Ramjet engines are installed on anti-aircraft guided missiles, cruise missiles, supersonic fighter-interceptors. Subsonic ramjet engines are used in helicopters (installed at the ends of the main rotor blades). Pulsating jet engines have little thrust and are intended only for aircraft at subsonic speeds. During the 2nd World War of 1939-45, these engines were equipped with V-1 projectiles.
RD in most cases are used on high-speed aircraft.
Liquid-propellant rocket engines are used on launch vehicles of spacecraft and spacecraft as marching, braking and control engines, as well as on guided ballistic missiles. Solid-propellant rocket engines are used in ballistic, anti-aircraft, anti-tank, and other military missiles, as well as on launch vehicles and spacecraft. Small solid propellant engines are used as boosters for aircraft takeoff. Electric rocket engines and nuclear rocket engines can be used in spacecraft.
However, this mighty trunk, the principle of direct reaction, gave life to a huge crown of the "family tree" of the family of jet engines. To get acquainted with the main branches of its crown, crowning the "trunk" of the direct reaction. Soon, as can be seen from the figure (see below), this trunk is divided into two parts, as if split by a lightning strike. Both new trunks are equally decorated with mighty crowns. This division occurred due to the fact that all "chemical" jet engines are divided into two classes, depending on whether they use ambient air for their work or not.
One of the newly formed trunks is the class of air-breathing engines (VRD). As the name suggests, they cannot operate outside of the atmosphere. That's why these engines are the backbone of modern aviation, both manned and unmanned. WFDs use atmospheric oxygen to burn fuel; without it, the combustion reaction in the engine will not proceed. But still, turbojet engines are currently the most widely used.
(TRD), installed on almost all modern aircraft without exception. Like all engines that use atmospheric air, turbojet engines need a special device to compress the air before it enters the combustion chamber. After all, if the pressure in the combustion chamber does not significantly exceed atmospheric pressure, then the gases will not flow out of the engine with more speed It's the pressure that pushes them out. But at a low exhaust velocity, the thrust of the engine will be small, and the engine will consume a lot of fuel, such an engine will not find application. In a turbojet engine, a compressor is used to compress the air, and the design of the engine largely depends on the type of compressor. There are engines with axial and centrifugal compressors, axial compressors can have fewer or more compression stages thanks to using our system, be one-two-stage, etc. To drive the compressor, the turbojet engine has a gas turbine, which gave the name to the engine. Due to the compressor and turbine, the design of the engine is very complex.
Air-jet engines without compressors are much simpler in design, in which the necessary pressure increase is carried out in other ways, which have names: pulsating and ramjet engines.
In a pulsating engine, this is usually done by a valve grill installed at the engine inlet, when a new portion of the fuel-air mixture fills the combustion chamber and a flash occurs in it, the valves close, isolating the combustion chamber from the engine inlet. As a result, the pressure in the chamber rises, and the gases rush out through the jet nozzle, after which the whole process is repeated.
In a compressorless engine of another type, a ramjet, there is not even this valve grid and the pressure in the combustion chamber rises as a result of dynamic pressure, i.e. deceleration of the oncoming air flow entering the engine in flight. It is clear that such an engine can only work when aircraft already flies at a sufficiently high speed, it will not develop thrust in the parking lot. But at a very high speed, 4-5 times the speed of sound, the ramjet develops very high thrust and consumes less fuel than any other "chemical" jet engine under these conditions. That's why ramjet motors.
The peculiarity of the aerodynamic scheme of supersonic aircraft with ramjet engines (ramjet engines) is due to the presence of special accelerating engines that provide the speed necessary to start stable operation of the ramjet. This makes the tail part of the structure heavier and requires the installation of stabilizers to ensure the necessary stability.
The principle of operation of a jet engine.
At the heart of modern powerful jet engines of various types is the principle of direct reaction, i.e. the principle of creating a driving force (or thrust) in the form of a reaction (recoil) of a jet of "working substance" flowing out of the engine, usually hot gases.
In all engines, there are two processes of energy conversion. First, the chemical energy of the fuel is converted into thermal energy of the combustion products, and then the thermal energy is used to perform mechanical work. These engines include piston engines cars, diesel locomotives, steam and gas turbines of power plants, etc.
Consider this process in relation to jet engines. Let's start with the combustion chamber of the engine, in which a combustible mixture has already been created in one way or another, depending on the type of engine and the type of fuel. This can be, for example, a mixture of air and kerosene, as in a turbojet engine of a modern jet aircraft, or a mixture of liquid oxygen and alcohol, as in some liquid rocket engines, or, finally, some kind of solid propellant for powder rockets. The combustible mixture can burn, i.e. enter into a chemical reaction with a rapid release of energy in the form of heat. The ability to release energy during a chemical reaction is the potential chemical energy of the molecules of the mixture. The chemical energy of molecules is related to the features of their structure, more precisely, the structure of their electron shells, i.e. the electron cloud that surrounds the nuclei of the atoms that make up the molecule. As a result of a chemical reaction, in which some molecules are destroyed, while others are formed, a rearrangement of the electron shells naturally occurs. In this restructuring, it is the source of released chemical energy. It can be seen that only substances that, during a chemical reaction in the engine (combustion), emit a sufficiently large amount of heat, and also form a large amount of gases, can serve as fuels for jet engines. All these processes take place in the combustion chamber, but let's dwell on the reaction not at the molecular level (this has already been discussed above), but at the "phases" of work. Until combustion has begun, the mixture has a large supply of potential chemical energy. But then the flame engulfed the mixture, another moment - and the chemical reaction is over. Now instead of molecules combustible mixture the chamber is filled with more densely "packed" molecules of combustion products. The excess binding energy, which is the chemical energy of the combustion reaction that has taken place, has been released. Molecules possessing this excess energy almost instantly transferred it to other molecules and atoms as a result of frequent collisions with them. All molecules and atoms in the combustion chamber began to randomly, chaotically move at a much higher speed, the temperature of the gases increased. So there was a transition of the potential chemical energy of the fuel into the thermal energy of the combustion products.
A similar transition was carried out in all other heat engines, but jet engines fundamentally differ from them in relation to the further fate of hot combustion products.
After hot gases have formed in the heat engine, containing large thermal energy, this energy must be converted into mechanical energy. After all, engines serve to make mechanical work, to “move” something, to put it into action, it doesn’t matter whether it is a dynamo, please add drawings of a power plant, a diesel locomotive, a car or an airplane.
In order for the thermal energy of gases to be converted into mechanical energy, their volume must increase. With such an expansion, the gases do the work for which their internal and thermal energy is expended.
In the case of a piston engine, expanding gases press on a piston moving inside the cylinder, the piston pushes the connecting rod, which already rotates the crankshaft of the engine. The shaft is connected to the rotor of a dynamo, the driving axles of a diesel locomotive or car, or the propeller of an aircraft - the engine performs useful work. IN steam engine, or a gas turbine, gases, expanding, force the wheel connected to the turbine shaft to rotate - there is no need for a transmission crank mechanism, which is one of the great advantages of the turbine
Gases expand, of course, in a jet engine, because without it they do not do work. But the expansion work in that case is not spent on the rotation of the shaft. Associated with the drive mechanism, as in other heat engines. The purpose of a jet engine is different - to create jet thrust, and for this it is necessary that a jet of gases - combustion products flow out of the engine at a high speed: the reaction force of this jet is the thrust of the engine. Consequently, the work of expanding the gaseous products of fuel combustion in the engine must be spent on accelerating the gases themselves. This means that the thermal energy of gases in a jet engine must be converted into their kinetic energy - the random chaotic thermal motion of molecules must be replaced by their organized flow in one direction common to all.
For this purpose, one of the most important parts of the engine, the so-called jet nozzle, serves. No matter what type a particular jet engine belongs to, it is necessarily equipped with a nozzle through which hot gases flow out of the engine at great speed - the products of fuel combustion in the engine. In some engines, gases enter the nozzle immediately after the combustion chamber, for example, in rocket or ramjet engines. In others, turbojets, the gases first pass through a turbine, to which they give up part of their thermal energy. It consumes in this case to drive the compressor, which serves to compress the air in front of the combustion chamber. But anyway, the nozzle is the last part of the engine - gases flow through it before leaving the engine.
The jet nozzle can have various shapes, and, moreover, a different design, depending on the type of engine. The main thing is the speed with which the gases flow out of the engine. If this outflow velocity does not exceed the speed with which sound waves propagate in the outflowing gases, then the nozzle is a simple cylindrical or narrowing pipe section. If the outflow velocity must exceed the speed of sound, then the nozzle is given the shape of an expanding pipe or, first, narrowing, and then expanding (Love's nozzle). Only in a tube of such a shape, as theory and experience show, is it possible to disperse the gas to supersonic speeds, to step over the "sonic barrier".
Jet engine diagram
The turbofan engine is the most widely used jet engine in civil aviation.
The fuel entering the engine (1) is mixed with compressed air and burned in the combustion chamber (2). The expanding gases rotate high-speed (3) and low-speed) turbines, which, in turn, drive the compressor (5), pushing air into the combustion chamber, and fans (6), driving air through this chamber and directing it to the exhaust pipe. By displacing air, fans provide additional thrust. An engine of this type is capable of developing thrust up to 13,600 kg.
Conclusion
The jet engine has many remarkable features, but the main one is as follows. A rocket does not need land, water, or air to move, as it moves as a result of interaction with gases formed during the combustion of fuel. Therefore, the rocket can move in airless space.
K. E. Tsiolkovsky is the founder of the theory of space flights. Scientific proof of the possibility of using a rocket for flights into outer space, beyond the earth's atmosphere and to other planets of the solar system was given for the first time by the Russian scientist and inventor Konstantin Eduardovich Tsiolkovsky
Bibliography
Encyclopedic Dictionary of the Young Technician.
Thermal Phenomena in Technology.
Materials from the site http://goldref.ru/;
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Jet motion is a process in which one of its parts is separated from a certain body at a certain speed. The force that arises in this case works by itself, without the slightest contact with external bodies. Jet propulsion was the impetus for the creation of a jet engine. The principle of its operation is based precisely on this force. How does such an engine work? Let's try to figure it out.
Historical facts
The idea of using jet thrust, which would make it possible to overcome the force of gravity of the Earth, was put forward in 1903 by the phenomenon of Russian science - Tsiolkovsky. He published a whole study on the subject, but it was not taken seriously. Konstantin Eduardovich, having survived the change in the political system, spent years of work to prove to everyone that he was right.
Today there are a lot of rumors that the revolutionary Kibalchich was the first in this matter. But the will of this man by the time of the publication of the works of Tsiolkovsky was buried along with Kibalchich. In addition, it was not a full-fledged work, but only sketches and sketches - the revolutionary could not bring a reliable basis for theoretical calculations in his works.
How does reactive force work?
To understand how a jet engine works, you need to understand how this force works.
So, imagine a shot from any firearm. This good example reactive force. A jet of hot gas, which was formed during the combustion of the charge in the cartridge, pushes the weapon back. The more powerful the charge, the stronger the return will be.
And now imagine the process of ignition of a combustible mixture: it takes place gradually and continuously. This is exactly what the principle of operation of a ramjet engine looks like. A rocket with a solid propellant jet engine works in a similar way - this is the simplest of its variations. Even novice rocket modellers are familiar with it.
As a fuel for jet engines, black powder was first used. Jet engines, the principle of which was already more advanced, required fuel with a base of nitrocellulose, which was dissolved in nitroglycerin. In large units that launch rockets that put shuttles into orbit, today they use a special mixture of polymer fuel with ammonium perchlorate as an oxidizing agent.
The principle of operation of the RD
Now it is worth understanding the principle of operation of a jet engine. To do this, consider the classic - liquid engines, which have hardly changed since the time of Tsiolkovsky. These units use fuel and an oxidizer.
As the latter, liquid oxygen or nitric acid is used. Kerosene is used as fuel. Modern cryogenic-type liquid engines consume liquid hydrogen. When oxidized with oxygen, it increases the specific impulse (by as much as 30 percent). The idea that hydrogen could be used was also born in Tsiolkovsky's head. However, at that time, due to the extreme explosiveness, it was necessary to look for another fuel.
The principle of operation is as follows. The components enter the combustion chamber from two separate tanks. After mixing, they turn into a mass, which, when burned, releases a huge amount of heat and tens of thousands of atmospheres of pressure. The oxidant is fed into the combustion chamber. fuel mixture as it passes between the double walls of the chamber and the nozzle, it cools these elements. Further, the fuel, heated by the walls, will enter the ignition zone through a huge number of nozzles. The jet, which is formed with a nozzle, breaks out. Due to this, a pushing moment is provided.
Briefly, the principle of operation of a jet engine can be compared with a blowtorch. However, the latter is much simpler. In the scheme of its work there are no different auxiliary systems engine. And these are compressors needed to create injection pressure, turbines, valves, as well as other elements, without which a jet engine is simply impossible.
Despite the fact that liquid engines consume a lot of fuel (fuel consumption is approximately 1000 grams per 200 kilograms of cargo), they are still used as marching units for launch vehicles and shunting units for orbital stations, as well as other spacecraft.
Device
A typical jet engine is arranged as follows. Its main nodes are:
Compressor;
combustion chamber;
Turbines;
Exhaust system.
Let's consider these elements in more detail. The compressor consists of several turbines. Their job is to suck in and compress air as it passes through the blades. The compression process increases the temperature and pressure of the air. Part of this compressed air fed into the combustion chamber. In it, air is mixed with fuel and ignition occurs. This process further increases the thermal energy.
The mixture exits the combustion chamber high speed and then expands. Then it follows another turbine, the blades of which rotate due to the action of gases. This turbine, connected to the compressor located in front of the unit, sets it in motion. Air heated to high temperatures, exits through exhaust system. The temperature, already high enough, continues to rise due to the throttling effect. Then the air comes out completely.
aircraft motor
Aircraft also use these engines. So, for example, turbojet units are installed in huge passenger liners. They differ from the usual ones in the presence of two tanks. One contains the fuel and the other the oxidizer. While a turbojet engine carries only fuel, air blown from the atmosphere is used as an oxidizer.
Turbojet engine
The principle of operation of an aircraft jet engine is based on the same reactive force and the same laws of physics. The most important part is the turbine blades. The final power depends on the size of the blade.
It is thanks to the turbines that the thrust that is needed to accelerate the aircraft is generated. Each of the blades is ten times more powerful than ordinary automotive internal combustion engine. Turbines are installed after the combustion chamber where the pressure is highest. And the temperature here can reach one and a half thousand degrees.
Double-circuit RD
These units have a lot of advantages over turbojet ones. For example, significantly lower fuel consumption with the same power.
But the engine itself has a more complex design and more weight.
Yes, and the principle of operation of a bypass jet engine is slightly different. The air captured by the turbine is partially compressed and supplied to the first circuit to the compressor and to the second circuit to the fixed blades. The turbine works as a compressor. low pressure. In the first circuit of the engine, the air is compressed and heated, and then through the compressor high pressure fed into the combustion chamber. This is where the fuel mixes and ignites. Gases are formed that are fed to the high-pressure turbine, due to which the turbine blades rotate, which, in turn, supply rotational motion to the high-pressure compressor. The gases then pass through a low pressure turbine. The latter drives the fan and, finally, the gases get outside, creating traction.
Synchronous taxiways
These are electric motors. The principle of operation of a synchronous reluctance motor is similar to the operation of a stepper unit. Alternating current applied to the stator and creates a magnetic field around the rotor. The latter rotates due to the fact that it tries to minimize the magnetic resistance. These motors have nothing to do with space exploration and shuttle launches.
A jet engine is a device that creates the traction force required for movement by converting the internal energy of the fuel into the kinetic energy of the jet stream of the working fluid.
Jet engine classes:
All jet engines are divided into 2 classes:
- Air-jet - heat engines, using the energy of the oxidation of air obtained from the atmosphere. In these engines, the working fluid is represented by a mixture of combustion products with the remaining elements of the bled air.
- Rocket - engines that contain everything on board necessary components and are able to work even in vacuum.
Direct flow jet engine- the simplest in the VRD class in terms of design. The pressure increase required for the operation of the device is formed by braking the oncoming air flow.
The ramjet workflow can be briefly described as follows:
- Air enters the engine inlet at flight speed, its kinetic energy is converted into internal energy, the air pressure and temperature increase. At the inlet to the combustion chamber and along the entire length of the flow path, the maximum pressure is observed.
- Compressed air is heated in the combustion chamber by oxidation of the supplied air, while the internal energy of the working fluid increases.
- Further, the flow narrows in the nozzle, the working fluid reaches sonic speed, and again, when expanding, it reaches supersonic speed. Due to the fact that the working fluid moves at a speed exceeding the speed of the oncoming flow, a jet thrust is created inside.
IN constructively ramjet is limiting simple device. The engine has a combustion chamber, inside which fuel comes from fuel injectors and air from the diffuser. The combustion chamber ends with an entrance to the nozzle, which is narrowing-expanding.
The development of mixed solid fuel technology has led to the use of this fuel in ramjet engines. In the combustion chamber there is a fuel block with a central longitudinal channel. Passing through the channel, the working fluid gradually oxidizes the surface of the fuel and heats up itself. The use of solid fuel further simplifies the engine design: fuel system becomes unnecessary.
Mixed fuel in its composition in a ramjet engine differs from that used in a solid propellant rocket engine. If in rocket engine Since most of the fuel composition is occupied by an oxidizer, in ramjet it is used in small proportions to activate the combustion process.
The ramjet mixed fuel filler mainly consists of a fine powder of beryllium, magnesium or aluminum. Their heat of oxidation significantly exceeds the heat of combustion of hydrocarbon fuel. As an example of a solid-propellant ramjet, one can cite the propulsion engine of the P-270 Moskit cruise anti-ship missile.
The ramjet thrust depends on the flight speed and is determined based on the influence of several factors:
- The higher the flight speed, the greater will be the air flow passing through the engine path, respectively, large quantity oxygen will penetrate into the combustion chamber, which increases fuel consumption, heat and mechanical power motor.
- The greater the air flow through the engine path, the higher the thrust generated by the motor. However, there is a certain limit, the air flow through the motor path cannot increase indefinitely.
- As the flight speed increases, the pressure level in the combustion chamber increases. As a result, the thermal efficiency of the engine is increased.
- How more difference between the speed of the flight of the apparatus and the speed of passage of the jet stream, the greater the thrust of the engine.
The dependence of the thrust of a ramjet engine on the flight speed can be represented as follows: until the flight speed is much lower than the speed of the passage of the jet, the thrust will increase along with the growth of the flight speed. When the flight speed approaches the speed of the jet stream, the thrust begins to fall, having passed a certain maximum, at which optimal speed flight.
Depending on the flight speed, the following categories of ramjet engines are distinguished:
- subsonic;
- supersonic;
- hypersonic.
Each group has its own distinctive features designs.
Subsonic ramjet
This group of engines is designed to provide flights at speeds from 0.5 to 1.0 Mach. Air compression and braking in such engines occurs in a diffuser - an expanding channel of the device at the flow inlet.
These engines are extremely low efficiency. When flying at a speed of M = 0.5, the level of pressure increase in them is 1.186, which is why the ideal thermal efficiency for them is only 4.76%, and if we also take into account losses in real engine, this value will approach zero. This means that when flying at speeds M<0,5 дозвуковой ПВРД неработоспособен.
But even at the limiting speed for the subsonic range at M=1, the level of pressure increase is 1.89, and the ideal thermal coefficient is only 16.7%. These indicators are 1.5 times less than those of reciprocating internal combustion engines, and 2 times less than those of gas turbine engines. Gas turbine and reciprocating engines are also efficient for use when operating in a stationary position. Therefore, ramjet subsonic engines, in comparison with other aircraft engines, turned out to be uncompetitive and are not currently mass-produced.
Supersonic ramjets
Supersonic ramjet engines are designed for flights in the speed range 1< M < 5.
The deceleration of a supersonic gas flow is always performed discontinuously, and a shock wave is formed, which is called a shock wave. At the distance of the shock wave, the process of gas compression is not isentropic. Consequently, losses of mechanical energy are observed, the level of pressure increase in it is smaller than in an isentropic process. The more powerful the shock wave, the more the flow velocity at the front will change, respectively, the greater the pressure loss, sometimes reaching 50%.
In order to minimize pressure losses, compression is organized not in one, but in several shock waves with a lower intensity. After each of these jumps, there is a decrease in the flow velocity, which remains supersonic. This is achieved if the shock front is at an angle to the direction of the flow velocity. The flow parameters in the intervals between jumps remain constant.
In the last jump, the speed reaches a subsonic indicator, further deceleration and air compression processes occur continuously in the diffuser channel.
If the motor inlet is located in the area of undisturbed flow (for example, in front of the aircraft at the nose end or at a sufficient distance from the fuselage on the wing console), it is asymmetric and is completed with a central body - a sharp long "cone" emerging from the shell. The central body is designed to create oblique shock waves in the oncoming air flow, which provide compression and deceleration of air until it enters a special channel of the inlet device. The presented inlet devices are called conical flow devices, the air inside them circulates, forming a conical shape.
The central conical body can be equipped with a mechanical drive, which allows it to move along the axis of the engine and optimize the deceleration of the air flow at different flight speeds. These input devices are called adjustable.
When fixing the engine under the wing or from the bottom of the fuselage, that is, in the area of aerodynamic influence of aircraft structural elements, two-dimensional flow inlet devices are used. They are not equipped with a central body and have a rectangular cross section. They are also called mixed or internal compression devices, since external compression here takes place only with shock waves formed at the leading edge of the wing or nose end of the aircraft. Rectangular inlet adjustable devices are able to change the position of the wedges inside the channel.
In the supersonic speed range, the ramjet is more efficient than in the subsonic range. For example, at a flight speed of M=3, the degree of pressure increase is 36.7, which is close to that of turbojet engines, and the calculated ideal efficiency reaches 64.3%. In practice, these indicators are lower, but at speeds in the range of M = 3-5, the SPVJE is superior in efficiency to all existing types of SPVJ.
At an undisturbed air flow temperature of 273°K and an aircraft speed of M=5, the temperature of the working retarded body is 1638°K, at a speed of M=6 - 2238°K, and in real flight, taking into account shock waves and the action of the friction force, it becomes even higher.
Further heating of the working fluid is problematic due to the thermal instability of the structural materials that make up the engine. Therefore, the speed limit for the SPVRD is M=5.
Hypersonic ramjet engine
The category of hypersonic ramjet includes ramjet, which operates at speeds of more than 5M. As of the beginning of the 21st century, the existence of such an engine was only hypothetical: not a single sample was assembled that would pass flight tests and confirm the feasibility and relevance of its serial production.
At the entrance to the scramjet device, air deceleration is performed only partially, and during the rest of the stroke, the movement of the working fluid is supersonic. At the same time, most of the initial kinetic energy of the flow is retained; after compression, the temperature is relatively low, which makes it possible to release a significant amount of heat to the working fluid. After the input device, the flow part of the engine expands along its entire length. Due to the combustion of fuel in a supersonic flow, the working fluid is heated, it expands and accelerates.
This type of engine is designed for flights in the rarefied stratosphere. Theoretically, such an engine can be used on reusable spacecraft carriers.
One of the main problems in the design of scramjet engines is the organization of fuel combustion in a supersonic flow.
In different countries, several programs have been launched to create a scramjet, all of them are at the stage of theoretical research and pre-design laboratory studies.
Where are ramjets used
The ramjet does not operate at zero speed and low airspeeds. An aircraft with such an engine requires the installation of auxiliary drives on it, which can be a solid-fuel rocket booster or a carrier aircraft from which the aircraft with a ramjet is launched.
Due to the inefficiency of the ramjet at low speeds, it is practically inappropriate to use it on manned aircraft. Such engines are preferably used for unmanned, cruise, disposable combat missiles due to their reliability, simplicity and low cost. Ramjet engines are also used in flying targets. The competition in terms of the characteristics of the ramjet is only a rocket engine.
Nuclear ramjet
During the Cold War between the USSR and the USA, projects of ramjet engines with a nuclear reactor were created.
In such units, the energy source was not the chemical reaction of fuel combustion, but the heat generated by a nuclear reactor installed instead of a combustion chamber. In such a ramjet, air entering through the inlet device penetrates into the active region of the reactor, cools the structure and heats up to 3000 K itself. Then it flows out of the engine nozzle at a speed close to the speed of perfect rocket engines. Nuclear ramjet engines were intended for installation in intercontinental cruise missiles carrying a nuclear charge. Designers in both countries have created small-sized nuclear reactors that fit into the dimensions of a cruise missile.
In 1964, as part of the Tory and Pluto nuclear ramjet research programs, stationary firing tests of the Tory-IIC nuclear ramjet were conducted. The test program was closed in July 1964, and the engine was not flight tested. The alleged reason for curtailing the program could be the improvement in the configuration of ballistic missiles with rocket chemical engines, which made it possible to carry out combat missions without the involvement of nuclear ramjet engines.
A fan is located in front of the jet engine. It takes air from the external environment, sucking it into the turbine. In engines used in rockets, air replaces liquid oxygen. The fan is equipped with many specially shaped titanium blades.
They try to make the fan area large enough. In addition to air intake, this part of the system is also involved in engine cooling, protecting its chambers from destruction. Behind the fan is the compressor. It pressurizes air into the combustion chamber.
One of the main structural elements of a jet engine is the combustion chamber. In it, the fuel is mixed with air and ignited. The mixture ignites, accompanied by a strong heating of the body parts. The fuel mixture expands under the influence of high temperature. In fact, a controlled explosion occurs in the engine.
From the combustion chamber, the mixture of fuel and air enters the turbine, which consists of many blades. The jet stream with force presses on them and sets the turbine in rotation. The force is transmitted to the shaft, compressor and fan. A closed system is formed, the operation of which requires only a constant supply of the fuel mixture.
The last detail of a jet engine is a nozzle. A heated stream enters from the turbine here, forming a jet stream. Cold air is also supplied to this part of the engine from the fan. It serves to cool the entire structure. The airflow protects the nozzle collar from the harmful effects of jet blast, preventing parts from melting.
How a jet engine works
The working fluid of the engine is reactive. It flows out of the nozzle at a very high speed. This creates a reactive force that pushes the entire device in the opposite direction. The traction force is generated solely by the action of the jet, without any support on other bodies. This feature of the jet engine allows it to be used as a power plant for rockets, aircraft and spacecraft.
In part, the operation of a jet engine is comparable to the action of a jet of water flowing from a hose. Under enormous pressure, the liquid is fed through the sleeve to the narrowed end of the hose. The speed of the water exiting the hose is higher than inside the hose. This creates a back pressure force that allows the firefighter to hold the hose with only great difficulty.
The production of jet engines is a special branch of technology. Since the temperature of the working fluid here reaches several thousand degrees, engine parts are made from high-strength metals and those materials that are resistant to melting. Separate parts of jet engines are made, for example, from special ceramic compositions.
