Ramjet engine

Jet engine - generating device required for the movement of the thrust by converting internal energy into kinetic energy of the fuel jet of the working substance.

Classes jet engines:

All jet engines are divided into 2 class:

• Jet - heat engines, air oxidation using energy derived from the atmosphere. In these engines the working fluid is a mixture of combustion products with the rest of the bleed air.
• Rocket - engines on board which contains all the necessary components and are able to work even in a vacuum.

Ramjet engine - the easiest in the class structure of the WFD. Required to operate the device an increase in pressure generated by braking the oncoming airflow.

Workflow ramjet can be summarized as follows:

• In the engine inlet air enters from the flight speed, its kinetic energy is converted to the internal pressure and temperature rise. At the entrance to the combustion chamber and the entire length of the flow part there is a maximum pressure.

• Heating the compressed air in the combustion chamber occurs by oxidation of feed air with the working fluid internal energy increases.
• Further, the flow is narrowed in a nozzle, the working fluid reaches sound velocity, and again during the expansion - supersonic. Due to the fact that the operating body is moving at a speed exceeding the speed of the oncoming flow is created within the jet thrust.

In a constructive manner ramjet is an extremely simple device. In the composition of the engine has a combustion chamber into which fuel flows from the fuel nozzles and the air - from the diffuser. The combustion chamber inlet end to a nozzle which is tapered-expanding.

The development of solid propellant technology led to the use of this fuel in a ramjet. The combustor is a fuel block with a central longitudinal bore. Passing through the channel, the working fluid is gradually oxidizes the surface of the fuel heats itself. The use of solid fuel further simplifies the design of the engine comprising: a fuel system becomes unnecessary.

Mixed fuel in its composition differs from the ramjet in used in solid propellant. If most of the rocket motor fuel composition takes the oxidant, the ramjet is used in small proportions to activate the combustion process.

The filler mixed fuel ramjet preferably consists of a fine powder of beryllium, magnesium, or aluminum. Their heat of oxidation is much greater than the heat of combustion of hydrocarbon fuels. As an example of solid ramjet can lead boosters cruise anti-ship missile "P-270 Mosquito."

Ramjet thrust depends on the flight speed is determined based on the influence of several factors:

• The greater the airspeed indicator, the larger the flow rate of air passing through the path of the engine, a correspondingly large amount of oxygen will penetrate into the combustion chamber, which increases fuel consumption, thermal, and mechanical power of the motor.
• The greater the air flow through the path of the engine, the higher will be generated by the engine thrust. However, there is a certain limit, the air flow path through the motor can not be increased without limit.
• With increasing airspeed increases the level of pressure in the combustion chamber. This increases the thermal efficiency of the engine.
• The greater the difference between the flight speed of the machine and the speed of passage of the jet, the greater the thrust of the engine.

Dependence of thrust ramjet engine speed of flight is as follows: up to the moment when the speed of a much lower rate of passage of the jet, the thrust will increase along with the growth speed of flight. When the flight speed approaches the speed of the jet, thrust begins to fall, after passing a certain maximum, in which there is an optimal speed of flight.

Depending on the airspeed isolated ramjet such categories:

• subsonic;
• supersonic;
• hypersonic.

Each group has its own distinctive design features.

Subsonic ramjet

This group of engines is designed to provide flights at a rate equal to the 0,5 1,0 from the Mach number. Compressing air to such engine braking occurs in the diffuser - expanding channel device at the flow inlet.

These engines are extremely inefficient. 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 a real engine, this value will approach zero. This means that when flying at speeds M <0,5, the subsonic ramjet is inoperative.

But even at the maximum speed for the subsonic range at M = 1 level of pressure increase is 1,89, and the ideal thermal efficiency - all 16, 7%. These indicators 1,5 times less than that of piston internal combustion engines, and in 2 times less than that of gas turbine engines. Gas turbine and piston engines also effective for use when working in a stationary position. Therefore, once-through subsonic engines in comparison with other aircraft engines proved uncompetitive and is currently not commercially available.

Supersonic ramjet

Supersonic ramjet engines are designed for flights in the speed range 1 <M <5.

Braking gas supersonic flow is always carried out discontinuously, forming a shock wave, called a shock wave. At a distance of a shock wave process gas compression it is not isentropic. Consequently, there is a loss of mechanical energy, the level of increased pressure therein lower than in the isentropic process. The more powerful the shock wave, the greater the flow rate changes at the front, accordingly, greater loss of pressure, sometimes reaching 50%.

To minimize pressure loss, compression is not arranged in one and several shock waves with lower intensity. After each of these spikes decrease the flow rate, which remains supersonic. This is achieved if the shock front at an angle to the direction of flow rates. The flow parameters in the intervals between the jumps are constant.

In the last jump subsonic speed reaches index, the further process of braking and compressed air occurs continuously in the diffuser.

If the motor input is located in the region of the 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 equipped 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 stream, which provide compression and braking of air until it reaches the special channel of the input device. The presented input 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 that allows it to move along the axis of the engine and braking to optimize airflow at different flight speeds. Data input devices are called regulated.

When the engine is fixed under the wing or from the bottom of the fuselage, that is, in the region of aerodynamic influence of the elements of the aircraft structure, two-dimensional plane input devices are used. They are not equipped with a central body and have a transverse rectangular cross-section. They are also called intermixed or internal compression devices, since external compression takes place here only in shock waves formed at the leading edge of the wing or the nose end of the aircraft. Input adjustable devices of rectangular cross-section are able to change the position of the wedges inside the channel.

The supersonic ramjet engine speed range is more effective than in the subsonic. For example, on the flight speed of M = 3 degree of pressure increase is 36,7, which is close to the index of turbojet engines and the calculated efficiency reaches the perfect 64,3%. In practice, these figures are smaller, but at speeds in the range M = 3-5 SPVRD performance superior to all existing types of WFD.

When the temperature of the undisturbed air flow 273 ° K and aircraft speed M = 5 temperature of the working body is hindered 1638 ° K at a speed of M = 6 - 2238 ° K, and in real flight, taking into account shocks and the force of friction is 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 maximum speed for SPVRD is considered to be M = 5.

Hypersonic ramjet engine

The category of hypersonic ramjet engine is the ramjet, which operates at speeds of more 5M. At the beginning of the XXI century the existence of such an engine was just hypothetical: not collected a single sample, which would have passed flight tests and confirmed the feasibility and relevance of its serial production.

On entering the device scramjet air braking is performed only partially, and throughout the rest of the stroke movement of the body is supersonic. Much of the initial kinetic energy of the flow is retained after the compression temperature is relatively low, which allows the working fluid to release a significant amount of heat. After the input of the device running the engine over its entire length is extended. Due to combustion in a supersonic flow is heated working fluid, it expands and accelerates.

This type of motor designed for flight in the rarefied stratosphere. Theoretically, such an engine can be used on re-usable carriers spacecraft.

One of the main problems is to organize the construction of a scramjet combustion in supersonic flow.

Different countries have started several programs for the creation of a scramjet, all of them are at the stage of theoretical research and pre-laboratory studies.

Where applicable ramjet

Ramjet does not operate at zero speed and low-speed flight. The aircraft with this engine requires the installation of its auxiliary drives, the role of which can be a solid rocket boosters, or carrier aircraft, is being used to start the machine with a ramjet.

Due to the inefficiency of the ramjet engine at low speeds it almost inappropriate to use manned aircraft. These motors are preferably used for unmanned, winged, single-use military missiles due to its reliability, simplicity and cheapness. Ramjet engine is also used in the flying targets. Competition on the characteristics of a ramjet is only the rocket engine.

The nuclear ramjet

During the Cold War between the USSR and the USA, projects of ramjet air jet engines with a nuclear reactor were created.

In such units, the energy source was not the chemical reaction of burning fuel, but the heat produced by the nuclear reactor installed in place of the combustion chamber. In such a ramjet, the air coming through the inlet device penetrates into the active region of the reactor, cools the structure and heats itself to 3000 K. Then, it flows out of the engine nozzle at a speed close to the speed of the perfect rocket engines. Nuclear ramjets were intended for installation in intercontinental cruise missiles carrying a nuclear charge. Designers in both countries have created small-sized nuclear reactors that fit in the dimensions of a cruise missile.

In 1964, within the framework of nuclear ramjet research programs, Tory and Pluto conducted stationary fire tests of the Tory-IIC nuclear ramjet. The test program was closed in July 1964, the engine was not flight tested. The presumable reason for curtailing the program could be the improvement of the equipment of ballistic missiles with rocket chemical engines, which made it possible to carry out combat missions without involving nuclear ramjet engines.