In-line jet engine

Fire ram test in NASA laboratory

Ramjet engine ( ramjet ) - jet engine , is the simplest in the class of air jet engines (WFD) for the device. Refers to the type of direct reaction WFD, in which the thrust is created exclusively due to the jet stream flowing out of the nozzle . The pressure increase necessary for the engine to work is achieved by braking the oncoming air flow. The ramjet is not operable at low flight speeds, especially at zero speed, in order to bring it to operating power one or another accelerator is needed.

History

- the first manned vehicle with marching ramjet (first flight - November 19, 1946). Museum of Aviation and Space in Le Bourget

In 1913, the Frenchman received a patent for a ramjet engine.

The ramjet engine attracted designers by the simplicity of its device, but most importantly, by its potential ability to operate at hypersonic speeds and in the highest, most rarefied layers of the atmosphere, that is, in conditions in which other types of WFD are inoperative or ineffective. In the 1930s, experiments with this type of engine were carried out in the USA (William Avery), in the USSR ( F. A. Zander , B. S. Stechkin , Yu. A. Pobedonostsev ).

In 1937, the French designer received an order from the French government to develop an experimental aircraft with ramjet. This work was interrupted by the war and resumed after its completion. On November 19, 1946, the first ever manned flight of a manned vehicle with marching ramjet, . Then, over the course of 10 years , several more experimental devices of this series were manufactured and tested, including the manned and , and in 1957 the French government refused to continue these works, which was rapidly developing at that time the direction of turbojet engines seemed more promising.

Having a number of drawbacks for use on manned aircraft (zero thrust when stationary, low efficiency at low flight speeds), ramjet is the preferred type of propulsion for unmanned disposable shells and cruise missiles , due to its simplicity, and therefore, low cost and reliability. Since the 1950s, the United States has created a number of experimental aircraft and serial cruise missiles for various purposes with this type of engine.

In the USSR, from 1954 to 1960, OKB-301, under the leadership of General Designer S. A. Lavochkin , developed the “ Storm ” cruise missile, designed to deliver nuclear charges to intercontinental distances, and used the ramjet as a marching engine , developed by the group M. M Bondaryuk , who had characteristics unique for his time: efficient operation at speeds above M = 3 and at an altitude of 17 km . In 1957, the project entered the stage of flight tests, during which a number of problems were identified, in particular, with the accuracy of guidance that was to be solved, and this required time that was difficult to determine. Meanwhile, in the same year, the R-7 ICBM , which had the same purpose, developed under the leadership of S.P. Korolyov, was already in service. This cast doubt on the feasibility of further development of the Storm. The death of S. A. Lavochkin in 1960 finally buried the project.

Among the more modern domestic developments, one can mention anti-ship cruise missiles with marching ramjet ramps: P-800 Onyx and P-270 Moskit .

Principle of Operation

The ramjet workflow can be briefly described as follows. Air entering the engine inlet at a flight speed is inhibited (in practice, up to speeds of 30-60 m / s, which corresponds to a Mach number of 0.1-0.2), its kinetic energy is converted into internal energy - its temperature and pressure are rising.

Assuming that air is an ideal gas and the compression process is isentropic , the degree of pressure increase (the ratio of static pressure in the inhibited flow to atmospheric) is expressed by the formula:

{\frac {p}{p_{o}}}={\bigg (}1+{\frac {k-1}{2}}\cdot M_{n}^{2}{\bigg )}^{\frac {k}{k-1}}

{\ displaystyle {\ frac {p} {p_ {o}}} = {\ bigg (} 1 + {\ frac {k-1} {2}} \ cdot M_ {n} ^ {2} {\ bigg) } ^ {\ frac {k} {k-1}}}

(one)

Where

p

{\ displaystyle p}

- pressure in a completely inhibited flow;

p_{o}

{\ displaystyle p_ {o}}

- Atmosphere pressure;

M_{n}

{\ displaystyle M_ {n}}

- flight Mach number (ratio of flight speed to sound speed in the environment),

k

{\ displaystyle k}

- the adiabatic index for air is 1.4.

At the exit of the input device, at the entrance to the combustion chamber, the working fluid has a maximum pressure throughout the entire flow part of the engine.

Compressed air in the combustion chamber is heated by oxidation of the fuel supplied to it, the internal energy of the working fluid increases. Then the working fluid first, compressing in the nozzle , reaches sound speed, and then, expanding - supersonic, accelerates and expires at a speed greater than the speed of the oncoming stream, which creates reactive thrust.

The scheme of the ramjet device for liquid fuel:

counter flow of air;
central body;
input device;
fuel burner;
the combustion chamber;
nozzle;
jet stream.

The scheme of the device solid propellant ramjet

The dependence of ramjet thrust on flight speed is determined by several factors:

The higher the flight speed, the greater the air flow through the engine path, and hence the amount of oxygen entering the chamber, which allows increasing the fuel consumption and increasing the thermal, and with it the mechanical power of the engine.
The greater the air flow through the engine path, the higher the thrust created by it, in accordance with formula (3). However, the air flow through the engine path cannot grow unlimitedly. The area of each cross section of the engine must be sufficient to ensure the necessary air flow.
With increasing flight speed, in accordance with formula (1), the degree of increase in pressure increases $\beta ={\frac {p_{2}}{p_{1}}}$ ${\ displaystyle \ beta = {\ frac {p_ {2}} {p_ {1}}}}$ in the combustion chamber, which entails an increase in thermal efficiency , which for an ideal ramjet engine is expressed by the formula ^[1] :

\eta _{t}=1-{\frac {1}{\beta ^{\frac {k-1}{k}}}}

{\ displaystyle \ eta _ {t} = 1 - {\ frac {1} {\ beta ^ {\ frac {k-1} {k}}}}}

(2)

Prepared ramjet Thor of the Bloodhound missile. The input device and the entrance to the combustion chamber are clearly visible.

In accordance with formula (1), the smaller the difference between the flight speed and the jet velocity, the less engine thrust (ceteris paribus).

In general, the dependence of the ramjet thrust on the flight speed can be represented as follows: while the flight speed is significantly lower than the velocity of the jet stream, the thrust increases with increasing flight speed (due to increased air flow, pressure in the combustion chamber and thermal engine efficiency), and with the approach the flight speed to the velocity of the jet stream, the thrust ramjet thrust falls, having passed a maximum corresponding to the optimum flight speed.

Ramjet thrust

The ramjet thrust force is determined by the expression

P={\frac {dm_{a}}{dt}}\cdot (v_{e}-v)+{\frac {dm_{f}}{dt}}\cdot v_{e}

{\ displaystyle P = {\ frac {dm_ {a}} {dt}} \ cdot (v_ {e} -v) + {\ frac {dm_ {f}} {dt}} \ cdot v_ {e}}

(3)

Where $P$ ${\ displaystyle P}$ - traction force, $v$ ${\ displaystyle v}$ - flight speed $v_{e}$ ${\ displaystyle v_ {e}}$ - jet speed relative to the engine, ${\frac {dm_{f}}{dt}}$ ${\ displaystyle {\ frac {dm_ {f}} {dt}}}$ - second fuel consumption.

Secondary air consumption:

{\frac {dm_{a}}{dt}}=\rho \cdot {\frac {dV}{dt}}=\rho \cdot S\cdot {\frac {dl}{dt}}=\rho \cdot S\cdot v

{\ displaystyle {\ frac {dm_ {a}} {dt}} = \ rho \ cdot {\ frac {dV} {dt}} = \ rho \ cdot S \ cdot {\ frac {dl} {dt}} = \ rho \ cdot S \ cdot v}

,

Where

\rho

{\ displaystyle \ rho}

- air density (depending on height),

{\frac {dV}{dt}}

{\ displaystyle {\ frac {dV} {dt}}}

- the volume of air that enters the air intake of the ramjet per unit time,

S

{\ displaystyle S}

- the cross-sectional area of the inlet of the air intake,

v

{\ displaystyle v}

- flight speed.

The second mass flow rate of the working fluid for the ideal case, when the fuel is completely burned and oxygen in the combustion process is completely used, is calculated using the stoichiometric coefficient:

{\frac {dm}{dt}}={\frac {dm_{a}}{dt}}+{\frac {dm_{f}}{dt}}={\frac {dm_{a}}{dt}}+{\frac {1}{L}}\cdot {\frac {dm_{a}}{dt}}={\frac {dm_{a}}{dt}}\cdot (1+{\frac {1}{L}})

{\ displaystyle {\ frac {dm} {dt}} = {\ frac {dm_ {a}} {dt}} + {\ frac {dm_ {f}} {dt}} = {\ frac {dm_ {a} } {dt}} + {\ frac {1} {L}} \ cdot {\ frac {dm_ {a}} {dt}} = {\ frac {dm_ {a}} {dt}} \ cdot (1+ {\ frac {1} {L}})}

,

Where

{\frac {dm_{a}}{dt}}

{\ displaystyle {\ frac {dm_ {a}} {dt}}}

- second air consumption,

{\frac {dm_{f}}{dt}}

{\ displaystyle {\ frac {dm_ {f}} {dt}}}

- second fuel consumption,

L

{\ displaystyle L}

- stoichiometric coefficient of the mixture of fuel and air.

Design

Structurally, ramjet has an extremely simple device. The engine consists of a combustion chamber into which air enters from the diffuser and fuel from the fuel nozzles . The combustion chamber ends with the entrance to the nozzle , usually tapering-expanding .

With the development of mixed solid fuel technology, it began to be used in ramjet. A fuel checker with a longitudinal central channel is placed in the combustion chamber. The working fluid, passing through the channel, gradually oxidizes the fuel from its surface, and heats itself. The use of solid fuel further simplifies the ramjet design: the fuel system becomes unnecessary. The composition of the mixed fuel for ramjet differs from that used in solid rocket engines . If for the latter most of the fuel is an oxidizing agent, then for ramjet fuel it is added only in a small amount to activate the combustion process. The main part of the ramjet mixed fuel filler is finely divided aluminum , magnesium or beryllium powder, the heat of oxidation of which significantly exceeds the heat of combustion of hydrocarbon fuels. An example of a solid propellant ramjet can be a marching engine of the P-270 Mosquito anti-ship cruise missile.

Depending on the flight speed, ramjets are divided into subsonic , supersonic and hypersonic . This division is due to the design features of each of these groups.

Subsonic ramjet

Subsonic ramjets are designed for flights at speeds with a Mach number from 0.5 to 1. Braking and compression of air in these engines occurs in the expanding channel of the input device - the diffuser .

These engines are characterized by extremely low efficiency. When flying at a speed of M = 0.5, the degree of pressure increase in them (as follows from formula (1)) is 1.186, as a result of which their ideal thermal efficiency (in accordance with formula (2)) is only 4.76%, and with taking into account losses in a real engine, this value becomes almost equal to 0. This means that at flight speeds at M <0.5 ramjet ramps are practically inoperative. But even at the speed limit for the subsonic range, that is, when M → 1 , the degree of pressure increase is only 1.89, and the ideal thermal efficiency is only 16.7%, which is 1.5 times less than that of real piston ICEs, and half as much as gas turbine engines. In addition, both piston and gas turbine engines are effective in the field.

For these reasons, direct-flow subsonic engines proved to be uncompetitive in comparison with other types of aircraft engines and are not currently commercially available.

Supersonic ramjets

Supersonic ramjet ramjets (SPJRDs) are designed for flights in the range of Mach numbers 1 < M <5 .

Braking of a supersonic gas flow always occurs discontinuously (stepwise) - with the formation of a shock wave, also called a shock wave . The process of gas compression at the front of the shock wave is not isentropic, as a result of which there are irreversible losses of mechanical energy in it, and the degree of pressure increase in it is less than in the ideal - isentropic process. The more intense the shock wave, that is, the greater the change in the flow velocity at its front, the greater the pressure loss, which can exceed 50%.

The process of inhibition of supersonic flow in the input device of a conical flow, external compression with three shock waves. M - graph of the change in the Mach number in the stream; p is a graph of static pressure.

Unmanned reconnaissance Lockheed D-21B (USA). Ramjet with axisymmetric input device with a central body.

Air-to-Ground Flat Intake Compressors for Ramjet (France)

Pressure losses can be minimized due to the organization of compression not in one but in several (usually no more than 4) consecutive shock waves of lower intensity, after each of which (except the last), the flow velocity decreases, remaining supersonic. This is possible if all jumps (except the last) are oblique , the front of which is inclined to the flow velocity vector (an oblique shock wave is formed when a supersonic flow is encountered with an obstacle whose surface is inclined to the air flow velocity vector). In the intervals between jumps, the flow parameters remain constant. In the last jump (always direct - normal to the airflow velocity vector) the speed becomes subsonic, and further braking and compression of the air occurs continuously in the expanding diffuser channel.

If the engine input device is located in the undisturbed flow zone, for example, in the nose end of the aircraft, or on the console at a sufficient distance from the fuselage, it is axisymmetric and provided with a central body - a long sharp “cone” protruding from the shell, the purpose of which consists in creating oblique shock waves in the oncoming flow system, which provide braking and compression of the air even before it enters the channel of the input device - the so-called external compression . Such input devices are also called conical flow devices , because the air flow in them has a conical shape. The conical central body can be equipped with a mechanical drive, allowing it to be displaced along the axis of the engine, thereby optimizing the braking of the air flow at different flight speeds. Such input devices are called adjustable.

When installing the engine on the lower (side) wall of the fuselage, or under the wing of the aircraft, that is, in the zone of aerodynamic influence of its elements, two-dimensional flat input devices with a rectangular cross section without a central body are usually used. The system of shock waves in them is provided due to the internal shape of the channel. They are also called internal or mixed compression devices , since external compression partially takes place in this case, too, in the shock waves formed at the nasal end and / or at the leading edge of the wing of the aircraft. Rectangular adjustable input devices have wedges that change their position inside the channel.

In the supersonic speed range, ramjet is much more effective than in the subsonic. For example, at a speed of M = 3 for an ideal ramjet, the degree of pressure increase according to formula (1) is 36.7, which is comparable to the performance of the highest pressure compressors of turbojet engines, and the thermal efficiency theoretically, according to formula (2), reaches 64.3%. In real ramjet engines, these indicators are lower, but even taking into account losses, in the range of flight Mach numbers from 3 to 5 SPRDs are superior in efficiency to all other types of airjet engines.

When braking the oncoming air flow, it not only contracts, but also heats up, and its absolute temperature during complete braking (in the isentropic process) is expressed by the formula:

T=T_{o}\cdot (1+{\frac {k-1}{2}}\cdot M_{n}^{2})

{\ displaystyle T = T_ {o} \ cdot (1 + {\ frac {k-1} {2}} \ cdot M_ {n} ^ {2})}

(four)

where T _о is the temperature of the ambient undisturbed flow. At M = 5 and T _о = 273 K (which corresponds to 0 ° C), the temperature of the inhibited working fluid reaches 1638 K , at M = 6 - 2238 K , and taking into account friction and shock waves in the real process, it is even higher. Moreover, further heating of the working fluid due to fuel combustion becomes problematic due to the restrictions imposed by the thermal stability of the structural materials of which the engine is made. Therefore, the speed corresponding to M = 5 is considered to be the limiting one for SPVRD.

Hypersonic ramjet

Experimental hypersonic aircraft X-43 (artist's drawing)

Illustration of gas-dynamic processes in a flat scramjet with a nozzle Air compression occurs in two shock waves: the external one, formed at the nasal end of the apparatus, and the internal one, at the front edge of the engine lower wall. Both jumps are oblique, and the flow velocity remains supersonic.

A hypersonic ramjet ( scramjet , the English term is scramjet ) is a ramjet operating at flight speeds above M = 5 (the upper limit is not exactly set).

At the beginning of the XXI century, this type of engine is experimental: there is not a single sample that has passed flight tests, confirming the practical feasibility of its serial production.

Inhibition of the air flow in the inlet of the scramjet engine occurs only partially, so that throughout the rest of the path the movement of the working fluid remains supersonic. In this case, most of the initial kinetic energy of the flow is retained, and the temperature after compression is relatively low, which allows the working fluid to inform a significant amount of heat. The flow part of the scramjet expands along its entire length after the input device. Fuel is introduced into the supersonic flow from the walls of the engine flow passage. Due to the combustion of fuel in a supersonic flow, the working fluid heats up, expands and accelerates, so that its outflow speed exceeds the flight speed.

The engine is designed for flights in the stratosphere . The possible purpose of an aircraft with a scramjet is the lowest level of a reusable spacecraft carrier.

The organization of fuel combustion in a supersonic flow is one of the main problems in creating a scramjet.

There are several scramjet development programs in different countries, all at the stage of theoretical research or pre-design experiments.

Scope

The ramjet is not operational at low flight speeds, much less at zero speed. To achieve the initial speed at which it becomes effective, an apparatus with this engine needs an auxiliary drive, which can be provided, for example, by a solid propellant rocket accelerator , or by a carrier aircraft (accelerator aircraft), from which the ramjet engine is launched.

The inefficiency of ramjet engines at low flight speeds makes it practically not applicable on manned aircraft with a non-nuclear propulsion system ^[2] , but for unmanned, including military (in particular cruise missiles ), single-use, flying in the speed range 2 < M <5 , due to its simplicity, cheapness and reliability, it is preferable. Also ramjets are used on flying targets. The main competitor of ramjet in this niche is a rocket engine .

Samples of cruise missiles with marching ramjet.
SAM Bristol Bloodhound (UK)
SAM CIM-10 Bomarc (USA)
Ship SAM RIM-8 Talos (USA)
Air-to-air missile Meteor (European Union)
BraMos anti-ship cruise missile (India-Russia)
Anti-ship cruise missile P-270 "Mosquito" (Russia)
Yakhont anti-ship cruise missile (Russia).
The 2P24 launcher as part of the Krug SAM , equipped with two 3M8 SAMs (Russia)

Nuclear ramjet

Nuclear ramjet " Pluto " (USA)

In the second half of the 1950s, during the Cold War era, projects of ramjet with a nuclear reactor were developed in the USA and the USSR.

The energy source of these ramjet engines (unlike other WFMs) is not the chemical reaction of fuel combustion, but the heat generated by a nuclear reactor in the heating chamber of the working fluid. The air from the inlet device in such ramjet passes through the reactor core, cooling it, heats itself up to the operating temperature (about 3000 K ), and then flows out of the nozzle at a speed comparable to the flow rates for the most advanced chemical rocket engines ^[3] . Possible destination of an aircraft with such an engine:

intercontinental cruise missile launch vehicle;
single-stage aerospace aircraft.

In both countries, compact, low-resource nuclear reactors were created that fit into the dimensions of a large missile. В США по программам исследований ядерного ПВРД «Pluto» и «Tory» в 1964 году были проведены стендовые огневые испытания ядерного прямоточного двигателя «Tory-IIC» (режим полной мощности 513 МВт в течение пяти минут с тягой 156 кН ). Лётные испытания не проводились, программа была закрыта в июле 1964 года. Одна из причин закрытия программы — совершенствование конструкции баллистических ракет с химическими ракетными двигателями, которые вполне обеспечили решение боевых задач без применения схем с сравнительно дорогостоящими ядерными ПВРД.

Тем не менее, ядерный ПВРД перспективен как двигательная система для одноступенчатых воздушно-космических самолётов и скоростной межконтинентальной тяжёлой транспортной авиации. Этому способствует возможность создания ядерного ПВРД, способного работать на дозвуковых и нулевых скоростях полёта в режиме ракетного двигателя, используя бортовые запасы рабочего тела. То есть, например, воздушно-космический самолёт с ядерным ПВРД стартует (в том числе взлетает), подавая в двигатели рабочее тело из бортовых (или подвесных) баков и, уже достигнув скоростей от М = 1 , переходит на использование атмосферного воздуха.

В России, по сделанному президентом В. В. Путиным в начале 2018 года заявлению, «состоялся успешный пуск крылатой ракеты с ядерной энергоустановкой ». ^[four]

Literature

Работы по ПВРД и крылатым ракетам дальнего действия с ПВРД в СССР (1947—1960)
Теория и расчёт воздушно-реактивных двигателей: Учебник для вузов / В. М. Акимов, В. И. Бакулев, Р. И. Курзинер, В. В. Поляков, В. А. Сосунов, С. М. Шляхтенко; Ed. С. М. Шляхтенко. - 2nd ed., Revised. and add. — М.: Машиностроение, 1987.
Абрамович Г. Н. Прикладная газовая динамика. - 4th ed. — М.: Наука , Главная редакция физико-математической литературы, 1976.
ПВРД — задание на завтра

Notes

↑ Яковлев К. П. Краткий физико-технический справочник. Т. 3. - М., Физматлит, 1962. - с. 138
↑ Начиная с (Франция, 1950 год) по настоящее время было создано около десятка экспериментальных самолётов с ПВРД (главным образом, в США), в серийное производство так и не поступивших, за исключением Lockheed SR-71 Blackbird с гибридным ТРД/ПВРД Pratt & Whitney J58 , выпущенного в количестве 32 изделий .
↑ Андрей Суворов. Ядерный след // Популярная механика . — 2018. — № 5 . — С. 88-92 .
↑ Путин презентовал новейшие стратегические ракеты: никакие системы ПРО нам не помеха . ВГТРК.

[1] Яковлев К. П. Краткий физико-технический справочник. Т. 3. - М., Физматлит, 1962. - с. 138

[autogenerated1-2] Начиная с (Франция, 1950 год) по настоящее время было создано около десятка экспериментальных самолётов с ПВРД (главным образом, в США), в серийное производство так и не поступивших, за исключением Lockheed SR-71 Blackbird с гибридным ТРД/ПВРД Pratt & Whitney J58 , выпущенного в количестве 32 изделий .

[POP201805-3] Андрей Суворов. Ядерный след // Популярная механика . — 2018. — № 5 . — С. 88-92 .

[4] Путин презентовал новейшие стратегические ракеты: никакие системы ПРО нам не помеха . ВГТРК.

In-line jet engine

History

Principle of Operation

Ramjet thrust

Design

Subsonic ramjet

Supersonic ramjets

Hypersonic ramjet

Scope

Nuclear ramjet

See also

Literature

Notes

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