Ramjet
Overview
 
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. Ramjets therefore require some other propulsion system to accelerate the vehicle to a speed where the ramjet begins to produce thrust.
Encyclopedia
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. Ramjets therefore require some other propulsion system to accelerate the vehicle to a speed where the ramjet begins to produce thrust. Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

 3. This type of jet can operate up to speeds of Mach 6.

Ramjets can be particularly useful in applications requiring a small and simple engine for high speed use, such as missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

s, while weapon designers are looking to use ramjet technology in artillery shells to give added range: it is anticipated that a 120-mm mortar shell, if assisted by a ramjet, could attain a range of 22 mi (35.4 km). They have also been used successfully, though not efficiently, as tip jets on helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

 rotors.

Ramjets are frequently confused with pulsejets, which use an intermittent combustion, but ramjets employ a continuous combustion process, and are a quite distinct type of jet engine.

History

René Lorin

The ramjet was invented in 1913 by French inventor René Lorin
René Lorin
René Lorin , a graduate of the Ecole Centrale Paris, invented the ramjet. In 1908 he patented a subsonic ramjet design.Lorin published the principles of a ramjet in articles in the journal L'Aérophile from 1908 to 1913, expressing the idea that the exhaust from internal combustion engines could be...

, who was granted a patent for his device. Attempts to build a prototype failed due to inadequate materials.

Albert Fonó

In 1915 Hungarian inventor Albert Fonó
Albert Fonó
Albert Fonó , was a Hungarian mechanical engineer who was one of the early pioneers of turbojet and ramjet propulsion and was first to patent a ramjet engine in 1928, ....

 devised a solution for increasing the range of artillery
Artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

, comprising a gun-launched projectile which was to be united with a ramjet propulsion unit, thus giving a long range from relatively low muzzle velocities, allowing heavy shells to be fired from relatively lightweight guns. Fonó submitted his invention to the Austro-Hungarian Army
Austro-Hungarian Army
The Austro-Hungarian Army was the ground force of the Austro-Hungarian Dual Monarchy from 1867 to 1918. It was composed of three parts: the joint army , the Austrian Landwehr , and the Hungarian Honvédség .In the wake of fighting between the...

 but the proposal was rejected. After World War I, Fonó returned to the subject of jet propulsion, in May 1928 describing an "air-jet engine" which he described as being suitable for high-altitude supersonic aircraft, in a German patent application. In an additional patent application he adapted the engine for subsonic speed. The patent was finally granted in 1932 after four years of examination (German Patent No. 554,906, 1932-11-02).

Gorgon IV

The US Navy developed a series of air-to-air missiles under the name of "Gorgon" using different propulsion mechanisms, including ramjet propulsion. The ramjet Gorgon IVs, made by Glenn Martin, were tested in 1948 and 1949 at Naval Air Station Point Mugu
Naval Air Station Point Mugu
Naval Base Ventura County Point Mugu or NBVC Point Mugu is a military airbase located in Point Mugu, Ventura County, California, United States. Due to realignment actions which occurred in 2000, the base is now part of Naval Base Ventura County , a consolidated organization that also includes...

. The ramjet engine itself was designed at the University of Southern California and manufactured by the Marquardt Aircraft Company. The engine was 7 feet long and 20 inches in diameter and was positioned below the missile (see photo).

Fritz Zwicky

The eminent Swiss astrophysicist was research director at Aerojet
Aerojet
Aerojet is an American rocket and missile propulsion manufacturer based primarily in Rancho Cordova, California with divisions in Redmond, Washington, Orange, Gainesville and Camden, Arkansas. Aerojet is owned by GenCorp. They are the only US propulsion company that provides both solid rocket...

 and holds many patents in jet propulsion. U.S. Patent 5121670 is for the Ram Accelerator and U.S. Patent 4722261 is the Extendable Ram Cannon. The U.S. Navy would not allow Fritz Zwicky to publicly discuss his own invention, U.S. Patent 2,461,797 for the Underwater Jet, a ram jet that performs in a fluid medium.TIME July 11, 1955 chronicles Fritz Zwicky's work in the "Missed Swiss" and the "Underwater Jet" in the March 14, 1949 issue.

Soviet Union

In the Soviet Union, a theory of supersonic ramjet engines was presented in 1928 by Boris S. Stechkin. Yuri Pobedonostsev, chief of GIRD
GIRD
The Moscow-based Group for the Study of Reactive Motion was a Soviet research bureau founded in 1931 to study various aspects of rocketry . In 1933 it was incorporated into the Reaction-Engine Scientific Research Institute .-History:...

's 3rd Brigade, carried out a great deal of research into ramjet engines. The first engine, the GIRD-04, was designed by I.A. Merkulov and tested in April 1933. To simulate supersonic flight, it was fed by air compressed to 200 atmospheres, and was fueled with hydrogen. The GIRD-08 phosphorus-fueled ramjet was tested by firing it from an artillery cannon. These shells may have been the first jet-powered projectiles to break the speed of sound.

In 1939, Merkulov did further ramjet tests using a two-stage rocket, the R-3. In August of that year, he developed the first ramjet engine for use as an auxiliary motor of an aircraft, the DM-1. The world's first ramjet-powered airplane flight took place in December 1939, using two DM-2 engines on a modified Polikarpov I-15. Merkulov designed a ramjet fighter "Samolet D" in 1941, which was never completed. Two of his DM-4 engines were installed on the YaK-7 PVRD fighter, during World War II. In 1940, the Kostikov-302 experimental plane was designed, powered by liquid fuel rocket for take-off and ramjet engines for flight. That project was cancelled in 1944.

In 1947, Mstislav Keldysh
Mstislav Keldysh
Mstislav Vsevolodovich Keldysh was a Soviet scientist in the field of mathematics and mechanics, academician of the USSR Academy of Sciences , President of the USSR Academy of Sciences , three times Hero of Socialist Labor , fellow of the Royal Society of Edinburgh . He was one of the key figures...

 proposed a long-range antipodal bomber
Keldysh bomber
The Keldysh bomber was a Soviet design for a rocket-powered sub-orbital bomber aircraft which drew heavily upon work carried out by Eugen Sänger and Irene Bredt for the German Silbervogel project.- Development :...

, similar to the Sänger-Bredt bomber, but powered by ramjet instead of rocket. In 1954, NPO Lavochkin and the Keldysh Institute began development of a trisonic ramjet-powered cruise missile, Burya
Burya
The Burya was a trisonic, intercontinental cruise missile developed by the Lavochkin design bureau , based on a request for proposal issued by the Soviet government in 1954. The purpose of the design competition was to develop a cruise missile capable of delivering a nuclear payload to the United...

. This project competed with the R-7 ICBM being developed by Sergei Korolev, and was cancelled in 1957.

Germany

In 1936 Hellmuth Walter
Hellmuth Walter
Hellmuth Walter was a German engineer who pioneered research into rocket engines and gas turbines...

 constructed a test engine powered by natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

. Theoretical work was carried out at BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...

 and Junkers
Junkers
Junkers Flugzeug- und Motorenwerke AG , more commonly Junkers, was a major German aircraft manufacturer. It produced some of the world's most innovative and best-known airplanes over the course of its fifty-plus year history in Dessau, Germany. It was founded there in 1895 by Hugo Junkers,...

 as well as the DFL
DFL
The initialism DFL can stand for the following:*Danmarks Lærerforening, the Danish Union of Teachers*Degree of Financial Leverage Leverage *David Florida Laboratory, a Canadian aerospace laboratory...

. In 1941 Eugen Sänger
Eugen Sänger
Eugen Sänger was an Austrian-German aerospace engineer best known for his contributions to lifting body and ramjet technology.-Early career:...

 of the DFL proposed a ramjet engine with a very high combustion chamber temperature. He constructed very large ramjet pipes with 500 millimetres (19.7 in) and 1000 millimetres (39.4 in) diameter and carried out combustion tests on lorries and on a special test rig on a Dornier Do 17Z
Dornier Do 17
The Dornier Do 17, sometimes referred to as the Fliegender Bleistift , was a World War II German light bomber produced by Claudius Dornier's company, Dornier Flugzeugwerke...

 at flight speeds of up to 200 m/s (655 ft/s). Later, with petrol becoming scarce in Germany due to wartime conditions, tests were carried out with blocks of pressed coal dust which were not successful due to slow combustion.

France

In France the works of René Leduc
René Leduc
René Leduc was a French engineer who is much acclaimed for his work on ramjets. In 1949 he created the first aircraft to fly under the power of ramjets alone, the Leduc 0.10....

 were notable. Leduc's Model, the Leduc 0.10 was one of the first ramjet-powered aircraft to fly, in 1949.

The Nord 1500 Griffon
Nord 1500 Griffon
-See also:-Bibliography:*Taylor, John W.R. Jane's Pocket Book of Research and Experimental Aircraft, London, Macdonald and Jane's Publishers Ltd, 1976. ISBN 0356 08409 4.- External links :* *****...

 reached Mach 2.19 in 1958.

Engine cycle


The Brayton cycle is a thermodynamic cycle
Thermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...

 that describes the workings of the gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engine, basis of the airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 and others. It is named after George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...

 (1830–1892), the American engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...

 who developed it, although it was originally proposed and patented by Englishman John Barber
John Barber (engineer)
John Barber was an English coalmaster and inventor. He was born in Nottinghamshire, but moved to Warwickshire in the 1760s to manage collieries in the Nuneaton area. For a time he lived in Camp Hill House, between Hartshill and Nuneaton, and later lived in Attleborough...

 in 1791. It is also sometimes known as the Joule
James Prescott Joule
James Prescott Joule FRS was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work . This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The...

 cycle
.

Design

A ramjet is designed around its inlet. An object moving at high speed through air generates a high pressure region in front and a low pressure region to the rear. A ramjet uses this high pressure in front of the engine to force air through the tube, where
it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

.

A ramjet is sometimes referred to as a 'flying stovepipe', a very simple device comprising an air intake, a combustor, and a nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

. Normally the only moving parts are those within the turbopump
Turbopump
A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...

, which pumps the fuel to the combustor in a liquid-fuel ramjet. Solid-fuel ramjets are even simpler.

By way of contrast, a turbojet uses a gas turbine driven fan
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 to compress the air further. This gives greater compression and efficiency and far more power at low speeds, where the ram effect is weak, but is also more complex, heavier and expensive, and the temperature limits of the turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

 section limit the top speed and thrust at high speed.

Inlet

Ramjets try to exploit the very high dynamic pressure within the air approaching the intake lip. An efficient intake will recover much of the freestream stagnation pressure
Stagnation pressure
In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...

, which is used to support the combustion and expansion process in the nozzle.

Most ramjets operate at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 flight speeds and use one or more conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...

 (or oblique) shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s, terminated by a strong normal shock, to slow down the airflow to a subsonic velocity at the exit of the intake. Further diffusion is then required to get the air velocity down to a suitable level for the combustor.

Subsonic ramjets do not need such a sophisticated inlet since the airflow is already subsonic and a simple hole is usually used. This would also work at slightly supersonic speeds, but as the air will choke at the inlet, this is inefficient.

The Inlet is divergent, to provide a constant inlet speed of Mach 0.5.

Combustor

As with other jet engines the combustor's job is to create hot air. It does this by burning a fuel with the air at essentially constant pressure. The airflow through the jet engine is usually quite high, so sheltered combustion zones are produced by using flame holder
Flame holder
A flame holder is a component of a jet engine designed to help maintain continual combustion.All continuous-combustion jet engines require a flame holder. A flame holder creates a low-speed eddy in the engine to prevent the flame from being blown out...

s
that stop the flames from blowing out.

Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

 fuel:air ratios, which implies a combustor exit stagnation temperature
Stagnation temperature
In thermodynamics and fluid mechanics, stagnation temperature is the temperature at a stagnation point in a fluid flow. At a stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy and is added to the local static enthalpy...

 of the order of 2400 K for kerosene. Normally the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds/altitudes. Usually a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...

, during turns. Other flame stabilization techniques make use of flame holders, which vary in design from combustor cans to simple flat plates, to shelter the flame and improve fuel mixing. Overfuelling the combustor can cause the normal shock within a supersonic intake system to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and net thrust.

Nozzles

The propelling nozzle
Propelling nozzle
A propelling nozzle is the component of a jet engine that operates to constrict the flow, to form an exhaust jet and to maximise the velocity of propelling gases from the engine....

 is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust.

For a ramjet operating at a subsonic flight Mach number, exhaust flow is accelerated through a converging nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

. For a supersonic flight Mach number, acceleration is typically achieved via a convergent-divergent nozzle
De Laval nozzle
A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...

.

Performance and control

Ramjets have been run from as low as 45 m/s (162 km/h) upwards. Below about Mach 0.5 they give little thrust and are highly inefficient due to their low pressure ratios.

Above this speed, given sufficient initial flight velocity, a ramjet will be self-sustaining. Indeed, unless the vehicle drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 is extremely high, the engine/airframe combination will tend to accelerate to higher and higher flight speeds, substantially increasing the air intake temperature. As this could have a detrimental effect on the integrity of the engine and/or airframe, the fuel control system must reduce engine fuel flow to stabilize the flight Mach number
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

 and, thereby, air intake temperature to reasonable levels.

Due to the stoichiometric combustion temperature, efficiency is usually good at high speeds (Mach 2-3), whereas at low speeds the relatively poor compression ratio means that ramjets are outperformed by turbojets, or even rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

s.

Ramjet Types

Ramjets can be classified according to the type of fuel, liquid or solid; and the booster.

In a liquid fuel ramjet (LFRJ) hydrocarbon fuel (typically) is injected into the combustor ahead of a flameholder which stabilises the flame resulting from the combustion of the fuel with the compressed air from the intake(s). A means of pressurizing and supplying the fuel to the ramcombustor is required which can be complicated and expensive. Aérospatiale-Celerg
Aérospatiale
Aérospatiale was a French aerospace manufacturer that built both civilian and military aircraft, rockets and satellites. It was originally known as Société Nationale Industrielle Aérospatiale...

 have designed an LFRJ where the fuel is forced into the injectors by an elastomer bladder which inflates progressively along the length of the fuel tank. Initially the bladder forms a close-fitting sheath around the compressed air bottle from which it is inflated, which is mounted lengthwise in the tank. This offers a lower cost approach than a regulated LFRJ requiring a turbopump and associated hardware to supply the fuel.

A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The first ramjet-powered missiles used external boosters, usually solid-propellant rockets, either in tandem, where the booster is mounted immediately aft of the ramjet, e.g. Sea Dart
Sea Dart missile
Sea Dart or Guided Weapon System 30 is a British surface-to-air missile system designed by Hawker Siddeley Dynamics and built by British Aerospace from 1977...

, or wraparound where multiple boosters are attached alongside the outside of the ramjet e.g. SA-4 Ganef. The choice of booster arrangement is usually driven by the size of the launch platform. A tandem booster increases the overall length of the system whereas wraparound boosters increase the overall diameter. Wraparound boosters will usually generate higher drag than a tandem arrangement.

Integrated boosters provide a more efficient packaging option since the booster propellant is cast inside the otherwise empty combustor. This approach has been used on solid, for example SA-6 Gainful
SA-6 Gainful
The 2K12 "Kub" mobile surface-to-air missile system is a Soviet low to medium-level air defence system designed to protect ground forces from air attack. "2К12" is the GRAU designation of the system...

, liquid, for example ASMP
Air-Sol Moyenne Portée
The Air-Sol Moyenne Portée is a French air-launched nuclear missile. Part of the Force de frappe, in French nuclear doctrine it is the last-resort "warning shot" prior to a full-scale employment of strategic nuclear weapons...

, and ducted rocket, for example Meteor
MBDA Meteor
Meteor is an active radar guided beyond-visual-range air-to-air missile being developed by MBDA. It is to equip the Eurofighter Typhoons of the UK's Royal Air Force , Germany's Luftwaffe, Spain's Ejército del Aire and Italy's Aeronautica Militare Italiana, the British and Italian F-35s , the...

, designs. Integrated designs are complicated by the different nozzle requirements of the boost and ramjet phases of flight. Due to the higher thrust levels of the booster a different shaped nozzle is required for optimum thrust compared to that required for the lower thrust ramjet sustainer. This is usually achieved via a separate nozzle which is ejected after booster burnout. However, designs such as Meteor feature nozzleless boosters. This offers the advantages of elimination of the hazard to launch aircraft from the ejected boost nozzle debris, simplicity, reliability, and reduced mass and cost, although this must be traded against the reduction in performance compared with that provided by a dedicated booster nozzle.

Integral rocket ramjet/ducted rocket

These are a slight variation on the ramjet where the supersonic exhaust from a rocket combustion process is used to compress and react with the incoming air in the main combustion chamber. This has the advantage of giving thrust even at zero speed.

In a solid fuel integrated rocket ramjet (SFIRR) the solid fuel is cast along the outer wall of the ramcombustor. In this case fuel injection is through ablation of the propellant by the hot compressed air from the intake(s). An aft mixer may be used to improve combustion efficiency. SFIRRs are preferred over LFRJs for some applications because of the simplicity of the fuel supply but only when the throttling requirements are minimal i.e. when variations in altitude or Mach number are limited.

In a ducted rocket a solid fuel gas generator produces a hot fuel-rich gas which is burnt in the ramcombustor with the compressed air supplied by the intake(s). The flow of gas improves the mixing of the fuel and air and increases total pressure recovery. In a Throttleable Ducted Rocket (TDR), also known as a Variable Flow Ducted Rocket (VFDR), a valve allows the gas generator exhaust to be throttled allowing control of the thrust. Unlike an LFRJ solid propellant ramjets cannot flameout
Flameout
A flameout refers to the failure of a jet engine caused by the extinction of the flame in the combustion chamber. It can be caused by a number of factors, including fuel exhaustion; compressor stall; insufficient oxygen supply; foreign object damage ; severe inclement weather; and mechanical...

. The ducted rocket sits somewhere between the simplicity of the SFRJ and the unlimited throttleability of the LFRJ.

Flight speed

Ramjets generally give little or no thrust below about half the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

, and they are inefficient (less than 600 seconds
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

) until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. Even above the minimum speed a wide flight envelope
Flight envelope
In aerodynamics, the flight envelope or performance envelope of an aircraft refers to the capabilities of a design in terms of airspeed and load factor or altitude. The term is somewhat loosely applied, and can also refer to other measurements such as maneuverability...

 (range of flight conditions), such as low to high speeds and low to high altitudes, can force significant design compromises, and they tend to work best optimised for one designed speed and altitude (point designs). However, ramjets generally outperform gas turbine based jet engine designs and work best at supersonic speeds (Mach 2–4). Although inefficient at slower speeds they are more fuel-efficient than rockets over their entire useful working range up to at least Mach 5.5.

The performance of conventional ramjets falls off above Mach 6 due to dissociation and pressure loss caused by shock as the incoming air is slowed to subsonic velocities for combustion. In addition, the combustion chamber's inlet temperature increases to very high values, approaching the dissociation limit at some limiting Mach number.

Air turboramjet

Another example of this is the Air TurboRamjet (ATR) which has a compressor powered by a gas heated via a heat exchanger within the combustion chamber.

Scramjets

Ramjets always slow the incoming air to a subsonic velocity within the combustor. Scramjet
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...

s, or "supersonic combustion ramjet" are similar to ramjets, but some of the air goes through the entire engine at supersonic speeds. This increases the stagnation pressure recovered from the freestream and improves net thrust. Thermal choking of the exhaust is avoided by having a relatively high supersonic air velocity at combustor entry. Fuel injection is often into a sheltered region below a step in the combustor wall. Although scramjet engines have been studied for many decades it is only recently that small experimental units have been flight tested and then only very briefly (e.g. the Boeing X-43
Boeing X-43
The X-43 is an unmanned experimental hypersonic aircraft with multiple planned scale variations meant to test various aspects of hypersonic flight. It was part of NASA's Hyper-X program. It has set several airspeed records for jet-propelled aircraft....

).

As of May, 2010, this engine has been tested to attain Mach 5 for 200 seconds on the X-51A Waverider.

In the scramjet, the ram air is not slowed to subsonic speeds for combustion and as a result, shocks are not encountered and pressure loss is avoided.

Precooled engines

A variant of the pure ramjet is the 'combined cycle' engine, intended to overcome the limitations of the pure ramjet. One example of this is the SABRE engine; this uses a precooler, behind which is ramjet and turbine machinery.

The ATREX
ATREX
The ATREX engine developed in Japan is an experimental precooled jet engine that works as a turbojet at low speeds and a ramjet up to mach 6.0....

 engine developed in Japan is an experimental implementation of this concept. It uses liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 fuel in a fairly exotic single-fan arrangement. The liquid hydrogen fuel is pumped through a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 in the air-intake, simultaneously heating the liquid hydrogen, and cooling the incoming air. This cooling of the incoming air is critical to achieving a reasonable efficiency. The hydrogen then continues through a second heat exchanger position after the combustion section, where the hot exhaust is used to further heat the hydrogen, turning it into a very high pressure gas. This gas is then passed through the tips of the fan providing driving power to the fan at sub-sonic speeds. After mixing with the air it is burned in the combustion chamber.

The Reaction Engines Scimitar
Reaction Engines Scimitar
The Reaction Engines Scimitar is a derivative of the SABRE engine technology, but intended for airliners , rather than space launch applications. Consequently, most of the Scimitar engine technology is similar to Sabre but designed for much longer life...

 has been proposed for the LAPCAT
LAPCAT
LAPCAT was a 36 month European FP6 study to examine ways to produce engines for a Mach 4-8 hypersonic aircraft. The project ended in April 2008...

 hypersonic airliner, and the Reaction Engines SABRE for the Reaction Engines Skylon spaceplane.

Nuclear powered ramjets

During the Cold War
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...

 the United States designed and ground-tested a nuclear-powered ramjet called Project Pluto
Project Pluto
Project Pluto was a United States government program to develop nuclear powered ramjet engines for use in cruise missiles. Two experimental engines were tested at the United States Department of Energy Nevada Test Site in 1961 and 1964.-History:...

. This system used no combustion – a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 heated the air instead. The project was ultimately canceled because ICBMs seemed to serve the purpose better, and because a low-flying radioactive missile could cause problems for any allied soldiers.

J58

The SR-71's Pratt & Whitney J58
Pratt & Whitney J58
The Pratt & Whitney J58 was a jet engine used on the Lockheed A-12, and subsequently on the YF-12 and SR-71 Blackbird aircraft. The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. The J58 was a single-spool turbojet engine with an afterburner...

 engines act as turbojet-assisted ramjets at high-speeds (Mach 3.2).

Ionospheric ramjet

The upper atmosphere above about 100 km contains monatomic oxygen that has been produced by the sun through photochemistry. A concept was created by NASA for recombining this thin gas back to diatomic molecules at orbital speeds to power a ramjet.

Bussard ramjet

The Bussard ramjet is a space drive concept that is intended to fuse
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 interstellar wind and exhaust it at high speed from the rear of the vehicle.

Aircraft using ramjets

  • Hiller Hornet (a ramjet-powered helicopter)
  • Focke-Wulf Triebflügel
  • Leduc experimental aircraft
    Leduc experimental aircraft
    |-See also:- External links :* http://xplanes.free.fr/stato/stato-2.html...

  • Lockheed D-21
    Lockheed D-21/M-21
    The Lockheed D-21 was an American Mach 3+ reconnaissance drone. The D-21 was initially designed to be launched from the back of its M-21 carrier aircraft, a variant of the Lockheed A-12 aircraft. Development began in October 1962...

  • Lockheed X-7
    Lockheed X-7
    -See also:-External links:*...

  • Nord 1500 Griffon
    Nord 1500 Griffon
    -See also:-Bibliography:*Taylor, John W.R. Jane's Pocket Book of Research and Experimental Aircraft, London, Macdonald and Jane's Publishers Ltd, 1976. ISBN 0356 08409 4.- External links :* *****...

  • Republic XF-103
    Republic XF-103
    The Republic XF-103 was an American project to develop a high speed interceptor aircraft capable of destroying Soviet bombers. Despite a prolonged development, it never progressed past the mock-up stage.-Development:...

  • SR-71 Blackbird
    SR-71 Blackbird
    The Lockheed SR-71 "Blackbird" was an advanced, long-range, Mach 3+ strategic reconnaissance aircraft. It was developed as a black project from the Lockheed A-12 reconnaissance aircraft in the 1960s by the Lockheed Skunk Works. Clarence "Kelly" Johnson was responsible for many of the...

     (Turbojet engines that function as ramjets at mach 1+ speeds.)

Missiles using ramjets

  • Bomarc
    Bomarc Missile Program
    The CIM-10 Bomarc was the only surface-to-air missile ever deployed by the United States Air Force. All other U.S. land-based SAMs were and are under the control of the United States Army....

  • BrahMos
    BrahMos
    BrahMos is a stealth supersonic cruise missile that can be launched from submarines, ships, aircraft or land. It is a joint venture between Republic of India's Defence Research and Development Organisation and Russian Federation's NPO Mashinostroeyenia who have together formed BrahMos Aerospace...

  • MBDA Meteor
    MBDA Meteor
    Meteor is an active radar guided beyond-visual-range air-to-air missile being developed by MBDA. It is to equip the Eurofighter Typhoons of the UK's Royal Air Force , Germany's Luftwaffe, Spain's Ejército del Aire and Italy's Aeronautica Militare Italiana, the British and Italian F-35s , the...

  • Bristol Bloodhound
  • Bendix RIM-8 Talos
    RIM-8 Talos
    The Bendix RIM-8 Talos was a long-range naval surface-to-air missile, and was among the earliest surface-to-air missiles to equip United States Navy ships. The Talos used radar beam riding for guidance to the vicinity of its target, and semiactive radar homing for terminal guidance...

  • North American SM-64 Navaho
    SM-64 Navaho
    The North American SM-64 Navaho was a supersonic intercontinental cruise missile project built by North American Aviation. The program ran from 1946 to 1958 when it was cancelled in favor of intercontinental ballistic missiles...

  • P-270 Moskit
  • Akash missile
    Akash missile
    Akash is India's medium range surface-to-air missile defense system developed by the Defence Research and Development Organisation and Bharat Electronics Limited as part of the Integrated Guided Missile Development Program. The missile can target aircraft up to 30 km away, at altitudes up to...

  • Sea Dart missile
    Sea Dart missile
    Sea Dart or Guided Weapon System 30 is a British surface-to-air missile system designed by Hawker Siddeley Dynamics and built by British Aerospace from 1977...

  • 2K11 Krug
  • MBDA ASMP
    Air-Sol Moyenne Portée
    The Air-Sol Moyenne Portée is a French air-launched nuclear missile. Part of the Force de frappe, in French nuclear doctrine it is the last-resort "warning shot" prior to a full-scale employment of strategic nuclear weapons...


External links

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