Ramjet

L'Autre Monde: ou les États et Empires de la Lune (Comical History of the States and Empires of the Moon) (1657) was the first of three satirical novels written by Cyrano de Bergerac that are considered among the first science fiction stories. Arthur C Clarke credited this book with conceiving of the ramjet,[3] and being the first fictional example of a rocket-powered space flight.

The ramjet was conceived in 1913 by French inventor René Lorin, who was granted a patent for his device. Attempts to build a prototype failed due to inadequate materials.[4] His patent FR290356 showed a piston internal combustion engine with added 'trumpets' as exhaust nozzles.

In 1915, Hungarian inventor Albert Fonó devised a solution for increasing the range of artillery, 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, but the proposal was rejected.[5] 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 granted in 1932 after four years of examination (German Patent No. 554,906, 1932-11-02).[6]

In the Soviet Union, a theory of supersonic ramjet engines was presented in 1928 by Boris Stechkin. Yuri Pobedonostsev, chief of GIRD'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 20,000 kilopascals (200 atm), 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. That August, 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 1940, 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 a liquid fuel rocket for take-off and ramjet engines for flight. That project was cancelled in 1944.

In 1947, Mstislav Keldysh proposed a long-range antipodal bomber, 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 Mach 3 ramjet-powered cruise missile, Burya. This project competed with the R-7 ICBM being developed by Sergei Korolev, and was cancelled in 1957.

On 1 March 2018 President Vladimir Putin announced Russia had developed a nuclear powered ramjet cruise missile capable of extended long range flight. It has been designated 9M730 "Burevestnik" (Petrel) by Russia and has the NATO reporting name SSC-X-9 "Skyfall".[7] On 9 August 2019, an explosion and release of radioactive material was recorded at the State Central Navy Testing Range. Recovery efforts were underway to raise a test article which had landed in the White Sea during testing in 2018 when the nuclear power source of the missile detonated and killed 5 researchers.[8]

In 1936, Hellmuth Walter constructed a test engine powered by natural gas. Theoretical work was carried out at BMW and Junkers, as well as DFL. In 1941, Eugen Sänger of DFL proposed a ramjet engine with a very high combustion chamber temperature. He constructed very large ramjet pipes with 500 millimetres (20 in) and 1,000 millimetres (39 in) diameter and carried out combustion tests on lorries and on a special test rig on a Dornier Do 17Z at flight speeds of up to 200 metres per second (720 km/h). Later, with petrol becoming scarce in Germany due to wartime conditions, tests were carried out with blocks of pressed coal dust as a fuel (see e.g. Lippisch P.13a), which were not successful due to slow combustion.[9]

An AQM-60 Kingfisher, the first production ramjet to enter service with the US military

Stovepipe (flying/flaming/supersonic) was a popular name for the ramjet during the 1950s in trade magazines such as Aviation Week & Space Technology[10] and other publications such as The Cornell Engineer[11] and the Journal Of The American Rocket Society.[12] The simplicity implied by the name came from a comparison with the turbojet engine which also has, together with the inlet, combustor and nozzle of a ramjet, complex and expensive spinning turbomachinery (compressor and turbine).

The US Navy developed a series of air-to-air missiles under the name of "Gorgon" using different propulsion mechanisms, including ramjet propulsion on the Gorgon IV. The ramjet Gorgon IVs, made by Glenn Martin, were tested in 1948 and 1949 at Naval Air Station Point Mugu. The ramjet engine itself was designed at the University of Southern California and manufactured by the Marquardt Aircraft Company. The engine was 2.1 metres (7 ft) long and 510 millimetres (20 in) in diameter and was positioned below the missile.

In the early 1950s the US developed a Mach 4+ ramjet under the Lockheed X-7 program. This was developed into the Lockheed AQM-60 Kingfisher. Further development resulted in the Lockheed D-21 spy drone.

In the late 1950s the US Navy introduced a system called the RIM-8 Talos, which was a long range surface-to-air missile fired from ships. It successfully shot down several enemy fighters during the Vietnam war, and was the first ship-launched missile to destroy an enemy aircraft in combat. On 23 May 1968, a Talos fired from USS Long Beach shot down a Vietnamese MiG at a range of about 105 kilometres (65 mi). It was also used as a surface-to-surface weapon and was modified to destroy land-based radars.

Using the technology proven by the AQM-60, In the late 1950s and early 1960s the US produced a widespread defense system called the CIM-10 Bomarc, which was equipped with hundreds of nuclear armed ramjet missiles with a range of several hundred miles. It was powered by the same engines as the AQM-60, but with improved materials to withstand the longer flight times. The system was withdrawn in the 1970s as the threat from bombers was reduced.

In April 2020, the U.S. Department of Defense and the Norwegian Ministry of Defense jointly announced their partnership to develop advanced technologies applicable to long range high-speed and hypersonic weapons. The Tactical High-speed Offensive Ramjet for Extended Range (THOR-ER) program completed a solid fuel ramjet (SFRJ) vehicle test in August 2022.[13]

A Bloodhound on display at the RAF Museum, Hendon, London.

In the late 1950s and early 1960s the UK developed several ramjet missiles.

A project called Blue Envoy was supposed to equip the country with a long range ramjet powered air defense against bombers, but the system was eventually cancelled.

It was replaced by a much shorter range ramjet missile system called the Bloodhound. The system was designed as a second line of defense in case attackers were able to bypass the fleet of defending English Electric Lightning fighters.

In the 1960s the Royal Navy developed and deployed a ramjet powered surface to air missile for ships called the Sea Dart. It had a range of 65–130 kilometres (40–80 mi) and a speed of Mach 3. It was used successfully in combat against multiple types of aircraft during the Falklands War.

Eminent Swiss astrophysicist Fritz Zwicky was research director at Aerojet and holds many patents in jet propulsion. U.S. patents 5121670 and 4722261 are for ram accelerators. 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 magazine reported Fritz Zwicky's work in the articles "Missed Swiss" on 11 July 1955[14] and "Underwater Jet" in the 14 March 1949 issue.[15]

Leduc 010

In France, the works of René Leduc 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 reached Mach 2.19 (745 m/s; 2,680 km/h) in 1958.

The diffuser is that part of the ramjet which converts the high velocity of the air approaching the intake into high (static) pressure required for combustion. High combustion pressures minimize wasted thermal energy which appears in the exhaust gases[16] (by reducing entropy rise during heat addition[17]).

Subsonic and low-supersonic ramjets use a pitot-type entrance for the inlet to capture air. This is followed by a widening internal passage (subsonic diffuser) to achieve a lower subsonic velocity which is required at the combustor. At low supersonic speeds a normal (plane) shock wave forms in front of the inlet.

For higher supersonic speeds the pressure loss through a normal shock wave in front of the inlet becomes prohibitive and a protruding spike or cone has to be used to produce oblique shock waves in front of a final normal shock which now occurs at the inlet entrance lip. The diffuser in this case consists of two parts, the supersonic diffuser, with its shock waves external to the inlet, followed by the internal subsonic diffuser.

At higher speeds still, part of the supersonic diffusion has to take place internally so there are external and internal oblique shock waves. The final normal shock has to occur in the vicinity of a minimum flow area known as the throat, which is followed by the subsonic diffuser.

As with other jet engines, the combustor has to raise the temperature of the air by burning fuel. This takes place with a small pressure loss. The air velocity entering the combustor has to be low enough such that continuous combustion can take place in sheltered zones provided by flame holders.

Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric fuel: air ratios, which implies a combustor exit stagnation temperature of the order of 2,400 K (2,130 °C; 3,860 °F) for kerosene. Normally, the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds and altitudes. Usually, a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch 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. Over-fuelling the combustor can cause the final (normal) shock in the diffuser to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and thrust.

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

Subsonic ramjets accelerate exhaust flow with a nozzle. Supersonic flight typically requires a convergent–divergent nozzle.

Bristol Thor ramjet modified for display purposes. Two Thor engines were used on the Bristol Bloodhound missile

Although ramjets have been run as slow as 45 metres per second (160 km/h),[18] below about Mach 0.5 (170 m/s; 610 km/h) 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 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 and, thereby, air intake temperature to reasonable levels.

Due to the stoichiometric combustion temperature, efficiency is usually good at high speeds (around Mach 2 – Mach 3, 680–1,000 m/s, 2,500–3,700 km/h), whereas at low speeds the relatively poor pressure ratio means the ramjets are outperformed by turbojets, or even rockets.

An air turboramjet has a compressor powered by a gas heated via a heat exchanger within the combustion chamber.

Ramjet diffusers slow the incoming air to a subsonic velocity before it enters the combustor. Scramjets are similar to ramjets, but the air flows through the combustor at supersonic speed. 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. The Boeing X-43 was a small experimental ramjet[24] which achieved Mach 5 (1,700 m/s; 6,100 km/h) for 200 seconds on the X-51A Waverider.[25]

Standing oblique detonation ramjets (Sodramjets) replace the diffusive ramjet combustion with an oblique detonation. See also: Shcramjet Criteria for hypersonic airbreathing propulsion and its experimental verification Oblique Detonation Wave Ramjet

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 the ramjet and turbine machinery.

The ATREX engine developed in Japan is an experimental implementation of this concept. It uses liquid hydrogen fuel in a fairly exotic single-fan arrangement. The liquid hydrogen fuel is pumped through a heat exchanger 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 to provide driving power to the fan at subsonic speeds. After mixing with the air, it is burned in the combustion chamber.

The Reaction Engines Scimitar has been proposed for the LAPCAT hypersonic airliner, and the Reaction Engines SABRE for the Reaction Engines Skylon spaceplane.

During the Cold War, the United States designed and ground-tested a nuclear-powered ramjet called Project Pluto. This system, intended for use in a cruise missile, used no combustion; a high-temperature, unshielded nuclear reactor heated the air instead. The ramjet was predicted to be able to fly at supersonic speeds for months. Because the reactor was unshielded, it was dangerous to anyone in or around the flight path of the low-flying vehicle (although the exhaust itself wasn't radioactive). The project was ultimately cancelled because ICBMs seemed to serve the purpose better.[26]

This type of engine could be used for the exploration of planetary atmospheres such as the one of Jupiter.[27]

The upper atmosphere above about 100 kilometres (62 mi) contains monatomic oxygen 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.[28]

The Bussard ramjet is a spacecraft propulsion concept intended to fuse interstellar wind and exhaust it at high speed from the rear of the vehicle.

An afterburning turbojet or bypass engine can be described as transitioning from turbo to ramjet mode if it can attain a flight speed at which the engine pressure ratio (epr) has fallen to one. The turbo afterburner then acts as a ramburner.[29] The intake ram pressure is present at entry to the afterburner but is no longer augmented with a pressure rise from the turbomachinery. Further increase in speed introduces a pressure loss due to the presence of the turbomachinery as the epr drops below one.

A notable example was the propulsion system for the Lockheed SR-71 Blackbird with an epr= 0.9 at Mach 3.2.[30] The thrust required, airflow and exhaust temperature, to reach this speed came from a standard method for increasing airflow through a compressor running at low corrected speeds, compressor bleed, and being able to increase the afterburner temperature as a result of cooling the duct and nozzle using the air taken from the compressor rather than the usual, much hotter, turbine exhaust gas.[31]