Ramjet Engine – How It Works & Its Evolution
Simple ramjet operation, with Mach numbers of flow
A ramjet is a type
of airbreathing jet engine that requires forward motion of engine to
provide air for combustion. Ramjets work most efficiently at supersonic
speeds around Mach 3 or (2,300 mph; 3,700 km/h) and
can operate up to Mach 6 (4,600 mph; 7,400 km/h). Ramjets can be
particularly useful in missiles where they
require a compact mechanism with high-speed. Weapons designers are investigating ramjet
technology for use in artillery shells to increase range; a
120 mm ramjet-assisted mortar shell is thought to be able to travel
35 km (22 mi). They have been used, though not efficiently,
as tip-jets on the ends of helicopter rotors.
History
France
Leduc 010
Cyrano de
Bergerac
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 the ramjet, and as the
first fictional example of rocket-powered space flight.
René Lorin
The ramjet was designed in 1913
by French inventor René Lorin, who got a patent (FR290356) for his device.
He could not test his invention due to the unavailability of adequate equipment
since there was no way at the time for an aircraft to go fast enough for a
ramjet to function properly. His patent showed a piston internal
combustion engine with added 'trumpets' as exhaust nozzles, expressing the idea
that the exhaust from internal combustion engines could be directed into
nozzles to create jet propulsion.
René Leduc
René Leduc’s works were
notable. Leduc's Model, the Leduc 0.10 was one of the first
ramjet-powered aircraft to fly, in 1949.
Nord Aviation
The Nord 1500 Griffon reached
Mach 2.19 (745 m/s; 2,680 km/h) in 1958.
Austria-Hungary
Albert Fonó
In 1915, Hungarian inventor Albert
Fonó devised a solution to increase range of artillery, comprising a
gun-launched projectile 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. in May 1928 he
described an "air-jet engine" as 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 (German Patent No. 554,906, 1932-11-02).
Soviet Union
Kh-31 missile ramjet exhaust
In 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 research. 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 bar,
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 the R-3, a two-stage rocket. 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 took off in December 1940, using two DM-2
engines on a modified Polikarpov I-15. Merkulov designed a ramjet fighter
"Samolet D" in 1941. 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
working on a long-range antipodal bomber, same as Sänger-Bredt bomber, but
powered by ramjet. In 1954, NPO Lavochkin with Keldysh Institute started
developing, Burya; a Mach 3 ramjet-powered cruise missile. This
project competed with the R-7 ICBM developed by Sergei Korolev,
but was cancelled in 1957.
Japan
Several ramjets were designed,
built, and ground-tested at the Kawasaki Aircraft Company's facility in Gifu
during the Second World War. Company officials claimed, in December 1945, that
these domestic initiatives were uninfluenced by parallel German developments.
One post-war U.S. intelligence assessment described the Kawasaki ramjet's
centrifugal fuel disperser as the company's "most outstanding
accomplishment ... eliminat[ing] a large amount of the fuel injection system
normally employed." Because of excessive vibration, the engine was
only intended for use in rocket, or catapult-launched pilotless aircraft.
Preparations for flight testing ended with the Japanese surrender in August
1945.
Germany
In 1936, Hellmuth Walter constructed
a test engine powered by natural gas. Theoretical work was carried out
at BMW, Junkers, and DFL. In 1941, Eugen Sänger of DFL
proposed a ramjet engine with a high combustion chamber temperature. He built
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, as petrol became scarce in Germany, tests
were carried out with blocks of pressed coal dust as a fuel, which were not
successful due to slow combustion.
United States
AQM-60
Kingfisher first production ramjet to enter service with US military
Stovepipe
(flying/flaming/supersonic) was a popular name for ramjet during 1950s in trade
magazines such as Aviation Week & Space Technology and
other publications like The Cornell Engineer. The simplicity
implied by the name came from a comparison with the turbojet engine
which employs relatively complex and expensive spinning turbomachinery. 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 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 early 1950s the US developed a Mach 4+ ramjet
under the Lockheed X-7 program that was developed into Lockheed
AQM-60 Kingfisher. Further development resulted in the Lockheed D-21 spy
drone.
In the late 1950s the US Navy
introduced RIM-8 Talos, which was a long-range surface-to-air missile fired
from ships. It successfully shot down enemy fighters during the Vietnam
War, and was the 1st ship-launched missile to destroy an enemy aircraft in
combat. On 23 May 1968, a Talos fired from USS Long Beach shot
down a North Vietnamese MiG from 105 kms or 65 miles away. It was
also used as a surface-to-surface weapon and was modified to destroy land-based
radars.
Using technology proven by
AQM-60, USA produced CIM-10 Bomarc, a widespread defense system. It 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 ensure longer flight
times. The system was withdrawn in the 1970s as the threat from bombers
subsided.
THOR-ER
In April 2020, the U.S.
Department of Defense and the Norwegian Ministry of Defense partnered to
develop advanced technologies applicable to long range high-speed and
hypersonic weapons. Tactical High-speed Offensive Ramjet for Extended Range
(THOR-ER) program completed a Solid Fuel Ram Jet (SFRJ) vehicle test
in August 2022.
Dual-mode ramjet
In 2023, General Electric demonstrated
a ramjet with rotating detonation combustion. It is a turbine-based
combined-cycle engine that incorporates a gas turbine; a rotating detonation
engine; a ramjet; a scramjet.
United Kingdom
Upper engine is a ramjet on the Bloodhound missile
In late 1950s, 60s, and early 70s,
the UK developed several ramjet missiles. Blue Envoy was supposed to equip UK
with a long-range ramjet powered air defense against bombers, but it was
cancelled and replaced by Bloodhound, a short-range ramjet missile system. Bloodhound
was designed as a second line of defense in case attackers bypassed the fleet
of defending English Electric Lightning fighters. In 1960s, Royal
Navy developed and deployed Sea Dart, a ramjet powered surface to air
missile for ships with range of 65–130 kms and speed of Mach 3. It was used
successfully in combat against multiple types of aircraft during Falklands War.
Fritz Zwicky
Fritz Zwicky, the eminent Swiss
astrophysicist was research director at Aerojet and holds many
patents in jet propulsion. Patents US 5121670 and US 4722261 are
for ram accelerators. U.S. Navy would not allow Zwicky to publicly discuss
his invention, US 2461797 is for Underwater Jet, a ramjet that
performs in fluid medium.
Design
A typical ramjet
The first part of a ramjet is its
diffuser (compressor) in which the forward motion of the ramjet is used to
raise the pressure of its working fluid (air) as required for combustion. Air
is compressed, heated by combustion and expanded in a thermodynamic cycle known
as the Brayton cycle, before being passed through a nozzle to accelerate
it to supersonic speeds and generate forward thrust.
Ramjets are much less complex
than turbojets or turbofans, requiring only an air intake, a combustor,
and a nozzle to be built. Additionally, ramjets have little to no
moving parts - liquid-fuel ramjets have only a fuel pump, whilst solid-fuel
ramjets lack even this. By comparison, a turbojet uses a compressor
driven by a turbine, which generates its own compressed air (i.e. ram air
in a ramjet) in order to generate thrust.
Construction
Diffuser
The diffuser converts the high
velocity of the air approaching the intake into high (static) pressure required
for combustion. High combustion pressures minimize entropy rise during heat
addition, thus minimizing wasted thermal energy in the exhaust gases.
Subsonic
and low-supersonic ramjets use a pitot-type opening for the inlet. This is
followed by a widening internal passage (subsonic diffuser) to achieve a lower
subsonic velocity that is required at the combustor. At low supersonic speeds a
normal (planar) shock wave forms in front of the inlet.
For higher supersonic speeds the
pressure loss through the shock wave becomes prohibitive and a protruding spike
or cone is used to produce oblique shock waves in front of a final normal shock
that occurs at the inlet entrance lip. The diffuser in this case consists of
two parts: the supersonic diffuser, with 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, requiring 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.
Combustor
As with other jet engines,
combustor raises the air temperature by burning fuel. This takes place with a
small pressure loss. Air velocity entering the combustor has to be low enough so
that continuous combustion can take place in sheltered zones provided by flame
holders. A ramjet combustor can safely operate at stoichiometric fuel:air
ratios. This implies that 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, matching 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 flat
plates, to shelter the flame and improve fuel mixing. Over-fueling 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 airflow and
thrust.
Nozzles
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, 2 Thor engines were used on Bristol Bloodhound missile
Performance and
control
Although ramjets have been run as
slow as 45 metres per second (160 km/h; 100 mph),[22] below
about Mach 0.5 (170 m/s; 610 km/h; 380 mph) they give
little thrust and are highly inefficient due to their low pressure ratios. Above
this speed, given sufficient initial flight velocity, a ramjet is
self-sustaining. Unless the vehicle drag is extremely high, the
engine/airframe combination tends to accelerate to higher and higher flight
speeds, substantially increasing the air intake temperature. As this could
damage the engine and/or airframe integrity, the fuel control system must reduce
fuel flow to stabilize speed and, thereby, air intake temperature. 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, 1,500–2,300 mph), whereas at low speeds the
relatively low pressure means the ramjets are outperformed by turbojets and rockets.
Control
Ramjets can be classified
according to the type of fuel, either liquid or solid; and the booster.
Liquid fuel
In a liquid fuel ramjet (LFRJ),
hydrocarbon fuel (typically) is injected into the combustor ahead of a
flameholder. The flameholder stabilizes the flame with the compressed air from
the intake(s). A means of pressurizing and supplying the fuel to the ram combustor
is required, which can be complicated and expensive. This propulsion system was
first perfected by Yvonne Brill during her stint at Marquardt
Corp. Aérospatiale-Celerg designed an LFRJ where the fuel is forced into
the injectors by an elastomer bladder that 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 pump system to supply the fuel.
Take-off
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, or
wraparound where multiple boosters are attached around the outside of the
ramjet, e.g. 2K11 Krug. The choice of booster arrangement is usually
driven by the size of the launch platform. A tandem booster increases the
length of the system, whereas wraparound boosters increase the diameter.
Wraparound boosters typically 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-Fuel
Ramjets (SFRJ) based missiles like in 2K12 Kub, liquid like in ASMP,
and ducted rocket like in Meteor. Integrated designs are complicated by
the different nozzle requirements of the boost and ramjet flight phases. Due to
the booster's higher thrust levels, a differently 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 nozzle-less boosters. This
offers advantage of elimination of hazard to launch aircraft from the boost
debris, simplicity, reliability, and reduced mass and cost, although it
must be traded against reduction in performance of a dedicated booster nozzle.
Integral rocket
ramjet/ducted rocket
A slight variation on the ramjet
uses supersonic exhaust from a rocket combustion process to compress and react
with the incoming air in the main combustion chamber. It has 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 ram combustor. 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 due to
simplicity of fuel supply, but only when throttling requirement are minimal,
i.e. when variations in altitude or speed are limited.
In a ducted rocket, a solid fuel
gas generator produces a hot fuel-rich gas which is burnt in the ram combustor
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, also known as a variable flow ducted rocket, a
valve allows the gas generator exhaust to be throttled allowing thrust control.
Unlike an LFRJ, solid propellant ramjets can’t flame out. The ducted
rocket sits somewhere between the simplicity of the SFRJ and LFRJ's unlimited
speed control.
Flight speed
Ramjets generally give little or
no thrust below about half the speed of sound, and they are inefficient (specific
impulse of less than 600 seconds) until the airspeed exceeds 1,000 kmph
(280 m/s; 620 mph) due to low compression ratios. It takes from
300mph (485km/h) at sea level onwards to work effectively. Even above the
minimum speed, a wide flight envelope (range of flight conditions),
such as low to high speeds and low to high altitudes, can force significant
design compromises, and tend to work best optimized 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 6 (2,000 m/s; 7,400 km/h). 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.
Related engines
Air turboramjet
Recreated schematic of an air turboramjet, featuring
1. compressor
2. gearbox
3. hydrogen and oxygen lines
4. gas generator
5. turbine
6. ram burner fuel injector
7. main combustor
8. nozzle
The air turboramjet engine is a
combined cycle engine that merges aspects of turbojet and ramjet
engines. The turboramjet is a hybrid engine that essentially consists of a
turbojet mounted inside a ramjet. The turbojet core is mounted inside a duct
that contains a combustion chamber downstream of the turbojet nozzle. The
turboramjet can be run in turbojet mode at takeoff and during low-speed flight
but then switch to ramjet mode to accelerate to high Mach numbers.
Supersonic-combustion
ramjets (scramjets)
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 combustor at supersonic speed.
This increases pressure recovered from the streaming air and improves net
thrust. Thermal choking of 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. Boeing X-43 was a
small experimental ramjet that achieved Mach 5 (1,700 m/s;
6,100 km/h) for 200 seconds on the X-51A Waverider.
Standing oblique
detonation ramjets (Sodramjets)
A shock-induced combustion ramjet engine (abbreviated as shcramjet; also called oblique detonation wave engine; also called standing oblique detonation ramjet (sodramjet); or simply shock-ramjet engine) is a concept of air-breathing ramjet engine, proposed to be used for hypersonic and/or single-stage-to-orbit propulsion application.
Pre-cooled
engines
A variant of the ramjet is the
'combined cycle' engine, intended to overcome the ramjet's limitations. One
example of this is the SABRE engine, which 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 single-fan arrangement. The liquid fuel is pumped
through a heat exchanger in the air intake, simultaneously heating
the fuel and cooling the incoming air. This cooling is critical to efficient
operation.
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 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 was proposed for the
LAPCAT hypersonic airliner, and the Reaction Engines SABRE was
proposed for the Reaction Engines Skylon spaceplane.
Nuclear-powered
ramjet
United States
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. 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 vehicle flight path (although its exhaust wasn't radioactive). The
project was ultimately cancelled because ICBMs seemed to serve the
purpose better. This type of engine could be used
for the exploration of planetary atmospheres such as Jupiter's.
Russia
On 1 March 2018 President
Vladimir Putin announced a nuclear-powered ramjet cruise missile capable of
extended long-range flight. It was designated 9M730 "Burevestnik"
(Petrel) and has the NATO reporting name SSC-X-9 "Skyfall". 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 that had landed in the White Sea during testing
in 2018 when the nuclear power source of the missile detonated and killed
5 researchers.
Ionospheric
ramjet
The upper atmosphere above about
100 kms (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.
Bussard ramjet
An artist's
conception of a Bussard ramjet
A major component of an actual
ramjet, a miles-wide electromagnetic field is invisible.
Bussard ramjet
in motion
1. Interstellar
medium
2. Collect
and compress hydrogen
3. Transport
hydrogen beside the payload
4. Thermonuclear
fusion
5. Engine
nozzle
6. Flue gas
jet
Bussard ramjet is a
theoretical method of spacecraft propulsion for interstellar
travel. A fast-moving spacecraft scoops up hydrogen from the interstellar
medium using an enormous funnel-shaped magnetic field (ranging from few kms
to many thousands of kms in diameter). The hydrogen is compressed until thermonuclear
fusion occurs, which provides thrust to counter the drag created by the
funnel and energy to power the magnetic field. The Bussard ramjet can thus be
seen as a ramjet variant of a fusion rocket.
Ramjet mode for
an afterburning turbojet
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 ram-burner. 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 propulsion
system for Lockheed SR-71 Blackbird with an EPR=0.9 at Mach 3.2. 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.
Aircraft using
ramjets
• AQM-60
Kingfisher
• Focke-Wulf
Super Lorin
• Focke-Wulf
Ta 283
• Focke-Wulf
Triebflügel
• Hiller
Hornet
• Leduc
experimental aircraft
• Lockheed
D-21
• Lockheed
X-7
• NHI
H-3 Kolibrie
• Nord
1500 Griffon
• Republic
XF-103
• Škoda-Kauba
P14
Missiles using
ramjets
• 2K11
Krug
• 2K12
Kub
• ASM-3
• Astra
• Bloodhound
(missile)
• BrahMos
• CIM-10
Bomarc
• Orbital
Sciences GQM-163 Coyote
• Hsiung
Feng III
• Kh-31
• MBDA
ASMP
• MBDA
Meteor
• P-270
Moskit
• P-800
Oniks
• R-77PD
• R-77ME
• Bendix
RIM-8 Talos
• Sea
Dart
• North
American SM-64 Navaho
• Solid
Fuel Ducted Ramjet
• YJ-12
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Ramjet Engine – How It Works & Its Evolution
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