A
ramjet, sometimes referred to as a
stovepipe jet, or an
athodyd, is
a form of
jet engine 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 require considerable forward speed to operate well, and as
a class work most efficiently at speeds around
Mach 3. This type of jet can operate up to
speeds of at least
Mach 5.
Ramjets can be particularly useful in applications requiring a
small and simple engine for high speed use; such as
missiles, 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 .. They have also been used successfully,
though not efficiently, as
tip jets on
helicopter 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, 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 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. 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).
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'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-7PVRD 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 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 trisonic ramjet-powered cruise missile,
Burya. This project competed with the
R-7 ICBM being developed by
Sergei
Korolev, and was cancelled in 1957.
Germany
In 1936
Hellmuth Walter constructed
a test engine powered by
natural gas.
Theoretical work was carried out at
BMW and
Junkers as well as the
DFL. In 1941
Eugen
Sänger of the DFL proposed a ramjet engine with a very high
combustion chamber temperature. He constructed very large ramjet
pipes with and 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 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
Leduc 022
Nord Aviation 1500 Griffon II
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 in 1958.
Design
A typical ramjet
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, whereit 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.
A ramjet is sometimes referred to as a 'flying stovepipe', a very
simple device comprising an air intake, a combustor, and a
nozzle. Normally the only moving parts are those
within the
turbopump, 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 to compress the
air further. This gives greater efficiency and far more power at
low speeds, where the ram effect is weak, but is also more complex,
heavier and more expensive, and the temperature limits of the
turbine section limits 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,
which is used to support the combustion and expansion process in
the nozzle.
Most ramjets operate at
supersonic flight
speeds and use one or more
conical
(or oblique)
shock waves, 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 level suitable for the
combustor.
Subsonic intakes on ramjets are
relatively simple
Subsonic ramjets don't 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 combustors 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 holders that stop the flames blowing
out.
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 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, 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 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. For a supersonic flight Mach number,
acceleration is typically achieved via a
convergent-divergent nozzle.
Performance and control
Ramjets have been run from as low as 45 m/s (100 mph) 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 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 sensible 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
rockets.
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 pressurising and
supplying the fuel to the ramcombustor is required which can be
complicated and expensive.
Aerospatiale-Celerg 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, 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, liquid, for example
ASMP, and ducted rocket,
for example
Meteor, 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. 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, and they are
inefficient (less than 600
seconds)
until the airspeed exceeds 1000 km/h (600 mph) due to low
compression ratios. 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 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.
Related engines
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.
Scramjets, 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).
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 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 providing
driving power to the fan at sub-sonic speeds. After mixing with the
air it is burned in the combustion chamber.
Nuclear powered ramjets
During the
Cold War the United States
designed and ground-tested a nuclear-powered ramjet called
Project Pluto. This system used no combustion
- a
nuclear reactor 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 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 interstellar wind and exhaust it
at high speed from the rear of the vehicle.
See also
References
Aircraft using ramjets
Missiles using ramjets
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