this is the nose of a Rafale this is the nose of the Eurofighter typhoon this is the Gripen e
this is the Sukhoi 30mki and this is the mig-35 did you notice anything
i am sure that many among you have recognized the globes on the nose of the planes in the intro they
are infrared search and track sensors the earth is a type of sensor that uses the temperature
of the target to generate a track for the pilot in a way similar to what a radar does
the Americans after having snubbed it for a few decades are coming back to it
the f-35 has one and the f-18 and the f-15 have been given infrared the search and track
pods the fact that these sensors are becoming increasingly integrated in the air combat tactics
is only one of the reason why stealth plays do exist now if you are thinking that stealth is
about raiders you are right but today we are going to talk about infrared stealth
infrared is the electromagnetic radiation emitted by an object in well normal condition the hotter
the object the more radiation it emits it is more useful for detection than visible light because
having a longer wavelength it can penetrate clouds and fog better also at night when light is absent
the bodies still emit infrared radiation the radar looks for reflected energy from the target but the
infrared searching track uses the infrared radiation to create images and extract information
from them to create a track on the pilot's head or on a presentation screen obviously there are
other devices that make use of infrared radiation and probably the most common use is for infrared
homing we have already covered in the past how the earth and the for what looking infrared and
infrared homing do actually work and i suggest to view those videos to better understand them
in fact in this video we are discussing how to make life difficult for all those devices
infrared radiation for practical purposes is usually divided in three bands the near infrared
has a wavelength between 0.7 and 1.5 micrometer it is emitted by the sun it is reflected from
the aircraft external surfaces the medium infrared delays between 1.5 and 6 micrometers
at short wavelength the aircraft reflects the sun radiation but at longer wavelengths above roughly
three micrometers the thermal signature of the hot parts and the exhaust plume
become prevalent all air-to-air missiles have sensors particularly sensitive in the higher part
of the medium infrared the far long infrared refers to wavelengths between
7 and 14 micrometers at these frequencies the main source of radiation is the earth
and the aircraft either directly emits the radiation or reflects the earth illumination
earth's tend to use these frequencies because they are the most effective in penetrating fog
and dust the infrared signature of a plane is the combination of all the emissions and reflection
in all these bands the signature is always present and since the sources are the sun the earth or the
plane itself all infrared sensors can identify a target remaining totally passive radars emit
energy and this energy can in principle be detected by the target but infrared sensor just
receive energy from the target so there is no way for a target itself to understand if it has been
spotted the infrared signature of an aircraft depends heavily from the aspect since the main
emitters of infrared radiation are the engines and the exhaust plume it is easier to detect a plane
from the rear modern earths have ranges comparable to radars if the target is showing the tail but
they are drastically reduced if the target is closing toward the sensor showing just the frontal
surfaces however frontal surfaces are subject to aerodynamic heating a plane traveling around Mach
1.5 has temperatures on the nose and the wing leading edges around 100 degrees Celsius these
hot parts are hot enough to be used for detection all the modern infrared homing missiles can lock
on this this hot areas ok I believe it is clear enough that the plane in flight is quite a good
source of infrared radiation now the problem is how do we hide it let's start from the aerodynamic
heating the easy one one solution is use the fuel as a heat sink to cool the surface down
it is actually also useful to cool the electronics and in particular the jammers that can become very
hot this is an effective solution for the wing leading edge but it may become unpractical for
other hot surfaces like the air intakes cowlings or the radome which is usually the hottest point
the f-35 uses this approach on the wing and the vertical empenages actually the air scoop on the
top right of the fuselage provides fresh air to the fuel air heat exchanger to keep the fuel cool
and to be fair the f-35 has a particular and innovative system to manage the thermal loads but
this is a story for another time however it might be fun to know that on the earlier f35 variants
there was the necessity to open the weapon base every now and then to refresh the inside i think
it has been fixed by now another solution is to use infrared suppressive skin coating
these are materials that have a very low emissivity at the same temperature they emit less
radiation than for example aluminum or the normal aromatic paints unconfirmed news say that the
f-22 and the f-45 coating can reduce the infrared emission in the far infrared of about 50 percent
and these coatings are also reported to change the spectral composition of the emitted infrared they
emit more energy at those wavelengths where the atmosphere absorbs the radiation the most in fact
the atmosphere is not uniformly transparent to infrared like it is in the visible light
more or less there are some ranges of wavelengths that are absorbed by water or co2 if the infrared
is emitted in those windows it is quickly absorbed by the atmosphere just goes away
a major source of infrared emissions are those parts of the fuselage that are heated
by the engines the heating is usually a serious problem not only for the knitted diaphragm but
also for the structural stress created by the high temperature in fact the rear of the fuselage where
the engines are located is the area where the heat resistant materials like titanium are used
more often to reduce the infrared signature the only solution is to cool these parts too modern
jets usually drain some air from the low pressure stage of the compressor to inject it around the
hottest part of the engine more to cope with the structural heating than the stealth to be honest
the f-35 which was designed for stealth but also has a very hot engine while it has two
large underwing scoops to refresh the area around the engine gone are the days when the temperature
inside the jet fighter flying at high altitude went 20 or 30 degrees below zero now engines and
electronics create the opposite problem and the inside of a fighter needs to be cooled down chuck
higger in his autobiography tells how he was able to store a fur coat as a present for his wife in a
small closet on a p80 and how the coat arrived undamaged despite the freezing temperatures
the hottest part of the whole plane is the nozzle and you may imagine it is the part that is very
difficult to cool the f-35 is basically unique in its attempt blowing the cold air that has
previously flowed inside the plane to cool the engine all around the nozzle there is a
small gap between the fuselage panels and the nozzle and it is where the air is blowing from
around the f-22 nozzles there are some small gaps visible but it i was unable honestly to
confirm if a similar system is installed on the plane from the rear aspect also the inner parts
of the engine are visible through the nozzle and they are by far the strongest infrared emitter
since reducing the temperature in the turbine will severely limit the engine performance a
way to reduce the infrared signature is using a cooled blocker the blocker is a plate inserted
in the central part of the exhaust that covers part of the turbine if the plate is cooled the
infrared signature of the turbine is reduced the f-22 and the f-35 both seem to have one
the exhaust plume is a powerful infrared emitter it is a large feature filled with
hot gas that stands out against the sky or the ground colder background and even more so when
afterburners easy news since both temperature and size increase actually one of the reasons why the
super cruise is so important among the others is that it reduces the need to use the after burner
among other things this also means a reduced infrared signature it is actually possible
to do something to reduce the exhaust plume infrared signature the most effective solution
is reducing the exhaust temperature and there are a few ways to do that the most radical is
to reduce the exhaust gas temperature by using a turbofan engine rather than a turbo jack it is
not widely known but modern military jet engines are low bypass ratio turbofans a turbofan is an
engine where only part of the air goes through the inner hot core a fraction of the air goes through
a fan that acts as a very low compressor stage and it accelerates the flow around the engine without
warming it up two flows mix downstream the turbine and the cold flow mixing with the hot flow has
the effect of lowering the temperature at the nozzle reducing the temperature in the
turbine is not advisable because it would greatly reduce the engine performance while a turbo fan
may increase the engine performance the bypass ratio is the ratio between the air that goes
through the fan and the air that goes through the core military combat planes usually have a
low bypass ratio to privilege thrust and high speed performance while civilian or transport
planes engines have a high bypass ratio placing fuel efficiency first this methodology to reduce
the plume temperature has the constraint of the engine design and the cooling of the plume is
almost a side effect the name in the coffin is that the use of the afterburner actually
nullifies the cooling for this reason stealth aircraft used design solutions to accelerate
the mixing of the plume with the surrounding air for example the f-117 and the b2 have thin
slit-like exhausts that produce an exhaust layer rather than a plume which tend to be disrupted and
broken up closer to the nozzle the f-22 exhaust up to a point works in a similar manner the f-35
other than the air blown around the nozzle has eject outer rim and some vents in the nozzle walls
that other than being useful for radar stealth help mixing the plume with the atmospheric gas
another way to reduce the conspicuousness of the plume is to geometrically hide it behind
the tail planes at least from some points of view the paradigm of this approach is the a10
the nozzle and the initial section of the exhaust plume are very well hidden behind the vintage
tail of the aircraft the view from below and from the side is obstructed quite effectively leaving a
limited arc of visibility from behind and from above since the a10 main enemy is expected to
be the shoulder launch surface-to-air missile so it makes sense to block the view from below
other aircraft have a similar approach even if it is less radical in particular f22 f35 and f18
have the nozzle in a position well advanced in respect to the trailing edge of the tail surfaces
this unusual geometry helps concealing the hottest parts of the plane from the side at bait
not as effectively as it is done on the a-10 European modern designs actually seem not to be
overly concerned with this point while Russian designs are somewhat in between the case of
the b2 is particularly interesting because the nozzles and initial section of the plume are on
the wing upper surface effectively screening from the lower hemisphere of the plane and
for a plane that is expected to fly at high altitude relying on stealth well it makes sense
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